FWD Calibration Center and Operational

Improvements: Redevelopment of the

Calibration Protocol and Equipment

Publication No. FHWA-HRT-07-040 october 2011

Research, Development, and Technology

Turner-Fairbank Highway Research Center

6300 Georgetown Pike

McLean, VA 22101-2296

Long-Term Pavement Performance Program

www.fhwa.dot.gov/pavement/ltpp

Transportation Pooled Fund Study TPF 5(039)

www.pooledfund.org
FOREWORD

This report details the critical updates to the Strategic Highway Research Program falling weight 

deflectometer (FWD) calibration procedure. This has led to the revision of the American 

Association of State Highway and Transportation Officials R32-09 calibration procedure.

(1) The 

report describes the procedural improvements and provides information on how to calibrate an 

FWD according to the updated procedure. Appendices provide the updated protocol for FWD 

calibration, drawings and specifications of the hardware needed, an updated specification for the 

pavement deflection data exchange (PDDX) protocol, and a flowchart for the new calibration 

software, WinFWDCal. This report will be of interest to engineers who perform structural 

evaluation of pavements. This final report covers the initial 2-year phase I and the subsequent 

task orders 1 and 2 of the pooled fund study TPF-5(039).

Jorge E. Pagán-Ortiz

Director, Office of Infrastructure 

Research and Development

Notice

This document is disseminated under the sponsorship of the U.S. Department of Transportation 

in the interest of information exchange. The U.S. Government assumes no liability for the use of 

the information contained in this document.

The U.S. Government does not endorse products or manufacturers. Trademarks or 

manufacturers’ names appear in this report only because they are considered essential to the 

objective of the document.

Quality Assurance Statement

The Federal Highway Administration (FHWA) provides high-quality information to serve 

Government, industry, and the public in a manner that promotes public understanding. Standards 

and policies are used to ensure and maximize the quality, objectivity, utility, and integrity of its 

information. FHWA periodically reviews quality issues and adjusts its programs and processes to 

ensure continuous quality improvement.
TECHNICAL REPORT DOCUMENTATION PAGE

1. Report No.

FHWA-HRT-07-040

2. Government Accession No. 3. Recipient’s Catalog No.

4. Title and Subtitle

Falling Weight Deflectometer Calibration Center and 

Operational Improvements: Redevelopment of the 

Calibration Protocol and Equipment

5. Report Date

October 2011

6. Performing Organization Code: 

7. Author(s)

Lynne H. Irwin, David P. Orr, and Daniel Atkins

8. Performing Organization Report No. 

CLRP Report No. 2010-03

9. Performing Organization Name and Address

Cornell University Local Roads Program

416 Riley-Robb Hall

Ithaca, NY 14853

10. Work Unit No.

11. Contract or Grant No.

DTFH61-04-C-00041

12. Sponsoring Agency Name and Address

Office of Infrastructure Research and Development

Federal Highway Administration

6300 Georgetown Pike

McLean, VA 22101-2296

13. Type of Report and Period Covered

Final Report, September 2004–October 2010

14. Sponsoring Agency Code

HRDI-30

15. Supplementary Notes

The Contracting Officer’s Technical Representatives (COTR) are Eric Weaver (2004–2008) and Jane Jiang 

(2008–2011).

16. Abstract

The objective of this project is to upgrade the existing falling weight deflectometer (FWD) calibration system to 

make calibration sustainable for the next decade without a loss of quality while ensuring that any new procedures 

are compatible with all brands of FWDs sold in the United States. This involves upgrading the hardware and 

software used in calibration to take advantage of improvements in technology. The primary result of the FWD 

calibration procedure is to obtain gain factors used to correct FWD load and deflection sensor data to agree with 

the calibration instrumentation. 

Changes incorporated in the new FWD calibration procedure include replacing the reference linear variable 

differential transducer with an accelerometer for deflection sensor calibration, developing a stiff multiple-sensor 

stand to allow calibration of all deflection sensors simultaneously, updating the calibration software to a modern

programming language with the ability to electronically read data from each brand of FWD, and using modern 

data acquisition techniques to eliminate sensitivity problems from the older Strategic Highway Research 

Program FWD calibration protocol. The time to complete an FWD calibration was greatly reduced. This report 

outlines the updated procedures and provides details about the equipment and methods needed to perform the 

updated protocol. 

17. Key Words

Deflection measurement, Falling weight 

deflectometer, FWD, Backcalculation, Calibration, 

PDDX

18. Distribution Statement

No restrictions. This document is available through the 

National Technical Information Service, Springfield, VA 

22161.

19. Security Classif. (of this report)

Unclassified

20. Security Classif. (of this page)

Unclassified

21. No. of Pages

268

22. Price

Form DOT F 1700.7 (8-72) Reproduction of completed page authorized
ii

SI* (MODERN METRIC) CONVERSION FACTORS 

APPROXIMATE CONVERSIONS TO SI UNITS

Symbol When You Know Multiply By To Find Symbol 

LENGTH

in inches 25.4 millimeters mm 

ft feet 0.305 meters m 

yd yards 0.914 meters m 

mi miles 1.61 kilometers km 

AREA 

in2 square inches 645.2 square millimeters mm2

ft2 square feet 0.093 square meters m2

yd2 square yard 0.836 square meters m2

ac acres 0.405 hectares ha

mi2 square miles 2.59 square kilometers km2

VOLUME 

fl oz fluid ounces 29.57 milliliters mL 

gal gallons 3.785 liters L 

ft3 cubic feet 0.028 cubic meters m3 

yd3 cubic yards 0.765 cubic meters m3 

NOTE: volumes greater than 1000 L shall be shown in m3

MASS oz ounces 28.35 grams g

lb pounds 0.454 kilograms kg

T short tons (2000 lb) 0.907 megagrams (or "metric ton") Mg (or "t") 

TEMPERATURE (exact degrees) o

F Fahrenheit 5 (F-32)/9 Celsius o

C 

or (F-32)/1.8 

ILLUMINATION 

fc foot-candles 10.76 lux lx

fl foot-Lamberts 3.426 candela/m2 cd/m2

FORCE and PRESSURE or STRESS 

lbf poundforce 4.45 newtons N 

lbf/in2 poundforce per square inch 6.89 kilopascals kPa 

APPROXIMATE CONVERSIONS FROM SI UNITS 

Symbol When You Know Multiply By To Find Symbol 

LENGTH mm millimeters 0.039 inches in 

m meters 3.28 feet ft 

m meters 1.09 yards yd 

km kilometers 0.621 miles mi 

AREA mm2 square millimeters 0.0016 square inches in2 

m2 square meters 10.764 square feet ft2 

m2 square meters 1.195 square yards yd2 

ha hectares 2.47 acres ac

km2 square kilometers 0.386 square miles mi2 

VOLUME 

mL milliliters 0.034 fluid ounces fl oz 

L liters 0.264 gallons gal 

m3 cubic meters 35.314 cubic feet ft3 

m3 cubic meters 1.307 cubic yards yd3 

MASS 

g grams 0.035 ounces oz

kg kilograms 2.202 pounds lb

Mg (or "t") megagrams (or "metric ton") 1.103 short tons (2000 lb) T 

TEMPERATURE (exact degrees) o

C Celsius 1.8C+32 Fahrenheit oF 

ILLUMINATION 

lx lux 0.0929 foot-candles fc 

cd/m2 candela/m2 0.2919 foot-Lamberts fl

FORCE and PRESSURE or STRESS 

N newtons 0.225 poundforce lbf 

kPa kilopascals 0.145 poundforce per square inch lbf/in2

*SI is the symbol for the International System of Units. Appropriate rounding should be made to comply with Section 4 of ASTM E380. 

(Revised March 2003)
iii

TABLE OF CONTENTS

CHAPTER 1. INTRODUCTION................................................................................................ 1

SOURCES OF ERRORS IN FALLING WEIGHT DEFLECTOMETER 

MEASUREMENTS ................................................................................................................ 1

Seating Errors...................................................................................................................... 1

Random Errors.................................................................................................................... 2

Bias Errors .......................................................................................................................... 2

Significance of Errors......................................................................................................... 3

DEVELOPMENT OF THE FIRST SHRP CALIBRATION PROTOCOL..................... 3

REDEVELOPMENT OF THE PROTOCOL...................................................................... 4

Pooled Fund Study Tasks.................................................................................................... 4

ADDITIONAL ACCOMPLISHMENTS OF THE POOLED FUND STUDY ................. 5

Quality Assurance Procedures............................................................................................ 6

Adoption of the Calibration Procedures by AASHTO ....................................................... 6

Revision of the PDDX Standard......................................................................................... 6

Development of an Informational Video ............................................................................ 7

Long-Term Calibration Center Support Stewardship......................................................... 7

CHAPTER 2. OVERVIEW OF THE NEW FWD CALIBRATION PROCEDURE ............ 9

WINFWDCAL.......................................................................................................................... 9

CALIBRATION OF THE REFERENCE DEVICES ....................................................... 11

SIGNAL CONDITIONER AND DATA ACQUISITION SYSTEM ............................... 11

FWD CALIBRATION.......................................................................................................... 12

Step 1: Setup ..................................................................................................................... 12

Step 2: FWD Deflection Sensor Calibration..................................................................... 12

Step 3: FWD Load Cell Calibration.................................................................................. 13

Step 4: Documentation...................................................................................................... 13

RELATIVE CALIBRATION OF THE DEFLECTION SENSORS ............................... 13

Monthly Calibration of the Deflection Sensors................................................................ 13

SETTING UP NEW FWD CALIBRATION CENTERS.................................................. 14

UPGRADING THE EXISTING SHRP CALIBRATION CENTERS............................. 15

TRAINING AND OPERATOR CERTIFICATION......................................................... 16

CHAPTER 3. DEVELOPMENT OF THE NEW PROTOCOL............................................ 17

FWD SPECIFICATIONS AND CALIBRATION ISSUES.............................................. 17

Dynatest® .......................................................................................................................... 18

JILS................................................................................................................................... 18

Carl Bro............................................................................................................................. 20

KUAB ............................................................................................................................... 20

Issues Encountered with the SHRP FWD Calibration Protocol....................................... 21

EQUIPMENT AND PROCEDURAL UPDATES ............................................................. 23

Data Acquisition ............................................................................................................... 23

Drop Sequence .................................................................................................................. 26

Recommended Peak Deflection........................................................................................ 27

Choosing and Using an Accelerometer in FWD Calibration............................................ 27

Hysteresis of Silicon Designs Accelerometer................................................................... 30
iv

Temperature and Electromagnetic Interference Sensitivity.............................................. 32

Double Integration of Accelerometer Signal .................................................................... 33

Integration Principles........................................................................................................ 37

Implementation ................................................................................................................. 39

Multiple Sensor Stand....................................................................................................... 41

Reference Load Cell ......................................................................................................... 42

Software Redevelopment.................................................................................................. 43

COMPARATIVE ACCURACY OF THE OLD AND NEW PROCEDURES ............... 45

Influence on Deflection Sensor Gain Factors................................................................... 45

Influence on Load Cell Gain Factors................................................................................ 46

Significance of the Differences......................................................................................... 48

PRECISION AND BIAS STATEMENTS FOR FWD CALIBRATION......................... 48

Load Cell........................................................................................................................... 49

Precision and Bias Statement for Dynatest® Load Calibration......................................... 49

Geophone Calibration ....................................................................................................... 49

Precision and Bias Statement for Dynatest® Geophone Calibration ................................ 49

QA PROCEDURES AND OPERATOR CERTIFICATION........................................... 49

CHAPTER 4. CONCLUSIONS AND RECOMMENDATIONS ........................................... 51

CONCLUSIONS ................................................................................................................... 51

RECOMMENDATIONS...................................................................................................... 51

APPENDIX A. FWD CALIBRATION PROTOCOL............................................................. 55

INTRODUCTION................................................................................................................. 55

Frequency of Calibration .................................................................................................. 56

WinFWDCal...................................................................................................................... 56

ANNUAL CALIBRATION PROCEDURE ....................................................................... 56

Personnel........................................................................................................................... 56

Equipment Preparation and Setup Before Calibration...................................................... 57

FWD Calibration Procedure ............................................................................................. 60

MONTHLY CALIBRATION PROCEDURE ................................................................... 65

Equipment Preparation...................................................................................................... 66

Procedure Overview.......................................................................................................... 66

Monthly Calibration of the Deflection Sensors................................................................ 66

ANNEX 1. PRE-CALIBRATION CHECKLIST .................................................................... 70

ANNEX 2. SPECIAL PROCEDURES FOR CALIBRATING A DYNATEST® FWD ....... 71

ANNEX 3. SPECIAL PROCEDURES FOR CALIBRATING A JILS FWD....................... 73

ANNEX 4. SPECIAL PROCEDURES FOR CALIBRATING A KUAB FWD.................... 75

ANNEX 5. SPECIAL PROCEDURES FOR CALIBRATING A CARL BRO FWD.......... 78

ANNEX 6. SPECIAL PROCEDURES FOR ON-SITE CALIBRATIONS........................... 80

ANNEX 7. REFERENCE LOAD CELL CALIBRATION PROCEDURE .......................... 83

INTRODUCTION................................................................................................................. 83

FREQUENCY OF CALIBRATION................................................................................... 83

EQUIPMENT........................................................................................................................ 83

Equipment Calibration...................................................................................................... 84

Equipment Preparation...................................................................................................... 84

Calibration Procedure ....................................................................................................... 86
v

Data Analysis.................................................................................................................... 87

Entering the Coefficients in WinFWDCal ........................................................................ 88

APPENDIX B. SUGGESTIONS FOR SUCCESSFUL ANNUAL CALIBRATIONS......... 89

APPENDIX C. PDDX STANDARD VERSION 2.0 ................................................................ 93

INTRODUCTION................................................................................................................. 93

PDDX VERSION 2.0 OVERVIEW..................................................................................... 93

Description of Each Data Section and Data Entry Lines.................................................. 94

STANDARD PDDX FILE FORMAT................................................................................. 94

Pavement Deflection Data Exchange File Section ........................................................... 94

Operations Information Section........................................................................................ 95

Units Section..................................................................................................................... 96

Device Information Section .............................................................................................. 98

Device Configuration Section........................................................................................... 99

Device Calibration Section ............................................................................................. 101

Location Identification Section....................................................................................... 104

Data Configuration Section............................................................................................. 105

Test Location Section ..................................................................................................... 106

SPECIAL FORMAT FOR FWD CALIBRATION RECORD FILES .......................... 108

FWD Calibration Center Information Section .............................................................. 109

SPECIAL FORMAT FOR FWD CALIBRATION DATA EXCHANGE FILES........ 115

Input Data Requirements for WinFWDCal..................................................................... 115

Output Data from WinFWDCal ...................................................................................... 116

APPENDIX D. QA REVIEWS................................................................................................ 119

REVIEW PROCESS .......................................................................................................... 120

Facility Review ............................................................................................................... 120

Operator Review ............................................................................................................. 120

APPENDIX E. SPECIFICATIONS AND DRAWINGS ....................................................... 133

SPECIFICATIONS............................................................................................................. 133

COMPLETE DRAWING LIST ........................................................................................ 137

ACCELEROMETER BOX ............................................................................................... 139

BALL-JOINT ANCHOR ................................................................................................... 147

GEOPHONE CALIBRATION STAND ........................................................................... 154

SEISMOMETER CALIBRATION STAND.................................................................... 164

GEOPHONE ADAPTERS................................................................................................. 177

REFERENCE LOAD CELL ............................................................................................. 183

DATA ACQUISITION SYSTEM ..................................................................................... 195

APPENDIX F. HARDWARE USE AND INSTALLATION GUIDE.................................. 207

OVERVIEW/SCOPE ......................................................................................................... 207

FACILITIES ....................................................................................................................... 208

TEST PAD........................................................................................................................... 208

REFERENCE LOAD CELL ............................................................................................. 209

Parts and Tools................................................................................................................ 209

Calibration....................................................................................................................... 210

Use .................................................................................................................................. 211
vi

CONCRETE ANCHOR INSTALLATION ..................................................................... 212

Parts and Tools................................................................................................................ 212

Installation....................................................................................................................... 213

BALL-JOINT ANCHOR ................................................................................................... 216

Parts and Tools................................................................................................................ 216

Assembly......................................................................................................................... 218

ACCELEROMETER BOX ............................................................................................... 220

Parts and Tools................................................................................................................ 220

Assembly......................................................................................................................... 221

Calibration Platter Assembly .......................................................................................... 224

Accelerometer Storage.................................................................................................... 224

Accelerometer Calibration.............................................................................................. 224

CALIBRATION STANDS................................................................................................. 225

Parts and Tools................................................................................................................ 225

Assembly......................................................................................................................... 226

Location of Accelerometer Box...................................................................................... 227

Deflection Sensors.......................................................................................................... 227

Using the Stand............................................................................................................... 231

DATA ACQUISITION SYSTEM ..................................................................................... 233

Parts and Tools................................................................................................................ 233

Cable Assembly .............................................................................................................. 233

Connection Breakdown................................................................................................... 233

Vishay 2310 Signal Conditioner Settings....................................................................... 234

COMPUTER ....................................................................................................................... 235

APPENDIX G. SOFTWARE FLOW CHARTS.................................................................... 237

ACKNOWLEDGEMENTS ..................................................................................................... 251

REFERENCES.......................................................................................................................... 253
vii

LIST OF FIGURES

Figure 1. Illustration. Random and systematic (bias) errors........................................................... 1

Figure 2. Screen shot. WinFWDCal software program FWD calibration screen......................... 10

Figure 3. Screen shot WinFWDCal software program collect drop data screen.......................... 10

Figure 4. Photo. Calibration system hardware set ........................................................................ 14

Figure 5. Map. Calibration center locations in the United States................................................. 16

Figure 6. Photo. Dynatest® model 8000 trailer-mounted FWD.................................................... 18

Figure 7. Photo. JILS-20T truck-mounted FWD.......................................................................... 20

Figure 8. Photo. Carl Bro FWD.................................................................................................... 20

Figure 9. Photo. KUAB FWD with sensors aligned toward the tow vehicle ............................... 21

Figure 10. Photo. FWD calibration beam and inertial block ........................................................ 22

Figure 11. Photo. Box for DAQ wiring protection ....................................................................... 24

Figure 12. Photo. Silicon Designs model 2220 accelerometer..................................................... 28

Figure 13. Photo. Accelerometer box on calibration platter......................................................... 28

Figure 14. Equation. Nonlinear conversion of voltage to acceleration......................................... 29

Figure 15. Graph. Hysteresis effect in a -1 g field........................................................................ 31

Figure 16. Graph. Hysteresis effect in a +1 g field....................................................................... 32

Figure 17. Graph. Scale factor temperature shift for accelerometer............................................. 33

Figure 18. Graph. Raw accelerometer output from a Dynatest® FWD impulse........................... 34

Figure 19. Graph. First integration (velocity) from a Dynatest® FWD impulse........................... 34

Figure 20. Graph. Second integration (deflection) from a Dynatest® FWD impulse................... 35

Figure 21. Graph. Raw accelerometer output from a JILS FWD impulse ................................... 35

Figure 22. Graph. First integration (velocity) from a JILS FWD impulse ................................... 36

Figure 23. Graph. Second integration (deflection) from a JILS FWD impulse............................ 36

Figure 24. Equation. Acceleration due to gravity and the FWD load pulse ................................. 37

Figure 25. Equation. Relative acceleration after removing Earth’s gravity ................................. 37

Figure 26. Equation. Velocity as a function of time..................................................................... 38

Figure 27. Equation. Deflection as a function of time.................................................................. 38

Figure 28. Equation. Velocity with initial vibration..................................................................... 38

Figure 29. Graph. Comparison of sensor stand performance ....................................................... 42

Figure 30. Graph. Deflection sensor final gain values produced with the old and new 

protocols........................................................................................................................................ 46

Figure 31. Graph. Individual value plot of load cell final gain values from the old and 

new protocols................................................................................................................................ 47

Figure 32. Graph. Load cell calibration history from 1994 to 2006............................................. 47

Figure 33. Photo. Attachment of a Dynatest® geophone in the stand........................................... 72

Figure 34. Photo. Attachment of a JILS geophone in the stand ................................................... 74

Figure 35. Photo. KUAB geophone parts..................................................................................... 76

Figure 36. Photo. Attachment of a KUAB geophone in the stand................................................ 77

Figure 37. Photo. Attachment of a Carl Bro geophone in the stand ............................................. 79

Figure 38. Photo. On-site FWD calibration at Hawaii Department of Transportation 

Materials Lab ................................................................................................................................ 81

Figure 39. Photo. Locating a place to install the ball-joint floor anchor ...................................... 82

Figure 40. Equation. Load cell calibration algorithm................................................................... 87

Figure 41. Illustration. Typical load cell and deflection sensor configuration ............................. 99
viii

Figure 42. Illustration. Example of a typical test location.......................................................... 106

Figure 43. Illustration. Pre-QA review questionnaire................................................................. 122

Figure 44. Illustration. Calibration operator QA review form.................................................... 123

Figure 45. Illustration. Center facilities QA review form........................................................... 128

Figure 46. Illustration. Center operator certificate of compliance.............................................. 131

Figure 47. Photo. Accelerometer box ......................................................................................... 139

Figure 48. Illustration. CLRP-AB01 accelerometer box assembly ............................................ 142

Figure 49. Illustration. CLRP-AB02 accelerometer box bottom................................................ 143

Figure 50. Illustration. CLRP-AB03 accelerometer box top ...................................................... 144

Figure 51. Illustration. CLRP-AB04 calibration platter ............................................................. 145

Figure 52. Illustration. CLRP-AB05 accelerometer wiring........................................................ 146

Figure 53. Photo. Ball-joint anchor............................................................................................. 147

Figure 54. Illustration. CLRP-BJ01 ball-joint anchor assembly ................................................ 149

Figure 55. Illustration. CLRP-BJ02 clamp ................................................................................. 150

Figure 56. Illustration. CLRP-BJ03 clamp base ......................................................................... 151

Figure 57. Illustration. CLRP-BJ04 base bar.............................................................................. 152

Figure 58. Illustration. CLRP-BJ05 rest stop.............................................................................. 153

Figure 59. Photo. Geophone calibration stand............................................................................ 154

Figure 60. Illustration. CLRP-GCS01 geophone stand assembly .............................................. 156

Figure 61. Illustration. CLRP-GCS02 geophone stand side rail................................................. 157

Figure 62. Illustration. CLRP-GCS03 geophone top shelf......................................................... 158

Figure 63. Illustration. CLRP-GCS04 geophone stand bottom shelf ......................................... 159

Figure 64. Illustration. CLRP-GCS05 geophone stand sensor shelf .......................................... 160

Figure 65. Illustration. CLRP-GCS06 geophone stand accelerometer shelf .............................. 161

Figure 66. Illustration. CLRP-GCS07 geophone stand handle holder........................................ 162

Figure 67. Illustration. CLRP-GCS08 geophone stand connector pin........................................ 163

Figure 68. Photo. KUAB seismometer calibration stand............................................................ 164

Figure 69. Illustration. CLRP-SCS01 seismometer stand assembly .......................................... 166

Figure 70. Illustration. CLRP-SCS02 seismometer stand side rail............................................. 167

Figure 71. Illustration. CLRP-SCS03 seismometer stand top shelf ........................................... 168

Figure 72. Illustration. CLRP-SCS04 seismometer stand sensor shelf ...................................... 169

Figure 73. Illustration. CLRP-SCS05 seismometer stand standoff ............................................ 170

Figure 74. Illustration. CLRP-SCS06 seismometer stand bottom shelf ..................................... 171

Figure 75. Illustration. CLRP-SCS07 seismometer stand handle holder.................................... 172

Figure 76. Illustration. CLRP-SCS08 seismometer stand connector pin.................................... 173

Figure 77. Illustration. CLRP-SCS09 seismometer stand shelf subassembly ............................ 174

Figure 78. Illustration. CLRP-SCS10 seismometer stand accelerometer shelf .......................... 175

Figure 79. Illustration. CLRP-SCS11 seismometer stand accelerometer shelf 

subassembly ................................................................................................................................ 176

Figure 80. Illustration. CLRP-GA01 geophone adapter assemblies........................................... 179

Figure 81. Illustration. CLRP-GA02 Carl Bro adapter............................................................... 180

Figure 82. Illustration. CLRP-GA03 JILS adapter..................................................................... 181

Figure 83. Illustration. CLRP-GA04 KUAB adapter ................................................................. 182

Figure 84. Illustration. CLRP-LC01 load cell assembly ............................................................ 185

Figure 85. Illustration. CLRP-LC02 load cell body ................................................................... 186

Figure 86. Illustration. CLRP-LC03 base plate .......................................................................... 187
ix

Figure 87. Illustration. CLRP-LC04 measuring link .................................................................. 188

Figure 88. Illustration. CLRP-LC05 cover plate ........................................................................ 189

Figure 89. Illustration. CLRP-LC06 foot.................................................................................... 190

Figure 90. Illustration. CLRP-LC07 guide fingers..................................................................... 191

Figure 91. Illustration. CLRP-LC08 interior connector wiring .................................................. 192

Figure 92. Illustration. CLRP-LC09 strain gauge layout............................................................ 193

Figure 93. Illustration. CLRP-LC10 bridge wiring .................................................................... 194

Figure 94. Illustration. CLRP-DAQ01 Vishay 2310 to DAQ cable ........................................... 198

Figure 95. Illustration. CLRP-DAQ01A Vishay 2310B BNC to DAQ cable ............................ 199

Figure 96. Illustration. CLRP-DAQ01B Vishay 2310B BNC to DAQ cable (stock parts)........ 200

Figure 97. Illustration. CLRP-DAQ02 Vishay to load cell cable ............................................... 201

Figure 98. Illustration. CLRP-DAQ03 accelerometer signal cable ............................................ 202

Figure 99. Illustration. CLRP-DAQ04 pushbutton to KUSB DAQ cable and cable wiring ...... 203

Figure 100. Illustration. CLRP-DAQ06 KUSB protection wiring ............................................. 204

Figure 101. Illustration. CLRP-DAQ07 KUSB enclosure rear panel......................................... 205

Figure 102. Illustration. CLRP-DAQ08 KUSB enclosure front panel ....................................... 206

Figure 103. Photo. Calibration system components ................................................................... 208

Figure 104. Photo. Reference load cell assembly....................................................................... 210

Figure 105. Photo. Bottom view of the reference load cell ........................................................ 211

Figure 106. Photo. Reference load cell positioned under the FWD load plate........................... 212

Figure 107. Photo. Concrete anchor ........................................................................................... 213

Figure 108. Photo. Concrete anchor installation step 3 .............................................................. 214

Figure 109. Photo. Concrete anchor installation step 6 .............................................................. 215

Figure 110. Photo. Concrete anchor installation step 7 .............................................................. 215

Figure 111. Photo. Parts and tools for ball-joint assembly......................................................... 217

Figure 112. Photo. Proper alignment of ball-joint screws.......................................................... 218

Figure 113. Photo. Assembly of the ball-joint anchor................................................................ 219

Figure 114. Photo. Parts for accelerometer box assembly.......................................................... 222

Figure 115. Photo. Accelerometer box assembly ....................................................................... 223

Figure 116. Photo. Accelerometer box attached to calibration platter ....................................... 224

Figure 117. Photo. Equipment for calibration stand assembly ................................................... 226

Figure 118. Photo. Calibration stand with handles, bubble level, and pushbutton..................... 227

Figure 119. Photo. Attachment of a JILS or Carl Bro geophone to the stand ............................ 228

Figure 120. Photo. Placement of the accelerometer box on the geophone stand........................ 229

Figure 121. Photo. Attachment of a Dynatest® geophone to the stand....................................... 229

Figure 122. Photo. KUAB geophone adapter equipment ........................................................... 230

Figure 123. Photo. Attachment of a KUAB geophone adapter .................................................. 230

Figure 124. Photo. Seismometer stand with sensors attached .................................................... 231

Figure 125. Photo. Coupling the calibration stand and ball-joint anchor................................... 232

Figure 126. Illustration. FWD data acquisition system components and connections............... 234

Figure 127. Illustration. WinFWDCal startup flowchart............................................................ 237

Figure 128. Illustration. WinFWDCal start new calibration flowchart ...................................... 238

Figure 129. Illustration. WinFWDCal open and delete interim calibrations flowchart.............. 239

Figure 130. Illustration. WinFWDCal main menu flowchart.................................................... 240

Figure 131. Illustration. WinFWDCal calibrate accelerometer flowchart.................................. 241

Figure 132. Illustration. WinFWDCal edit setup data flowchart................................................ 242
x

Figure 133. Illustration. WinFWDCal determine trigger and number of drops flowchart......... 243

Figure 134. Illustration. WinFWDCal calibrate FWD load cell flowchart................................. 244

Figure 135. Illustration. WinFWDCal calibrate FWD sensors flowchart .................................. 245

Figure 136. Illustration. WinFWDCal center documentation flowchart .................................... 246

Figure 137. Illustration. WinFWDCal monthly calibration flowchart ....................................... 247

Figure 138. Illustration. WinFWDCal search for file and convert to PDDX input flowchart.... 248

Figure 139. Illustration. WinFWDCal review time history by clicking on drop cell 

flowchart ..................................................................................................................................... 249

Figure 140. Illustration. Quit WinFWDCal flowchart................................................................ 250
xi

LIST OF TABLES

Table 1. Hardware cost estimate for a new FWD calibration center............................................ 15

Table 2. FWD manufacturer specifications.................................................................................. 17

Table 3. Suggested dynamic load levels for reference calibration ............................................... 58

Table 4. FWD calibration center equipment................................................................................. 59

Table 5. FWD calibration data reporting requirements................................................................ 61

Table 6. Monthly calibration sensor positions by set for a nine-sensor FWD.............................. 67

Table 7. Monthly calibration sensor positions by set for a seven-sensor FWD ........................... 68

Table 8. Geophone positions in single-column stand (nine sensors)............................................ 91

Table 9. KUAB seismometer positions in double column stand (seven sensors) ........................ 91

Table 10. Unit systems for English and metric PDDX files......................................................... 96

Table 11. Roles and responsibilities during QA review ............................................................. 119

Table 12. Calibration operator compliance levels for QA review.............................................. 121

Table 13. Parts required for one hardware set ............................................................................ 133

Table 14. Complete drawing list................................................................................................. 138

Table 15. Fabricated parts required for BM-AB accelerometer box assembly .......................... 140

Table 16. Hardware items required for BM-AB accelerometer box assembly........................... 140

Table 17. Vendor contact information for BM-AB accelerometer box assembly...................... 141

Table 18. Hardware costs for BM-AB accelerometer box ......................................................... 141

Table 19. Fabricated parts required for the BM-BJ ball-joint anchor assembly......................... 147

Table 20. Hardware items required for the BM-BJ ball-joint anchor assembly......................... 148

Table 21. Vendor contact information........................................................................................ 148

Table 22. Hardware costs for BM-BJ ball-joint anchor.............................................................. 148

Table 23. Fabricated parts required for BM-GCS geophone calibration stand assembly .......... 154

Table 24. Hardware items required for BM-GCS geophone calibration stand assembly........... 155

Table 25. Vendor contact information for BM-GCS geophone calibration stand...................... 155

Table 26. Hardware costs for BM-GCS geophone calibration stand.......................................... 155

Table 27. Fabricated parts required for BM-SCS seismometer calibration stand assembly ...... 164

Table 28. Hardware items required for BM-SCS seismometer calibration stand assembly....... 165

Table 29. Vendor contact information for BM-SCS seismometer calibration stand 

assembly...................................................................................................................................... 165

Table 30. Hardware costs for BM-SCS seismometer calibration stand assembly...................... 165

Table 31. Fabricated parts for use as BM-GA geophone adapters............................................. 177

Table 32. Hardware items required for use of BM-GA geophone adapters............................... 177

Table 33. Vendor contadct information for BM-GA geophone adapters................................... 178

Table 34. Hardware costs for BM-GA geophone adapters......................................................... 178

Table 35. Fabricated parts for the BM-LC reference load cell................................................... 183

Table 36. Equipment and hardware items required for the BM-LC reference load cell ............ 183

Table 37. Vendor contact information for the BM-LC reference load cell ................................ 184

Table 38. Cable diagrams required for BM-DAQ ...................................................................... 195

Table 39. Equipment and hardware items required for BM-DAQ ............................................. 196

Table 40. Vendor contact information for BM-DAQ ................................................................. 197

Table 41. Parts and tools for load cell calibration ...................................................................... 209

Table 42. Parts and equipment for concrete anchor install......................................................... 212

Table 43. Tools and equipment for ball-joint assembly ............................................................. 216
xii

Table 44. Parts and tools for accelerometer box assembly......................................................... 220

Table 45. Equipment for calibration stand assembly.................................................................. 225

Table 46. Parts and equipment for data acquisition.................................................................... 233

Table 47. Settings for the Vishay 2310 signal conditioner......................................................... 234

Table 48. Minimum requirements for computer hardware......................................................... 235
xiii

LIST OF ABBREVIATIONS

AASHTO American Association of State Highway and Transportation Officials

AMRL AASHTO Materials Reference Laboratory

ANOVA Analysis of variance

ASCII American Standard Code for Information Interchange

ASTM American Society for Testing and Materials

Caltrans California Department of Transportation

CDOT Colorado Department of Transportation

CLRP Cornell Local Roads Program

DAQ Data acquisition board

EMI Electromagnetic interference

FHWA Federal Highway Administration

FWD Falling weight deflectometer

HWD Heavy weight deflectometer

INDOT Indiana Department of Transportation

ISA Industry standard(s) architecture

LTPP Long-term pavement performance

LVDT Linear variable differential transducer

MDT Montana Department of Transportation

MEMS Microelectromechanical systems

micron (μm) One millionth of a meter or 0.0039 mil

mil One thousandth of an inch or 25.4 microns

Mn/DOT Minnesota Department of Transportation

NIST National Institute of Standards and Technology

PCI Peripheral component interconnect

PDDX Pavement deflection data exchange

PennDOT Pennsylvania Department of Transportation

QC/QA Quality control/quality assurance

SHRP Strategic Highway Research Program

TxDOT Texas Department of Transportation

USB Universal serial bus

1

CHAPTER 1. INTRODUCTION

SOURCES OF ERRORS IN FALLING WEIGHT DEFLECTOMETER 

MEASUREMENTS

Deflection data from falling weight deflectometers (FWDs) are used for structural evaluation of 

pavements and can be used to calculate remaining pavement life, load-supporting capacity, and 

the required thickness of structural overlays. The results of these calculations can be used to 

manage pavement systems at the project or network level.

Various mechanistic models of pavements are used in analysis including the elastic layer theory, 

the finite element theory, the discrete element theory, etc. These analytical methods utilize the 

moduli of elasticity of the pavement layers as an input. Pavement deflections, combined with 

backcalculation methods, can provide the layer moduli. However, the accuracy of the deflections 

has a major influence on the accuracy of the backcalculated moduli and subsequent analyses.(2)

FWD data have the following sources of error:

• Seating errors.

• Random errors.

• Bias or systematic errors.

Repeated deflection measurements plot as a bell-shaped frequency distribution as shown in

figure 1. 

Figure 1. Illustration. Random and systematic (bias) errors.

Seating Errors

When an FWD first occupies a new test point, seating errors arise from the mechanical 

placement of the sensors (i.e., geophones or seismometers) on the pavement. Because of 
Redevelopment of the FWD Calibration Protocol and Equipment

2

roughness and loose debris on the road surface, the sensors are not always seated firmly. 

Deflection sensors can be seated fairly easily by dropping the FWD mass two times without 

recording data. It is seldom necessary to perform more than two drops to achieve good seating. 

However, extensively cracked pavements, such as alligator cracking, may pose a seating problem 

regardless of the number of seating drops. 

In addition to the deflection sensors, the FWD load plate must also be seated squarely on 

the pavement. For most types of FWDs, all that is needed is to keep the load plate swivel 

well lubricated.

Random Errors

Random errors are mainly attributable to the digitization of the analog (voltage) signal from the 

deflection sensors. Analog-to-digital devices convert the voltage so it can be read and processed 

by a computer. Using an operating system (also known as the field program) provided by the 

FWD manufacturer, the digitized signal is converted to displacement in engineering units (mils, 

microns, and other units). Digital conversions include a small amount of error, typically one or 

two least significant bits. Equipment manufacturers state that the random error is on the order of 

0.04–0.08 mil (1–2 μm) per reading. The magnitude of the error varies from one drop to the next 

independent of the peak amplitude.

Thus, each deflection reading includes a small component of random error. Because they are 

random, some of the errors are positive and some are negative. For any drop, it is impossible to 

know the magnitude of the random error or whether it is positive or negative. However, by 

averaging the results of several deflection peaks taken at essentially the same load level on the 

same test point, the random error of the average is reduced (but not totally eliminated).

To avoid changing the properties of the pavement by performing too many drops on a single 

point, typically no more than three to five drops per load level are taken and averaged. If the 

random error in one measurement is X, the random error of the average of n drops is X/n

0.5. 

Specifically, the random error of the average is equal to the random error of one observation 

divided by the square root of the number of observations. For example, by performing and 

averaging four drops, the random error can be reduced by half (since 4 = 2 ).

The random error exhibited by a set of deflection sensors is quantified by the calibration 

procedure outlined in this report.

Bias Errors

Bias errors (also known as systematic errors) arise from the need to calibrate the sensors and are 

directly proportional to readings. For example, if the deflection doubles, the bias error in the 

reading will be approximately twice as large in magnitude. Bias errors can occur for any number 

of reasons; however, regardless of the reason, FWD manufacturers generally state that they will 

comprise no more than ±2 percent of the total reading. Therefore, bias errors could be zero, 

slightly positive, or slightly negative. Unlike random errors, they will be constant (as a 

percentage of the reading) for a given sensor from one drop to the next.
Chapter 1. Introduction

3

The main reason for performing FWD calibration is to reduce the bias error of each load and 

deflection sensor using the procedure outlined in this report. The procedure uses a reference 

sensor (i.e., a reference load cell for load calibration and a reference accelerometer for deflection 

calibration) that is independently calibrated to a high degree of accuracy and reliability. The peak 

result from the FWD is compared with the peak recorded by the reference sensor. The ratio 

(reference device divided by the FWD reading) becomes the calibration factor (also referred to as 

the gain factor).

Deflections must be fairly large to be able to detect bias error. In figure 1, if the measured 

deflection is close to the true deflection, the random error will mask the bias error. For example, 

if measured deflection is 2 mil (50 μm) and the bias error is 2 percent, the bias error is only 

0.04 mil (1 μm). With a random error of 0.08 mil (2 μm), the bias error is so small that it is 

nearly impossible to detect using statistical methods. 

Similarly, there are limits to how much bias error can be reduced, even with a large deflection. 

In the calibration procedure, there is typically an average deflection of about 20 mil (500 μm). 

When the bias error is reduced to 0.3 percent, the difference between the measured deflection 

and the true deflection is only about 0.06 mil (1.5 μm). Again, in this range, bias error is difficult 

to detect reliably, so 0.3 percent is effectively the practical lower limit that FWD calibration 

can achieve. 

Significance of Errors

In 1988, Irwin et al. reported on the effect of random deflection errors on backcalculated layer 

moduli.(2) They found that random error, without any additional bias error, could cause the 

backcalculated modulus of the surface layer to be off by a factor of two or more. The effect on 

base and subgrade layer moduli was less dramatic.

When a deflection is greater than 4 mil (100 μm), a 2 percent bias error in an uncalibrated FWD 

will be larger than the ±0.08-mil (±2-μm) random error. Thus, the combined effect of random 

and bias errors could be substantially more detrimental to the backcalculated moduli than 

random error alone. 

DEVELOPMENT OF THE FIRST SHRP CALIBRATION PROTOCOL

In 1988, the dramatic effect of FWD errors on backcalculated moduli led the Strategic Highway 

Research Program (SHRP) to begin developing an FWD calibration procedure with the goal of 

reducing the bias error to as close to 0.3 percent as possible. This goal was achieved by the 

SHRP/LTPP FWD Calibration Protocol.

(3) Reducing the bias error to 0.3 percent ensures that a 

0.08-mil (2-μm) random error is larger than the bias error for deflections up to 26 mil (667 μm). 

Deflections measured on most good-quality roads are seldom that large, especially for sensors at 

or beyond 12 inches (300 mm) from the load plate. For a calibrated FWD, random error, which is 

not preventable, controls the accuracy of backcalculated layer moduli in most circumstances.

Four regional FWD calibration centers were established by SHRP in 1992 and were operated by 

the State transportation departments in Pennsylvania (PennDOT), Texas (TxDOT), Minnesota

(Mn/DOT), and Nevada. The western center in Nevada was later moved to Colorado. Since 
Redevelopment of the FWD Calibration Protocol and Equipment

4

their inception, the demand for FWD calibration services has risen steadily.

(4) In August 2001, 

the American Association of State Highway and Transportation Officials (AASHTO) 

Subcommittee on Materials adopted a resolution supporting the continued operation of the 

calibration centers.(5)

Because of changes in technology, the original calibration hardware and software became 

obsolete. There was a particular need to update the FWDREFCL software.

(6) It was originally 

written as a disk operating system (DOS) program and utilized a Metrabyte DAS-16G data 

acquisition board (DAQ). DOS has been replaced by Microsoft Windows, and the industry 

standard architecture (ISA) bus used by the Metrabyte board has been phased out by the 

computer industry.

Orr and Wallace reported that the 1992 SHRP calibration procedure had several shortcomings.

(4)

Among them were the following: 

• The procedure was not well adapted to all types of FWDs.

• The procedure was a slow process, requiring nearly a full day to complete.

• Movement of the inertial block holding the linear variable differential transducer (LVDT) 

was sometimes a problem.

• Manual data entry was time consuming and prone to typing errors.

The procedure was not field portable; hence, the FWDs had to go to a calibration center, and the 

need for out-of-State travel posed a problem for some agencies.

REDEVELOPMENT OF THE PROTOCOL

Pooled Fund Study Tasks

In 2002, the Federal Highway Administration (FHWA) initiated a pooled fund study, 

TPF-5(039), with the financial support of 17 State transportation departments (see the 

acknowledgements section) to overcome the problems noted above. Overall, the primary goal of 

the study was to modernize and streamline the calibration procedure compared to the earlier 

SHRP approach without reducing the accuracy and precision of the results.

To meet this goal, the various FWD manufacturers and FWD calibration centers were contacted 

for suggestions and ideas. Based on their suggestions as well as those from annual meetings of 

the FWD User Group, a series of tasks was developed. The tasks accomplished during the pooled 

fund study were as follows: 

• A universal calibration protocol was developed. Equipment and software were redesigned 

to work with all four brands of FWDs currently sold in the North American market.

• The calibration process was expedited. The time to perform a calibration was reduced to 

less than 3 h.
Chapter 1. Introduction

5

• New hardware and procedures were developed that sped up the calibration process and 

overcame the beam movement issue. The old procedure used a linear variable differential 

transducer (LVDT) as the reference deflection sensor; however, at certain times of the 

year, movement of the inertial system holding the LVDT was a problem. To correct this 

problem, the LVDT was replaced with an accelerometer whose signal was double 

integrated to obtain deflection.

• A Microsoft Windows-based software program was developed to perform all activities 

of FWD calibration. The entire FWD calibration procedure is now controlled by 

WinFWDCal. This software provides a dual-unit option in metric and U.S. customary 

units. Data checks for quality control (QC) are built into the software. Electronic data 

transfer speeds up the calibration process. WinFWDCal also provides a mechanism for 

storing data for historical review. Software updates have been issued quarterly since the 

original release in November 2006. Version 2 of the software was released in May 2010. 

ADDITIONAL ACCOMPLISHMENTS OF THE POOLED FUND STUDY

The four original SHRP calibration centers have been outfitted with new equipment, and their 

personnel have been trained and certified. In addition, two new calibration centers have been 

established, operated by the Montana Department of Transportation (MDT) and the California 

Department of Transportation (Caltrans). The calibration center operated by the Indiana 

Department of Transportation (INDOT) has also been updated, but it is not available to 

outside agencies. 

While they are not part of the pooled fund study project, calibration centers have also been set up 

at other manufacturing facilities: Foundation Mechanics Inc. (JILS) in El Segundo, CA; 

Dynatest® in Starke, FL; and Carl Bro in Kolding, Denmark. Additionally, the calibration center 

operated by Main Roads Western Australia in Perth, Australia, has been updated, and new 

calibration centers have been established at the Australian Road Research Board in Melbourne

and Brisbane, Australia, and at Fugro PMS in Hamilton, New Zealand.

The new equipment and procedure are fully portable. Onsite calibrations conducted by certified 

technicians have been performed in Hawaii and Chile, and nine different FWDs have been 

calibrated at remote locations in Australia. Four agencies (Dynatest®, Foundation Mechanics Inc.

(JILS), Carl Bro, and Fugro PMS) are currently offering onsite calibrations, and others have 

expressed interest in doing so.

A spinoff product of the project arose from the need to electronically transfer calibration data 

from the FWD computer to the calibration computer. Each different type of FWD has its own 

native file format, and the formats are not interchangeable. This incompatibility led to the 

development of PDDXconvert, which is a software program that converts each native FWD data 

file to the pavement deflection data exchange (PDDX) standard (see appendix C).

(7,8) It is used 

integrally with WinFWDCal to seamlessly transfer FWD data to the calibration computer. 

PDDXconvert can also be used in a standalone mode in support of other software, such as 

those used for backcalculation. For example, the MODTAG backcalculation program uses 

PDDXconvert to process all types of FWD data to determine pavement layer moduli.
Redevelopment of the FWD Calibration Protocol and Equipment

6

Quality Assurance Procedures

Quality assurance (QA) procedures have been developed to annually recertify the calibration 

center operators. Procedures and checklists have been applied successfully and updated several 

times during four annual cycles of QA visits. Currently, all FWD calibration centers are operated 

by certified technicians.

At the conclusion of the pooled fund project in October 2010, the AASHTO Materials Reference 

Laboratory (AMRL) in Gaithersburg, MD, took over the responsibility of certifying calibration 

technicians. In preparation of this responsibility, the AASHTO personnel worked and trained 

with Cornell University staff for more than 1 year.

Adoption of the Calibration Procedures by AASHTO

The SHRP FWD calibration protocol was originally adopted by AASHTO as AASHTO R32-

03.(9) The AASHTO R32-03 procedure has been revised to reflect the changes brought about by 

the pooled fund study, and in January 2009, the AASHTO Subcommittee on Materials approved 

and adopted the revised procedure. The revised procedure was published as AASHTO R32-09.(1) 

There have been additional revisions and improvements in the calibration procedure since 2009. 

As of October 2010, a revised AASHTO R32-09 procedure was under review by AASHTO, and 

it is expected to be published in 2011.(1)

The AASHTO R33-03 for annual calibration of the reference load cell has also been updated.(10)

It was submitted for review and approval by AASHTO in May 2010. As of October 2010, it was

under review by AASHTO, and it is expected to be published in 2011.(10)

Revision of the PDDX Standard

In April 1998, AASHTO adopted a standard for PDDX.(7) The initial intent of the standard was 

to facilitate electronic input of FWD data to the DARWin pavement design program. At the 2001 

meeting of the FWD User Group in Gulfport, MS, all four FWD manufacturers confirmed they 

would provide an output file in their field programs that complied with the PDDX standard.

As noted previously, the pooled fund study used the PDDX standard output as the means for 

electronic exchange of FWD calibration data. However, several problems became evident. First, 

the PDDX file input requirements for DARWin were not fully compliant with the 1998 AASHTO 

PDDX standard. In addition, the AASHTO PDDX standard did not provide a placeholder for all 

of the FWD identification data that are needed for FWD calibration.

To overcome these problems and continue with the project, the PDDXconvert computer program 

was created.

(8) In addition, a revised version of the PDDX standard was created to ensure 

compatibility of the PDDX files and satisfy the original intent of the AASHTO PDDX 

standard (see appendix C).
Chapter 1. Introduction

7

Development of an Informational Video

An 11-min video was produced to familiarize FWD owners with the new calibration procedure 

and encourage State transportation departments to utilize the calibration facilities. The video is 

available at the Long-Term Pavement Performance (LTPP) Web site, and CD-ROM copies can 

also be obtained from FHWA.

Long-Term Calibration Center Support Stewardship 

Responsibility for the long-term support for the calibration centers is being transferred to 

AMRL. From March 2009 to June 2010, training was provided to two AMRL technicians 

and their supervisor. The training placed emphasis on the QA visit and operator certification 

processes (see appendix D). Each of the nine calibration centers was jointly visited one or more 

times by both AMRL and Cornell personnel. AMRL was slated to take over the QA visit 

responsibility starting in late 2010. Cornell expects to continue to support the centers’ needs for 

technical assistance, reference load cell calibrations, and software upgrades for at least 1–2 years.

9

CHAPTER 2. OVERVIEW OF THE NEW FWD CALIBRATION PROCEDURE

Philosophically, the new AASHTO calibration procedure is similar to the old SHRP procedure; 

however, the manner in which the new procedure is carried out is different. The basic steps of 

the procedure are as follows:

1. Independently calibrated reference transducers are used to calibrate the load and deflection 

measurement systems on an FWD. This is called reference calibration.

2. After reference calibration, relative calibration of the deflection sensors is used to refine the 

calibration factors (also known as gain factors). 

3. The gain factors are checked for acceptability and then entered into the FWD operating 

system (i.e., the field program) in the FWD computer. 

4. The gain factors are used as multipliers to the internal results from the FWD. 

Few changes were made in the load system calibration procedure, which is carried out in a

similar manner to the old SHRP protocol. However, more flexibility is allowed in the number of 

load levels used.

The deflection sensors are calibrated simultaneously by placing them in a rigid columnar stand 

along with a reference accelerometer. This expedites the procedure, as the old SHRP protocol 

specified performing reference calibration one sensor at a time. Data from the accelerometer are 

double integrated to yield a precise peak reference deflection.

Relative calibration is performed in the same stand as reference calibration, and the procedure 

checks the quality of the calibration. Sensors are rotated in the stand to remove any bias due to 

position. Only one rotation is required, which also helps expedite the procedure. 

The new protocol requires two operators: the FWD system operator and the calibration system 

operator. The protocol takes less than 3 h to perform, provided the FWD is in working condition. 

The detailed FWD calibration protocol is given in appendix A. 

WINFWDCAL

The AASHTO protocol is conducted using a new software program, WinFWDCal.

(11) The 

program reads input from the FWD computer and produces all required outputs. The Microsoft 

Windows user interface is graphically oriented, showing the time history data for each drop 

during reference calibration. Typical screens are shown in figure 2 and figure 3.

Data from FWD are transferred electronically to the calibration computer using a flash drive, 

floppy disk, or CD, which enhances accuracy and speeds up the calibration process. WinFWDCal

utilizes an FWD data file format conversion program called PDDXconvert.

(8) The program reads 

data files from all types of FWDs in their native format and converts them to an AASHTO 

standard PDDX format.
Redevelopment of the FWD Calibration Protocol and Equipment

10

Figure 2. Screen shot. WinFWDCal software program FWD calibration screen.

Figure 3. Screen shot WinFWDCal software program collect drop data screen.

Data are checked during acquisition to ensure accurate, high-quality results. A calibration 

certificate is produced, and, in instances where prior calibration results are available for a 

particular FWD, the history is reviewed as part of the acceptance process. At the completion of 

the calibration, data are retained for historical and forensic analyses. 
Chapter 2. Overview of the New FWD Calibration Procedure

11

CALIBRATION OF THE REFERENCE DEVICES

The two reference calibration devices (load and displacement) are calibrated against independent 

reference systems. The reference load cell is calibrated annually with a National Institute of 

Standards and Technology (NIST) traceable universal testing machine. A load algorithm is 

developed using a polynomial statistical fit between a corrected load in pounds from the 

universal testing machine and the output from the reference load cell in volts. This adjusts for the 

typical nonlinear response of the reference load cell, allowing it to achieve accuracy approaching 

0.1 percent of full scale. In the new procedure, the calibrated range of the reference load cell is 

also increased from 20,000 lbf (89 kN) to 24,000 lbf (107 kN) to accommodate a higher preload 

force from the FWD. A study performed by Orr and Wallace in 2002 confirmed the need to 

calibrate the reference load cell annually.(4)

The reference accelerometer is calibrated daily using the Earth’s gravitational field. Daily 

calibration accounts for temperature, hysteresis, and other known variations in the response of 

the reference accelerometer. Nonlinear characteristics of the accelerometer are corrected using 

factors provided by the manufacturer. However, gravitational differences between regional 

calibration centers can result in a small difference in the results of daily calibration. The 

maximum difference in gravity constants between the Pennsylvania and Colorado calibration 

centers (at the lowest and highest elevations) can result in a difference of about 0.1 percent or 

less than 0.02 mil (0.5 μm) for a deflection of 20 mil (500 μm).(12)

SIGNAL CONDITIONER AND DATA ACQUISITION SYSTEM

The Vishay 2310 signal conditioner is still used in the new protocol. The major change is the use 

of a Keithley model KUSB-3108 universal serial bus (USB) DAQ rather than the industry ISA

bus Metrabyte board. The USB DAQ is an improvement since it is small and can be shipped with 

the reference load cell for its annual calibration. In addition, the newer USB board uses a 16-bit

conversion of the analog signal rather than the older 12-bit ISA board.

Data acquisition is controlled by WinFWDCal via commands to the DAQ. If the DAQ is not 

plugged into the computer when WinFWDCal is running, the program runs in a “simulation 

mode” that allows data for previously completed calibrations to be reviewed.

The DAQ has been upgraded by use of triggering. By using buffers, the rise of the first major 

peak on the time history triggers data collection (both before and after the peak); this improves 

the ability to detect the FWD impulse.
Redevelopment of the FWD Calibration Protocol and Equipment

12

FWD CALIBRATION

The new protocol provides two types of FWD calibration: monthly and annual calibration. 

Monthly calibration is performed by the FWD operator at any suitable location to verify that the 

FWD is working correctly. WinFWDCal can assist with data evaluation. Annual calibration, 

which historically has been performed at calibration centers, can also be performed onsite 

wherever an FWD is located. The calibration equipment is sufficiently portable and can be 

transported to the FWD. A certified technician must perform the annual calibration whether 

it is performed at a center or onsite. Annual calibration of an FWD is completed in four 

steps: (1) setup, (2) FWD deflection sensor calibration, (3) FWD load cell calibration, and 

(4) documentation. WinFWDCal must be used in the process. 

Step 1: Setup

Setup involves inputting information about the FWD and its current calibration settings. 

The FWD calibration operator inputs details about the calibration, and the FWD owner 

determines the number of load levels to be used. During setup, the FWD is used to determine 

the recommended number of drops needed for each load level and the trigger level for the 

reference accelerometer. The reference accelerometer is also calibrated daily during setup. A 

special calibration stand is used for this purpose.

Drop Sequence

Under the old SHRP reference calibration procedure, the drop sequence was fixed at 20 total 

drops (5 drops at 4 load levels). The number of drops required was based on having a 

16-mil (400-μm) deflection at the largest load level used in the calibration, but many of the 

existing FWD calibration centers cannot achieve this deflection during different times of the 

year because of seasonal variations in the response of the test slab. 

During setup, WinFWDCal determines the minimum number of drops needed to meet the 

precision requirements on the day of calibration when the FWD is in position. Determining a 

minimum also allows the number of load levels to be selected as needed for different brands of 

FWDs. Some FWDs can be calibrated more easily using only three load levels rather than four. 

Step 2: FWD Deflection Sensor Calibration

The FWD deflection sensors are calibrated in two sequential procedures: reference and relative 

calibration. Reference calibration uses a series of replicate drops to determine a relationship 

between the deflections recorded with the reference accelerometer and the deflection sensors. 

All of the FWD sensors are placed into a columnar calibration stand with the reference 

accelerometer in the middle. The multiple drop sequence is repeated after the sensors are 

inverted relative to the accelerometer to eliminate a small bias in the stand. In the case of KUAB 

seismometers, there are two columns instead of one. The calibration stand is rotated 180 degrees 

in the ball-joint anchor to eliminate the bias in the stand. 

At least two trials are performed to ensure repeatability and improve precision. Data collected 

from each trial are used to determine the relationship between the reference accelerometer and 
Chapter 2. Overview of the New FWD Calibration Procedure

13

the FWD deflection sensors. The slope of the regression line that is forced through (0, 0) is used 

to define an interim gain factor for each sensor. This interim gain is stored internally in 

WinFWDCal and used as part of the second procedure, relative calibration, to provide the final 

gain factors for the deflection sensors. 

Next, 40 drops are performed in each of the two sensor arrangements used during reference 

calibration. This eliminates any bias from the stand and provides a large number of drops that are 

used in a statistical analysis of variance (ANOVA) to improve the precision of the FWD gain 

values. Relative calibration is also a means of detecting any anomalous behavior of the 

deflection sensors.

Step 3: FWD Load Cell Calibration

The FWD load cell is calibrated with a series of replicate drops from at least three different load 

levels. Data collected from each trial are used to determine the relationship between the 

reference load cell and the FWD load cell. The slope of the regression line that is forced through 

(0, 0) is used to modify the existing gain of FWD. At least two trials are performed to ensure 

repeatability and improve precision. 

Step 4: Documentation

The final step in annual calibration of an FWD is to review the data and document the calibration 

through a series of QA data checks. WinFWDCal alerts the user to perform additional tests or 

repeat the calibration if needed. 

WinFWDCal produces a certificate of calibration along with an electronic data file containing the 

calibration gains. The electronic file is transferred to the FWD operator, and the new gain 

settings are manually input into the FWD. This process may be automated in the future when the 

FWD operating systems have a means to read the electronic file.

RELATIVE CALIBRATION OF THE DEFLECTION SENSORS

The major difference in relative calibration when using the new annual calibration protocol is the 

elimination of the need to fully rotate the sensors in the calibration stand. Since the new stand is 

very stiff, the sensors only need to be inverted around a common average point to eliminate bias 

due to position in the stand. Although this constitutes a relatively small time savings, the process 

is much easier to perform than the full rotation needed in the SHRP procedure. 

Monthly Calibration of the Deflection Sensors

Frequent relative calibration of the deflection sensors will ensure that FWD is operating 

properly. It should be completed at the home base on a monthly basis using a relative calibration 

stand provided by the FWD manufacturer. Relative calibration involves multiple drops and full 

rotation of all sensors in all positions in the stand. By rotating the sensors through all positions, 

any bias due to position in the stand is removed. If a sensor needs to be replaced, the relative 

calibration procedure can be used to compare it with the remaining sensors to determine a 

temporary gain factor for the new sensor. 
Redevelopment of the FWD Calibration Protocol and Equipment

14

SETTING UP NEW FWD CALIBRATION CENTERS

The pooled fund study allowed for the creation of two new FWD calibration centers. These 

centers are located in Helena, MT, and Davis, CA, and they will be operated by MDT and 

Caltrans/University of California Davis. New equipment and training were provided at both 

locations, and both centers were fully operational in summer 2009.

In addition, two new calibration centers were created at the Foundation Mechanics, Inc. (JILS) 

manufacturing facility in El Segundo, CA, and at the Carl Bro manufacturing facility in Kolding, 

Denmark (these centers are not part of the pooled fund study). Two new centers have also been 

established in Melbourne and Brisbane, Australia, operated by the Australian Road Research 

Board. Installation and training has been provided, along with operator certification.

Sites for new FWD calibration centers must meet certain minimum specifications. Calibration 

center needs are well known and are based upon experience gained from the first 15 years of 

FWD calibration. Appendix F provides details about setting up the equipment found in 

figure 4, but certain basic operational issues must be discussed prior to ordering equipment and 

installing a test pad.

Figure 4. Photo. Calibration system hardware set.

Calibrating an FWD at a center controls or eliminates as many variables as possible by using the 

actual ground wave created by the FWD. Choosing an appropriate site at the center to install the 

test pad is important. A site should be large enough for the FWD and tow vehicle to be placed 

entirely indoors during calibration, which allows the calibrations to be performed year round. If 
Chapter 2. Overview of the New FWD Calibration Procedure

15

possible, a trailer-mounted FWD should be driven into the facility prior to construction to ensure 

that the location is appropriate. In addition to the space, the site should have enough room to 

store and secure the FWD calibration equipment, including the testing stands, load cell, 

electronics, and the calibration computer. The computer can be a laptop or a desktop, but it 

should be dedicated to FWD calibration.

Once a site has been chosen, the test pad can be installed. Test pads are recommended to provide 

consistent deflections. If the test pads are built well, deflections will be consistent from season to

season. Test pads can also be designed to provide larger deflections than might be found on 

normal industrial floors. Test pads that are 12 by 15 ft (4.0 by 4.5 m) are preferred, but 12- by 

12-ft (4- by 4-m) pads are acceptable if space is limited. Test pad slabs must be accessible by all 

brands and models of FWDs. Space around the test pad should be sufficient for personnel to 

move and to allow the FWD to be maneuvered. A larger area is better, and at least 5 ft (2.5 m) of 

clear space is recommended on all sides. Table 1 lists the estimated cost for a new FWD 

calibration center.

Table 1. Hardware cost estimate for a new FWD calibration center.

Component Notes Cost1

Accelerometer box AB-01

Includes Silicon Designs Model 2220-5 

accelerometer and cables $1,600

Geophone stand GCS-01

Includes hardware for 10 Carl Bro and 

10 JILS//Dynatest® geophone adapters $1,600

Geophone adapters

Includes 10 adapters to mount KUAB 

geophones in GCS-01 stand $200

Seismometer stand SCS-01 For up to 10 KUAB seismometers $1,600

Floor mount BJ-01 Includes ball-joint and mounting clamp $1,200

Signal conditioner Vishay 2310B and 2310-A20 $2,200

Data acquisition system

Keithley KUSB-3108, with cables to signal 

conditioner and computer $1,250

Load cell

Includes calibration certificate, cable, and 

shipping case ±$11,000

Computer Laptop model with four USB ports ±$1,500

Total $22,150 1

Prices current as of February 2009. Prices do not include shipping, installation, or training.

UPGRADING THE EXISTING SHRP CALIBRATION CENTERS

The pooled fund study provided support for upgrading all existing calibration centers in the 

United States. These included the centers operated by the Colorado Department of 

Transportation (CDOT), INDOT, Mn/DOT, PennDOT, and TxDOT (see figure 5). Installation 

and training has been provided to all centers. In addition, the calibration center operated by 

Dynatest® in Starke, FL, and the center operated by Main Roads Western Australia were also 

upgraded, although they are not part of the pooled fund study. The only existing center that has 

not been upgraded as of November 2010 is the center operated by the South African Bureau 

of Standards.
Redevelopment of the FWD Calibration Protocol and Equipment

16

Figure 5. Map. Calibration center locations in the United States.

As a part of the upgrade, old beams and concrete blocks were removed, and new hardware 

equipment listed was installed (see table 1 and appendix E). Centers continued to use their 

existing test pads but with the new ball-joint anchor located close to the rear edge. New laptop 

computers were provided in most locations. During a 3-day training program, operators 

demonstrated their proficiency with the new procedure and were certified.

TRAINING AND OPERATOR CERTIFICATION

Calibration operators require training to use the equipment and software. Although training and 

certification only takes 3 days, it is recommended that new calibration operators also perform 

several trial calibrations prior to their first calibration for a client.

At the conclusion of the training, operators demonstrate their proficiency at carrying out the 

protocol detailed in appendix A, which is the same as the AASHTO R 32-09 procedure.(1) A 

certificate is issued to each operator who passes this review, and the certificate must be renewed 

annually during a QA visit. The procedures and checklists used during the QA visit are included 

in appendix D. All FWD calibrations must be conducted or supervised directly by a certified 

calibration technician.
17 

CHAPTER 3. DEVELOPMENT OF THE NEW PROTOCOL

FWD SPECIFICATIONS AND CALIBRATION ISSUES 

A primary goal of the updated calibration protocol is universal compatibility with the four brands 

of FWDs used in North America. Interviews conducted with the FWD manufacturers, calibration 

operators, some owners, and other FWD experts determined what problems were encountered 

during calibration with the old SHRP procedure. The following sections discuss calibration of 

the four brands of FWDs as well as known issues associated with their calibration. Table 2

provides an overview of the technical specifications for all four brands of FWDs. 

Table 2. FWD manufacturer specifications.

FWD Brand JILS Carl Bro Dynatest® KUAB

Trailer or truck mounted Both Both Both Trailer

FWD typical 

maximum load

20,000 lbf 

(89 kN)

34,000 lbf 

(150 kN)

30,000 lbf 

(130 kN)

16,000 lbf 

(71 kN)

Heavy weight deflectometer 

(HWD) typical maximum load

50,000 lbf 

(222 kN)

56,200 lbf 

(250 kN)

54,000 lbf 

(240 kN)

135,000 lbf 

(600 kN)

Typical pulse time range 20–34 ms 20–30 ms 25–30 ms 23 ms1

Typical maximum deflection

80 mil

(2,032 μm)

87 mil

(2,200 μm)

78 mil

(2,000 μm)

78 mil

(2,000 μm)

Optional maximum deflection None None 

98 mil

(2,500 μm)

197 mil

(5,000 μm)

Deflection sensor types Geophone Geophone Geophone 

Geophone, 

seismometer

Typical number of deflection 

sensors 7 9 9 7 

Maximum number of deflection 

sensors 9 18 15 None specified

Deflection sensor mounting Spring coupling Spring coupling

Spring and 

magnetic coupling Spring coupling

Typical load plate diameter

12 inches 

(305 mm)

11.7 inches 

(300 mm)

11.7 inches 

(300 mm)

11.7 inches 

(300 mm)

Optional load plate diameter

17.82 inches 

(457 mm)

17.55 inches 

(450 mm)

17.55 inches 

(450 mm)

17.55 inches 

(450 mm)

Data taking trigger type

About 

triggering2 Proximity sensor Proximity sensor See note3 

When trigger occurs At peak load2

Just before mass 

strikes

Just before mass 

strikes See note3

1

KUAB specifications are for rise time, not pulse duration time.

2

JILS constantly monitors and buffers the load cell signal. When a peak is detected by the data acquisition system, it then uses the 

buffered data to look 20 ms before the peak load was recorded.

3

KUAB has used four different triggering techniques: (1) data recording at all times, (2) end switch activated between 0.39 and 

1.17 inches (10 and 30 mm) before the buffers strike the load plate, (3) end switch activated about 0.39 inches (10 mm) after the 

weight is released, and (4) activation when the magnet holding the weight turns off.
Redevelopment of the FWD Calibration Protocol and Equipment

18

Dynatest®

The first Dynatest® FWD was manufactured in Denmark in 1976, and the first one was sent to 

the United States in 1981.(13) Dynatest® manufactures two varieties of FWDs, the model 8000 

FWD and the model 8081 HWD. Both systems are trailer-mounted with 11.7-inch (300-mm)

loading plates (with an optional 17.55-inch (450-mm) plate available) and can have between 

7 and 15 deflection sensors. Figure 6 shows a model 8000 FWD. A truck-mounted FWD unit has 

recently been developed.

Figure 6. Photo. Dynatest® model 8000 trailer-mounted FWD.

Dynatest® FWDs consist of a hydraulically controlled single-mass system. Weights can be 

removed or added to change the magnitude of the load pulse. The standard equipment places one

deflection sensor directly under the load plate, and at least six additional deflection sensors are 

placed along a raise-lower bar. The geophone sensor bar runs from the load plate toward the tow 

vehicle. Additional deflection sensors can be positioned on a rear or transverse extension bar. 

The deflection sensors are velocity transducers (geophones), and the deflections are calculated 

using a single integration of the velocity response.

The HWD normal load levels exceed the usual 6,000-, 9,000-, 12,000-, and 16,000-lbf (27-, 

40-, 53-, and 71-kN) range specified in the original SHRP calibration procedure. In order for the 

HWD to achieve the preferred load levels, all attached weights must be removed. This causes 

some concern about whether the calibration is valid for the HWD since it is not normally used 

with all weights removed. 

However, both load and deflection calibration procedures under the old and the new protocols 

have very tight tolerance requirements for sensor linearity. This assures that the sensors are 

performing linearly over the calibration range. 

JILS
Chapter 3. Development of the New Protocol

19

The first JILS FWD was made in 1987 in the United States.(14) JILS manufactures the following 

four varieties of FWDs:

• JILS-20C: A single-axle trailer-mounted FWD.

• JILS-20: A two-axle trailer-mounted FWD.

• JILS-20T: A truck-mounted FWD (see figure 7).

• JILS-20HF: A trailer mounted high-force model designed for airports.

All JILS FWDs use a 12-inch (305-mm) rigid steel loading plate and have an optional 

12-inch (305-mm) segmented or 18-inch (457-mm) rigid loading plate. The deflection sensors 

are velocity transducers, and their response is single-integrated to determine the deflection. A 

unique feature of a JILS FWD is its ability to report both the first and second peaks of each drop. 

The second peak is not calibrated in the AASHTO R32-09 procedure.

(1)

Previously, the lack of a swivel on the load plate has made calibration difficult in some locations

(although the manufacturer has recently announced that a swivel is now an available option). 

Without a swivel, the calibration operator must ensure that the load plate is squarely seated on 

the reference load cell without any gaps around the perimeter. Since the standard JILS load plate 

is slightly larger than the 11.7-inch (300-mm) diameter of the reference load cell, adjustments 

have been made to the original design of the reference load cell. The guide fingers have also 

been adjusted to accommodate the JILS load plate.

Two additional issues posed challenges for calibrating JILS using the SHRP procedure. The 

machine uses a large mass with a short drop height. As a result, it was difficult to achieve 

optimal triggering at the low drop height due to the short free fall time as well as mechanical 

vibration. Additionally, JILS applies a large static preload on the pavement. This frequently 

causes clipping of the load response at the high drop height. These problems have been reduced 

in the new AASHTO R32-09 procedure.(1)
Redevelopment of the FWD Calibration Protocol and Equipment

20

Figure 7. Photo. JILS-20T truck-mounted FWD.

Carl Bro

Carl Bro manufactures both a double-axle trailer-mounted FWD and a vehicle-mounted FWD.(15)

The two loading plate options are a four-segmented 11.7-inch (300-mm)-diameter plate or a 

17.55-inch (450-mm) plate. A single mass is used and controlled hydraulically. The minimum 

number of deflection sensors is 9, and the maximum is 18. One sensor is used to measure 

deflection through the center of the load plate, while the remaining sensors can be placed on the 

raise-lower bar oriented toward the tow vehicle or on the side or rear using a T-beam. The 

deflection sensors are velocity transducers, and their response is single integrated to determine 

the deflection. Figure 8 shows an example of a Carl Bro FWD.

Figure 8. Photo. Carl Bro FWD.

KUAB

KUAB has been manufacturing FWDs since 1976, and the first one was sent to the United States 

in 1988.(14) KUAB manufactures both trailer-mounted and vehicle-mounted FWDs that are either

single- or dual-mass. Most KUAB FWDs use seismometers to measure deflections. Velocity 

transducers (geophones) are available as an option for some models. The overall number of 

deflection sensors is unlimited according to the specification, although the KUAB-50 model has 

seven sensors. The sensors can be aligned either toward the tow vehicle (see figure 9) or away 

from it. KUAB has the option of either a four-part segmented load plate with an 11.7-inch 

(300-mm) or 17.55-inch (450-mm) diameter, or it has a rigid load plate with a 5.85-inch 

(150-mm), 11.7-inch (300-mm), or 17.55-inch (450-mm) diameter. Figure 9 shows a KUAB 

FWD with geophone sensors.
Chapter 3. Development of the New Protocol

21

Figure 9. Photo. KUAB FWD with sensors aligned toward the tow vehicle.

For the KUAB FWD with deflection sensors positioned to the rear of the load plate, positioning 

the FWD load plate close enough to the LVDT during the SHRP calibration procedure was 

difficult. This made it hard to trigger the data acquisition system and achieve the desired 

deflections during geophone reference calibration. About triggering, which is discussed in more 

detail in a later section, has reduced that issue.

Issues Encountered with the SHRP FWD Calibration Protocol

In addition to the FWD-specific issues cited above, several more general problems were 

encountered while using the old SHRP procedure and equipment. These included excessive 

beam movement, difficulty in triggering, and difficulty in getting sufficiently large deflections. 

Electrical problems included excessive noise in the signal and insufficient power from the 

battery charger. Also, data acquisition problems affected the speed and accuracy of 

FWD calibration. 
Redevelopment of the FWD Calibration Protocol and Equipment

22

Beam Movement Problems

Beam movement was measured using one of the deflection sensors from the FWD. Figure 10

shows the beam and block combination used in the SHRP protocol. After reference calibration of 

each deflection sensor, beam movement was determined from the FWD time histories. If beam 

movement was 0.12 mil (3 μm) or more when the sensor being calibrated peaked out, the 

calibration was repeated. This problem seemed to be aggravated by water in the soil beneath 

the test pad, making it a seasonal issue. This forced the FWD owner to return and attempt to 

calibrate at a later date. It also caused some of the calibration centers to shut down for periods of 

time (particularly in the spring). 

Figure 10. Photo. FWD calibration beam and inertial block.

The SHRP calibration protocol required at least 16 mil (400 μm) of deflection at the 16,000-lbf

(71.2-kN) load level during deflection sensor calibration. This meant that the load plate needed 

to be close to the end of the beam without touching it. Due to geometric constraints, it was 

difficult to achieve the necessary load plate proximity for several models of FWDs and HWDs.

Data Acquisition Problems

The most common data acquisition problems fell into the following two categories: FWD brandspecific problems and general data problems.

KUAB and JILS FWDs had triggering problems, especially at low load levels. The SHRP 

procedure used the response of the test pad to trigger the data acquisition system. When the 

calibration software detected the vibration in the test pad due to the release of the FWD mass, the 

system began to acquire data. Because of the nature of its mass release system, KUAB FWD did 

not cause a measurable vibration at the time of release. As a result, the data acquisition system 

often failed to begin acquiring data. Conversely, JILS produced a lot of vibration when the mass 

was released, and this often triggered data acquisition too early.
Chapter 3. Development of the New Protocol

23

Data transfer from the FWD was a problem with the old SHRP method because while the 

calibration system software collected and stored data from the reference calibration 

instrumentation, the data collected from the FWD had to be printed and typed into the calibration 

system. This increased the probability of transcription errors and took a considerable amount 

of time.

The software program FWDCAL 4 was written to perform relative calibration data analysis using 

the SHRP method. Because the program could only read output from older Dynatest® FWDs, a 

spreadsheet with macros was written to work with additional data formats. However, the 

spreadsheet did not cover all FWD types and system configurations. 

The lack of a common data file format was the main problem. Because each brand of FWD 

outputs data was in a different native format, calibration operators were required to be experts in 

reading FWD data. Differences in data file formats occasionally required manual calculations to 

convert FWD plate pressure to load units. Some but not all of the different brands of FWDs

have the ability to directly produce the AASHTO standard PDDX data format.(7) All FWD 

manufacturers have promised to provide the PDDX output option. Some FWDs use a separate 

converter program to change their native data format into PDDX format. Unfortunately, the 

original AASHTO standard format did not provide all of the data needed for FWD calibration.

Other Known Problems

Legislative and financial restrictions limited some State transportation departments from sending 

people and equipment out of State to one of the FWD calibration centers, preventing them from 

getting their FWDs calibrated to the SHRP/ LTPP standard. Included in this group were FWDs 

from other countries such as Canada. 

EQUIPMENT AND PROCEDURAL UPDATES

This section discusses the research that led to the development of the new FWD calibration 

protocol. The detailed procedure is included in appendix A. Specifications for each item of 

calibration equipment mentioned are provided in appendix E.

Data Acquisition

The Keithley KUSB-3108 DAQ replaces the Metrabyte model DAS-16G in the new protocol. 

The Keithley board offers the advantages of USB 2.0 connectivity, which is universally 

compatible with any computer with USB 1.0 or 2.0 ports and does not require the now obsolete 

ISA bus.

Initially, the possibility of using an internal peripheral component interconnect (PCI) card was 

explored. The PCI specification is slowly being phased out in favor of the faster PCI express 

slot; however, PCI cards are not backwards compatible with the new standard. An external 

device such as the KUSB-3108 can be easily installed by connecting it to the calibration 

computer via a standard USB cable. USB standards are backwards compatible and are a common 

feature of all modern computers.
Redevelopment of the FWD Calibration Protocol and Equipment

24

A feature specific to the chosen model was the availability of drivers compatible with the 

Microsoft Visual Basic® language in which the WinFWDCal software was written, eliminating 

the need to develop custom drivers. Keithley supplies a driver set that is compatible with 

Microsoft Windows XP and Windows Vista. The software has been used successfully in both 

operating systems.

Protection of DAQ Cabling

One problem became evident as researchers began using the KUSB-3108 DAQ—the cables 

are attached to the box via a screw terminal system that leaves the wires vulnerable to being 

pulled out. Several calibration center operators reported having problems with the wires 

becoming loose.

An aluminum box was found to be well suited to provide some protection for the wiring

(see figure 11). DAQ is held in the box with a strip of Velcro®. Holes are drilled in both ends 

for connections to the USB port and the input and output channels. The box has a lid, and it can 

be stored with the lid on. However, the lid should be removed during use so that the DAQ can 

render an accurate measure of ambient temperature. Details about the box and the related 

hardware are provided in table 39 of appendix E.

Figure 11. Photo. Box for DAQ wiring protection.
Chapter 3. Development of the New Protocol

25

About Triggering

Modern data acquisition equipment allows the use of a technique called “about triggering” to 

determine when an event has occurred. Using buffers, about triggering allows the collection of 

data before and after a user-specified threshold value is reached. This means that the data are 

taken about or around the time when the trigger level is reached. The readings are continuously 

stored in buffers by WinFWDCal. 

Currently, WinFWDCal uses 50 buffers with 300 readings per buffer. Data are collected at a 

speed of 15,000 readings per second. When each buffer is full, the data in it are scanned to 

determine if the threshold level (trigger) was exceeded. If it was, the data in the buffer, along 

with several preceding and following buffers, are saved to create the complete dataset. If not, the 

oldest data are discarded, and a new set of data is collected and reviewed.

One second of data is saved for analysis: one-third of 1 s before the trigger and two-thirds of 

1 s after. With a reading rate of 15,000 Hz, this means 5,000 readings are taken before, and 

10,000 readings are taken after the trigger point. This includes data from before the release of the 

mass to the several bounces after the first main pulse.

Incoming data are read as voltages. Converting the voltages to engineering units requires the 

sensors (the accelerometer and reference load cell) to be calibrated. The procedures for sensor 

calibration will be discussed in following sections.

About triggering has streamlined the data-taking process and eliminated the need to have 

separate hardware to detect the trigger point. It has also eliminated the triggering problem 

mentioned previously with the KUAB FWD load pulse.

For load calibration, the trigger level must be set high enough to avoid triggering by the preload 

force but low enough to trigger when the lowest drop level is used. When 10 consecutive 

readings exceed the trigger level, the about trigger point is set. Using 10 readings eliminates false 

triggers caused by voltage spikes.

In the case of a Dynatest® or Carl Bro FWD, the preload is about 1,000 lbf (4.5 kN). For a JILS

FWD, the preload can be as high as 2,500 lbf (11 kN), while the KUAB preload level is very 

small. Setting the trigger level at 3,000 or 4,000 lbf (13 or 18 kN) is generally correct for load 

calibration since the smallest dynamic peak load is usually 6,000 lbf (26 kN).

Setting the appropriate acceleration trigger level requires finding an optimum between the pulse 

peak from the lowest and highest load levels. Research has shown that this must be tailored for 

each type of FWD. The pulse durations, and thus, the accelerations, differ from one type of FWD 

to another. The position of the FWD load plate face-to-face with the accelerometer in the 

calibration stand is variable. As a result, so are the peak accelerations. The response 

characteristics of the test pad can also vary from one day to another at the same location, so the 

trigger level must be determined immediately prior to each FWD calibration.

When the FWD is in place with the accelerometer on the deflection sensor calibration stand, one 

drop at the low drop height is analyzed to determine an appropriate trigger level for the 

accelerometer. WinFWDCal performs the analysis. 
Redevelopment of the FWD Calibration Protocol and Equipment

26

Drop Sequence

When the original SHRP FWD reference calibration procedure was developed, the specified 

drop sequence required five replicate drops at four load levels. Experience with the initial four 

and later eight Dynatest® FWDs used by SHRP showed that the drop sequence was well suited to 

check the linearity of the sensors, and 20 drops provided enough data to fit a regression with a 

good level of confidence. 

However, it has been noted that some FWDs had a problem with this drop sequence. For 

example, JILS uses a heavier mass and a shorter drop distance, and the lowest load level 

(6,000 lbf (26 kN)), had a much shorter free-fall time than the Dynatest® FWD. Vibration 

associated with the release of the mass did not have enough time for damping before the mass 

struck the load plate. Calibration operators reported problems with excess noise messages at the 

low, and even the second, load level. 

This issue was overcome by relying on the experience from performing hundreds of FWD 

calibrations over the past 15 years. Rather than working with a single fixed drop sequence, 

researchers were able to allow some flexibility depending on the daily response of the test pad. 

The solution involves a tradeoff between the number of load levels used and the number of 

replicate drops required to achieve the desired level of precision.

Either three or four load levels may be used, depending on the preference of the FWD owner or 

the recommendation of the manufacturer. WinFWDCal determines the required number of 

replicate drops per load level. The number of drops must be the same at each load level to 

simplify data collection and analysis. It is also recommend that the same drop sequence be used 

for both load and deflection sensor calibration.

The goal is for the standard error of the final calibration factors to be no more than 0.003 

(i.e., 0.3 percent). Experience with all types of FWDs has shown that the expected random 

measurement error for one drop is seldom more than ±0.08 mil (±2 μm) deflection and ±20 lbf 

(±0.09 kN) load. Deflection is the more critical of the two to achieve the desired standard error, 

so it controls the number of replicate drops.

When the FWD is in place with the accelerometer on the deflection sensor calibration stand, one 

drop at the low drop height and one at the high drop are analyzed to determine the minimum 

number of drops needed to achieve the desired precision. WinFWDCal performs the analysis. 

A total of at least 18 drops is necessary to ensure the minimum statistical power. This requires 

a minimum of six replicate drops at three load levels or five replicates at four levels. 

It was determined that 10 replicate drops per load level were a practical upper limit. If the 

required number of replicates exceeds 10, the FWD should be repositioned to obtain larger 

deflections. A larger range of deflections between the lowest and the highest drop height will 

result in fewer replicate drops needed. Therefore, switching from three to four load levels will 

usually reduce the required number of drops. 

It is important that the drop sequence be determined for each FWD calibration. The routine 

accounts for the position of the FWD load plate with respect to the sensor stand, and the current 
Chapter 3. Development of the New Protocol

27

properties of the test pad are also factored. The dynamics of the FWD load pulse are considered 

to reduce the likelihood of excess noise messages. The ability of the software to determine the 

drop sequence based on the local daily conditions provides flexibility.

Recommended Peak Deflection

As noted in chapter 1, it is important that the peak deflection be sufficiently large to allow a clear 

distinction between the bias error and the random error. Research has shown that a peak 

deflection of 20 mil (500 μm) at a force of 16,000 lbf (71 kN) would adequately achieve that 

goal. Lesser deflections as little as 12 mil (300 μm) may suffice, but the required number of 

drops increases as peak deflection decreases. 

It was found that a test area that is built with a 5-inch (125-mm) fiber-reinforced concrete 

surface, a 5.85-inch (150-mm) aggregate base, and a 120–200-inch (3–5-m) soft clay subgrade 

will achieve the target deflection level. The deflection profile near the edge of the test pad is 

variable, and deflections may initially go up before they go down near the edge of the pad. As a 

result, it is best to avoid placing the sensor stand too close to the edge of the test pad.

The calibration stand should be placed 11.7–17.55 inches (300–450 mm) from the edge of the 

test pad. It is important that the perimeter of the concrete is not bonded to the surrounding floor. 

To maintain a near-constant deflection over long periods of time, it is recommended to 

completely wrap the clay subgrade in neoprene or heavy polyethylene sheeting to resist drying 

of the material.

AASHTO R32-09 no longer requires a special test pad to be built to carry out the FWD 

calibration procedure.(1) Any good-quality concrete pavement that will provide a peak deflection 

that does not require more than 10 replicate drops per load level is sufficient. This makes the 

calibration procedure portable. The limit on the required number of drops, not the minimum 

amplitude of the peak deflection, determines whether a site can be used for FWD calibration.

However, as a general rule, the optimal peak deflection is at the high load level of 16 mil

(300 μm) or more.

WinFWDCal will also check each drop to ensure that the maximum acceleration does not exceed 

5 g. If this limit is exceeded, the data are invalid.

Choosing and Using an Accelerometer in FWD Calibration

After reviewing many accelerometers, one was discovered with excellent frequency response and 

shock resistance. Peak accelerations experienced during the development phase showed that the 

accelerometer needed to have a range of at least 3 g. The Silicon Designs model 2220-005 ±5 g

accelerometer was chosen (see figure 12). The device offers an optimum combination of low 

noise, high sensitivity, and excellent shock resistance. It is small in size and is housed inside a 

shielded aluminum box. 
Redevelopment of the FWD Calibration Protocol and Equipment

28

Figure 12. Photo. Silicon Designs model 2220 accelerometer.

Most accelerometers are not very shock resistant, and dropping them is usually fatal. Due to 

hydraulic damping, specifications for the Silicon Designs accelerometer indicate that the 5-g unit 

is resistant to a 2,000-g shock. During the research, an accelerometer box was unintentionally 

dropped onto a concrete floor from a height of 30 inches (760 mm), and it survived without 

any measurable effect on its calibration. Figure 13 shows an accelerometer box on a 

calibration platter.

Figure 13. Photo. Accelerometer box on calibration platter.

The Silicon Designs accelerometer is a microelectromechanical systems (MEMS) device. As a 

result, it produces a signal due to the acceleration of Earth’s gravity. This is termed the zero bias 

by the manufacturer. It is advantageous because it allows researchers to use Earth’s gravity to 

calibrate the accelerometer.
Chapter 3. Development of the New Protocol

29

A Vishay 2310 signal conditioner is used to power the accelerometer. The signal conditioner has 

a six-pole, low-pass Butterworth filter that is set at 1,000 Hz to protect against aliasing by 

removing unwanted high-frequency electrical noise. The same signal conditioner is also used 

with the reference load cell, thereby saving on hardware costs.

Calibration of the Accelerometer

The Silicon Designs accelerometer is a nonlinear device. The output is slightly nonlinear with 

respect to the acceleration. In order to overcome this, the manufacturer defines the following 

relationship:

Figure 14. Equation. Nonlinear conversion of voltage to acceleration.

Where:

g = Measured acceleration in gravitational units.

V = Accelerometer output in volts.

a, b = Coefficients of polynomial from manufacturer’s calibration at 50 Hz.

This relationship is used to convert the accelerometer response (in volts) to engineering units 

(in gravitational units). The second- and third-order terms contribute a relatively small amount to 

the conversion.

The accelerometer response is also slightly affected by ambient temperature. This only affects 

the zero bias, b, and the first order slope, a1. Daily calibration of the accelerometer to a known 

acceleration (Earth’s gravity) is used to calibrate the output and determine the bias and slope a1

at ambient temperature.

Daily accelerometer calibration is straightforward. The accelerometer is calibrated on a flat and

level plate that is also used to store the accelerometer when it is not being used (see figure 13). 

First, 1s of accelerometer readings (15,000 readings) is taken in a +1 g gravity field. This process 

is repeated 10 times. Next, the accelerometer (in its protective box) is inverted, and 10 more sets 

of readings are taken in a -1 g field. The accelerometer is immediately inverted back into the 

+1 g field, and 10 more sets are read to verify that the hysteresis effect (discussed later) did not 

cause a significant change in the resulting output.

The standard deviation of each second of data is examined to confirm that the accelerometer was 

being held steady during the test. Results are also compared to historical data for the 

accelerometer to ensure that the results are typical. Data from the calibration are used to 

determine a daily value for the zero bias, b, in gravitational units, and the slope, a1, in 

gravitational units per volt. These results include the effect of the gain setting in the signal 

conditioner, temperature effects, hysteresis effects, aging of the accelerometer, and other factors. 
Redevelopment of the FWD Calibration Protocol and Equipment

30

Data Checks During Accelerometer Calibration

Several data checks are made during the daily calibration of the accelerometer. Some are 

performed to ensure that the device is working within normal parameters, and others are used to 

look for changes from one accelerometer calibration to the next. The data checks are as follows:

• Time differential during data collection: To minimize the hysteresis effect during 

accelerometer calibration, the maximum allowable time to turn over the box and take 

10 s of readings in the -1 g position is 20 s. Generally, most operators can accomplish 

the flip in 6–8 s. 

• Change in slope: The slope of voltage versus acceleration (the inverse of a1) should be 

relatively constant over time even with temperature changes. Generally, a maximum of 

0.25 percent change from one calibration to the next has been observed. A maximum 

change of 1.0 percent is currently being allowed in WinFWDCal. 

• Change in offset: The change in the zero offset (bias) of the accelerometer is monitored. 

Currently, a maximum change of 0.02 percent is allowed from one calibration to the next.

• Temperature change: The KUSB DAQ has a temperature measurement system. The 

temperature at the time of accelerometer calibration is recorded, and the temperature 

thereafter is monitored. A temperature change less than 18 °F (10 °C) during calibration 

is allowed. Normally, the temperature is constant in an indoor calibration facility.

• Overall temperature: The behavior of the accelerometer was examined in a 

temperature sensitivity study. This occurred over a range of temperatures within 

the expected range in service and within the allowable range of the device from 5 to 

131 °F (-15 to 55 °C). 

Hysteresis of Silicon Designs Accelerometer

The Silicon Designs model 2220 accelerometer encounters problems with hysteresis. Every time 

the accelerometer is inverted in Earth’s gravity, the zero bias changes. The magnitude of this 

change increases exponentially with time and occurs whether or not the accelerometer is 

powered; thus, it is a materials science problem rather than an electrical one. This accelerometer 

drift could present a biased nonrandom error. If it is not accounted for, hysteresis can lead to a 

calibration error in excess of 1 percent. 

The magnitude of the error changes over time when the accelerometer is at rest in Earth’s 

gravitation. To overcome this, the accelerometer needs to be kept upright in the +1 g gravity field 

as much as possible. It should only be flipped upside down during daily calibration. If the time 

for flip calibration is kept short, the effect on the overall daily calibration is small.

Hysteresis does not affect the dynamic response of the accelerometer during the FWD pulse 

because it stays in a +1 g overall gravity field. Since Earth’s gravity (a static response) is used to 

calibrate the dynamic response of the sensor, hysteresis must be accounted for as part of the 

accelerometer calibration. 
Chapter 3. Development of the New Protocol

31

Figure 15 and figure 16 show the hysteresis effect measured for one accelerometer. An 

accelerometer was left in a +1 g condition overnight. Readings were taken for 15 min and were 

very stable (not shown). Next, the accelerometer was inverted to a -1 g condition, and the voltage 

output was recorded every second for 30 min. The accelerometer was then returned to a +1 g

gravity field, and additional measurements were taken for 15 min (see figure 15).

Each figure shows an immediate change in one direction, with a signal change on the order of 

1–1.5 mV. This is followed by a slower long-term change in the opposite direction. If inversion 

time (during daily flip calibration) is kept under 20 s, the effect on the accelerometer voltage is 

less than 0.035 percent (about 0.5 mV).

The rate of drift due to hysteresis diminishes with time. After 6 days at room temperature, the 

accelerometer is completely stable. If the accelerometer is stored upside down for a long period, 

it takes about 24 h to equilibrate it in the upright position to within 0.1 percent of the stable 

response. For this reason, the accelerometer should be stored in a +1 g field at all times.

As a general rule, if the accelerometer is inverted for a brief period of time, it should be left in 

the upright position for an equal length of time to re-equilibrate it. Note that in figure 16, the 

accelerometer was inverted from the orientation in figure 15, and the time scale was continued.

Figure 15. Graph. Hysteresis effect in a -1 g field.
Redevelopment of the FWD Calibration Protocol and Equipment

32

Figure 16. Graph. Hysteresis effect in a +1 g field.

Temperature and Electromagnetic Interference Sensitivity

The feasibility of using an accelerometer as a reference device for calibrating FWD deflection 

sensors depends in part on the effects of the environment. If the device is affected too much by 

temperature or by interference from electromagnetic fields, then the accelerometer could fall 

short of the ±0.3 percent accuracy required for deflection sensor calibration.

Silicon Designs specifies a scale factor (slope a1) temperature shift between -250 and 

+150 ppm/°C for model 2220-005 (see figure 17). Two accelerometers (serial numbers 277 and 

278) were calibrated over a range of temperatures, and the change in the output voltage was 

determined. Results show that if the maximum allowable error due to temperature change was 

0.1 percent, the maximum acceptable temperature shift was 18 °F (10 °C).

Electromagnetic interference (EMI) has been shown to affect accelerometer measurements 

when the fields move with respect to the device. Aluminum and steel were tested for use as 

shielding materials. While a grounded steel box was able to completely defeat the effect of 

electromagnetic interference (EMI) on the accelerometer, an aluminum box had optimal results 

in fields above 1 G (0.0001 T) and was deemed sufficient. Even large magnetic fields had a 

limited effect on the accelerometer readings. A maximum change of 0.04 percent in the voltage 

output was observed when several high-powered magnets were placed next to the accelerometer. 

It should be noted that the aluminum box also helps insulate the accelerometer from temperature 

effects. Temperature measured at the DAQ can be expected to move more rapidly than inside the 

aluminum box, provided the box is not in direct sunlight. 
Chapter 3. Development of the New Protocol

33

Figure 17. Graph. Scale factor temperature shift for accelerometer.

Double Integration of Accelerometer Signal

Using an accelerometer as a reference sensor in deflection measurement requires the output to be 

double integrated from the accelerometer to convert to displacement. This is a challenging 

problem in part because releasing the FWD mass before the mass strikes the load plate can leave 

a legacy of ongoing damped vibration in the pavement at the moment the mass strikes.

When considering integration for any purpose, it is necessary to remember that the first integral 

of a constant is a ramp. That is, ∫A dt = At + B. The second integral of a constant is a parabola,

∫∫A dt = 1⁄2 At2 + Bt + C, where A, B, and C are constants, and t is time. In instrumentation 

engineering terminology, the ramp is the drift, and the parabola is the nonlinear drift. 

Before performing the integration calculations, several boundary conditions and assumptions 

must be defined. Figure 18 shows the acceleration trace from a Dynatest® FWD impulse after 

converting from volts to gravitational units. The release of the mass occurs around 40 ms, and 

the strike of the mass on the load plate occurs just after 300 ms.

The baseline in figure 18 is on +1.0 g because the MEMS accelerometer responds to Earth’s

gravity. This zero bias can be measured by taking a burst of readings before the FWD mass is 

released. The readings are examined to ensure that no unwanted vibrations are occurring just 

before the drop. Subsequent acceleration data can be corrected to remove the zero bias before 

integration (additional details on this process are given in the next section). Figure 19 and 

figure 20 show the resulting integration of the accelerometer trace, first to velocity and then 

to displacement.

Figure 21 through figure 23 show a similar set of results for a JILS machine. The peak force for 

both examples was about 16,000 lbf (70 kN). It is evident that different types of FWDs have 
Redevelopment of the FWD Calibration Protocol and Equipment

34

different vibration characteristics both before and especially after the falling mass strikes the 

load plate, which is a challenge to accommodate in the integration routines. A small but 

acceptable amount of baseline drift can be seen in figure 20 and figure 23.

Figure 18. Graph. Raw accelerometer output from a Dynatest® FWD impulse.

Figure 19. Graph. First integration (velocity) from a Dynatest® FWD impulse.
Chapter 3. Development of the New Protocol

35

Figure 20. Graph. Second integration (deflection) from a Dynatest® FWD impulse.

Figure 21. Graph. Raw accelerometer output from a JILS FWD impulse.
Redevelopment of the FWD Calibration Protocol and Equipment

36

Figure 22. Graph. First integration (velocity) from a JILS FWD impulse.

Figure 23. Graph. Second integration (deflection) from a JILS FWD impulse.
Chapter 3. Development of the New Protocol

37

Integration Principles

The deflection sensor calibration stand experiences a unidirectional acceleration, a(t), in the 

vertical (z-coordinate) direction. This is measured continuously using the accelerometer that is 

attached to the stand. The digitized data represent the vector sum of two accelerations: a constant 

acceleration, ao, due to Earth’s gravity and a variable acceleration, a'(t), due to the impact of the 

falling mass. 

The mass is released at time tr, and the time of impact of the mass is ti, which occurs after the 

release of the mass. Peak deflection zpeak is at tpeak (marked in figure 20 and figure 23), which 

occurs after the mass strikes the load plate. Thus, tr < ti < tpeak. The goal is to determine zpeak as 

accurately as possible.

When the mass goes into free fall, the force of the FWD load plate on the pavement is reduced 

by the weight of the mass. This causes the pavement to spring up slightly. The KUAB FWD has 

an outrigger system that supports the mass; hence, there is no change of force at the load plate. 

The JILS FWD uses a pair of air bags to apply a large preload on the load plate. It also uses a 

larger mass and a shorter free fall distance than the other types of FWDs, so the damping time 

between tr and ti is shorter (about 175 ms in figure 23).

If the extraneous vibrations due to the release of the mass are damped out before ti, the combined 

acceleration after ti is as follows:

Figure 24. Equation. Acceleration due to gravity and the FWD load pulse.

Where:

a(t) = Acceleration over time t.

ao = Constant acceleration due to Earth’s gravity.

a'(t) = Acceleration due to the FWD mass strike at ti.

The constant, ao, is measured by averaging a burst of 3,000 readings taken immediately before 

the mass is released (at that time, a'(t) = 0). If the calibration stand is not exactly vertical or if the 

temperature has changed since the accelerometer was last calibrated, ao will include these 

effects. Since ao can be measured precisely and its value is subtracted from all subsequent 

accelerometer readings taken before and after the mass is released, this leaves a relative 

acceleration, a'(t) = a(t) - ao. Thus, the equation in figure 24 becomes the following:

Figure 25. Equation. Relative acceleration after removing Earth’s gravity.
Redevelopment of the FWD Calibration Protocol and Equipment

38

The relative acceleration in figure 25 can be integrated to find the velocity as a function of time, 

v(t) (see figure 26). 

Figure 26. Equation. Velocity as a function of time.

By examining the data, the starting point of the integration can be chosen on the interval tr < ti so 

that t = 0 and a'(0) = 0. If v(0) = 0 (i.e., there is no initial velocity of the pavement at t = 0), then 

constant c1 = 0. Hence, the deflection over time z(t) becomes the following:

Figure 27. Equation. Deflection as a function of time.

Again, t = 0 is chosen so that v(t) = 0, and if z(0) = 0, then c2 = 0. The starting point for 

integration in figure 26 and figure 27 will most likely not be at the same reading number in the 

data file. If there is also a time-variable acceleration ao(t) and/or a time-variable velocity vo(t) 

(e.g., residual vibration in the pavement caused by the decrease in force when the mass is 

released or due to the FWD vibrating after the release) at the start of integration, then figure 26

becomes the following:

Figure 28. Equation. Velocity with initial vibration.

Where:

v(t) = Velocity over time.

vo(t) = Initial time-variable pavement velocity (vibration).

ao(t) = Initial time-variable pavement acceleration (vibration).

The accelerometer measures the combined vector a(t) = ao + ao(t) + a'(t). ao can still be measured 

at a time when ao(t) + a'(t) = 0. A starting point can also be chosen for the integration such that 

ao(0) + a'(0) = 0. However, v(0) is not likely to be zero at t = 0, and vo(t) and ao(t) are unknown 

functions, so it becomes difficult to solve the equation in figure 28. It is even more difficult to 

define z(t).

If there is vibration present at ti but it is still assumed that figure 26 and figure 27 are applicable, 

the constants will be nonzero. It is not possible to separate ao(t) and a'(t) in the accelerometer 

data, and vo(t) is an undefined function. Thus, z(t) will contain nonlinear drift. 

The vibration functions vo(t) and ao(t) may be nearly fully damped at the starting point t = 0, in 

which case, the drift may be small, and the error of the peak deflection zpeak may also be small 

and possibly of random magnitude. However, if the vibrations are renewed and enhanced when 

the falling mass strikes the load plate at ti, then vo(t) and ao(t) may be significantly nonzero, and 

the drift and the error in zpeak may become large. 
Chapter 3. Development of the New Protocol

39

The best approach is to only perform the double integration in an environment where the 

vibration due to the release of the FWD mass is essentially zero. Then, both ao(t) and vo(t) will be 

zero, or nearly so, and will contain little if any drift. To ensure that condition is met, the 

accelerometer signal can be analyzed during each drop for a short period before ti and all drops 

that show excessive vibration can be rejected.

Research for this study has shown that it is important for the FWD to be level and seated 

squarely on the pavement to minimize undesired vibrations. Several seating drops (no data 

recorded) should be performed before data collection to establish good contact of the load plate. 

For the Dynatest® and Carl Bro FWDs, it is necessary to lubricate the load plate swivel, and it 

must be free to rotate. For the JILS FWD, which does not have a swivel, it is important to verify 

that the load plate is properly seated while the mass is in free fall. This can be accomplished 

using a sheet of paper, and the procedure for checking proper seating is explained in more detail 

in annex 3 of appendix A.

Implementation

WinFWDCal is used to implement data acquisition and double integration. It also provides 

several QC checks during data processing. An earlier section discussed setting the trigger level. 

The following steps are taken when the calibration operator initiates an FWD drop during 

reference calibration. 

Measuring ao

Before the computer screen turns green around the perimeter, which is a signal to the FWD 

operator to release the mass, the computer takes a burst of 3,000 readings at 15,000 Hz 

(200-ms read time) and computes the average (ao) and standard deviation. If the standard 

deviation is larger than 0.005 V, a popup message alerts the user that the predrop vibration 

exceeds the acceptable level. 

The readings are converted to gravitational units, and the vector is checked to ensure that all 

readings are between 0.98 and 1.02 g. According to NIST, the acceleration of gravity (1 g) 

equals approximately 32.2 ft/s2(9.80665 m/s2).

(16) In Minneapolis, MN, gravity is approximately 

32.2 ft/s2 (9.80585 m/s2); in Denver, CO, it is approximately 32.1 ft/s2 (9.79615 m/s2); and in 

College Station, TX, it is approximately 32.1 ft/s2 (9.79324 m/s2). Gravity is not less than 

0.9985 g in any of these locations. If the data are out of range, the accelerometer is not working 

correctly, and a popup message alerts the calibration operator. Data for the drops are saved in the 

time history file.

Finding the Trigger Point

Incoming readings from the accelerometer are continuously scanned for the trigger level. When 

10 consecutive readings are over the trigger level, the trigger is satisfied. This process avoids a 

false trigger due to a voltage spike or other anomaly. 

After the trigger is satisfied, at least 10,000 more readings are collected and saved in buffers. 

Accelerometer readings measured in volts are then transferred from buffers to a program array 

such that the trigger point is the 5,000th reading out of 15,000. Thus, 0.333 s of data are ahead of 
Redevelopment of the FWD Calibration Protocol and Equipment

40

the trigger, and 0.666 s are after the trigger. The raw data in volts, the absolute acceleration in 

gravitational units, and the relative acceleration in gravitational units are saved in the time 

history file for the drop.

Finding the Time of Impact, the Time of Peak, and the Pulse Rise Time

The time of impact, ti, is determined by working backward in time from the trigger point. 

Researchers seek to find the transition point from where the deflection time history trace is steep 

to where it is level by examining the standard deviation of sets of 25 readings. In figure 23, ti is 

located at the first X around 300 ms.

The standard deviation of the previous 25 readings is calculated and placed in an array. Moving 

back one point, the standard deviation of the next set of 25 readings is calculated. Similarly, the 

running standard deviation of consecutive sets of 25 points is tabulated. 

When the standard deviation falls below 0.020 g, it indicates the time of impact. A secondary 

check is made to assure that the acceleration at that point is not less than -0.030 g. If it is not 

satisfied, researchers must continue to work backward until both criteria are satisfied.

The time of peak, tpeak, occurs at the first maximum reading after the trigger point. Pulse rise 

time can be calculated by subtracting ti from tpeak. Rise time is reported on screen for each drop 

by WinFWDCal. 

Finding the Starting Points for Integration

The starting point for integration of the acceleration data to velocity is determined by working 

backward from ti. The first set of three consecutive readings where the acceleration is within 

±0.0030 g is found. The starting point for acceleration integration is assigned as the middle 

reading of the three. The reading number is recorded in the time history file.

The starting point for integration of velocity data to deflection is accomplished in a similar 

manner using the velocity data array. Working back from ti, the first set of three consecutive 

readings where the velocity is within ±30 μm/s is found. The starting point for velocity 

integration is assigned as the middle reading of the three. The reading number is recorded in the 

time history file.

Generally, the two starting points occur at different reading numbers. For both the acceleration 

and the velocity data arrays, if a starting point cannot be located within 50 ms ahead of ti, a 

popup error message indicates excessive vibration before the strike of the mass. In this case, the 

data for the drop should be rejected, and the drop should be repeated.

Performing the Integration

Integration involves computing the area under a curve. Over a period of 1 s, 15,000 readings are 

taken, so the readings are 0.0667 ms apart. The digital integration is computed by averaging two 

adjacent readings and multiplying by the time interval. The running sum is computed by adding 

these areas forward and backward from the starting point. By definition, the sum at the starting 

point is zero. 
Chapter 3. Development of the New Protocol

41

Multiple Sensor Stand

The development of a multiple-sensor stand contributed to the project’s goals of expediting the 

calibration process and creating a universally compatible procedure. The multiple-sensor stand 

can save time during calibration by eliminating the need to reference calibrate deflection sensors 

individually. The design of the stand allows it to be positioned close to the load plate for all 

brands and models of FWDs. 

Under the new calibration protocol, the sensors only need be repositioned once during reference 

and relative calibration by performing a vertical inversion with respect to the location of the 

accelerometer. This ensures unbiased results from positions on the lower half of the stand to 

those at the top. In the case of a KUAB seismometer FWD, horizontal rotation of the stand 

removes the effect of the two columns of the stand. 

Photos of the two stand designs are provided in figure 59 and figure 68 of appendix E. Details 

about installing and using the stands are provided in appendix F.

Column Design

After examining three different geometries of multiple-sensor stands, the vertical column 

or “ladder” design was found to be the best option. Its welded aluminum construction is 

lightweight, and it is stiff enough to ensure consistent deflections for all positions in the 

stand. Additionally, it is the least affected by the rolling motion of deflection waves. 

For the new AASHTO R32-09 protocol, it was necessary to develop two different stands, one for 

geophones and another for the seismometers used by KUAB FWDs.(1) A total of 10 geophones 

or seismometers can be simultaneously calibrated using the new stands, and the same stand can 

be used for both reference and relative calibration.

Ball-Joint Anchor

To ensure accurate measurement with the columnar stand design, it is necessary to anchor the 

stand firmly to the test pad. Early tests on a variety of prototype stand designs involving an 

uncoupled anchor showed that testers could not provide enough downward force to keep the 

stand in contact with the test pad and produce consistent results. The graph in figure 29 shows 

the results of the tests.

Coupling the anchor to the floor is only an issue when performing the single inversion procedure. 

Monthly calibration with full rotation allows the use of an uncoupled stand since all sensors are 

in all positions in the stand, and the effects of position in the stand are accounted for. 
Redevelopment of the FWD Calibration Protocol and Equipment

42

Two methods of attaching the calibration stand to the floor were investigated. One approach was 

to bolt the stand directly to the pad with one or two anchors. The other method was to connect 

the stand through a ball-joint clamp bolted to the pad (see figure 53 in appendix E). The singlehole direct anchor was acceptable in terms of unattributed (or residual) error and range of 

deflections, as was the ball-joint anchor. The ball-joint anchor was selected over the direct 

anchor for the following reasons: 

• The ball-joint makes it possible to keep the stand vertical during testing. 

• Having a person hold the stand dampens the vibration. With the direct anchor and 

without a ball-joint, it is possible for the stand to be used without being held, which

increased the variability of the results.

• Direct anchored stands would be more prone to damage if hit by the FWD trailer 

or truck.

Figure 29. Graph. Comparison of sensor stand performance.

Reference Load Cell

The reference load cell has been modified in two ways. The thickness of the lid was increased 

from 0.625 to 1 inch (16 to 25.4 mm) to better resist bending and permanent deformation. The 

guide fingers were moved back slightly from an 11.7-inch (300-mm) diameter to an 11.89-inch 

(305-mm) diameter to accommodate the slightly larger load plate on JILS FWD.
Chapter 3. Development of the New Protocol

43

When calibrating the reference load cell, the maximum load was increased from 20,000 lbf 

(89 kN) used in the SHRP protocol to 24,000 lbf (107 kN). This change was made to 

accommodate the higher preload pressure used by the JILS FWD. The calibration of the 

reference load cell should be performed by a qualified agency in accordance with 

AASHTO R33-03.

(10)

A new design for the reference load cell with a significantly higher load capacity for use with 

HWDs was investigated. The design used an array of three commercially sold load cells with a 

combined load capacity of 90,000 lbf (400 kN). However, the concept was not successful 

because each of the three load cells had a slightly different sensitivity in terms of force per volt. 

When combined with slight variations in the flatness of the test pad and small eccentricities of 

loading from FWD, the resulting calibration factor was sensitive to the orientation of the 

reference load cell. 

Overcoming this problem would require that each load cell have its own signal conditioner. The 

DAQ would read each channel, and the computer would sum the three loads. This made the 

endeavor prohibitively expensive and too complicated, so the project was abandoned.

Software Redevelopment

One task was to update the existing software to a modern Microsoft Windows®-based graphical 

user interface and eliminate the need for manual data entry. While there will always be ongoing 

updates to any software as complicated as WinFWDCal, the original goal was met.

The programming language for WinFWDCal is Microsoft Visual Basic 6.0. It was chosen 

because it did not require a complete rewrite of the original code, which used Microsoft 

QuickBasic language. Also, the KUSB data acquisition card had drivers for Microsoft 

Visual Basic 6.0. 

The update of the program started in June 2005 with the conversion of the FWDREFCL code to 

Microsoft Visual Basic 6.0. By the time this was completed, the accelerometer and data 

acquisition systems had been chosen, and the proof of concept for the double integration method 

had been accomplished using LabVIEW in October 2005. 

A draft Microsoft Visual Basic program that performed some of the basic data acquisition 

functions and analysis was used during visits to the FWD calibration centers in April and 

May 2006. The objective of the visits was to test the software with all four types of FWDs. 

Comments by the calibration operators and other people who participated were incorporated

into the program. 

Results with WinFWDCal were checked using spreadsheets and Minitab statistical software, as 

well as by hand to confirm their validity.(16)

The beta version of WinFWDCal was vetted at Cornell University prior to its release to the four 

regional calibration centers in late November and early December 2006. Periodic updates of 

WinFWDCal have been issued on a roughly quarterly basis. Version 2.0 of the software was

released in December 2009. It incorporates some user-suggested improvements that make the 
Redevelopment of the FWD Calibration Protocol and Equipment

44

calibration setup easier to navigate. Version 2.0 also provides software support for monthly 

relative calibration using the manufacturer-provided calibration stands.

Electronic Data Transfer

A problem was encountered with transferring data from an FWD to WinFWDCal. Each type of 

FWD has a different native file format, and several manufacturers have many different formats. 

The AASHTO PDDX file standard was chosen as the preferred format for data transfer after 

discussion with the FWD manufacturers, calibration operators, and other FWD users.(7) This led 

to the development of a stand-alone program, PDDXconvert. WinFWDCal uses PDDXconvert as 

an intermediary for all data transfers. 

A data file is copied in native format onto a USB flash drive or a floppy disk to transfer data

from the FWD to the calibration computer. This eliminates the manual entry of data into the 

calibration software, which, in turn, saves time and reduces human error as a factor in the 

calibration procedure. PDDXconvert reads the file and converts it to a common format that 

WinFWDCal can read and use. 

PDDXconvert does not solve all of the problems of electronic data entry to WinFWDCal. More 

data than just the sensor readings are needed for FWD calibration. Information such as the FWD 

model identification, FWD operator name, sensor serial numbers, current sensor gain factors, and 

last calibration date are needed for record keeping and for the certificate of calibration. A revised 

and more comprehensive standard is needed for the PDDX file format. A recommended new 

standard, based on experience working with various FWD native file formats and many user 

needs, is provided in appendix C.

Two FWD calibration data exchange file formats are described in appendix C. One format 

provides a suggested file name and contents for exchange of information from FWD that is 

needed by WinFWDCal. The other details the layout of an electronic file that is created by 

WinFWDCal that can transfer the calibration results to the FWD. The FWD manufacturers need 

to provide the means to create the first file and to read the second one.

Historical Database of FWD Calibrations

During the development of the new calibration protocol, each of the existing FWD calibration 

centers in the United States was asked to provide records of all calibrations performed there, and 

data were combined into a database of historical results. The data were reviewed to see if annual 

calibration is warranted. The results showed that changes in the sensor gains occur continuously, 

and annual calibration is needed to ensure that accurate data are collected by FWDs.(4)

The review found that some deflection sensors have gain settings outside the recommended 

range of 0.98 to 1.02 (which comes from the manufacturer’s specifications). However, many of 

these sensors are stable from year to year. Since the calibration factors correct the measurement 

errors, there is no need to replace sensors that are stable.
Chapter 3. Development of the New Protocol

45

Stability over time is more important than whether the gain factor falls between 0.98 and 1.02. 

Evaluation of the historical database showed that an individual gain factor does not change more 

than 1 percent from one calibration to the next (typically 1 year apart). This result is used as the 

first criterion in evaluating and accepting the results of an FWD calibration. 

When data for a particular FWD are available for 4 years or more, the long-term data can also be 

used as part of the calibration acceptance criteria. From the historical database, it was found that

an individual gain factor should change no more than 0.3 percent per year, provided that at least 

three calibrations were done in the 4-year period. 

The long-term allowance for change (0.1 percent) is less than that for a single year (1 percent) 

because short-term change is usually more volatile than long-term change. Given that the random 

variability of two repeat calibrations is 0.3 percent, it makes sense that a long-term series of 

calibrations would show less variability.

In spite of the existence of the historic database, it will be difficult to implement the long-term 

acceptance criterion until the new AASHTO procedure has been in use for 4 years or longer for 

several reasons. Although the new calibration procedure is similar to the old SHRP procedure, 

there are many differences. Beam movement was a factor in the old procedure, and it has been 

eliminated in the new procedure. Additionally, there are only a small number of FWDs that have

4 years of calibration history, and essentially all of those machines were Dynatest® FWDs. 

COMPARATIVE ACCURACY OF THE OLD AND NEW PROCEDURES

The objective of creating the new procedure was to make improvements without sacrificing the 

quality of the calibration. In order to compare the results of the old and new procedures, two 

large sets of replicate calibrations were collected—one using the older SHRP protocol and the 

other using the newly developed AASHTO R32-09 method.(1)

The same FWD was used for all of the calibrations. The data using the old procedure were taken 

in early 2006, and the new procedure was used in late 2006. The FWD sensor gain factors that 

were in place before the data collection began were left in place throughout all of the testing. 

Influence on Deflection Sensor Gain Factors

Nine calibrations of the nine geophone sensors were completed using each protocol. The overall 

average gain obtained with the new AASHTO protocol was 0.26 percent higher than the average 

gain using the older SHRP protocol. The difference was statistically significant. The variance of 

the calibrations was the same for each protocol.

Figure 30 shows a comparison of the results for the two protocols. The upper half of the graph 

shows the final gain with the old SHRP protocol, and the lower half shows the results with the 

new AASHTO protocol. 
Redevelopment of the FWD Calibration Protocol and Equipment

46

Figure 30. Graph. Deflection sensor final gain values produced with the old 

and new protocols.

Influence on Load Cell Gain Factors

In total, 20 load cell calibrations were completed with the old SHRP protocol, and 17 were 

completed with the new AASHTO procedure. Figure 31 shows the load cell final gains for the 

old and new protocols. The average final gain increased by 0.4 percent. The 95th percentile 

confidence interval on the difference in the final gain was 0.27 percent. Thus, the difference was 

statistically significant. The variance of the calibrations was the same for each protocol.

This difference was more than expected. Unlike the hardware and procedural changes that may 

have affected the deflection sensor gains, there was no significant change in the load cell 

calibration procedure in the new protocol. The long-term history of the load cell calibrations for

the FWD was researched going back to 1994 to see if there was any explanation for the gain 

change by 0.4 percent.

The gain factor history is shown in figure 32. The year-to-year changes are much more volatile 

than the long-term trend, which is normal. The calculated trend line is essentially flat. The set of 

calibrations in early 2006 which used the old SHRP procedure tended to fall mostly below the 

trend line. The data for the new procedure tended to fall mainly above the trend line. 
Chapter 3. Development of the New Protocol

47

Figure 31. Graph. Individual value plot of load cell final gain values

from the old and new protocols.

Figure 32. Graph. Load cell calibration history from 1994 to 2006.

Therefore, both protocols may have accurately reported the gain factors that were applicable at 

the time. It is believed that the response of the FWD load cell actually changed during the course 

of the year. The magnitude of the change found in 2006 was consistent with the changes that 

have been observed in earlier years. Other interpretations of the data are possible, but from a 

practical point of view, the results by the new procedure are credible.
Redevelopment of the FWD Calibration Protocol and Equipment

48

Significance of the Differences

The differences in the calibration results using the two protocols were statistically significant. 

One conclusion from this may be that the new AASHTO protocol yields different results than the 

old SHRP protocol, but in practical terms, the differences are small. For example, for a 

deflection of 20 mil (500 μm), the average difference in the two protocols is only 0.05 mil

(1.3μm). It is questionable whether such a small difference has practical meaning, even if the 

statistics indicate it is significant. Additional differences include the following:

• Beam movement up to 0.12 mil (3 μm) was tolerated using the old protocol.

• The new protocol completely eliminated that problem, which could account for the 

difference in the two protocols.

• The standard deviation for multiple calibrations is ±0.3 percent.

• Some real change in the sensor response could have occurred over the course of the year.

It is possible that the new procedure is more accurate than the old SHRP procedure. The 

differences are consistent with the expected standard deviation for multiple calibrations. It is also 

encouraging that the variances were the same by each method. From a practical perspective, the 

small differences are not a concern. 

PRECISION AND BIAS STATEMENTS FOR FWD CALIBRATION

The pooled fund study focused on removing any possible bias error in the gain factors. Research 

found that no meaningful bias comes from either the load or deflection calibration procedures. 

The data cited in the previous section was used to formulate a precision and bias statement 

according to ASTM E-177.(17)

Using ASTM E-177 terminology, repeatability is defined as the overall variation expected at an 

individual FWD calibration center for all operators. Reproducibility is defined as the expected 

variation between single tests performed at multiple FWD calibration centers. Thus, repeatability 

was studied, but there is no information about reproducibility.

The series of calibration trials using the new procedure were used to define the precision and 

bias. The Dynatest® FWD was used for these tests. Multiple operators were involved to see if 

there was any difference between calibration operators. The calibrations were completed as 

quickly as possible (over a period of approximately 3 weeks) to minimize possible changes 

in the FWD.

Since this testing was on a single Dynatest® FWD at one location, the results should not be 

applied directly to all brands of FWDs or to multiple centers. However, the historical information 

about FWD calibration collected by Orr and Wallace and reviewed as part of this research shows 

that similar results can be expected for most other Dynatest® FWDs.(4)
Chapter 3. Development of the New Protocol

49

Load Cell

In total, 17 load cell calibration trials were completed by 3 FWD calibration operators. No 

differences between operators were found. The overall standard deviation of the final calibration 

factor was 0.00152. The 95 percent repeatability using a student's t-test used a multiplier 

of 2.11.

Precision and Bias Statement for Dynatest® Load Calibration

The 95 percent repeatability of the final gain factor for Dynatest® load calibration is 

0.321 percent. There is no bias expected in the final gain factor. No reproducibility statement

is available at this time.

Geophone Calibration 

Nine geophone calibration trials were completed by three calibration operators. No differences 

between operators were found. The overall pooled standard deviation of the final calibration 

factors for all nine geophones was 0.00245. By using the pooled standard deviation, the normal 

distribution multiplier of 1.96 may be used.

Precision and Bias Statement for Dynatest® Geophone Calibration

The 95 percent repeatability of the final gain factor for the Dynatest® geophone calibration is 

0.480 percent. There is no bias expected in the final gain factor. No reproducibility statement is 

available at this time.

QA PROCEDURES AND OPERATOR CERTIFICATION

A key support function under this pooled fund study is the annual QA and certification reviews 

at each center. These visits help keep each operator informed about the changes in the protocol, 

equipment, and software. They also ensure that the calibration procedures are being carried out 

correctly. The annual visits also promote communication and feedback, helping identify needed 

improvements in the software and the protocol based on operator experience.

QA procedures were developed in the early 1990s beginning with SHRP to be used annually at 

the FWD calibration centers. The procedures were reviewed and updated as part of the support 

function of the pooled fund study. During March and April 2008, the first QA visits were 

conducted. A report for each separate visit was sent to the calibration center and copied to 

FHWA. Each calibration operator received a certificate of compliance, which was valid for the 

following year. The certificate conveys that the person performing the calibration is

knowledgeable about the procedure.

Two checklists have been developed for use during the QA reviews. One is for reviewing the 

facilities and equipment, and the other is for reviewing the operator’s performance. The findings 

are reviewed with the calibration operator, and they become part of the QA visit report. Details 

about the QA procedures and examples of the checklists are included in appendix D.

51

CHAPTER 4. CONCLUSIONS AND RECOMMENDATIONS

CONCLUSIONS

The primary objectives of the pooled fund study have been achieved because the new protocol 

greatly increased the speed of FWD calibration without reducing the accuracy or ease of 

calibration. WinFWDCal is a Microsoft Windows®-based software program, and it guides all 

aspects of the new calibration procedure. 

The overall philosophy for calibration of the load cell and deflection sensors has not changed 

from the old SHRP procedure. Reference calibration and relative calibration are still the 

foundation on which the new protocol was developed. The major changes include the use of the 

self-referencing accelerometer, the updated data acquisition system, the WinFWDCal computer 

program, and the new multisensor deflection sensor calibration stands. 

A small but statistically significant shift in final calibration (gain) factors is evident when 

comparing results from the old SHRP procedure to the new AASHTO R32-09 procedure.(1) The 

average gain factor for the deflection sensors increased by less than 0.3 percent, while the 

average gain factor for the reference load cell increased by 0.4 percent. 

It is not possible to determine which procedure is correct, but these results are close to the 

statistical repeatability of the procedure, and they should not be a cause for concern. The new 

AASHTO procedure eliminates the beam movement problem that was inherent in the SHRP 

procedure. As a result, it is likely to be more accurate. 

The new procedure works well with all brands of FWDs. However, it is expected that there will 

continue to be a need for improvements as changes occur in FWDs, computer technology, and 

Microsoft Windows®. For example, the new Dynatest® truck-mounted FWD did not exist when 

the new AASHTO protocol was initially developed. In addition, access to the Carl Bro FWD was 

very limited during the development stage.

RECOMMENDATIONS

1. Review and update the calibration acceptance criteria.

There are still a few instances in WinFWDCal where the internal QC criteria are not 

decisively defined. Using available data along with best judgment, initial tolerances were 

chosen to get the procedure into use. Over time, use of the new AASHTO R32-09 protocol 

has provided additional data to help refine the critical criteria. Release of WinFWDCal

Version 2.0 includes changes that were created due to experience.

The following criteria should be reexamined as additional data become available:

• The allowable change for daily accelerometer calibration.

• The allowable change in gain factors from one calibration year to the next. 
Redevelopment of the FWD Calibration Protocol and Equipment

52

• The allowable rate of change per year in gain factors for FWDs with at least 4 years of 

calibration history. (It will be mid2012 before sufficient data are available to allow 

reconsideration of this criterion.)

• The allowable noise in the accelerometer signal in the interval before the release of the 

FWD mass.

• The allowable noise in the accelerometer signal in the interval just preceding the impact 

of the falling mass.

Additional criteria may require review as researchers gain experience applying the 

WinFWDCal program. Feedback from the calibration center operators has been a great 

source of information.

2. Continue to provide support, QA, and operator certifications for the calibration centers.

The calibration operators need ongoing support for several reasons. If they are experiencing 

issues calibrating an FWD, they need to communicate with people who can help them solve 

those problems. They need to know whether their equipment is working properly and if their 

results are believable. They also need a contact to offer suggestions for procedural 

improvements and help implement them.

The annual QA reviews and operator certification are an important part of the center support. 

They provide a mechanism for two-way communication about changes in the protocol, 

equipment, and software. They also help identify problems that the centers are encountering.

Since 2004, the pooled fund study has provided this support. Additionally, the SHRP 

calibration procedure was first introduced in 1992, and the Cornell Local Roads Program

(CLRP) has provided general support to the centers ever since. 

In fall 2010, AMRL provided QA reviews and operator certifications. As a result, the basic 

support was assured. Careful coordination and communication between AMRL and CLRP is

necessary to maintain the other aspects of general support.

3. Develop a means for interim certification of new calibration operators.

The retirement and promotion of certified calibration operators seldom occurs just before a 

QA visit is due. Since the calibration operator must be certified in order to calibrate FWDs, 

this loss of personnel often requires a temporary shutdown of the center until the new 

operator can be trained and certified. It is costly and seldom possible to immediately arrange 

for new operator training and certification, which is a 3-day process.

To avoid a prolonged closure period, a process whereby a certified operator can train and 

attest to the proficiency of the new operator should be developed. When the operator 

certifying agency (i.e., AMRL) receives notice, a temporary certificate should be issued 

covering the reminder of the pervious operator’s certification period. 
Chapter 4. Conclusions and Recommendations

53

This process avoids a situation where the QA schedule gets out of sync. When the regularly 

scheduled time for the QA visit arrives, the temporary certificate can be replaced with a 

regular certificate, and the normal schedule is maintained. A similar procedure should be 

defined for new calibration operators in overseas centers.

4. Continue to upgrade WinFWDCal and PDDXconvert and release revised versions.

Since the first public release of WinFWDCal in November 2006, several major releases of 

the software have gone out, and numerous small “bug fixes” have been made. While the rate 

of problem identification from the users has slowed down, programming support will be 

needed in the future. 

With any software package, there is a continual need for updates. Recommendations from the 

calibration centers and FWD owners need to be incorporated into the software as soon as 

possible to reduce the frustration of working with or around a bug.

5. Provide an electronic means to transfer the results (gain factors) of the calibration into the 

FWD computer.

The new calibration procedure provides the means for electronic transfer of data from 

FWD to the calibration computer via a PDDX file format. The new procedure also outputs 

the calibration results electronically in the same file format. Electronic data transfer avoids 

the errors that come from manual entry. Currently, it is not possible to read the electronic 

output file and update the FWD operating system (field program) with the new calibration 

factors, calibration date, etc. This will require the assistance and cooperation of the 

FWD manufacturers.

6. Improve the electronic data transfer procedures.

PDDXconvert only partially meets the needs for FWD calibration. Some of the necessary 

input data from FWD must still be entered manually or left out entirely. Also, several types 

of FWDs have their input data scattered among several different files.

The manufacturers need to agree on and provide a standard output file format from the FWD 

operating system that would overcome the input/output problem. It would put all needed data 

in a single file and in a harmonized format. This will require the assistance and cooperation 

of FWD manufacturers.

55

APPENDIX A. FWD CALIBRATION PROTOCOL

INTRODUCTION

This appendix describes the new FWD calibration protocol. It is the basis of the current version 

of the AASHTO R32-09 procedure, Standard Practice for Calibrating the Load Cell and 

Deflection Sensors for a Falling Weight Deflectometer.

(1) AASHTO R32-09 overrules any 

discrepancies with appendix A.

In May 2010, additional revisions to AASHTO R32-09 were submitted for review and approved 

by AASHTO. It is anticipated that the revised procedure will be published as AASHTO R32-11.

This appendix is supplemental to the forthcoming AASHTO R32-11, and it provides additional 

background information about the FWD calibration procedures.

This protocol is written for use with the four types of FWDs currently manufactured or sold in 

the United States. It is not applicable to the calibration of lightweight deflectometers or cyclic 

loading pavement testing equipment. Due to differences in design of the four types of FWDs, 

there are some special considerations for each type, which are described in the annexes.

This protocol does not cover the calibration of other parts of FWDs including temperature probes 

and distance-measuring instruments. The manufacturers’ recommended procedures should be 

used to calibrate those devices.

This protocol contains two procedures: annual calibration and monthly calibration. The 

deflection sensors must be removed from their holders on the FWD and installed in a calibration 

stand for either procedure. 

Annual calibration can either be performed at a FWD calibration center or at a site where the 

FWD is located. The same equipment and procedures are used at both locations. A certified 

technician is required to perform the procedure. Annual calibration involves two steps: reference 

calibration and relative calibration. In reference calibration, the FWD’s deflection and load 

transducers are calibrated against independently calibrated reference devices. In relative 

calibration, the deflection sensors are compared to each other. 

In monthly calibration, only relative calibration of the deflection sensors is completed at any 

suitable location using the calibration stand supplied by the FWD manufacturer. A certified 

technician is not required. Monthly calibration is completed for verification of the accuracy of 

the deflection sensors and occasionally when a sensor must be replaced. The procedure used for 

monthly relative calibration is different than for annual relative calibration.

The annual and monthly procedures result in gain factors or dynamic calibration factors 

which are entered into the FWD software as multipliers. When the FWD raw measurements 

are multiplied by the gain factors, the result is a value which has been corrected to agree with 

the calibration instrumentation. It is necessary to have a place to enter the gain factors 

in the FWD operating system software (also known as the field program) provided by 

the manufacturer.
Redevelopment of the FWD Calibration Protocol and Equipment

56

Frequency of Calibration

Annual calibration of the FWD load cell and deflection sensors should be performed at least 

once per year and as soon as possible after a sensor has been replaced. 

Monthly calibration should be performed on the deflection sensors at least once per month and 

immediately after a deflection sensor has been replaced.

WinFWDCal

The calibration protocol as described herein has been automated in a software package named 

WinFWDCal. It can be used for both annual and monthly calibration. Use of the program is 

required to carry out the procedure. 

ANNUAL CALIBRATION PROCEDURE

For annual calibration, the load cell and deflection sensors are calibrated with the goal of 

adjusting the accuracy (i.e., systematic error) of the devices to ±0.3 percent or better. 

Personnel

Annual calibration requires two people to perform the procedure. One person is the FWD 

operator, who is responsible for assuring that the FWD is in proper working order for the 

calibration. During calibration, the operator controls the FWD and removes and replaces the 

sensors in their holders.

The other person is the calibration operator and is a certified technician. The calibration operator 

ensures that the calibration equipment is maintained and calibrated as needed. During calibration, 

this person is responsible for operating the calibration computer and the specialized software 

used in the calibration of the FWD. In addition, the calibration operator is responsible for 

providing the documentation of the calibration exercise.

Before beginning a calibration, the FWD operator should present a signed checklist documenting 

the steps taken in preparation for the calibration, indicating certain preferences concerning the 

way the calibration should be performed (see annex 1 of appendix A). The FWD operator is 

responsible for programming the FWD computer to carry out the requested procedure. The FWD 

operator should provide the history of past calibration results for the FWD.

During calibration, moving the sensors and operating the specialized equipment is a shared 

responsibility, with the calibration operator having primary control over the calibration 

equipment. The FWD operator is responsible for transferring the FWD data from the FWD 

computer to the calibration computer in a format that can be read electronically (including 

providing a means for the transfers).
Appendix A. FWD Calibration Protocol

57

After completion of the procedure, the calibration operator should provide the FWD operator 

with a certificate of calibration that lists the final gain factors for the load cell and each 

deflection sensor. The FWD operator should enter the final gain factors in the FWD computer

and maintain a cumulative history of calibration results in the FWD computer as well as a history 

of calibration results for the FWD at the calibration center.

Equipment Preparation and Setup Before Calibration

During setup, FWD-specific information will need to be transferred from the FWD operating 

system (e.g., the FWD field program computer files) to the calibration computer. The annexes in 

this appendix describe the procedure for obtaining this information for each type of FWD.

FWD

The FWD needs to be in good operating condition prior to performing a calibration. A wellmaintained FWD is easier to calibrate and less prone to mechanical and electrical problems 

during calibration and general use. A checklist to help the FWD operator prepare the FWD for 

calibration is provided in annex 1. It should be filled out in advance, and a signed copy of the 

completed checklist needs to be provided to the calibration operator.

Before beginning any calibration work and throughout the entire calibration period, there should 

not be any data filters turned on in the FWD operating system. The FWD operator is required to 

verify that all smoothing or filtering has been turned off.

Prior to calibration, the FWD should be warmed up using the standard operating procedure 

for the particular brand of FWD. After the FWD is moved into position for calibration, it is 

important that it is level to minimize unwanted vibrations. Trailer-mounted FWDs should also be 

level to avoid unwanted vibrations.

FWD Drop Sequence

The FWD mass and drop heights/load levels should be set up to produce loads within 

±10 percent of the suggested loads shown in table 3. The FWD should be calibrated using three 

or four load levels. If only three load levels are used, the highest three load levels shown in 

table 3 should be used.

It is the FWD operator’s prerogative to specify the load levels that will be used for reference 

calibration. Other load levels may be substituted for those suggested in table 3. The range of 

loads used should reflect that which FWD normally uses in daily operation. However, in no 

instance should the combination of static plate load plus maximum dynamic load exceed 

24,000 lbf (106 kN). This limitation is required to protect the reference load cell and the 

concrete pavement used in the calibration procedure.
Redevelopment of the FWD Calibration Protocol and Equipment

58

During setup, the minimum number of drops at each load level will be determined by 

WinFWDCal based on the deflection response of the concrete pavement or test pad. More than 

the minimum number of drops may be used without exceeding 10 drops per load level. The 

FWD operator should program the drop sequence in the FWD computer, progressing from the 

lowest to the highest load level. The same number of drops should be used at each load level, and 

the same drop sequence should be used for both load and deflection sensor calibration. 

Table 3. Suggested dynamic load levels for reference calibration.

FWD Brand Load Level 1 Load Level 2 Load Level 3 Load Level 4

Carl Bro

6,000 lbf

(27 kN)

9,000 lbf

(40 kN)

12,000 lbf

(53 kN)

16,000 lbf

(72 kN)

Dynatest®

6,000 lbf

(27 kN)

9,000 lbf

(40 kN)

12,000 lbf

(53 kN)

16,000 lbf

(72 kN)

JILS

9,000 lbf

(40 kN)

12,000 lbf

(53 kN)

15,000 lbf

(67 kN)

18,000 lbf

(80 kN)

KUAB

6,000 lbf

(27 kN)

9,000 lbf

(40 kN)

12,000 lbf

(53 kN)

16,000 lbf 

(72 kN)

Note: The metric and U.S. customary values in this table are not exactly the same. FWD should be 

calibrated in one unit system or the other. The values in the table are rounded with intervals that are 

approximately equally spaced.

The software may inform users that the deflections are either too large or too small to satisfy the 

precision requirements for reference calibration. The vertical accelerations are also checked to 

ensure that they do not exceed ±5 g. If the deflection or the acceleration is too large, the FWD 

should be moved further away from the sensor stand. If the deflection is too small and if three 

load levels are used, users should try four load levels. If the deflection is still too small and the 

FWD cannot be moved closer to the sensor stand, users should try a sequence of higher load 

levels, if possible. If this does not solve the problem, users should find an alternative location to 

calibrate the deflection sensors.

FWD Calibration Equipment

Table 4 provides information on the equipment needed to perform the load cell and deflection 

sensor calibration. Detailed information for all components is found in appendix E of this report.

Both reference and relative calibration of all sensors should be performed on concrete pavement. 

The concrete floor area, or optional concrete test pad, should be in good condition with little or 

no cracking. The ball-joint base should be attached firmly to the concrete with two anchor bolts 

to hold the sensor stand in direct contact with the concrete. Additionally, the sensor stand should 

be clamped tightly in the base. Slippage, rocking, or vibration between the stand and the base or 

between the base and the concrete is not allowed. The ball-joint should rotate with slight friction.

Reference Load Cell Calibration

The reference load cell should be calibrated at least once per year in accordance with the 

AASHTO R33-03 procedure (see annex 7 of appendix A).(10)
Appendix A. FWD Calibration Protocol

59

Table 4. FWD calibration center equipment.

Equipment Notes

Reference load cell with signal 

cable

30,000 lbf (133 kN) maximum capacity; calibrated annually 

to 24,000 lbf (106 kN) using AASHTO R33-03(10)

Reference accelerometer with 

signal cable

± 5 g maximum acceleration range, calibrated on the day 

of use

Vishay 2310 or 2310B signal 

conditioner with power cable

Amplifies the output from the reference devices before 

analog to digital conversion

Keithley KUSB-3108 data 

acquisition board with cables

Converts the analog output signal into a 16-bit digital value; 

connected to Vishay and calibration computer

Calibration computer and 

WinFWDCal

Stores the analog-to-digital (A/D) output and performs the 

calculations needed for the calibration

Accelerometer calibration platform

Performs daily calibration of the accelerometer. Also used 

to store accelerometer in a +1 g field

Geophone calibration stand and 

hardware sets

Designed to be used with Dynatest®, JILS, and Carl Bro 

geophones; also used with KUAB geophones

Seismometer calibration stand Designed to be used with KUAB seismometers

Ball-joint base and anchor

Used with both the geophone and seismometer calibration 

stands

Protective shipping case

For storage and shipping of reference load cell, Vishay 

signal conditioner, Keithley DAQ, and related cables

Isolated concrete test pad

Designed to generate pavement deflections in the desired 

range; a test pad is recommended but not required

Accelerometer Calibration

The accelerometer is calibrated in Earth’s gravity prior to use for relative calibration to 

determine the daily response of the accelerometer. This calibration should be repeated after 8 h 

have elapsed.

The accelerometer, mounted in an aluminum box, should be calibrated using the calibration 

platform. The platform should be carefully adjusted using the bubble level to assure that the 

accelerometer is aligned with Earth’s gravity. The accelerometer needs to be calibrated in both 

±1 g fields by inverting the box briefly. Care must be taken to avoid dropping the accelerometer 

during the calibration process because the shock may cause damage. 

WinFWDCal will guide the calibration operator through the accelerometer calibration procedure 

to calculate the calibration coefficients. The accelerometer box should not be in the -1 g

orientation for more than 30 s during the calibration process to minimize the effect of hysteresis 

on the readings. If it is inverted for a longer period of time, the accelerometer calibration process 

needs to be stopped. Next, the box should be placed upright in a +1 g field for at least twice as 

long as it was inverted (up to a maximum of 24 h) to return it to equilibrium. For example, if the 

box is upside down for 1 min, it should be allowed to equilibrate for at least 2 min before 

repeating the calibration of the accelerometer. 
Redevelopment of the FWD Calibration Protocol and Equipment

60

The accelerometer calibration is slightly temperature sensitive. As a result, it is important that the 

accelerometer is calibrated shortly before its use. Temperature is monitored continuously by 

WinFWDCal. The program will alert the calibration operator if the temperature changes by more 

than ±18 °F (±10 °C). If this occurs, the accelerometer needs to be recalibrated.

FWD Calibration Procedure

It is recommended to complete that deflection sensor calibration first, followed by the load cell 

calibration. WinFWDCal should be used to collect data and make the associated QA checks. 

Table 5 shows the calibration data reporting requirements and the sources of the data. Most of 

the data are read electronically by the calibration computer from files copied from the FWD 

operating system. All of the data can be entered or updated manually using WinFWDCal.

Deflection Sensor Calibration

Deflection sensor reference calibration consists of two trials where all of the sensors are 

calibrated simultaneously in a special stand (see appendix F). The position of the sensors in 

the stand is inverted between the trials. This is followed by two relative calibration trials using 

the same stand, where the sensors are inverted once more. Spare deflection sensors should 

not be calibrated unless they have separate dedicated signal-conditioning channels in the 

FWD microprocessor. 

For reference and relative calibration, the sensor stand should be manually held and kept vertical 

(as indicated by the bubble level) while data are being collected so the accelerometer box will be 

correctly aligned with Earth’s gravity. The sensor stand should be supported by the rest stop 

between trials. 

Reference Calibration

Deflection sensor reference calibration consists of performing at least two trials in which all of 

the sensors are calibrated simultaneously in the sensor stand. The position of the sensors in the 

stand is inverted between each trial. Spare deflection sensors should not be calibrated unless the 

FWD has separate dedicated signal-conditioning channels.
Appendix A. FWD Calibration Protocol

61

Table 5. FWD calibration data reporting requirements.

Data Item Mode of Entry Source

Center Information

Calibration center location Automatic Center configuration file

Calibration operator name Automatic Center configuration file

Date and time of calibration Automatic Calibration computer

Temperature at calibration Automatic Data acquisition system 

Reference load cell serial number Automatic Center configuration file

Reference load cell calibration 

constants Automatic Center configuration file

Reference load cell calibration date Automatic Center configuration file

FWD Information 

FWD owner Manual FWD operator

FWD operator name PDDX file1 FWD computer

FWD Serial/ID number PDDX file FWD computer

FWD manufacturer PDDX file FWD computer

FWD load cell serial number PDDX file FWD computer or operator

FWD deflection sensor serial 

numbers PDDX file FWD computer or operator

Initial gain factor for FWD load cell PDDX file FWD computer

Initial gain factors for FWD 

deflection sensors PDDX file FWD computer

History of previous calibration 

results History file FWD computer

Calibration Data

Unit system used in calibration Manual FWD operator

Number of load levels Manual FWD operator

Number of replicate drops Computed WinFWDCal

Reference load cell readings Computed WinFWDCal

FWD load cell readings PDDX file FWD computer

Reference accelerometer readings Computed WinFWDCal

FWD deflection readings PDDX file FWD computer

Interim gain factors from reference

calibration Computed WinFWDCal

FWD relative calibration data PDDX file FWD computer

Adjustment factors from relative 

calibrations Computed WinFWDCal

Final gain factors Computed WinFWDCal 1

Indicates that the PDDX file is created by WinFWDCal from FWD native output.

The deflection sensors should be placed in the sensor stand and centered on the reference 

accelerometer. WinFWDCal displays a diagram showing how to arrange the sensors in the stand 

(see table 8 and table 9 in appendix B).
Redevelopment of the FWD Calibration Protocol and Equipment

62

The drop sequence for the first reference calibration trial should be performed. The operators 

should review and accept or reject the data for each drop. WinFWDCal will graphically display 

the deflection time history data after each drop.

Excessive drift from the baseline shown on the time history graph should be cause for rejection 

of a drop. The software will alert the user when drift is excessive.

At the conclusion of each trial, the user should transfer the data electronically from the FWD to 

the calibration computer and review the results. For each sensor, WinFWDCal will regress the 

FWD output (independent variable) versus the reference deflection sensor (dependent variable) 

forced through zero. The slope of the regression line for each sensor, when multiplied times 

the initial gain factor, gives the reference gain factor. The slope for an individual sensor is 

acceptable if its standard error does not exceed 0.0020. The trial is acceptable if the standard 

errors for all sensors do not exceed 0.0020.

If the first trial is acceptable, the user should continue with the second trial. The sensors in 

the stand should be inverted before the second trial according to the diagram displayed

by WinFWDCal. If any trial is not acceptable, the data should be rejected and the trial is 

repeated. The user should investigate the reason why the standard error exceeded 0.0020 and 

correct it, if possible, before repeating the procedure. The user must verify that all sensors are 

held firmly in the stand.

Interim Gain Factor Acceptance Criteria

After two reference calibration trials have been accepted, WinFWDCal will calculate the average 

reference gain factor for each sensor and display the results as the interim gain factors (one gain 

factor for each deflection sensor).

WinFWDCal will calculate the difference in the reference gain factors for each sensor between 

the two trials. If the difference for each sensor is no more than 0.005, then the reference 

calibration test is complete. If any of the differences is greater than 0.005, two additional trials

should be performed. The user should accept the results and continue but note if any of the 

differences is more than 0.005. The average of the reference gain factors for all trials for each 

sensor should be reported as the interim gain factors.

Relative Calibration

Relative calibration is followed by reference calibration and uses the same sensor stand. Two 

trials are performed. For each trial, 40 drops are applied from the highest drop height used in 

reference calibration. The sensors should not be repositioned in the sensor stand before the 

first trial.

WinFWDCal will adjust the FWD data collected in the relative calibration using the interim 

calibration factors internally. The user should not enter the interim factors in the FWD operating 

program before performing relative calibration.

To begin, the user should perform the drop sequence for the first relative calibration trial. At 

the conclusion of each trial, the data should be transferred electronically from the FWD to the 
Appendix A. FWD Calibration Protocol

63

calibration computer, and the results should be reviewed. For each sensor, WinFWDCal will 

calculate the means ratio. The means ratio multiplied times the interim gain factor gives the 

relative gain factor.

WinFWDCal performs an ANOVA test for the data and reports the standard error. The trial is 

acceptable if the standard error is not more than 0.12 mil (3 μm) and there are no extreme 

outliers in the data.

WinFWDCal will display a plot of the data for the 40 drops. The graph should be scanned to 

detect extreme outliers (i.e., due to a loose sensor in the stand). An extreme outlier would appear 

substantially outside the normal range of the deflection data.

If the standard error is greater than 0.12 mil (3 μm) or if there are extreme outliers in the data, 

the first trial is not acceptable, the data should be rejected, and the trial should be repeated. 

Note that the sensors in the stand should not be repositioned. The reason why the data are 

unacceptable should be investigated and corrected, if possible, before the procedure is repeated.

However, if the first trial is acceptable, the user should continue with the second trial. The 

sensors in the stand should be inverted before the second trial according to the diagram displayed 

by WinFWDCal. After two trials have been accepted, WinFWDCal will calculate the average 

relative gain factor for each sensor and report the results as the final gain factors, completing the

deflection sensor calibration procedure.

Note that if two acceptable relative calibration trials cannot be obtained after performing four 

trials, no further effort should be made to calibrate the deflection sensors. 

Load Cell Reference Calibration

If the reference load cell has not been calibrated within the past 12 months, it should be 

recalibrated in accordance with AASHTO R33-03.

(10)

Reference load cell calibration consists of at least two trials. The FWD load plate should not be 

raised at any time during the procedure. To begin, users should perform the drop sequence for 

the first reference calibration trial. After, they should review and accept or reject the data for 

each drop. WinFWDCal will graphically display the deflection time data after each drop.

At the conclusion of each trial, the data should be transferred electronically from the FWD to the 

calibration computer, and the results should be reviewed. WinFWDCal will regress the FWD 

output (independent variable) versus the reference load cell (dependent variable) forced through 

zero. The slope of the regression line for each sensor, when multiplied times the initial gain 

factor, gives the reference gain factor. The slope for the trial is acceptable if its standard error is 

not more than 0.0020.

If the first trial is acceptable, the user should continue with the second trial. However, if the first 

trial is not acceptable, the data should be rejected, and the trial should be repeated. The user 

should investigate the reason why the standard error exceeded 0.0020 and should correct it, if 

possible, before repeating the procedure. The load cell should sit squarely under the FWD load 

plate and squarely on the concrete. 
Redevelopment of the FWD Calibration Protocol and Equipment

64

Gain Factor Acceptance Criteria

After two trials have been accepted, WinFWDCal will calculate the average reference gain factor 

and report the results as the final gain factor. If the range of the two reference gain factors does 

not exceed 0.003, then the final gain factor should be accepted, completing the load cell 

calibration procedure. 

However, if the results of the first two trials are outside the acceptable range, a third 

reference calibration trial should be performed. If the standard deviation of the gain factors 

for three acceptable trials does not exceed 0.003, then the results of the three trials should be 

averaged and reported as the final gain factor for the load cell, thus completing the load cell 

calibration procedure.

If the standard deviation of the three trials exceeds 0.003, the reference load cell calibration 

procedure should be repeated, yielding a fourth reference gain factor. If the standard deviation 

of all calibrations (four acceptable trials) does not exceed 0.003, the average of all four results 

should be reported as the final gain factor for the load cell, and the load cell calibration 

procedure is complete. 

If acceptable results cannot be obtained after performing four trials, no further effort should be 

made to calibrate the load cell. 

Evaluation and Acceptance of Final Results

WinFWDCal will perform the needed calculations. The data should be evaluated as follows.

1. The final gain factors from this calibration should be compared to the corresponding gain 

factors from the previous calibration (i.e., the initial gain factors). There should not be more 

than a 1 percent difference for each individual deflection sensor and for the load cell. If this 

criterion is satisfied, the final gain factor for the sensor is accepted. However, if this criterion 

is not satisfied for a sensor, it should be evaluated according to the next criterion.

2. The final gain factor for the sensor should fall within the range of 0.980 to 1.020. If this

criterion is satisfied, the final gain factor for the sensor should be accepted. However, if this 

criterion is not satisfied for the sensor, it should be evaluated according to the next criterion.

3. If a historical record of previous calibrations according to this procedure is available for the 

sensor for 4 years or more and there are at least three previous calibration results over this 

time period, then the best fit time rate of change of the final gain factor for the sensor should 

not exceed 0.3 percent per year. If this criterion is satisfied, the final gain factor for the 

sensor is accepted.
Appendix A. FWD Calibration Protocol

65

Certificate of Calibration

If the final calibration results meet the acceptance criteria for all sensors according to one or 

more of the three criteria listed above, the calibration operator should provide the FWD operator

with a certificate of calibration listing the final gain factors for the load cell and each deflection 

sensor. These factors should be entered into the FWD computer. An output file in PDDX format 

is provided by WinFWDCal to facilitate electronic data transfer.

Report and Retention of Data

The FWD operator should be provided with an electronic copy and a hardcopy of the calibration 

results. The FWD calibration report should consist of the following:

• The certificate of calibration or a summary page of the results if the calibration was 

not successful.

• An electronic copy of the complete record of the annual calibration including the data 

transferred from the FWD.

• A pre-calibration checklist.

• Any other records should be annotated with the FWD unit identification 

(e.g., manufacturer's serial number or agency ID) and the calibration date.

Calibration records should be retained by the calibration center for at least 5 years.

MONTHLY CALIBRATION PROCEDURE

Monthly calibration serves two purposes. First, it is a means to verify that the deflection sensors 

are functioning properly and consistently. Second, it can also be used to replace a damaged 

sensor, providing a short-term gain factor for the replacement sensor until an annual calibration

can be performed.

Monthly calibration uses a calibration stand provided by the FWD manufacturer. The deflection 

sensors are stacked vertically in the stand so that all of the sensors are subjected to the same 

pavement deflection. Position in the stand may have an effect on the deflection readings. To 

compensate for this, the sensors are rotated through all positions in the stand. This rotation 

procedure is different from the relative calibration procedure performed for annual calibration.

Relative calibration relies on collecting a large amount of data that can be averaged to reduce 

the significance of random measurement errors. Deflections in excess of 20 mil (500 μm) are 

needed for the results to be accurate. WinFWDCal does the statistical data analysis to compute 

adjustment ratios and final gain factors from the data. 

Since a large number of drops are involved, the properties of the pavement materials may change 

due to compaction or liquefaction during the procedure. However, all of the sensors are equally 

affected, and as long as the effect is not too extreme, the adjustment ratios are still accurate.
Redevelopment of the FWD Calibration Protocol and Equipment

66

Some FWDs may have less than seven or more than nine active deflection sensors. If they 

do, these procedures should be modified to simultaneously calibrate the actual number of 

active sensors.

Equipment Preparation

FWD

The FWD should be in good operating condition prior to calibration. A well-maintained FWD is 

easier to calibrate and less prone to mechanical and electrical problems during calibration and 

during general use. A checklist to help the FWD operator prepare the FWD for calibration is 

provided in annex 1. 

Before beginning any calibration work and throughout the entire calibration period, there 

should not be any data filters in operation in the FWD operating system. The FWD operator

must verify that all smoothing or filtering has been turned off. Additionally, prior to calibration, 

the FWD should be warmed up using the standard operating procedure for the particular 

FWD brand.

Other Equipment

Additional calibration equipment includes the FWD calibration stand provided by 

the manufacturer.

Procedure Overview 

Replicate deflection readings should be taken with the sensors assembled in the calibration stand. 

With the sensors in a particular position in the stand, two unrecorded seating drops followed by 

five recorded drops constitute a set. The deflection sensors should be rotated through the various 

positions in the calibration stand in a prescribed way. The rotation procedure is shown in table 6

for a nine-sensor system and table 7 for a seven-sensor system. The total number of sets of data 

is equal to the number of sensors on the FWD. 

The test point (the location where the FWD load plate is positioned) should be “conditioned”

before beginning the calibration procedure to reduce the significance of the set in the data 

analysis. The warm-up drops should be used for this purpose. 

Monthly Calibration of the Deflection Sensors

Follow the WinFWDCal on-screen instructions for setup of the procedure as follows:

1. Remove all the deflection sensors from their holders on the FWD. For an FWD with n

sensors, number the sensors from D1 to Dn with respect to their normal position on the 

FWD. The center position is sensor number “1” on the Dynatest®, Carl Bro, and JILS FWDs 

and sensor number “0” on the KUAB FWD. In either case, the center sensor should be 

defined as D1 for this procedure. 
Appendix A. FWD Calibration Protocol

67

2. Label the levels on the sensor stand from “A” to “G” or “I” as appropriate. The top level is 

labeled “A.”

3. Position the deflection sensors in the stand for the first set as shown in table 6 or table 7.

4. Position the sensor stand vertically. Mark the location where it rests so that it can be 

relocated precisely on the same spot. This should be performed by gluing a washer to the 

pavement or by chipping a small divot in the pavement with a chisel or a screwdriver.

5. Select the FWD drop height and the distance from the loading plate to the sensor stand to 

yield deflections near 20 ±4 mil (500 ±100 μm). If deflections in this range cannot be 

achieved, choose another location. A concrete pavement on a relatively weak subgrade will 

usually yield the required deflection.

6. Lower the FWD loading plate. If the FWD operating system allows, do not raise the loading 

plate or move the FWD during relative calibration testing.

7. Make two seating drops (no data recorded) for each set followed by five replicate drops 

(for which data should be recorded).

8. Rotate the sensors in the stand at the end of each set as shown in table 6 and table 7. The 

general progression is for the sensors to move from the bottom toward the top of the stand.

Note: The rotation must be performed as prescribed for the data analysis in WinFWDCal to 

work properly. If the direction of rotation is reversed, the calculations will be incorrect.

9. Record 45 drops for a 9-sensor FWD and 35 drops for a 7-sensor FWD.

Table 6. Monthly calibration sensor positions by set for a nine-sensor FWD.

Stand 

Position

Deflection Sensor Number in the Stand

1 2 3 4 5 6 7 8 9

A (top) D1 D2 D3 D4 D5 D6 D7 D8 D9

B D2 D3 D4 D5 D6 D7 D8 D9 D1

C D3 D4 D5 D6 D7 D8 D9 D1 D2

D D4 D5 D6 D7 D8 D9 D1 D2 D3

E D5 D6 D7 D8 D9 D1 D2 D3 D4

F D6 D7 D8 D9 D1 D2 D3 D4 D5

G D7 D8 D9 D1 D2 D3 D4 D5 D6

H D8 D9 D1 D2 D3 D4 D5 D6 D7

I 

(bottom) D9 D1 D2 D3 D4 D5 D6 D7 D8
Redevelopment of the FWD Calibration Protocol and Equipment

68

Table 7. Monthly calibration sensor positions by set for a seven-sensor FWD.

Stand 

Position

Deflection Sensor Number in the Stand

1 2 3 4 5 6 7

A (top) D1 D2 D3 D4 D5 D6 D7

B D2 D3 D4 D5 D6 D7 D1

C D3 D4 D5 D6 D7 D1 D2

D D4 D5 D6 D7 D1 D2 D3

E D5 D6 D7 D1 D2 D3 D4

F D6 D7 D1 D2 D3 D4 D5

G 

(bottom) D7 D1 D2 D3 D4 D5 D6

Monthly Calibration Data Analysis

The data file should be transferred electronically from the FWD to WinFWDCal for analysis. 

Options provided in the software indicate whether a normal data analysis or a special analysis is 

required for sensor replacement. 

Adjustment of Gain Factors—Normal Analysis

WinFWDCal will report the adjustment ratios and the gain factors for each deflection sensor. 

Since sensor replacement is not involved, the adjustment of the gain factors in the FWD 

operating system should be made only when those changes are both significant and verified to 

be necessary. The following guidelines should be used to evaluate the need to adjust the 

gain factors:

• If all of the adjustment ratios fall between 0.997 and 1.003, then they are not statistically 

significantly different from a ratio of 1.000. The calculated adjustments are trivial, the 

calibration test is complete, and no change of the gain factors should be made.

• When the adjustment ratios for one or more sensors fall outside of the range 0.997 to 

1.003, a second relative calibration trial should be performed. If the gain factors for each 

sensor in both trials agree within 0.003, the need for the adjustment has been verified. 

The gains should be adjusted for all sensors. The gain factors for each sensor for the two 

trials should be averaged and entered in the FWD operating system, and the calibration 

test is complete.

• If the difference in the gains for the two trials for one or more sensors is greater than 

0.003, two additional trials should be performed. If the second attempt also exceeds the 

allowable difference, the annual calibration procedure should be performed as soon 

as possible.
Appendix A. FWD Calibration Protocol

69

Adjustment of Gain Factors—Sensor Replacement

When replacing a damaged deflection sensor, the monthly calibration procedure should be used 

to determine a gain factor for the replacement sensor. The calculations are performed by 

WinFWDCal, and a gain factor is reported only for the replacement sensor.

Two relative calibration trials should be performed. If the two gain factors agree within 0.003, 

then the gain factors for the two trials should be averaged and entered in the FWD operating 

system, and the calibration test is complete. However, if the difference is greater than 0.003, two 

more relative calibration trials should be performed. WinFWDCal will report the average gain 

factor from the four trials and the standard deviation. If the standard deviation is not more than 

0.003, then the average gain factor should be entered in the FWD operating system, and the

calibration test is complete. If the standard deviation is more than 0.003, no further effort should 

be made to calibrate the replacement sensor, and the annual calibration procedure should be 

performed as soon as possible.

Report

The relative calibration report consists of all printouts from WinFWDCal annotated as necessary 

to explain any problems which may have occurred.
Redevelopment of the FWD Calibration Protocol and Equipment

70

ANNEX 1. PRE-CALIBRATION CHECKLIST

Fill out and bring this checklist with you to the calibration center. Your signature below 

indicates that you have met all of the pre-calibration requirements.

FWD Operator:

FWD ID Numbers:

(serial and/or model 

numbers)

FWD Manufacturer:

FWD Owner Agency:

 Inspect all connections, fittings, and cables and repair or replace those which are damaged. 

Damaged cables and bad connections can and will cause inaccuracies in deflection data.

 Assure that the load plate swivel is properly lubricated, if applicable, and that all bolts are tight. 

Refer to your owner’s manual for instructions. Use the 12-inch or 300-mm diameter load plate 

during calibration.

 Remove the rear sensor extension bar if it is currently installed.

 Clean and inspect all sensors and signal cables. (Deflection sensors are removed from their 

holders during calibration.) Fine grained emery cloth is useful for cleaning the magnetic bases 

of Dynatest® sensors. Remove all stones embedded under the load plate.

 Provide a USB thumb drive or a formatted 31⁄2" diskette for transfer of the FWD data to the 

calibration computer.

 Store your operating manuals in the FWD vehicle in case they are needed.

 Check the integrity of the batteries with a hydrometer or load tester. Clean the battery terminals 

and cables of corrosion.

 Check hydraulic fluid level(s) and assure that they are at the proper fill point. Inspect the 

hydraulic system for leaks. Replace the hoses if necessary.

 Select the system of units and the load levels to be used for calibration.

Unit system: Target load levels:

 US Customary(lbf) 1. ___________________ 2. ____________________

 Metric 3. ___________________ 4. ____________________

Adjust or calibrate the FWD to achieve the target levels within ±10 percent.

 Verify that the proper drop sequences are programmed into the FWD software for both 

reference and relative calibration. Name and save the setup files.

 Turn off filtering (smoothing) in the FWD operating system (if applicable).

 Have electronic data files or hardcopies from the previous calibration(s) available.

Operator’s signature:
Appendix A. FWD Calibration Protocol

71

ANNEX 2. SPECIAL PROCEDURES FOR CALIBRATING A DYNATEST® FWD

1. The FWD operating system (i.e., the field program) should be programmed to provide the 

output from the FWD in the F10, F20, or F25 format. All file extensions should be Fnn, 

where nn is the version number of the FWD field program. This format is needed so that 

PDDXconvert can read the FWD file for WinFWDCal.

2. Save an F25 file in addition to the Microsoft Access database file for use with 

PDDXconvert for FWDs using Dynatest®’s FWDWin as the operating system. 

3. Warm up the FWD off the calibration test pad. Save the data in a file named “Warmup.Fnn,”

where nn is the version number of the FWD field program. 

Note: During the setup procedure in WinFWDCal, the Warmup.Fnn file will be read 

electronically to get information on sensor serial numbers, initial gains, etc. 

4. Power the FWD with a battery charger. Without a charger, the FWD electrical system 

may not have enough power for a full calibration. Charging the FWD should be done in 

accordance with the manufacturer’s recommendations. Some chargers may cause electrical 

noise on the time history signal during reference calibration.

5. Attach the trailer to the tow vehicle during calibration for trailer-mounted FWDs.

6. For the Dynatest® FWD, it is possible to be within the tolerance for the highest load and have 

the drop height set too high. Before placing the reference load cell under the load plate and 

with the mass positioned at drop height four (the highest position), users must verify that 

there will be no interference between the catch mechanism and the brace between the two 

columns that surround the cylinders that raise and lower the load plate. The reference load 

cell is nearly 3.875 inches (99 mm) high, so the catch mechanism will be that much higher 

during load calibration. If the clearance is too small, the target for the fourth drop height 

should be repositioned to provide the needed clearance.

7. Remove the removable masses to get the peak load from the highest drop height into 

the range of 16,000 lbf (71kN) ±10 percent for the Dynatest® HWD. This will avoid 

exceeding the capacity of the reference load cell and avoid damaging the test pad due to 

excessive deflections.

8. Clean out all stones that are embedded in the rubber pad under the load plate. The FWD 

operator should complete this cleaning before arrival at the calibration center.

9. Assemble the calibration stand hardware before FWD arrives. The knurled knob should be 

placed above the shelf in the calibration stand (see figure 33). The knurled knob should only 

be tightened by hand. Do not use a wrench to attach the hardware.

10. Clean the bottom of the geophones and remove any small particles that will interfere with 

proper contact of the magnetic mounts. Emery paper does a good job of this. The FWD 

operator should complete this cleaning before arrival at the calibration center.
Redevelopment of the FWD Calibration Protocol and Equipment

72

11. Place a geophone on the knurled knob in the stand and try to rock it lightly back and forth. 

If there is any motion, reclean the bottom of the sensor. Each geophone must be securely 

fastened in the stand.

12. Place a wooden block under the FWD’s raise-lower bar during load cell calibration to reduce 

the noise and vibration. 

13. Ensure that the air in the large hydraulic cylinder has been bled out. Failure to bleed the air 

from the hydraulic system will result in vibration when the catch is released. This will delay 

the calibration due to excess noise messages. The FWD operator should perform the bleeding 

before arrival at the calibration center.

14. Verify that the new calibration gains have been entered correctly into the FWD field program 

by printing a summary of the relative gains. The relative gain is the value that is determined 

from the calibration. Do not change the reference gains because those are determined by 

the manufacturer.

Figure 33. Photo. Attachment of a Dynatest® geophone in the stand.
Appendix A. FWD Calibration Protocol

73

ANNEX 3. SPECIAL PROCEDURES FOR CALIBRATING A JILS FWD

1. The FWD operating system (i.e., the field program) should be programmed to provide output 

with DAT file extensions. This format is needed so that PDDXconvert can read the FWD file 

for the WinFWDCal software.

2. The JILS calibration operating system software will require the FWD operator to release 

each drop manually. It is used during reference calibration. JTEST is used during relative 

calibration. 

3. Warm up the FWD off the calibration test pad. Save the data in a file named 

“WARMUP.DAT.”

Note: During the setup procedure in WinFWDCal, there are three files from the FWD 

computer that need to be read electronically in order to obtain information about the FWD 

sensor serial numbers and gains. The WARMUP.DAT file contains the deflection sensor 

serial numbers, the SETUP.PAR file contains the deflection sensor current gains, and the 

JILS.CFG file contains the load cell current gain. 

4. Type the sensor gains or serial numbers manually into the WinFWDCal setup screen if the 

sensor gains or serial numbers cannot be read electronically.

5. Read the three configuration files and verify that each serial number is different. If they are 

not, rename the sensor numbers on the WinFWDCal setup screen with a number matching 

the sensor position on the FWD. For example, if all of the sensor serial numbers are 

“CHOP,” rename them “CHOPn” where n is the number of the sensor starting at 1 for the 

center sensor at the load plate.

6. Attach the trailer to the tow vehicle during calibration for trailer-mounted FWDs.

7. Shim under the wheels as needed to assure both vehicles are level. The tongue wheel can also 

be used to help level the trailer. Both the tow vehicle and the trailer must be level when the 

FWD load plate is on the floor.

8. Check that the truck-mounted FWDs are level when the load plate is down. Check this on the 

chassis with a carpenter's level. Shim under the wheels if necessary to get the vehicle level.

9. Check that the load plate is squarely seated on the reference load cell and on the concrete test 

pad. The JILS load plate does not have a swivel, so the load plate may only be making partial 

contact. When good contact is achieved, it will not be possible to slide a piece of copy paper 

under the load plate more than a fraction of an inch (a few millimeters) at any point around 

the perimeter of the plate. Apply several seating drops (no data recorded) before making 

this check.

10. Remove the geophone sensor bar attached to the back of the FWD to allow the FWD to get 

close to the ball-joint anchor and the geophone stand. 
Redevelopment of the FWD Calibration Protocol and Equipment

74

11. Attach each geophone in the calibration stand with the knurled knob below the shelf as 

shown in figure 34. The knurled knob should only be tightened by hand. Do not use a wrench 

to attach the hardware.

12. Check that the quick connectors for the geophones are working properly. If they are not 

making good electrical contact, the FWD may provide intermittent noise or a sensor may fail 

to respond at all. 

13. Power the FWD with a battery charger. Without a charger, the FWD electrical system may 

not have enough power for a full calibration. Charging the FWD should be performed in 

accordance with the manufacturer’s recommendations. Some chargers may cause electrical 

noise on the time history signal during reference calibration.

14. Verify that the new calibration gains have been entered correctly into the FWD field program 

by printing a copy of the WARMUP.DAT, SETUP.PAR, and JILS.CFG files. The deflection 

sensor serial numbers are in the line that begins “Sensors:” near the top of the file. The 

geophone gains are in the SETUP.PAR file in the line below the text “# Sensor channels EU 

conversion factors:” The load cell gain is in the JILS.CFG file. 

Figure 34. Photo. Attachment of a JILS geophone in the stand.
Appendix A. FWD Calibration Protocol

75

ANNEX 4. SPECIAL PROCEDURES FOR CALIBRATING A KUAB FWD

1. The FWD operating system (i.e., the field program) should be programmed to provide output 

of with FWD file extensions. This format is needed so that PDDXconvert can read the FWD 

file for the WinFWDCal software.

2. The FWD operating system software may require the FWD operator to release each drop 

manually. It may not be possible to program the desired drop sequence. In order to provide 

the pause needed after each drop during reference calibrations, the KUAB operating system

software should be set up to perform manual drops. 

3. During the setup procedure in WinFWDCal, the FVO.INI file from the FWD computer will 

be read electronically to get information on sensor serial numbers, initial gains, etc.

Note: If the sensor initial gains or serial numbers cannot be read electronically, type the 

values into the WinFWDCal setup screen.

4. Power the FWD with a battery charger. Without a charger, the FWD electrical system 

may not have enough power for a full calibration. Charging the FWD should be done in 

accordance with the manufacturer’s recommendations. Some chargers may cause electrical 

noise on the time history signal during reference calibration.

5. Attach the FWD trailer to the tow vehicle during calibration.

6. Conduct a static calibration of the seismometers before the reference calibration procedure is 

performed. The KUAB software will automatically file the static calibration factors.

7. It will only be possible to calibrate the 11.7-inch (300-mm) load cell on the KUAB. If the 

KUAB is outfitted with load cells with a diameter larger or smaller than 11.7 inches 

(300 mm), they should not be calibrated using the 11.7-inch (300-mm)-diameter reference 

load cell because the results would not be accurate.

8. Locate the FWD load plate as close as possible to the ball-joint anchor. This may require 

aligning the FWD beside the seismometer stand. Move the FWD into position before putting 

the seismometer stand in the ball-joint to avoid possible damage to the stand. The FWD 

should not touch the calibration stand during calibration. 

9. Use the special two-column seismometer stand for deflection sensor calibration for KUAB 

FWDs with seismometers. The sensors are mounted on standoffs that are the same size as 

the ones on the FWD. After the set screw has been tightened, the seismometer should be 

firmly attached, and it should not wobble or rock on the standoff. Do not over tighten the set 

screw because the hole may be stripped. Place the geophones in the calibration stand as 

diagrammed by WinFWDCal. 

Note: The deflection sensor that is mounted through the load plate (i.e., the center sensor) 

is called sensor number zero on KUAB, but it is sensor number 1 as far as WinFWDCal

is concerned.
Redevelopment of the FWD Calibration Protocol and Equipment

76

10. Keep the stand aligned perpendicular to the load pulse from FWD during calibration of the 

seismometers. The easiest way to do this is to stand facing the load plate of FWD.

11. Attach each geophone in the calibration stand with the knurled knob below the shelf after 

removing the deflection sensors from the FWD as shown in figure 34. KUABs outfitted with 

geophones use the same one-column calibration stand used for Dynatest® geophones. The 

knurled knob should only be tightened by hand. Do not use a wrench to attach the hardware.

12. Place the geophones in the calibration stand as diagrammed by WinFWDCal. A magnetic 

collar holds the KUAB geophone to the calibration stand as shown in figure 35 and figure 36.

A special magnetic cup holds the collar to the stand. 

13. Enter the final gain factors for the deflection sensors in the sharpcalfa variable within the 

applicable [sensor#] section of the FVO.INI file. “#” refers to the sensor number with the 

numbering starting at zero.

14. Enter the final gain factor for the load cell in the sharpcalfa variable within the [loadplate] 

section of the FVO.INI file, if available. If this section is not in the file, the load cell testing 

should be considered to be calibration verification only.

15. Verify that the new calibration gains have been entered correctly into the FWD field program 

by printing a copy of the FVO.INI file. 

Figure 35. Photo. KUAB geophone parts.
Appendix A. FWD Calibration Protocol

77

Figure 36. Photo. Attachment of a KUAB geophone in the stand.
Redevelopment of the FWD Calibration Protocol and Equipment

78

ANNEX 5. SPECIAL PROCEDURES FOR CALIBRATING A CARL BRO FWD

1. The FWD operating system (i.e., the field program) should be programmed to provide output 

with FWD file extensions. This format is needed so that PDDXconvert can read the FWD file 

for WinFWDCal.

2. The FWD operating system software may require the FWD operator to release each drop 

manually. It may not be possible to program the desired drop sequence. 

In order to provide the pause needed after each drop during reference calibration, the 

operating system software can be set up to perform manual drops without raising the plate 

only after performing the seating drops (no data recorded) and at least one recorded drop. 

The FWD operator should prepare a list of the drops needed ahead of time and be prepared to 

reject the first drop after the seating drops.

3. In order to obtain the sensor serial numbers and the initial gains, use the RoSy® software 

from Carl Bro to create a text file containing these values. The file should be named 

“HW_INI.TXT.”

Note: During the setup procedure in WinFWDCal, the HW_INI.TXT file will be read 

electronically to get information on sensor serial numbers, initial gains, etc.

4. Type the values of the sensor initial gains or serial numbers into the WinFWDCal setup 

screen if they cannot be read electronically.

5. Attach the trailer to the tow vehicle during calibration for trailer-mounted FWDs.

6. The Carl Bro FWD uses a generator on the trailer to provide the electrical power for 

operations. If possible, warm up the FWD outside and fully charge the batteries prior to 

moving the FWD indoors. This will allow the calibration to proceed without turning on the 

generator any more than necessary.

7. Locate the FWD load plate as close as possible to the ball-joint anchor. This may require 

placing the geophone stand inside the rear of the FWD frame. Move the FWD into position 

before putting the geophone stand in the ball-joint to avoid possible damage to the stand. The 

FWD should not touch the calibration stand during calibration.

8. Remove the deflection sensors from the FWD and attach each geophone in the calibration 

stand with the metric knurled knob below the shelf as shown in figure 37. The knurled knob 

should only be tightened by hand. Do not use a wrench to attach the hardware.

9. The Carl Bro operating system software does not have a specific place for the new gain 

factors, so the final gain factors must be multiplied by hand and input into the FWD 

operational software in the correct place. Follow the manufacturer’s recommendations for 

this step.
Appendix A. FWD Calibration Protocol

79

10. Verify that the new calibration gains have been entered correctly into the FWD field program 

by printing a copy of the updated HW_INI.TXT file. Use the RoSy® software from Carl Bro 

to create this text file.

Figure 37. Photo. Attachment of a Carl Bro geophone in the stand.
Redevelopment of the FWD Calibration Protocol and Equipment

80

ANNEX 6. SPECIAL PROCEDURES FOR ON-SITE CALIBRATIONS

The calibration protocol allows users to perform annual calibrations at locations other than a 

calibration center. The equipment is highly portable, and it may be advantageous to perform the 

calibration where the FWD is located rather than shipping the FWD to a center. This procedure 

is called on-site calibration because it is performed at a site preferred by the FWD owner.

The calibration procedure outlined in appendix A should be used without exception. It must be 

carried out by a certified calibration center operator as required in AASHTO R 32-09.(1)

A special test pad is not required to be constructed for on-site calibrations. In this regard, 

AASHTO R 32-09 indicates the following:

(1)

While an isolated test pad is recommended, it is not required, provided that all 

other facilities requirements, especially the minimum slab deflection requirement 

and sufficient slab damping, are achieved. The slab dimensions (4 by 5 m) are 

suggested, and other dimensions may be satisfactory ... In general, a concrete 

pavement on a relatively weak subgrade will yield the required deflection 

amplitude.

As previously noted in this report, the recommended peak deflection is 20 mil (500 μm), and the 

minimum recommended deflection is 12 mil (300μm). Peak deflection is a useful way to identify 

suitable locations for performing calibrations. However, deflection is not the limiting criterion. 

WinFWDCal checks for the following two conditions:

• The maximum acceleration must not exceed 5 g.

• The maximum number of replicate drops must not exceed 10 drops at each load level.

The difficult part of on-site calibration is finding a suitable place to mount the ball-joint anchor 

and the calibration stand. Figure 38 shows a calibration being performed at the Hawaii 

Department of Transportation in a breezeway between two buildings. The concrete pavement at 

this location was relatively thin, and when the FWD was close to the calibration stand, the 

maximum acceleration exceeded 5 g. To overcome this problem, the FWD trailer was moved 

about 24 inches (600 mm) in front of the stand, thereby reducing both the peak deflections and 

maximum accelerations.
Appendix A. FWD Calibration Protocol

81

Figure 38. Photo. On-site FWD calibration at Hawaii Department of Transportation 

Materials Lab.

Note that the FWD must be level during the calibration. It is also important to have the 

calibration equipment protected from the weather at all times. To assess the suitability of a 

location for FWD calibration, it may be helpful to use the Facilities Review Form found in

figure 45 in appendix D.

A procedure involving an FWD and one deflection sensor is used to find a place to install the 

ball-joint anchor. The sensor is removed from its holder and held in place by hand on the 

pavement. A firm downward pressure is required to keep the sensor in contact with the 

pavement. A concrete pavement works better than an asphalt pavement because it spreads the 

deflection basin over a wider area. The procedure is as follows:

1. Position the deflection sensor 3.9 inches (100 mm) from a joint in the concrete. Position 

the FWD so the rear of the load plate is 30 inches (750 mm) ahead of the deflection sensor 

(34 inches (850 mm) from the slab joint). If there is no pavement joint, draw a line across the 

pavement that can be used to measure from.

2. While manually holding the sensor on the, floor (see figure 39), perform one seating drop (no 

data recorded) at approximately 6,000 lbf (27 kN) followed by two drops at approximately 

16,000 lbf (70 kN). Record the peaks for both drops, and refer to it as position #1.

Note: If there is any concern that a 16,000-lbf (70-kN) load will damage the test pad, use a 

lesser load level and scale the measured deflection linearly to 16,000 lbf (70 kN).
Redevelopment of the FWD Calibration Protocol and Equipment

82

3. Repeat the above procedure with the deflection sensor positioned at 7.8, 11.7, 17.55, and 

23.4 inches (200, 300, 450, and 600 mm) from the rear of the test pad. Keep the spacing 

between the sensor and the rear of the FWD load plate constant at 29.25 inches (750 mm). 

Name the positions sequentially (#1, #2, #3, etc.).

4. Plot the peak deflection at 16,000 lbf (70 kN) versus distance from the joint or the line. Find 

the distance where the deflection is roughly 20 mil (500 μm). Install the two stainless steel 

inserts in the floor at that location as described in the Concrete Anchor Installation section in 

appendix E.

Care should be taken to avoid placing the ball-joint anchor too close to a joint or the edge of the 

pavement. It is often the case that the edge raises before it deflects downward. This can lead to 

vibration and excess noise problems in the data. Based on experience, a minimum distance of 

11.7 inches (300 mm) is recommended between the anchor and the edge of the pavement.

After the ball-joint anchor and the calibration stand have been installed (see figure 39), go 

through the “Set Trigger” and “Determine Number of Drops” routines in WinFWDCal. Adjust 

the position of FWD either closer to the calibration stand or further away as needed to get a 

successful reference calibration.

Once a suitable place for on-site calibration has been found, it can be used repeatedly for future 

annual FWD calibrations. To protect the two stainless steel inserts from filling up with dirt, it is 

wise to put an Allen® head set screw in each anchor flush with the top of the holes. 

Figure 39. Photo. Locating a place to install the ball-joint floor anchor.
Appendix A. FWD Calibration Protocol

83

ANNEX 7. REFERENCE LOAD CELL CALIBRATION PROCEDURE

INTRODUCTION

This appendix describes the new reference load cell calibration protocol in detail. It is the basis 

for the current version of the AASHTO R33-03 procedure.(10) AASHTO R33-03 overrules any 

discrepancies with this appendix.

The reference load cell is a custom-made precision instrument that is capable of measuring 

loads within ±0.3 percent or better. However, such a high degree of precision can only

be attained if this calibration procedure is followed exactly. It is essential that the reference 

load cell is calibrated using a universal testing machine that is properly maintained and 

accurately calibrated.

The reference load cell and its signal cable, the associated signal conditioner, and DAQ should 

be considered a system of instruments that should be calibrated and used together. The load cell 

should be calibrated to a maximum load of 24,000 lbf (100 kN).

This procedure is written with both U.S. customary and metric units shown. The calibration 

operator should choose one unit system and follow the procedure using the values shown. The 

values are not meant to be a direct conversion. The values are chosen to provide regular steps 

and ranges with whole numbers where possible.

This procedure has been automated and is included in the RefLCCal computer program.(18)

FREQUENCY OF CALIBRATION

Calibration of the reference load cell should be performed at least once per year. It should also be 

performed immediately after any of the machine screws that attach the load measurement links to 

the upper or lower plates are loosened. Calibration may also be necessary if the load cell fails to 

pass the unbalanced zero test during FWD annual calibration.

EQUIPMENT

The following list provides information on the necessary equipment for reference load 

cell calibration:

• Universal testing machine.

A static testing machine (hydraulic or screw-powered) with a load capacity of 100,000 lbf

(500 kN) or more should be used. Although the reference load cell will only be calibrated 

to a capacity of 100 kN (24,000 lbf), the higher capacity of the testing machine assures 

that the test frame will be adequately rigid. The testing machine should have several 

load ranges, among them a zero–30,000 lbf (zero–130 kN) range. Slightly different 

ranges (i.e., zero–24,000 lbf (zero–100 kN)) would be acceptable as long as the entire 

range of the load cell can be calibrated on a single range of the universal testing machine. 

Care must be taken to avoid overloading the reference load cell during calibration.
Redevelopment of the FWD Calibration Protocol and Equipment

84

Do not use a servo-controlled, closed-loop testing system for this purpose. In general,

such equipment does not provide a high degree of accuracy that is required for 

this calibration.

• Special wood/aluminum bearing blocks for placement above and under the reference 

load cell.

• Custom manufactured load cell with its signal cable, associated signal conditioner, and 

DAQ. The load cell has three measurement links contained in aluminum housing. 

• A Vishay 2310 signal conditioner, which should be the same signal conditioner that will 

be later used in the FWD calibration procedure.

• Keithley model KUSB-3108 DAQ, which should be the same A/D board that will be later 

used in the FWD calibration procedure.

• Pushbutton trigger for activating the data acquisition system.

Equipment Calibration 

The universal testing machine should be calibrated annually by a certified technician according

to ASTM E-74.(19) The calibrated machine should have a certified accuracy of 1 percent or 

better. The load indication system used for calibration should be traceable to NIST. The 

calibration certificate should be evaluated using a multinomial regression procedure to develop 

an adjustment algorithm (up to fifth order) that adjusts the indicated load on the universal testing 

machine to the corrected NIST traceable calibrated load. The load calculated by use of the 

adjustment algorithm is referred to herein as the adjusted load, while the load indicated on the 

testing machine dial is referred to as the indicated load.

The testing machine calibration coefficients and the date of calibration of the testing machine 

should be entered into the FWDCalCenterConfig.ini file used by WinFWDCal prior to 

calibrating a reference load cell.

The Vishay 2310 signal conditioner amplifier should be balanced according to the procedure 

described in the manufacturer’s instruction manual. With the signal input terminals shorted 

together, at gain 100, the alternating current (a/c) noise on the ±10-V output terminals should be 

1 mV or less.

Equipment Preparation

Load Cell Conditioning

A new load cell or one that has had the lid removed must be conditioned before being calibrated. 

1. Use a torque wrench to tighten the machine screws on the top and the bottom of the load cell 

to 100 lbf-inch (11.3 N-m). Apply at least 100 conditioning drops on the load cell from the 

16,000-lbf (72 kN) load level with the FWD. Remove the machine screws one at a time, 
Appendix A. FWD Calibration Protocol

85

apply medium strength Loctite® to the threads, and torque to precisely 100 lbf-inches 

(11.3 N-m).

2. Apply another 100 conditioning drops on the load cell from the 16,000-lbf (72-kN) load level 

with the FWD. Record the unbalanced zero for the load cell after each 25 drops. It should

change less than 1 mV during 25 load cycles. Continue applying additional load cycles until 

the unbalanced zero stabilizes, and apply at least 20 cycles of 24,000 lbf (100 kN) to the load 

cell with the universal testing machine. Apply additional cycles, if necessary, until the 

unbalanced zero stabilizes.

Equipment Inspection and Setup

1. Inspect the reference load cell carefully before calibration. Verify that the cable and the cable 

connectors fit and lock tightly and that there are no breaks in the wires. Verify that the 

machine screws on the load cell are tight.

2. Verify that one of the wood/aluminum bearing blocks has a ribbed rubber pad cemented to it. 

If the edges of the pads are loose, use automobile weatherstrip cement to reattach the pad. 

3. Install a spherically seated bearing block in the cross head of the universal testing machine.

4. Make the following settings on the front panel of the Vishay 2310 signal conditioner:

• Excitation switch ON.

• Excitation voltage set to 10 V.

• Filter set to 1,000 Hz.

• AC IN button fully extended (e.g., out).

• Set gain initially to 4.2 x 100.

Note: If the reference load cell has been previously calibrated, the initial gain value may 

be different. If so, set the Vishay gain to the most recent value.

• Auto balance switch off. 

Note: Verify that the tape playback switch on the rear panel of the signal conditioner is 

off. Position the signal conditioner and the computer several feet apart near the testing 

machine and attach them to the a/c line power.

5. The load cell and signal conditioner should be connected and powered for at least 30 min 

before performing the calibration procedure. This ensures the electronics are properly 

warmed up.
Redevelopment of the FWD Calibration Protocol and Equipment

86

Calibration Procedure

Perform three calibration trials according to the following procedure. WinFWDCal must be used 

in conjunction with the following step-by-step procedure:

1. Hook up all cables and warm up the equipment for at least 30 min. Turn on the computer and 

initialize WinFWDCal. If a hydraulic universal testing machine is used, turn the pump on and 

allow the machine to warm up for at least 30 min.

2. Place a wood/aluminum bearing block with no rubber pad in the center of the testing 

machine platen.

3. Place the reference load cell on top of the bearing block with the support feet down (i.e., in 

contact with the top surface of the lower bearing block).

4. Place the second bearing block on top of the load cell with the cemented rubber pad down 

(i.e., in contact with the top surface of the load cell).

5. Carefully align the edges of the load cell and the two bearing blocks, and center the system 

under the spherical loading block of the testing machine.

6. Set the testing machine on a range equal to or slightly larger than 24,000 lbf (100 kN). Apply 

an indicated load of 24,000 lbf (100 kN) to the load cell three times. Apply the load at a rate 

of approximately 5,000–10,000 lbf (22–44 kN) per minute.

7. Temporarily remove the upper wood/aluminum bearing block. Set the auto balance switch on 

the Vishay 2310 signal conditioner to off. Read and record the unbalanced zero voltage using 

the pushbutton. If this voltage is in excess of ±5 V, the load cell may have been damaged by 

yielding, and it should be returned to the manufacturer for repair. If the load cell has been 

previously calibrated, verify that the difference in the unbalanced zero voltage is no more 

than 100 mV, or 5 percent of the previous value.

8. Briefly push down the auto balance switch on the signal conditioner to the reset position and 

release it to the on position. Adjust the trim knob until the KUSB board reads 0.0 V.

9. Replace and align the upper bearing block rubber pad down. Apply a load of 24,000 lb 

(100 kN), and check the output of the load cell is between 97 and 99 percent of full scale on 

the data acquisition system. In the case of the Keithley KUSB-3108, this will be between 

-9.7 and -9.9 V. Release the load, and record the gain setting.

10. If the voltage is not in the correct range, adjust the gain knob on the signal conditioner in 

0.1 increments until the signal conditioner output is between 97 and 99 percent of full scale. 

(A gain of 4.20 x 100 is acceptable. A gain of 4.25 x 100 or 4.21 x 100 is not.) In no case 

should the output at 24,000 lb (100 kN) exceed 99 percent of full scale. The largest gain that 

does not produce an output above 99 percent of full scale should be used. 

11. If the gain setting is changed, repeat steps 7–9.
Appendix A. FWD Calibration Protocol

87

Note: When the load is released, the indicted voltage will not read exactly zero because it 

was zeroed before the upper bearing block was put in place. Do not re-zero the signal 

conditioner at this point.

12. Carefully zero the universal testing machine. Use the push-button trigger to record the 

reading at the zero load level.

13. Apply load at a rate of 1,000 lbf (5 kN) per minute. Use the pushbutton trigger to record 

the readings at 1,000 lbf (5 kN) intervals up to a maximum indicated load of 24,000 lbf 

(100 kN). While releasing the load, record a reading at 12,000 lbf (50 kN) and at zero 

pound loads.

14. Remove the upper bearing block. Use the pushbutton to record the signal conditioner 

calibration voltages for +B and -B shunt values on the Vishay. Set the auto balance switch to 

off and again record the unbalanced zero voltage. This reading should be within 10 mV of 

the earlier reading. If it is not, repeat the calibration procedure.

Data Analysis

WinFWDCal will perform the data analysis for each trial. It will use a stepup regression utility to 

calculate a fifth degree polynomial of the form as follows:

Figure 40. Equation. Load cell calibration algorithm.

Where:

Y = The adjusted load calculated from the universal testing machine indicating load including 

the weight of the upper bearing block.

V = The load cell voltage.

Ai = Coefficients determined by the regression.

Evaluate the results according to the following acceptance criteria:

• The standard error of the regression should be less than ±20 lbf (±0.1 kN).

• Each of the coefficients should be statistically significant.

After completion of at least three acceptable trials, WinFWDCal will pool the data for all 

three trials and determine regression coefficients based on the combined data, and the calibration 

is complete.

The final set of calibration coefficients should be evaluated according to the above two criteria. 

In addition, the three sets of data should be random, neither steadily increasing nor steadily 

decreasing. This should be verified by reviewing a plot of the residuals versus the fitted values 

from the regression. 
Redevelopment of the FWD Calibration Protocol and Equipment

88

Entering the Coefficients in WinFWDCal

The load cell coefficients should be entered in the FWDCalCenterConfig.ini file. Any of the 

coefficients that are not found to be significant should be entered as 0.0.

When the regression coefficients are entered in the FWDCalCenterConfig.ini file, the 

unbalanced zero, the +B and -B calibration factors, and the load cell signal conditioner 

gain factor should also be entered. This information is used to validate the load cell during 

FWD calibration.
89

APPENDIX B. SUGGESTIONS FOR SUCCESSFUL ANNUAL CALIBRATIONS

The following suggestions are not a part of the formal protocol, but they have been found to be 

helpful in the success of the procedure.

1. The deflection sensor calibration procedure will work better if the deflections are 16 mil

(400 μm) or more. It is not required that a special test pad be constructed, but if a regular 

concrete floor is used, locate the ball-joint base on a spot where the deflections are 

large enough. 

2. Before doing any calibrations, verify that the FWD computer and the calibration computer 

are registering the correct date and time. Reconcile any issues before proceeding.

3. Locate the calibration data acquisition system as close as possible to the FWD computer so 

that the two system operators will be able to converse easily. 

4. The signal conditioner and load cell should be connected and warmed up for at least 30 min 

before calibration. This will reduce the variability of the data during calibration. The signal 

conditioner and accelerometer only need to be warmed up an additional 5 min if the signal 

conditioner is already warm. If deflection sensor calibration is done first, the load cell can be 

warmed up while relative calibration is being performed.

5. Prior to calibration, the FWD should be warmed up using the standard operating procedure 

for the particular brand of FWD. This will reduce the variability of the data during 

calibration. Around 25 drops are needed for this purpose. More may be necessary if the 

FWD is colder than room temperature.

6. At the beginning of each programmed series of drops, add two seating drops (no data 

recorded) for which the data are not recorded. Seating drops are not required, but they will 

reduce the variability of the data during calibration. Seating drops are particularly needed 

after the deflection sensors have been moved in the calibration stand.

7. For load cell reference calibration, position the FWD so that the load plate is near the center 

of the calibration test pad. It is very important that the FWD is level when the calibration is 

performed. By doing load cell calibration on a different area of the test pad than the 

deflection sensor calibration, the life of the test pad is increased. It is acceptable to perform

load cell calibration on an area of concrete floor that is away from the test pad provided that 

the FWD is level.

8. Position the reference load cell beneath the FWD load plate, ensuring that the three guides 

are properly aligned around the plate. Do not loosen or remove the alignment fingers on the 

reference load cell. Zero the signal conditioner with the load plate high so that there is no 

external load on the reference load cell.

Note: For accurate results, it is important that the reference load cell be zeroed with the 

FWD load plate in the raised position. Also, the signal conditioner excitation and gain must 
Redevelopment of the FWD Calibration Protocol and Equipment

90

be set carefully to the levels at which the reference load cell was calibrated, as indicated in 

the WinFWDCal prerequisite information during setup.

9. Ensure that the FWD load plate is seated squarely on the reference load cell and on the 

concrete. To check this, perform a couple of seating drops and then try to slip a single 

sheet of copy paper under the load plate anywhere around the perimeter. It should not be 

possible to get the paper under the load plate or the three supports under the load cell any 

more than approximately 0.25 inches (6 mm). This will reduce the variability of the data 

during calibration. 

10. For deflection sensor calibration, the mounting bolts for the ball-joint base and the bolts that 

clamp the stand to the base must be tight. Vibration will cause excessive variability of the 

readings, leading to an inability to pass the data acceptance criteria. The bolts holding the 

ball-joint in its socket should be tight enough to hold the stand upright but still allow the 

ball-joint to rotate freely. 

11. The sensor stand connector pin (see figure 125) should be tightly attached to the stand. Apply 

Loctite® to the threads to assure that the pin will not turn. This will reduce the variability of 

the data during calibration

12. Verify that the deflection sensors are held firmly on the shelves of the sensor stands. This 

will reduce the variability of the data during calibration.

13. Attach the reference accelerometer box on the center shelf in the stand using two thumb 

screws. Press down on the box while tightening the screws to ensure that the box is seated 

firmly on the shelf. Try to keep the accelerometer aligned vertically in Earth’s gravity field 

at all times to avoid hysteresis in the accelerometer readings.

14. Place the geophones in the single-column calibration stand as shown in table 8. If only seven 

sensors are being calibrated, then the top two positions (A and B) should be empty in trial 1,

and the bottom two positions (I and J) should be empty in trial 2. An on-screen figure in 

WinFWDCal, similar to table 8, will show how the sensors should be placed in the stand.

Note: The goal is to have the sensors centered on the accelerometer and to invert them 

uniformly in the second trial. If additional calibration trials are needed, they should be 

performed in sets of two using the positioning shown in the table.

15. Place the KUAB seismometers in the double-column calibration stand as shown in

table 9. If nine sensors are being calibrated, put them in the top two positions. For trial 2, 

rotate the stand 180 degrees. Do not invert the sensors in the stand. The wide axis of the 

stand should be perpendicular to the wave coming from the FWD load plate. An on-screen 

figure in WinFWDCal, similar to table 9 will show how the sensors should be placed in 

the stand.

Note: For trial 1, stand behind the stand facing the FWD load plate with sensors D1, D3, etc., 

in the left column. For trial 2, rotate the stand vertically on the ball swivel so the column with 

sensors D1, D3, etc., is on the right.
Appendix B. Suggestions for Successful Annual Calibrations

91

Table 8. Geophone positions in single-column stand (nine sensors).

Stand Position Trial 1 Trial 2

A (top) Empty D9

B D1 D8

C D2 D7

D D3 D6

E D4 D5

Accelerometer Shelf Accelerometer Accelerometer

F D5 D4

G D6 D3

H D7 D2

I D8 D1

J (bottom) D9 Empty

Table 9. KUAB seismometer positions in double column stand (seven sensors).

Stand Position Trial 1 Trial 2

A (top) Empty Empty Empty Empty

B D1 D2 D2 D1

C 

(accelerometer) D3 D4 D4 D3

D D5 D6 D6 D5

E (bottom) D7 Empty Empty D7

16. The accelerometer calibration is slightly temperature sensitive. Temperature is monitored 

continuously by WinFWDCal; however, the measurement point is at the KUSB DAQ. It is 

important to keep the DAQ and the accelerometer box out of direct sunlight to avoid 

measurement errors or false warnings.

17. Use a gentle downward pressure on the handles of the calibration stand while the reference

and relative calibration data are being collected. At least half of the bubble on the level 

should be inside the black circle on the sight glass while data are being collected. Holding the 

stand consistently for each drop will reduce the standard error in the calibration trial. 

18. For load or deflection reference calibration, if either of the following conditions occurs, the 

calibration testing should be repeated after identifying the source of the following problems

and correcting them:

• Excessive noise at load levels of 9,000 lbf (40 kN) or more. The noise, due to electrical 

noise or mechanical vibrations, is of concern only if it results in an erroneous baseline 

value or an erroneous peak reading. The time history graphs provided by WinFWDCal

should be reviewed to determine if the noise is a concern before rejecting the data

for a drop.

• Standard deviations at any load levels that differ by more than a factor of three between 

the reference system data set and the FWD dataset.
Redevelopment of the FWD Calibration Protocol and Equipment

92

• Standard deviations less than 25 lbf (0.11 kN) for the load cell and 0.16 mil (4 μm) for 

the deflection sensors are acceptable regardless of the ratio. WinFWDCal will check the 

ratios and post an error message, if applicable.

19. When not in use, the load cell, accelerometer, signal conditioner, and other calibration 

equipment should be stored in a protected location. The accelerometer should be stored in a 

+1 g gravity field to eliminate hysteresis. To accomplish this, attach the accelerometer box to 

the calibration platform using the two thumb screws. Use the bubble level to adjust the

platform on the storage shelf.

20. In the event that the accelerometer is found to not be level after a period of storage, it will 

take at least 24 h at room temperature to eliminate most of the internal hysteresis. Full 

recovery can take 3–6 days, and the time period is longer at cold temperatures. This is true 

whether or not the accelerometer is powered by the signal conditioner.
93

APPENDIX C. PDDX STANDARD VERSION 2.0

INTRODUCTION

The purpose of the AASHTO PDDX standard is to foster a uniform file format for output from 

all types of pavement deflection devices, including FWDs. A harmonized file format allows 

programmers to develop deflection data analysis programs without the need to provide file 

readers for the many different native file formats that have been promulgated by equipment 

manufacturers over time. 

While the purpose of this specification is to define a harmonized data format, it is not intended to 

replace the native file formats of the various manufacturers. Both formats can be provided if the 

manufacturer chooses. Equipment owners have the option to choose the desired output file 

format. The PDDX format can be used for static, steady-state vibratory, and dynamic pulseloading devices, although the primary focus of the Version 2.0 standard is for pulse-loading 

devices (i.e., FWDs).

The PDDX format puts related data items into sections with descriptive names. It is similar to the 

.INI format used in Windows®-based configuration files. Each line in the file can either be a 

section heading or a data entry field with one or more values.

PDDX VERSION 2.0 OVERVIEW

Version 2.0 of the PDDX standard incorporates a few changes from Version 1.0. It reconciles the 

minor differences that have evolved between the PDDX standard and the input data needs of the 

AASHTO DARWin pavement design program. DARWin can read PDDX Version 2.0 files. The 

new version also adds a few data items that were inadvertently left out of Version 1.0. It was 

necessary to change the name of some data items from Version 1.0, which may require updating 

PDDX file readers. 

The following three types of file formats are described in detail below:

• PDDX file format.

• FWD calibration history file format.

• FWD calibration data exchange file formats.

The PDDX file format is used for reporting the results of routine FWD testing of roads 

and airfields. This format provides data that is collected at multiple test points in a line or a 

grid pattern. 

The FWD calibration record file format contains a historical record of the last calibration that 

was performed on the FWD. This may be an annual calibration possibly supplemented by 

monthly calibrations. The file contains information on the date and location where the calibration 

was performed. It also identifies the special equipment that was used in the calibration. Some 

sections from the PDDX file format are included in the calibration history file.
Redevelopment of the FWD Calibration Protocol and Equipment

94

Two FWD calibration data exchange file formats are provided. One file is created by the FWD 

computer and provides information that is needed to begin performing an annual calibration. The 

data are transferred electronically from the FWD computer to the calibration center computer. 

The second file contains the results of the calibration that can be transferred electronically 

from the calibration center computer to the FWD computer. Both of these files require the 

cooperation of the FWD manufacturers to provide a means for writing the first file and reading 

the second one.

Description of Each Data Section and Data Entry Lines

As discussed previously, the data format has a modular structure. Data elements of similar 

characteristics are grouped together in individual sections. The first line is the section descriptor 

contained in brackets. Each module contains a number of data items with its associated value or 

values. Both the added data items and the renamed data items have been identified in the 

descriptions in bold type.

STANDARD PDDX FILE FORMAT

Pavement Deflection Data Exchange File Section

This should be the initial section in the file. It identifies information related to the PDDX format 

or version that is being used. 

Section 

 Descriptor [Pavement Deflection Data Exchange File] 

PDDX Version Number 

Syntax: PDDXVersionNumber = number

Description: This entry denotes the current PDDX format version. Depending on the nature 

and scope of future revisions, the version number of PDDX may change. This is 

provided to allow for possible changes in the PDDX Standard in the future. 

Example: PDDXVersionNumber = 2.0 

Delimiter Symbol

Syntax: DelimiterSymbol = string

Description: A single American Standard Code for Information Interchange (ASCII) printable 

non-white space character denoting the delimiter symbol. The recommended 

delimiter symbols include the comma (ASCII character 44), exclamation mark 

(ASCII character 33), and concatenation symbol (ASCII character 124). 

Example: DelimiterSymbol = ,

Note: The delimiter symbol and the decimal symbol entries must be mutually exclusive. 

In the United States, the usual delimiter is the comma, and the usual decimal 

symbol is the period.
Appendix C. PDDX Standard Version 2.0

95

Decimal Symbol

Syntax: DecimalSymbol = string

Description: A single ASCII printable non-white space character denoting the decimal symbol. 

The internationally accepted decimal symbols include ASCII characters 44 

(comma) and 46 (period). 

Example: Decimal Symbol = .

Note: The decimal symbol and the delimiter symbol entries must be mutually exclusive. 

In the United States, the usual delimiter is the comma, and the usual decimal 

symbol is the period.

Operations Information Section

This section contains general information recorded once for informational purposes. These data 

are not used in the analysis of the deflection data; however, they may aid engineers with respect 

to data analysis by recording time of day and other pertinent information. 

Section 

 Descriptor [Operations Information]

File Name 

Syntax: FileName = string

Description: This entry identifies the directory location and filename of the PDDX file where 

the deflection testing data are stored. 

Example: FileName = C:\FWD\DATA\SAMPLE.DDX 

Software Version

Syntax: SoftwareVersion = string

Description: This entry identifies the software version of the FWD operational software.

Example: SoftwareVersion = 18.9 

Note: This is used by the LTPP database to identify which version of the field program 

is used to collect the data from FWD. This data field is not in the PDDX standard 

Version 1.0.

Start Date 

Syntax: StartDate = date

Description: Date at onset of testing in dd-mmm-yyyy date format. 

Example: StartDate = 28-Apr-2009 

Start Time 

Syntax: StartTime = time

Description: Time at the onset of testing in hour:minute:second in 24-h clock format. 

Example: StartTime = 07:41:18 

End Date 

Syntax: EndDate = date

Description: Date at completion of testing in dd-mmm-yyyy date format.

Example: EndDate = 29-Apr-2009 
Redevelopment of the FWD Calibration Protocol and Equipment

96

End Time 

Syntax: EndTime = time

Description: Time at completion of testing in hour:minute:second 24-h clock format. 

Example: EndTime = 15:24:12 

Operator Name 

Syntax: OperatorName = string

Description: Name of deflection device operator. 

Example: OperatorName = Joe Smith 

Weather Condition 

Syntax: WeatherCondition = string

Description: General description of weather conditions at testing site (e.g., sunny, cloudy, 

raining). 

Example: WeatherCondition = sunny 

Units Section

This section contains information regarding the units system. All units applicable to the PDDX 

file format are described in this section. Some units that are unique to FWD calibration are listed 

in the other file formats. 

Note that all units are listed in singular format. English and metric units for applicable data are 

summarized in table 10. All units used in one PDDX file must be of the same system of units. 

Thus, a file must use only metric or English measurements. Also, only one unit identifier per 

data element is allowed for each entry. All permissible unit identifiers are defined below and are 

case-insensitive. 

Table 10. Unit systems for English and metric PDDX files.

1

Parameter English System Units Metric System Units

Length Mil Micron 

Inch Millimeter

Foot Centimeter

Yard Meter

Mile Kilometer

Mass Pound Kilogram

Time Second Second

Force Pound-force KiloNewton

Kilopound-force Kilogram-force

Pressure Pound-force per square inch KiloPascal

Temperature Fahrenheit Celsius

Frequency Hertz Hertz

Angle Degree-minute-second Degree-minute-second 1

Some of the strings in the table are modified slightly from Version 1.0.
Appendix C. PDDX Standard Version 2.0

97

Section 

 Descriptor [Units] 

Load Plate Radius Units 

Syntax: LoadPlateRadiusUnits = string

Description: The units to be used to identify the radius of the load plate on an impulse load 

deflection device. The common units of measurement are inches and millimeters. 

Example: LoadPlateRadiusUnits = inch 

Load Units 

Syntax: LoadUnits = string

Description: Units used to measure the magnitude of the applied load (force or pressure) in 

pound-force, Newton, kiloPascal, or pound-force per square inch. Data with a 

different unit than the one chosen during calibration setup is converted based 

upon this data field. 

Example: LoadUnits = pound-force 

Deflection Units 

Syntax: DeflectionUnits = string

Description: Unit used to measure the magnitude of the pavement surface deflection. All 

deflection sensors must use the deflection unit specified by this entry. The typical 

units of measurement are mils or microns. Data with a different unit than the one 

chosen during calibration setup is converted based upon this data field.

Example: DeflectionUnits = mil 

Temperature Units 

Syntax: TemperatureUnits = string

Description: Unit used to measure temperature data. All temperature sensors must use the 

temperature unit specified by this entry. 

Example: TemperatureUnits = Fahrenheit 

Sensors Location Units 

Syntax: SensorsLocationUnits = string

Description: Units used to measure the longitudinal and transverse coordinates of deflection 

sensors with respect to the center of the load plate. 

Example: SensorsLocationUnits = inch 

GPS Units 

Syntax: GPSUnits = string, string, string

Description: Units used to measure global position using a global positioning system (GPS) 

device. Each GPS unit should be separated by the delimiter symbol. 

Example: GPSUnits = degree-minute-second, degree-minute-second, foot 
Redevelopment of the FWD Calibration Protocol and Equipment

98

Test Location Units 

Syntax: TestLocationUnits = string, string, string

Description: Units used to measure test location distances in an x, y, z coordinate system, with 

z denoting elevation. Common units are mile, foot, kilometer, and meter. Each 

test location unit should be separated by the delimiter symbol. 

Example: TestLocationUnits = mile, foot, foot 

Nominal Test Spacing Units 

Syntax: NominalTestSpacingUnits = string

Description: Units used to measure the nominal spacing between test points. 

Example: NominalTestSpacingUnits = foot 

Load Frequency Units 

Syntax: LoadFrequencyUnits = string

Description: Units used to measure the loading frequency of vibratory loading devices. 

Example: LoadFrequencyUnits = Hertz 

Drop History Data Frequency Units 

Syntax: DropHistoryDataFrequencyUnits = string

Description: Units used to measure the drop history data sampling rate. 

Example: DropHistoryDataFrequencyUnits = Hertz 

Joint Crack Width Units

Syntax: JointCrackWidthUnits = string

Description: Units used to measure the width of a joint or a crack. 

Example: JointCrackWidthUnits = millimeter

Note: This data field is not in the PDDX standard Version 1.0.

Device Information Section

This section contains information regarding the deflection device type, name, and model. Other 

entries include serial numbers for the overall device, the load plate, and the deflection sensors. 

Section 

 Descriptor [Device Information] 

Device Designation Name 

Syntax: DeviceDesignationName = string

Description: The name of the deflection device being used for testing. 

Example: DeviceDesignationName = KUAB 2m-FWD 

Device Model Number 

Syntax: DeviceModelNumber = string

Description: The model designation for the deflection device being used for testing. 

Example: DeviceModelNumber = 8714 
Appendix C. PDDX Standard Version 2.0

99

Device Serial Number 

Syntax: DeviceSerialNumber = string

Description: The serial number of the deflection device being used for testing. 

Example: DeviceSerialNumber = 245 

Load Cell Serial Number 

Syntax: LoadCellSerialNumber = string

Description: The serial number of the load cell being used for testing. 

Example: LoadCellSerialNumber = LC132 

Sensor Serial Numbers 

Syntax: SensorSerialNumbers = string(s)

Description: The serial numbers of each of the deflection sensors being used for testing listed 

in the same order as the x-axis distances defined in the device configuration 

section. Each serial number should be separated by the delimiter symbol. 

Example: SensorSerialNumbers = 30829, 30830, 30831, 30890, 30832, 30796, 30864 

Device Load Type 

Syntax: DeviceLoadType = string

Description: The type of loading device being used for testing (static, vibratory, impulse, 

or rolling). 

Example: DeviceLoadType = Impulse 

Device Configuration Section

This section contains information regarding the exact configuration of the deflection device at 

the time of testing. Figure 41 provides an illustration of a typical load cell and deflection

sensor configuration.

Figure 41. Illustration. Typical load cell and deflection sensor configuration.
Redevelopment of the FWD Calibration Protocol and Equipment

100

Section 

 Descriptor [Device Configuration] 

Load Plate Radius 

Syntax: LoadPlateRadius = number

Description: Radius of the load plate used during testing. Enter the radius in the units that have 

been previously specified. 

Example: LoadPlateRadius = 5.91 

Number of Deflection Sensors 

Syntax: NumberOfDeflectionSensors = number

Description: This entry denotes the number of deflection measuring sensors that are being used 

for collection of deflection data. 

Example: NumberOfDeflectionSensors = 9 

Deflection Sensor X-Axis Distances

Syntax: DeflectionSensorXAxisDistances = number(s) 

Description: This entry denotes the x-coordinate of each deflection sensor with respect to the 

center of the load plate. The x-direction is typically measured in the direction 

in which the deflection device is traveling. Negative distances are behind the 

load plate.

Example: DeflectionSensorXAxisDistances = 0, 12, 24, 36, 48, 60, 72, -12, 0 

Note: Generally, FWD collects data in the order of the data acquisition channels. 

Therefore, the x and y distances should correspond to the arrangement in which 

the deflection sensors are connected to the data acquisition channels. Thus, if 

channel 1 corresponds to the sensor in position (0,0) and channel 2 corresponds to 

position (12,0), they should be listed in that order. 

Deflection Sensor Y-Axis Distances 

Syntax: DeflectionSensorYAxisDistances = number(s) 

Description: This entry denotes the y-coordinate of each deflection sensor with respect to the 

center of the load plate. The y-direction is typically perpendicular to the direction 

in which the deflection device is traveling. Negative distances are to the right of 

the load plate.

Example: DeflectionSensorYAxisDistances = 0, 0, 0, 0, 0, 0, 0, 0, -12 

Note: Generally, FWD collects data in the order of the data acquisition channels. 

Therefore, the x and y distances should correspond to the arrangement in which 

the deflection sensors are connected to the data acquisition channels. Thus, if 

channel 1 corresponds to the sensor in position (0,0) and channel 2 corresponds to 

position (12,0), they should be listed in that order. 

Number of Temperature Sensors 

Syntax: NumberOtTemperatureSensors = number

Description: Number of sensors used to collect temperature data. 

Example: NumberOtTemperatureSensors = 3 
Appendix C. PDDX Standard Version 2.0

101

Temperature Sensor Use 

Syntax: TemperatureSensorUse = string(s)

Description: Description of the item for which each temperature sensor is recording the 

temperature at the time of testing. Options for the item description include air, 

pavement surface, pavement middepth, and pavement bottom. 

Example: TemperatureSensorUse = air, surface, middepth 

Load Frequency 

Syntax: LoadFrequency = number

Description: The loading frequency used for vibratory testing devices. Note that the frequency 

value should correspond with the previously entered load frequency unit. 

Example: LoadFrequency = 25 

Device Calibration Section

This section contains information regarding the equipment manufacturer's calibration of the 

deflection device as well as the results of the most recent annual calibration and monthly 

calibration. Annual calibration is conducted on FWDs for the load cell and deflection sensors to 

ensure that all readings are accurate within specified limits. 

Section 

 Descriptor [Device Calibration] 

Load Cell Manufacturer Calibration Date

Syntax: LoadCellManufacturerCalibrationDate = date

Description: Date of the most recent load cell calibration in dd-mmm-yyyy date format. 

Example: LoadCellManufacturerCalibrationDate = 27-Sep-1991 

Note: This calibration date is the FWD manufacturer’s load cell calibration. Usually 

done only upon delivery or major work on the machine, it is not the annual 

calibration date. This field is named load cell calibration date in Version 1.0.

Load Cell Manufacturer Calibration Factor

Syntax: LoadCellManufacturerCalibrationFactor = number

Description: Calibration factor obtained from the most recent calibration of the load plate 

load cell. 

Example: LoadCellManufacturerCalibrationFactor = 10956 

Note: This calibration factor is the FWD manufacturer’s calibration. Usually done only 

upon delivery or major work on the machine, it is not an annual calibration 

gain factor. This field is named load cell calibration factor in Version 1.0.
Redevelopment of the FWD Calibration Protocol and Equipment

102

Load Cell Manufacturer Calibration Intercept

Syntax: LoadCellManufacturerCalibrationIntercept = number

Description: The y-intercept of the linear relationship developed for calibration of the load cell. 

Example: LoadCellManufacturerCalibrationIntercept = 174 

Note: This calibration intercept is the FWD manufacturer’s calibration. Usually done 

only upon delivery or major work on the machine, it is not the annual calibration 

factor. This field is named load cell calibration intercept in Version 1.0.

Load Cell Annual Calibration Date 

Syntax: LoadCellAnnualCalibrationDate = date

Description: Date of the most recent reference calibration of the FWD load cell in dd-mmmyyyy date format. 

Example: LoadCellAnnualCalibrationDate = 27-Sep-2009 

Note: This is the date of the most recent annual calibration of the load cell. This data 

field is not in the PDDX standard Version 1.0.

Load Cell Annual Calibration Gain 

Syntax: LoadCellAnnualCalibrationGain = number

Description: The multiplier that adjusts the measured values from FWD to match the output 

from the reference load cell. 

Example: LoadCellAnnualCalibrationGain = 1.003

Note: This is the final gain factor for the FWD load cell determined by using the annual 

calibration procedure. This data field is not in the PDDX standard Version 1.0.

Sensor Static Calibration Date

Syntax: SensorStaticCalibrationDate = date

Description: Date of most recent deflection sensors static calibration in dd-mmm-yyyy date 

format. 

Example: SensorStaticCalibrationDate = 27-Apr-1992 

Note: The static calibrations are only done with a KUAB FWD. The static calibration of 

seismometers is performed through the use of a built-in micrometer on the top of 

each sensor. 

Sensor Static Calibration Factors

Syntax: SensorStaticCalibrationFactors = number(s) 

Description: Static calibration is conducted for deflection devices utilizing seismic 

displacement transducers (seismometers) for deflection measurement under a 

static condition (no pavement loading). 

Example: SensorStaticCalibrationFactors = 0.573, 0.553, 0.420, 0.300 

Note: The static calibrations are only done with a KUAB FWD. The static calibration of 

seismometers is performed through the use of a built-in micrometer on the top of 

each sensor. 
Appendix C. PDDX Standard Version 2.0

103

Sensor Manufacturer Calibration Date

Syntax: SensorManufacturerCalibrationDate = date

Description: Date of most recent deflection sensors dynamic calibration in dd-mmm-yyyy date 

format. 

Example: SensorManufacturerCalibrationDate = 27-Apr-1992 

Note: This is the manufacturer’s deflection sensor calibration date. This field is named 

sensor dynamic calibration date in Version 1.0. 

Sensor Manufacturer Calibration Factors

Syntax: SensorManufacturerCalibrationFactors = number(s)

Description: Dynamic calibration is conducted for deflection devices with seismometers to 

relate the sensor’s dynamic to static behavior. 

Example: SensorManufacturerCalibrationFactors = 1.050, 1.050, 1.050, 1.050 

Note: This is the FWD manufacturer’s deflection sensor calibration factor. For example, 

in the Dynatest® FWD, this is labeled the absolute gain factor. This field is named 

sensor dynamic calibration factor in Version 1.0.

Sensor Annual Calibration Date 

Syntax: SensorAnnualCalibrationDate = date

Description: Date of most recent deflection sensor annual calibration in dd-mmm-yyyy date 

format. 

Example: SensorAnnualCalibrationDate = 27-Apr-2009 

Note: This is the date that the deflection sensors were most recently calibrated using the 

annual calibration procedure. This field is named sensor reference calibration date 

in Version 1.0.

Sensor Monthly Calibration Date 

Syntax: SensorMonthlyCalibrationDate = date

Description: Date of most recent deflection sensor annual calibration in dd-mmm-yyyy date 

format. 

Example: SensorMonthlyCalibrationDate = 27-May-2009 

Note: This is the date that the deflection sensors were most recently calibrated using the 

monthly calibration procedure. This field is named sensor relative calibration date 

in Version 1.0.

Sensor Calibration Gains

Syntax: SensorCalibrationGains = number(s)

Description: The multiplier that adjusts the measured values from FWD to match the output 

from the reference accelerometer. 

Example: SensorCalibrationGain = 1.006, 1.003, 1.011, 0.998, 1.018, 0.999, 1.001 

Note: These are the final gain factors for the deflection sensors using the annual 

calibration procedure or the most recent monthly calibration procedure. This 

field is named sensor reference calibration gain in Version 1.0.
Redevelopment of the FWD Calibration Protocol and Equipment

104

DMI Device Calibration Date 

Syntax: DMIDeviceCalibrationDate = date

Description: Date of the most recent distance measuring instrument (DMI) device calibration 

in dd-mmm-yyyy date format. 

Example: DMIDeviceCalibrationDate = 27-Sep-2009 

DMI Device Calibration Factor 

Syntax: DMIDeviceCalibrationFactor = number

Description: Calibration factor obtained for the DMI device. 

Example: DMIDeviceCalibrationFactor = 2694.3 

Location Identification Section

This section contains information regarding the location and nature of the test site. Basic 

components of the test site are included such as the geographic location, client, and pavement 

feature section. 

Section 

 Descriptor [Location Identification] 

Site Name 

Syntax: SiteName = string

Description: Location of the project site. For highway testing, provide the name of the State, 

county, township, or city in which the project is located. On airport projects, 

provide the name of the airport.

Examples: SiteName = VA, Fairfax County

SiteName = O'Hare International Airport 

Facility Name 

Syntax: FacilityName = string

Description: Name of pavement facility or system being tested. 

Example: FacilityName = Interstate 95 

FacilityName = Runway 9L-27R 

Note: A facility is a collection of sections that have the same use (e.g., a single runway, 

street, or parking lot). 

Section Name 

Syntax: SectionName = string

Description: Name of the pavement section where deflection testing is conducted. 

Example: SectionName = Lorton Road to Newington Interchange 

SectionName = Section Al 

Note: A section is a pavement that has similar cross section, construction history, traffic 

level, and use that can be treated as a single unit for analysis purposes. 
Appendix C. PDDX Standard Version 2.0

105

Direction of Travel 

Syntax: DirectionOfTravel = string

Description: Indicates the direction of travel during testing. 

Example: DirectionOfTravel = Northbound 

Note: This is a user-defined entry that may be a compass direction, but it does not need 

be. Any designation, such as a runway end or street name, may be used to indicate 

direction of travel. 

Pavement Type 

Syntax: PavementType = string

Description: Type of pavement being tested (e.g., asphalt concrete (AC), Portland cement 

concrete (PCC), AC/PCC, flexible, rigid, composite). 

Example: PavementType = PCC 

Data Configuration Section

This section describes total test points on the project, their spacing, details of the measurements 

collected at each test point, and other data specific to each test point. 

Section 

 Descriptor [Deflection Data] 

Number of Test Locations 

Syntax: NumberOfTestLocations = number

Description: The number of test locations that are included in the current file. 

Example: NumberOfTestLocations = 10 

Nominal Test Pattern 

Syntax: NominalTestPattern = string

Description: The typical test pattern being used for testing—linear for highways and runways 

or grid for parking lots and aircraft parking aprons. 

Example: NorninalTestPattern = linear 

Nominal Test Spacing 

Syntax: NorninalTestSpacing = number

Description: The typical test spacing being used for testing; a typical test spacing exists in feet, 

meters, etc. 

Example: NorninalTestSpacing = 500 

Drop History Data Frequency 

Syntax: DropHistoryDataFrequency = number

Description: The rate at which readings are recorded for each sensor being sampled by 

the DAQ. 

Example: DropHistoryDataFrequency = 10,000 
Redevelopment of the FWD Calibration Protocol and Equipment

106

Number of Drop History Data Samples 

Syntax: NumberOfDropHistoryDataSamples = number

Description: Total number of readings obtained for each sensor during the recording of each 

time history. 

Example: NumberOfDropHistoryDataSamples = 300 

Test Location Section

The test location section indicates the number of current test points where “locationcounter” is an 

integer starting at 1 and ascending for each test point in the current file. Figure 42 provides an 

example of a typical test location.

Figure 42. Illustration. Example of a typical test location. 

Section 

 Descriptor [Test Location locationcounter]

Example: [Test Location 1] 

Note: The maximum number for locationcounter is NumberOfTestLocations. This 

section will be repeated sequentially for each test location.

Test Location 

Syntax: TestLocation = number, number, number

Description: Location of the test point in an x, y, z coordinate system with respect to a 

beginning reference point, where z is an elevation. On a roadway, the x-parameter 

may be the stationing or an electronic distance meter reading. 

Example: TestLocation = 62584, 0, 0 

GPS Location 

Syntax: GPSLocation = number, number, number

Description: Global location of the test point in an x, y, z coordinate system. 

Example: GPSLocation = 28.345, 74.345, 0 
Appendix C. PDDX Standard Version 2.0

107

Test Lane 

Syntax: TestLane = string

Description: Designation of the lane being tested. Test lane layouts are designated at the 

discretion of the engineer or FWD operator. 

Example: TestLane = outside lane, inner wheelpath 

Test Type 

Syntax: TestType = string

Description: Type of test being performed (e.g., basin, center, edge, corner). The latter three 

designations would mainly be used on concrete pavements.

Example: TestType = basin 

Drop History Type 

Syntax: DropHistoryType = string

Description: The type of drop history recorded at the test location (e.g., load only, deflections 

only, or load and deflections). 

Example: DropHistoryType = load and deflections 

Test Temperatures 

Syntax: TestTemperatures = number(s)

Description: The temperatures measured for the current test point. 

Example: TestTemperatures = 60.7, 94.2, 80.5 

Note: The measured temperatures are described in the [Device Configuration] section 

(e.g., air, surface, mid-depth, gradient average). 

Test Date

Syntax: TestDate = date

Description: Date at onset of testing in dd-mmm-yyyy date format. 

Example: TestDate = 12-Apr-2009 

Test Time 

Syntax: TestTime = time

Description: Time at the beginning of testing for the current test point in hour:minute:second 

24-h clock format. 

Example: TestTime = 07:41:18 

Test Comment 

Syntax: TestComment = string

Description: Any comment regarding the current test point entered manually by the operator. 

Example: TestComment = Longitudinal crack 

Number of Drops 

Syntax: NumberOfDrops = number

Description: The number of load drops made at each test point. 

Example: NumberOfDrops = 12 
Redevelopment of the FWD Calibration Protocol and Equipment

108

Drop Data 

Syntax: DropData_dropcounter = number(s) 

Description: The measured peak load and deflections for the current drop, where dropcounter 

is the number of the drop in the sequence. 

Example: DropData_l = 9194, 4.93, 4.14, 4.01, 3.65, 3.10, 2.50, 2.15 

Note: The maximum number for dropcounter is NumberOfDrops.

Drop History Data 

Syntax: DropHistoryData_dropcounter_samplecounter = number(s) 

Description: The time history load and deflections data for each drop, numbered sequentially 

as dropcounter_samplecounter. Each entry contains information similar in format 

to DropData_counterdrop when load and deflection are recorded. 

Example: DropHistoryData_1_5 = 1248, 1.15, 1.02, 0.98, 0.87, 0.79, 0.70, 0.62 

Note: The maximum number for samplecounter is NumberOfDropHistoryDataSamples.

SPECIAL FORMAT FOR FWD CALIBRATION RECORD FILES

At the conclusion of each FWD calibration, WinFWDCal creates a permanent electronic record 

of the calibration results in DDX file format. The file is named FWDCalibrationRecorddd-mmmyyyy.DDX (where dd is the date, mmm is the month abbreviation, and yyyy is the year (e.g., 12-

Feb-2009)). 

The file includes the following sections from the PDDX file format as defined above:

• Pavement Deflection Data Exchange File Section.

• Operations Information Section.

• Units Section.

• Device Information Section.

• Device Configuration Section.

• Device Calibration Section.

• A FWD Calibration Center Information Section, as defined below, is appended. 

The same file is created for both annual and monthly calibration. When retained together with 

the FWD and reference system data files, a complete record of the calibration is formed that 

allows the calibration results to be recreated and reviewed at any time using WinFWDCal.
Appendix C. PDDX Standard Version 2.0

109

FWD Calibration Center Information Section

This section includes information about the FWD calibration center personnel and equipment. It 

is created only for record keeping associated with FWD calibration. This section would not 

normally be included in the PDDX file format that is used for road and airfield data exchange. In 

order to be self-contained, some additional units of measurement are defined in this file.

Section 

 Descriptor [FWD Calibration Center Information] 

Calibration Center 

Syntax: CalibrationCenter = string

Description: The name of the FWD calibration center.

Example: CalibrationCenter = Colorado Department of Transportation

Calibration Center ID Code 

Syntax: CalCenterIDCode = number

Description: An ID code assigned to each FWD calibration center based upon the order of 

installation.

Example: CalCenterIDCode = 21

Calibration Center Operator

Syntax: CalibrationCenterOperator = string

Description: Name of the FWD calibration center operator.

Example: CalibrationCenterOperator = Edward Trujillo 

Calibration Center Operator Certification Date

Syntax: CalibrationCenterOperatorCertificationDate = date

Description: Date of last FWD calibration center operator certification and QA/QC visit. The 

certification is valid for 15 months after this date, and recertification must be 

scheduled within 12 months of this date.

Example: CalibrationCenterOperator = 27-Aug-08 

WinFWDCal Version Number

Syntax: WinFWDCalVersionNumber = string

Description: Version number of WinFWDCal software along with the release date. 

Example: WinFWDCalVersionNumber = 1.2.702 21 April 2008

Type of Calibration

Syntax: TypeOfCalibration = string

Description: Denotes the type of procedure that was used for the FWD Calibration 

(e.g., annual, monthly, or sensor replacement).

Example: TypeOfCalibration = Annual 
Redevelopment of the FWD Calibration Protocol and Equipment

110

Signal Conditioner Serial Number

Syntax: SignalConditionerSerialNumber = string

Description: Serial number of signal conditioner used in FWD calibration at a calibration 

center.

Example: SignalConditionerSerialNumber = 095889 

A to D Board Serial Number

Syntax: AtoDBoardSerialNumber = string

Description: Serial number of KUSB A/D board used in FWD calibration at a calibration 

center.

Example: AtoDBoardSerialNumber = 1142589

Universal Test Machine Name

Syntax: UniversalTestMachineName = string

Description: Name of universal calibration machine used to calibrate the reference load cell.

Example: UniversalTestMachineName = Baldwin Universal Testing Machine 

UniversalTestMachineSerialNumber = 040-1663 

Universal Test Machine Calibration Date

Syntax: UniversalTestMachineCalibrationDate = date

Description: Date of NIST traceable calibration of universal testing machine in dd-mmm-yyyy 

format.

Example: UniversalTestMachineCalibrationDate = 09-Sept-2008 

Reference Load Cell Name

Syntax: ReferenceLoadCellName = string

Description: Name assigned to reference load cell used in FWD calibration.

Example: ReferenceLoadCellName = CLRP001 

Reference Load Cell Calibration Date

Syntax: ReferenceLoadCellCalibrationDate = date

Description: Date of last calibration of reference load cell used in FWD calibration.

Example: ReferenceLoadCellCalibrationDate = 04-August-2004 

Reference Load Cell Excitation Volts

Syntax: ReferenceLoadCellExcitationVolts = number

Description: Excitation value in volts for reference load cell used in FWD calibration.

Example: ReferenceLoadCellExcitationVolts = 10.000 

Reference Load Cell Signal Conditioner Gain

Syntax: ReferenceLoadCellSignalConditionerGain = string

Description: Gain setting on signal conditioner used with reference load cell for FWD 

calibration.

Example: ReferenceLoadCellGain = 5.3 X 100 

Note: The first number is the setting on the knob of the signal conditioner. It is 

followed by the gain multiplier (pushbutton).
Appendix C. PDDX Standard Version 2.0

111

Reference Load Cell Unbalanced Zero

Syntax: ReferenceLoadCellUnbalancedZero = number

Description: The reading in volts with the load cell in an unbalanced condition.

Example: ReferenceLoadCellUnbalancedZero = 0.283 

Note: By comparing this value to the daily value, the unbalanced zero reading is used to 

ensure the load cell is undamaged and working properly.

Reference Load Cell +B Voltage

Syntax: ReferenceLoadCell+BVoltage = number

Description: The reading in volts with the load cell balanced first when a shunt resistance is 

used to produce a positive voltage on the balancing circuit. 

Example: ReferenceLoadCell+BVoltage = 1.772 

Note: By comparing this value to the daily value, the +B and -B readings are used to 

ensure the signal conditioner gain setting is correct. A load cell with a very large 

gain may need to use the +A and -A shunts on the signal conditioner. In these 

cases, the field names will have an A instead of a B in the field names. 

Reference Load Cell -B Voltage

Syntax: ReferenceLoadCell-BVoltage = number

Description: The reading in volts with the load cell balanced first when a shunt resistance is 

used to produce a negative voltage on the balancing circuit.

Example: ReferenceLoadCell-BVoltage = -1.772 

Note: By comparing this value to the daily value, the +B and -B readings are used to 

ensure the signal conditioner gain setting is correct. A load cell with a very large 

gain may need to use the +A and -A shunts on the signal conditioner. In these 

cases, the field names will have an A instead of a B in the field names. 

Reference Load Cell Calibration Units

Syntax: ReferenceLoadCellCalibrationUnits = string

Description: The units used in the calibration of the reference load cell (kiloNewton or 

pound-force).

Example: ReferenceLoadCellCalibrationUnits = pound-force

Reference Load Cell Calibration Coefficients

Syntax: ReferenceLoadCellCalibrationCoefficients = number(s)

Description: Polynomial coefficients used to convert volts into load in units listed under 

ReferenceLoadCellCalibrationUnits.

Example: ReferenceLoadCellCalibrationCoefficients = -98.2421e-01, 27.1714e-05, 

61.1421e-09, 00.0000e+00, 00.0000e+00 

Reference Load Cell Trigger Level

Syntax: ReferenceLoadCellTriggerLevel = number

Description: The load level in ReferenceLoadCellCalibrationUnits that is used to start autotriggering and collection of data during FWD calibration.

Example: ReferenceLoadCellTriggerLevel = 1,000 
Redevelopment of the FWD Calibration Protocol and Equipment

112

Reference Load Cell Daily Unbalanced Zero

Syntax: ReferenceLoadCellDailyUnbalancedZero = number

Description: The daily reading in volts with the load cell in an unbalanced condition taken as 

part of the prerequisites of calibration.

Example: ReferenceLoadCellDailyUnbalancedZero = 0.283 

Note: The unbalanced zero reading is used to ensure the load cell is undamaged and 

working properly.

Reference Load Cell Daily +B Voltage 

Syntax: ReferenceLoadCellDailyPBVoltage = number

Description: The daily +B voltage reading taken as part of the prerequisites of calibration. 

Together with the -B reading, these data are used to account for variations in the 

setting of the gain by the FWD calibration center operator.

Example: ReferenceLoadCellDailyPBVoltage = 1.773 

Reference Load Cell Daily -B Voltage 

Syntax: ReferenceLoadCellDailyMBVoltage = number

Description: The daily -B voltage reading taken as part of the prerequisites of calibration. 

Together with the +B reading, these data are used to account for variations in the 

setting of the gain by the FWD calibration center operator.

Example: ReferenceLoadCellDailyMBVoltage = -1.773 

Accelerometer Model Number

Syntax: AccelerometerModelNumber = string

Description: Manufacturer’s model number of accelerometer used in FWD deflection sensor 

calibration.

Example: AccelerometerModelNumber = 2220-005

Accelerometer Serial Number

Syntax: AccelerometerSerialNumber = string

Description: Manufacturer’s serial number of accelerometer used in FWD deflection sensor 

calibration.

Example: AccelerometerSerialNumber = 278 

Accelerometer Reference Calibration Date

Syntax: AccelReferenceCalibrationDate = date

Description: Date of calibration of reference accelerometer provided by manufacturer.

Example: AccelReferenceCalibrationDate = 07-Sep-2004

Accelerometer Reference +1G DC

Syntax: AccelRef+1GDC = number

Description: Reading of accelerometer in a +1G field (volts) provided by manufacturer.

Example: AccelRef+1GDC = 0.825
Appendix C. PDDX Standard Version 2.0

113

Accelerometer Reference -1G DC

Syntax: AccelRef-1GDC = number

Description: Reading of accelerometer in a -1G field (volts) provided by manufacturer.

Example: AccelerometerRef-1GDC = -0.777

Accelerometer Reference Calibration Coefficients

Syntax: AccelRefCalibrationCoefficients = number(s)

Description: Polynomial coefficients to convert output in volts into acceleration in g provided 

by manufacturer where the first coefficient listed is co.

Example: AccelRefCalibrationCoefficients = -4.52e-02, 1.241e + 00, 1.048e-03, -1.369e-04

Notes: The form of the equation is acceleration (g) = co + Σ(ci volts). 

Accelerometer Daily Calibration Date

Syntax: AccelDailyCalibrationDate = date

Description: Date of the last daily flip calibration in dd-mmm-yyyy format.

Example: AccelDailyCalibrationDate = 03-Aug-2005

Accelerometer Daily Calibration Time

Syntax: AccelDailyCalibrationTime = time

Description: The time of the last daily flip calibration of the accelerometer in hour:minute 

24-h clock format.

Example: AccelDailyCalibrationTime = 16:17 

Accelerometer Daily Calibration Temperature Unit

Syntax: AccelDailyCalibrationTemperatureUnit = string

Description: Units of temperature used when performing the last daily flip calibration of the 

accelerometer (Fahrenheit or Celsius).

Example: AccelDailyCalibrationTemperatureUnit = Fahrenheit

Accelerometer Daily Calibration Temp

Syntax: AccelDailyCalibrationTemp = number

Description: Temperature recorded during the last daily flip calibration of the accelerometer in 

units of AccelDailyCalibrationTemperatureUnit.

Example: AccelDailyCalibrationTemp = 67.5

Accelerometer Excitation Volts

Syntax: AccelExcitationVolts = number

Description: Excitation voltage for the reference accelerometer.

Example: AccelExcitationVolts = 10 

Accelerometer Signal Conditioner Gain

Syntax: AccelSignalConditionerGain = string

Description: Gain setting on signal conditioner used with for the reference accelerometer.

Example: AccelSignalConditionerGain = 2.0 X 1 

Note: The first number is the setting on the knob of the signal conditioner. It is 

followed by the gain multiplier (pushbutton).
Redevelopment of the FWD Calibration Protocol and Equipment

114

Accelerometer Daily Slope Factor

Syntax: AccelDailySlopeFactor = number

Description: Calculated relative difference in voltage between the reference calibration and the 

last daily flip calibrations. 

Example: AccelDailySlopeFactor = -0.50486 

Note: This value is multiplied by the output voltage to generate a relative voltage that 

can be used directly with the calibration factors from the accelerometer certificate 

(AccelerometerRefCalibrationCoefficients).

Accelerometer Historical Slope Factor Dates

Syntax: AccelHistoricalSlopeFactor = date, date, date, date

Description: Dates of the four accelerometer historical slope factors in dd-mmm-yyyy format. 

Example: AccelHistoricalSlopeFactorDates = 12-Jan-2009, 02-Feb-2009, 27-Feb-2009, 

18-Mar-2009

Accelerometer Historical Slope Factors

Syntax: AccelHistoricalSlopeFactor = number, number, number, number

Description: Historical listing of the previous four accelerometer slope factors. 

Example: AccelHistoricalSlopeFactor = -0 49856, -0 49857, -0 49853, -0 49850 

Note: These values are compared to the accelerometer daily slope factor during the 

FWD calibration to ensure the device is working properly. 

Accelerometer Daily Calibration +1G Voltage

Syntax: AccelDailyCalibration+1GVoltage = number

Description: Recorded voltage in a +1-G field during the last flip calibration of the 

accelerometer.

Example: AccelDailyCalibration+1GVoltage = 1.7598

Accelerometer Daily Calibration -1G Voltage

Syntax: AccelDailyCalibration-1GVoltage = number

Description: Recorded voltage in a -1-G field during the last flip calibration of the 

accelerometer.

Example: AccelDailyCalibration-1GVoltage = -1.7598

Accelerometer Daily Calibration +1G Flip Voltage

Syntax: AccelDailyCalibration+1GFlipVoltage = number

Description: Recorded voltage in a +1-G field after the -1 g reading during the last flip 

calibration of the accelerometer.

Example: AccelDailyCalibration+1GFlipVoltage = 1.7698

Note: The daily calibration readings are used to confirm that the hysteresis of the 

accelerometer is not significant and will not adversely affect the FWD calibration. 
Appendix C. PDDX Standard Version 2.0

115

Accelerometer Trigger Level

Syntax: AccelTriggerLevel = number

Description: The acceleration level in g units that is used to start auto-triggering and collection 

of data during FWD calibration.

Example: AccelTriggerLevel = 0.50

SPECIAL FORMAT FOR FWD CALIBRATION DATA EXCHANGE FILES 

The information in this section will assist FWD manufacturers to adapt their FWD operating 

systems (field programs) to be more compatible with the FWD annual and monthly calibration 

processes. This section defines the format for a minimum subset of the many data items listed in 

the previous sections that is needed to facilitate calibration. 

Electronic methods are used to transfer data between the FWD computer and the calibration 

computer. Some of this information is required just for record-keeping purposes, while other data 

items are used in the calibration calculations. Electronic data transfer helps reduce transcription 

errors and expedite FWD calibration. 

At the beginning of a calibration, the FWD computer needs to provide some information about 

the load cell, deflection sensor serial numbers, the current gain factors, the FWD operator's 

name, etc. The FWD operating program (field program) should be able to produce a file named 

FWDCalibrationInput dd-mmm-yyyy.DDX (where dd is the date, mmm is the month 

abbreviation, and yyyy is the year (e.g., 12-Nov-2009)). The contents of this file are 

defined below.

After an annual or monthly calibration is complete, the FWD operating program needs to be able 

to locate and read the results from a file named FWDCalibrationOutputdd-mmm-yyyy.DDX. 

The results should then be used to update the calibration data in the FWD computer. 

Input Data Requirements for WinFWDCal

The following is an example of the contents of FWDCalibrationInput dd-mmm-yyyy.DDX. Only 

the data sections and data items are listed here. A complete description of each item is provided 

in the previous sections.

[Pavement Deflection Data Exchange File]

PDDXVersionNumber = 2.0

DelimiterSymbol = ,

DecimalSymbol = .

[Operations Information]

FileName =D:\WINFWD\Temp\FWDCalibrationInput 20-Feb-2009.ddx

EndDate = 02-20-2009 

EndTime = 15:43

OperatorName = Daniel Atkins
Redevelopment of the FWD Calibration Protocol and Equipment

116

[Units]

LoadUnits = pound

DeflectionUnits = mil

[Device Information]

DeviceDesignationName = Dynatest® FWD

DeviceModelNumber = 8001

DeviceSerialNumber = 8001-001

LoadCellSerialNumber = 0331

SensorSerialNumbers = 3170, 3162, 3163, 3164, 3165, 3166, 3167, 3168, 3169

[Device Configuration]

NumberOfDeflectionSensors = 9

[Device Calibration]

LoadCellAnnualCalibrationDate = 20-Feb-2008

LoadCellAnnualCalibrationGain = 1.0006

SensorAnnualCalibrationDate = 20-Feb-2008

SensorCalibrationGains = 1.001, 0.999, 1.001, 0.999, 0.999, 0.998, 1, 0.996, 0.999

Output Data from WinFWDCal

The following is an example of the contents of FWDCalibrationOutput dd-mmm-yyyy.DDX. 

Only the data sections and data items are listed. A complete description of each item is provided 

in the previous sections.

[Pavement Deflection Data Exchange File]

PDDXVersionNumber = 1.0

DelimiterSymbol = ,

DecimalSymbol = .

[Operations Information]

FileName = C:\WinFWDCal\InterimDropData\FWDCalibrationOutput 21-Feb-2009.ddx

EndDate = 21-Feb-2009

EndTime = 15:43

OperatorName = Daniel Atkins

[Device Information]

DeviceDesignationName = Dynatest® FWD

DeviceModelNumber = 8001

DeviceSerialNumber = 8001-001

LoadCellSerialNumber = 0331

SensorSerialNumbers = 3170, 3162, 3163, 3164, 3165, 3166, 3167, 3168, 3169
Appendix C. PDDX Standard Version 2.0

117

[Device Configuration]

NumberOfDeflectionSensors = 9

[Device Calibration]

LoadCellAnnualCalibrationDate = 21-Feb-2009

LoadCellAnnualCalibrationGain = 1.0005

SensorAnnualCalibrationDate = 21-Feb-2009

SensorCalibrationGains = 1.002, 1.002, 1.003, 1.000, 1.000, 0.999, 1.002, 0.997, 1.001

[FWD Calibration Center Information]

CalibrationCenter = Cornell Local Roads

CalCenterIDCode = 21

CalibrationCenterOperator = David Orr

CalibrationCenterOperatorCertificationDate = 18-Nov-2008

WinFWDCalVersionNumber = Version 1.4.18 22 December 2008

TypeOfCalibration = Annual

119

APPENDIX D. QA REVIEWS

The purpose of a QA review is to achieve consistency of calibrations among centers. When the 

protocol is done correctly, the final gain factors for an FWD will be accurate within ±0.3 percent 

regardless of which center performs the calibration. QA reviews should be performed annually to 

ensure that the center is achieving the quality standards. In addition, the QA review provides the 

center with the most current information about the protocol.

Three people are needed to perform a QA review: the FWD calibration operator, an FWD 

operator, and the independent QA reviewer. Table 11 lists the roles and responsibilities for each 

person involved in the review. 

Table 11. Roles and responsibilities during QA review.

Personnel Role Definition

FWD calibration 

operator

Independently performs an FWD calibration in accordance with the 

most recent calibration protocol.

FWD operator Runs the FWD during the calibration.

QA reviewer

Observes the FWD calibration and assesses the proficiency of the 

calibration operator.

This appendix assumes a single FWD calibration operator is being reviewed, but multiple people 

may be certified in a single QA review. One day should be scheduled for each FWD calibration 

operator. While training is not part of the QA review, if a new operator needs to be trained 

during the same visit, up to an additional 2 days should be scheduled for that. 

Prior to the review, the calibration operator is provided with a pre-QA review questionnaire to 

assure a smooth and efficient review (see figure 43). An operator review form (see figure 44) and 

a facilities review form (see figure 45) are used during the review. Copies of these forms should 

also be sent out before the review. The purpose of each document is as follows:

• Pre-Trip Memorandum: Provides an overview and schedule for the review process. 

• Transmits checklists: Provides any specific details not covered by the other documents. 

• Pre-QA Review Questionnaire: Gathers data about the number and types calibrations 

performed since the last review. This is a useful way to know if the center has been able 

to perform calibrations and may be used to help solve problems the FWD calibration 

center is having.

• Operator Checklist: Defines key points that the QA reviewer will be looking for 

concerning the calibration operator’s implementation of the calibration protocol and use 

of the equipment and software.

• Facility Checklist: Defines key points that the QA reviewer will be looking for 

concerning the calibration facility and equipment.
Redevelopment of the FWD Calibration Protocol and Equipment

120

In order for the QA review to be efficient, it is important that the FWD is in good working 

condition. It is not required that the FWD passes the calibration in order for the calibration center

operator to demonstrate his/her proficiency and be certified. However, if the FWD is not working 

properly, it may not be possible to complete the review process.

REVIEW PROCESS

At the beginning of the review, the calibration operator, FWD operator, and QA reviewer meet to 

discuss the anticipated flow of the review process. This provides an opportunity to ask questions 

about the review, the calibration protocol, or the software and equipment. Problems or questions 

that have come up during the previous year can be reviewed and discussed. During this meeting, 

the QA reviewer should provide information about changes in the protocol or the software since 

the last review.

As part of the review, the QA reviewer will copy the last year of FWD calibration results from 

the calibration computer to a CD, a thumb drive, or a portable hard drive. This semiautomated 

process takes 15–45 min once it is started depending on how many files need to be copied. It can 

be done while the QA review meeting is taking place. After the files are copied, the QA reviewer 

will update the FWD calibration software and provide a backup copy of the updated software for 

the calibration operator. 

Facility Review

A review of the facility and the equipment determines compliance with the FWD calibration 

protocol. This assures that the equipment conforms to the protocol and that it is in good 

working condition. 

The QA reviewer checks that the equipment being used is compliant with the calibration 

protocol, the serial numbered equipment are correct, the reference devices are up to date on their 

calibrations, and the facility is able to perform FWD calibrations without electromagnetic 

interference or significant temperature changes during a typical FWD calibration. A checklist 

assists the reviewer by providing information on the important observation points. It is used as a 

record of the items reviewed.

Operator Review

The calibration operator is reviewed to assure that the protocol is being performed correctly. The 

calibration operator demonstrates proficiency with the protocol by calibrating an FWD. Three 

levels of operator compliance have been defined in table 12.

A checklist guides the QA reviewer through the important observation points. This includes the 

setup procedure in the WinFWDCal software, deflection sensor calibration, load cell calibration, 

and documentation and certification of the FWD calibration.
Appendix D. QA Reviews

121

Table 12. Calibration operator compliance levels for QA review.

Compliance Level Definition

Fully compliant

Operator fully understands and demonstrates sound implementation of 

the calibration protocol and uses the calibration equipment and software 

appropriately.

Partially compliant

Operator fully understands the calibration protocol. Some necessary 

changes in equipment, software use, and/or implementation of the 

calibration protocol may be required.

Noncompliant

Operator does not demonstrate an adequate understanding of the 

calibration protocol or its implementation.

Upon completion of the FWD calibration, the calibration operator and QA reviewer meet to 

discuss the review process and communicate any needed modifications. The FWD operator

should be included in the meeting if the QA reviewer feels that this would improve the quality of 

future calibrations. During the meeting, the QA reviewer informs the calibration operator of the 

compliance level. When the review is passed, the operator is issued a certificate of compliance. 

A sample certificate is provided in figure 46. 

A written report documenting the QA visit should be created as soon as possible after the visit, 

preferably within 2 weeks. The certificate of compliance is valid for 15 months, but the next QA 

visit must be scheduled within 12 months. This allows some flexibility in scheduling but ensures 

communication within 1 year.

Because AMRL took over responsibility for the QA visits in late 2010, it can be expected that 

some changes in procedures and checklists will occur. 
Redevelopment of the FWD Calibration Protocol and Equipment

122

Figure 43. Illustration. Pre-QA review questionnaire.
Appendix D. QA Reviews

123

Figure 44. Illustration. Calibration operator QA review form.
Redevelopment of the FWD Calibration Protocol and Equipment

124

Figure 44 (continued). Illustration. Calibration operator QA review form.
Appendix D. QA Reviews

125

Figure 44 (continued). Illustration. Calibration operator QA review form.
Redevelopment of the FWD Calibration Protocol and Equipment

126

Figure 44 (continued). Illustration. Calibration operator QA review form.
Appendix D. QA Reviews

127

Figure 44 (continued). Illustration. Calibration operator QA review form.
Redevelopment of the FWD Calibration Protocol and Equipment

128

Figure 45. Illustration. Center facilities QA review form.
Appendix D. QA Reviews

129

Figure 45 (continued). Illustration. Center facilities QA review form.
Redevelopment of the FWD Calibration Protocol and Equipment

130

Figure 45 (continued). Illustration. Center facilities QA review form.
Appendix D. QA Reviews

131

Figure 46. Illustration. Center operator certificate of compliance.

133

APPENDIX E. SPECIFICATIONS AND DRAWINGS

SPECIFICATIONS

1. General specifications

1.1 Description

The FWD calibration hardware set includes one accelerometer box assembly, one geophone 

calibration stand assembly, one seismometer calibration stand assembly, one reference load cell 

assembly, and one ball-joint anchor assembly. In some cases, only one of the stands is ordered 

for a hardware set. Drawings of each item are included by reference as part of this specification. 

Table 13 shows a list of drawings for each assembly.

Table 13. Parts required for one hardware set.

Assembly Drawing 

Number

Accelerometer box assembly CLRP-AB01

Ball-joint anchor assembly CLRP-BJ01

Geophone stand assembly1 CLRP-GCS01

Seismometer stand assembly1 CLRP-SCS01

Geophone adapters assembly CLRP-GA01

Reference load cell assembly CLRP-LC01 1

Indicates that in some cases, only one stand is ordered for a 

hardware set.

1.2 Materials

The required materials are described in the referenced drawings (see table 13).

1.3 Manufacturing Requirements/Conflicts

The contractor should provide all materials needed to manufacture one complete hardware set 

unless otherwise noted.

Additional manufacturing requirements are listed with the specifications for each individual 

part or subassembly. If there is a conflict between the drawings and the specifications, the 

specifications should govern.

1.4 Surface Smoothness

An industrial quality surface finish is desired. All machined surfaces should have a smoothness 

of 63 μ-inches (0.0016 mm) or less.
Redevelopment of the FWD Calibration Protocol and Equipment

134

1.5. Mill Surface Finish 

The mill surface finish on aluminum extrusions should be inspected for nicks, cuts, and dings, 

and a reasonable effort should be made to smooth out the defects before anodizing. Avoid 

removal of excessive amounts of material in this process.

1.6 Anodizing

Where anodizing is required on aluminum parts, all parts should be thoroughly cleaned and 

anodized according to MIL-A-8625F Type 2 Class 2 (Black). All parts should have a 

cosmetically homogeneous appearance after being anodized.

If for any reason parts must be stripped to remove the anodizing, a phosphoric chromic solution 

should be used, followed by surface polishing to remove pitting and achieve a 63-μ-inch 

(0.0016-mm) smoothness.

1.7 Loctite®

Where Loctite® is required, medium strength #242 Loctite® should be used.

1.8 Delivery/Acceptance

The contractor should provide a guaranteed delivery time. All parts should be assembled and 

delivered to the address provided by the purchasing agency.

The purchasing agency should have 7 days after delivery to check parts for proper alignment and 

fit as well as finish, as required. If the part or parts are not acceptable, the contractor should

replace them at no additional charge.

1.9 Method of Payment

Payment should be authorized upon acceptance of the complete order.

2.0 Accelerometer Box Assembly Specifications (Drawing CLRP-AB01)

2.1 Description

The accelerometer box assembly is designed to house a Silicon Designs Model 2220 

accelerometer as well as to provide some EMI shielding and additional physical protection. The 

box assembly mounts on the calibration platter, the geophone calibration stand, and the 

seismometer calibration stand by means of two #10-24 knurled head thumbscrews through the 

lip on the box.
Appendix E. Specifications and Drawings

135

2.2 Materials

All of the materials for the accelerometer box assembly are described in the referenced drawing 

and the bill of materials.

2.3 Manufacturing Requirements

The accelerometer box top and bottom, the box assembly, and calibration platter should be 

assembled together to ensure proper alignment prior to delivery to the customer. The 

accelerometer box top and bottom should also be fitted into the geophone calibration stand and 

the seismometer calibration stand, and the fit should be a firm coupling to both stands without 

any gaps or interference between mating surfaces.

Each aluminum part should be cleaned and then anodized in accordance with section 1.5. Upon 

delivery, the customer should finish assembly of the box by mounting the accelerometer and the 

Amphenol receptacle, mating the box top and bottom, mounting the bubble level, and attaching 

the leveling screws to the calibration platter.

3.0 Ball-joint Anchor Assembly Specifications (Drawing CLRP-BJ01)

3.1 Description

The ball-joint anchor assembly is the means by which the calibration stands are coupled to the 

test pad during FWD calibration. The clamp provides a solid mechanical connection to the 

calibration stand, and the Techno/Sommer KG60 ball-joint provides some rotational freedom,

making the stands easier to use as well as providing for some moment compensation.

3.2 Materials

All of the materials for the ball-joint anchor assembly are described in the referenced drawing 

and the bill of materials.

3.3 Manufacturing Requirements

Upon delivery, the customer should finish assembly of the anchor by attaching the base bar and 

the clamp base to the ball-joint. The clamp should be attached to the clamp base, and the rest 

stop should be mounted to the anchor assembly

4.0 Geophone Calibration Stand Assembly Specifications (Drawing CLRP-GCS01)

4.1 Description

The geophone calibration stand assembly is designed to hold up to 10 geophones from the 

Dynatest®, Carl Bro, KUAB, or JILS FWD. The accelerometer box (see section 2.0) mounts to 

the middle shelf in the stand. The stand is attached to the ball-joint anchor (see section 3.0) 

during the calibration procedure.
Redevelopment of the FWD Calibration Protocol and Equipment

136

4.2 Materials

All of the materials for the geophone calibration stand are described in the referenced drawing 

and the bill of materials.

4.3 Manufacturing Requirements

Prior to welding the stand, the parts should be cleaned and dry fitted together. A qualified

Tungsten inert gas (TIG) welder should perform all of the welding. After welding, the stand

should be cleaned and then anodized in accordance with section 1.5.

After delivery, the customer should finish assembly by adding handles and bubble levels as 

needed and use Loctite® to keep the connector pin in place.

5.0 Seismometer Calibration Stand Assembly (Drawing CLRP-SCS01)

5.1 Description

The seismometer calibration stand assembly is designed to hold up to 10 KUAB seismometers in 

a 2-column configuration. Additionally, the accelerometer box (see section 2.0) mounts to the 

middle shelf in the stand. The stand is attached to the ball-joint anchor (see section 3.0) during 

the calibration procedure.

5.2 Materials

All materials for the seismometer calibration stand assembly are described in the referenced 

drawing and the bill of materials.

5.3 Manufacturing Requirements

Prior to welding the stand, the parts should be cleaned and dry fitted together. A qualified TIG 

welder should perform all of the welding. After welding, the stand should be cleaned and 

anodized in accordance with section 1.5.

After delivery, the customer should finish assembly by adding handles and bubble levels as 

needed and use Loctite® to keep the connector pin in place.

6.0 Geophone Adapter Specifications

6.1 Description

The geophone adapters are used to couple three different types of geophones to the geophone 

calibration stand (see section 4.0) and allow for ease of movement of the geophones within 

the stand.
Appendix E. Specifications and Drawings

137

6.2 Materials

All materials for the geophone adapters are described in the referenced drawing and the bill 

of materials.

6.3 Manufacturing Requirements

The geophone adapters should be dry fitted into the geophone calibration stand sensor shelves to 

ensure a loose sliding fit in the slotted hole on each shelf.

7.0 Reference Load Cell Assembly (Drawing CLRP-LC01)

7.1 Description

The reference load cell is a custom load cell designed specifically for FWD calibration. The load 

cell allows all brands of FWD to be calibrated with a 11.7-inch (300-mm) load plate.

7.2 Materials

All materials for the reference load cell assembly are described in the referenced drawings and 

bill of materials.

7.3 Manufacturing Requirements

Prior to welding the base to the body, the parts should be cleaned and dry fitted together. A 

certified TIG welder should perform all of the welding. After welding, the load cell parts should

be cleaned and anodized in accordance with section 1.5, with the exception of the measuring 

links (Drawing CLRP-LC04), which should not be anodized.

After delivery, the customer should have the measuring links gauged by a qualified strain gauge

installer. Then, the load cell should be fully assembled, conditioned, and calibrated according to

AASHTO R33-03.

(10)

COMPLETE DRAWING LIST

Table 14 provides a complete drawing list for the accelerometer box, ball-joint anchor, geophone 

calibration stand, seismometer calibration stand, geophone adapters, reference load cell, and data 

acquisition cables. The drawings follow in the same order in this appendix.
Redevelopment of the FWD Calibration Protocol and Equipment

138

Table 14. Complete drawing list.

Drawing 

Number Part Name

CLRP-AB01 Accelerometer box assembly

CLRP-AB02 Accelerometer box bottom

CLRP-AB03 Accelerometer box top

CLRP-AB04 Calibration platter

CLRP-AB05 Accelerometer wiring

CLRP-BJ01 Ball-joint anchor assembly

CLRP-BJ02 Clamp

CLRP-BJ03 Clamp base

CLRP-BJ04 Base bar

CLRP-BJ05 Rest stop

CLRP-GCS01 Geophone stand assembly

CLRP-GCS02 Geophone stand side rail

CLRP-GCS03 Geophone stand top shelf

CLRP-GCS04 Geophone stand bottom shelf

CLRP-GCS05 Geophone stand sensor shelf

CLRP-GCS06 Geophone stand accelerometer shelf

CLRP-GCS07 Geophone stand handle holder

CLRP-GCS08 Geophone stand connector pin

CLRP-SCS01 Seismometer stand assembly

CLRP-SCS02 Seismometer stand side rail

CLRP-SCS03 Seismometer stand top shelf

CLRP-SCS04 Seismometer stand sensor shelf

CLRP-SCS05 Seismometer stand standoff

CLRP-SCS06 Seismometer stand bottom shelf

CLRP-SCS07 Seismometer stand handle holder

CLRP-SCS08 Seismometer stand connector pin

CLRP-SCS09 Seismometer stand shelf subassembly

CLRP-SCS10 Seismometer stand accelerometer shelf

CLRP-SCS11

Seismometer stand accelerometer shelf 

subassembly

CLRP-GA01 Geophone adapter assemblies

CLRP-GA02 Carl Bro adapter

CLRP-GA03 JILS adapter

CLRP-GA04 KUAB adapter

CLRP-LC01 Load cell assembly

CLRP-LC02 Load cell body

CLRP-LC03 Base plate

CLRP-LC04 Measuring link

CLRP-LC05 Cover plate

CLRP-LC06 Foot

CLRP-LC07 Guide fingers

CLRP-LC08 Interior connector wiring
Appendix E. Specifications and Drawings

139

Drawing 

Number Part Name

CLRP-LC09 Strain gauge layout

CLRP-LC10 Bridge wiring

CLRP-DAQ01 Vishay to DAQ cable

CLRP-DAQ01A Vishay 2310B BNC to DAQ cable

CLRP-DAQ01B

Vishay 2310B BNC to DAQ cable—

stock parts

CLRP-DAQ02 Vishay to load cell DAQ cable

CLRP-DAQ03 Accelerometer signal cable

CLRP-DAQ04 Pushbutton to KUSB DAQ cable

CLRP-DAQ06 KUSB protection wiring

CLRP-DAQ07 KUSB enclosure rear panel

CLRP-DAQ08 KUSB enclosure front panel

ACCELEROMETER BOX

Figure 47. Photo. Accelerometer box.
Redevelopment of the FWD Calibration Protocol and Equipment

140

Table 15. Fabricated parts required for BM-AB accelerometer box assembly.

Drawing Number Description Quantity

CLRP-AB02 Accelerometer box bottom 1

CLRP-AB03 Accelerometer box top 1

CLRP-AB04 Calibration platter 1

CLRP-AB05 Accelerometer wiring 1

Table 16. Hardware items required for BM-AB accelerometer box assembly.

Vendor Part Number Item Vendor Quantity

96877A209

Flat head Phillips machine screw, 18-8 

SS, 4-40 thread, 3/8-inch length, Mil Spec 

51959-15 McMaster-Carr® 4

91400A110

Pan head Phillips machine screw, 18-8 

SS, 4-40 thread, 1/2-inch length, Mil Spec 

51957-17 McMaster-Carr® 2

91737A072

Fillister head Phillips machine screw, 

18-8 SS, 4-40 thread, 1/4-inch length McMaster-Carr® 7

91746A876

Knurled head thumb screw, slotted, 18-8 

SS, 10-24 thread, 1/2-inch length, 

1

/4-inch head diameter, 1/2-inch length McMaster-Carr® 2

91755A205

Retaining washer, Nylon 6/6, #4 screw 

size, 7/64-inch inside diameter, 17/64-inch

outside diameter, 0.027–0.037-inch

thickness McMaster-Carr® 2

2198A85

Bull's-eye level, glass, surface mount, 

center circle and cross lines, 11/4-inch

base diameter, 7

/16-inch height McMaster-Carr® 1

23015T64

Leveling mount, polyethylene base, 

3

/8-inch 16 thread, 50 lbf maximum load, 

1-inch length McMaster-Carr® 3

98520A145

Locking wing nut, zinc alloy, nylon

insert, 3/8-inch 16 screw size, 11/2-inch

wing spread McMaster-Carr® 3

2220-005

Accelerometer, ± 5 g with calibration 

certificate Silicon Designs 1

654-PT02A106P Amphenol receptacle, PT02A-10-6P 

Mouser 

Electronics 1

1 inch = 25.4 mm

1 lbf = 4.45 N
Appendix E. Specifications and Drawings

141

Table 17. Vendor contact information for BM-AB accelerometer box assembly.

Vendor Web site Notes

McMaster-Carr® www.mcmaster.com

See Web site for specific contact 

information located in the “About Us”

section.

Silicon Designs www.silicondesigns.com

1445 NW Mall St, Issaquah, WA 98027, 

425-391-8329

Mouser Electronics www.mouser.com

See Web site for specific contact 

information located in the “Contact Us” 

section.

Incodema www.incodema.com

Incodema Inc.

407 Cliff Street, Ithaca, NY 14850

607-277-7070

Table 18. Hardware costs for BM-AB accelerometer box.

Part Quantity Supplier Cost1

Materials and 

machining 1 Incodema $912.00

Accelerometer 

(±5 g) 1 Silicon Designs $580.00

Miscellaneous

hardware 1 $38.00 1

Indicates the cost in February 2009.
Redevelopment of the FWD Calibration Protocol and Equipment

142

Figure 48. Illustration. CLRP-AB01 accelerometer box assembly.
Appendix E. Specifications and Drawings

143

Figure 49. Illustration. CLRP-AB02 accelerometer box bottom.
Redevelopment of the FWD Calibration Protocol and Equipment

144

Figure 50. Illustration. CLRP-AB03 accelerometer box top.
Appendix E. Specifications and Drawings

145

Figure 51. Illustration. CLRP-AB04 calibration platter.
Redevelopment of the FWD Calibration Protocol and Equipment

146

Figure 52. Illustration. CLRP-AB05 accelerometer wiring.
Appendix E. Specifications and Drawings

147

BALL-JOINT ANCHOR

Figure 53. Photo. Ball-joint anchor.

Table 19. Fabricated parts required for the BM-BJ ball-joint anchor assembly.

Drawing Number Description Quantity

CLRP-BJ02 Clamp 1

CLRP-BJ03 Clamp base 1

CLRP-BJ04 Base bar 1

CLRP-BJ05 Rest stop 1
Redevelopment of the FWD Calibration Protocol and Equipment

148

Table 20. Hardware items required for the BM-BJ ball-joint anchor assembly.

Vendor Part 

Number Item Vendor Quantity

KG-60 Ball-joint Techno/Sommer 1

91292A145

Socket head cap screw, 18-8 

stainless steel, M8 thread, 

16-mm length, 11/4-mm pitch McMaster-Carr® 6

91292A148

Socket head cap screw, 18-8 

stainless, M8 thread, 25-mm 

length, 11/4-mm pitch McMaster-Carr® 2

91292A135

Socket head cap screw, 18-8 

stainless steel, M6 thread, 

16-mm length, 1-mm pitch McMaster-Carr® 4

1 inch = 25.4 mm

Table 21. Vendor contact information.

Vendor Web site Notes

Techno/Sommer www.techno-sommer.com

See Web site for specific contact 

information located in the “Sales 

Representatives” section.

McMaster-Carr® www.mcmaster.com

See Web site for specific contact 

information located in the “About 

Us” section.

Incodema www.incodema.com

Incodema Inc.

407 Cliff Street, Ithaca, NY 14850

607-277-7070

Table 22. Hardware costs for BM-BJ ball-joint anchor.

Part Quantity Supplier Cost1

Materials and machining 1 Incodema $919.00

Techno/Sommer KG60 

Ball-joint 1 Techno/Sommer $143.00

Miscellaneous hardware 1 $31.00 1

Indicates the cost in February 2009.
Appendix E. Specifications and Drawings

149

Figure 54. Illustration. CLRP-BJ01 ball-joint anchor assembly.
Redevelopment of the FWD Calibration Protocol and Equipment

150

Figure 55. Illustration. CLRP-BJ02 clamp.
Appendix E. Specifications and Drawings

151

Figure 56. Illustration. CLRP-BJ03 clamp base.
Redevelopment of the FWD Calibration Protocol and Equipment

152

Figure 57. Illustration. CLRP-BJ04 base bar.
Appendix E. Specifications and Drawings

153

Figure 58. Illustration. CLRP-BJ05 rest stop.
Redevelopment of the FWD Calibration Protocol and Equipment

154

GEOPHONE CALIBRATION STAND

Figure 59. Photo. Geophone calibration stand.

Table 23. Fabricated parts required for BM-GCS geophone calibration stand assembly.

Drawing Number Item Quantity

CLRP-GCS02 Geophone stand side rail 2

CLRP-GCS03 Geophone stand top shelf 1

CLRP-GCS04 Geophone stand bottom shelf 1

CLRP-GCS05 Geophone stand sensor shelf 10

CLRP-GCS06

Geophone stand accelerometer

shelf 1

CLRP-GCS07

Geophone stand handle 

holder1 4

CLRP-GCS08

Geophone stand connector 

pin2 1 1

Identical to CLRP-SCS07. 2

Identical to CLRP-SCS08.
Appendix E. Specifications and Drawings

155

Table 24. Hardware items required for BM-GCS geophone calibration stand assembly.

Vendor Part 

Number Item Vendor Quantity

62385K65

Tapered handle, Smooth phenolic, 

3

/8-inch, 16 x 

1

/2-inch threaded stud, 

1-inch diameter McMaster-Carr® 2

2198A85

Glass surface-mount bull’s-eye 

level center circle and cross lines, 

11/4-inch base diameter, 7/16-inch

height McMaster-Carr® 1

91737A072

18-8 stainless steel fillister head 

Phillips machine screw 4-40 thread, 1

/4-inch length McMaster-Carr® 3

1 inch = 25.4 mm

Table 25. Vendor contact information for BM-GCS geophone calibration stand.

Vendor Web site Notes

McMaster-Carr® www.mcmaster.com

See Web site for specific contact 

information in the “About Us” section.

Incodema www.incodema.com

Incodema Inc.

407 Cliff Street, Ithaca, NY 14850

607-277-7070

Table 26. Hardware costs for BM-GCS geophone calibration stand.

Part Quantity Supplier Cost1

Materials and 

machining 1 Incodema $1407.00

Miscellaneous

hardware 1 $30.00 1

Indicates the cost in February 2009.
Redevelopment of the FWD Calibration Protocol and Equipment

156

Figure 60. Illustration. CLRP-GCS01 geophone stand assembly.
Appendix E. Specifications and Drawings

157

Figure 61. Illustration. CLRP-GCS02 geophone stand side rail.
Redevelopment of the FWD Calibration Protocol and Equipment

158

Figure 62. Illustration. CLRP-GCS03 geophone top shelf.
Appendix E. Specifications and Drawings

159

Figure 63. Illustration. CLRP-GCS04 geophone stand bottom shelf.
Redevelopment of the FWD Calibration Protocol and Equipment

160

Figure 64. Illustration. CLRP-GCS05 geophone stand sensor shelf.
Appendix E. Specifications and Drawings

161

Figure 65. Illustration. CLRP-GCS06 geophone stand accelerometer shelf.
Redevelopment of the FWD Calibration Protocol and Equipment

162

Figure 66. Illustration. CLRP-GCS07 geophone stand handle holder.
Appendix E. Specifications and Drawings

163

Figure 67. Illustration. CLRP-GCS08 geophone stand connector pin.
Redevelopment of the FWD Calibration Protocol and Equipment

164

SEISMOMETER CALIBRATION STAND

Figure 68. Photo. KUAB seismometer calibration stand.

Table 27. Fabricated parts required for BM-SCS seismometer calibration stand assembly.

Drawing Number Description Quantity

CLRP-SCS02 Seismometer stand side rail 2

CLRP-SCS03 Seismometer stand top shelf 1

CLRP-SCS04 Seismometer stand sensor shelf 4

CLRP-SCS05 Seismometer stand standoff 10

CLRP-SCS06 Seismometer stand bottom shelf 1

CLRP-SCS07 Seismometer stand handle holder1 4

CLRP-SCS08 Seismometer stand connector pin2 1

CLRP-SCS09 Seismometer stand shelf subassembly 4

CLRP-SCS10

Seismometer stand accelerometer

shelf 1

CLRP-SCS11

Seismometer stand accelerometer 

shelf subassembly 1 1

Identical to CLRP-SCS07. 2

Identical to CLRP-SCS08.
Appendix E. Specifications and Drawings

165

Table 28. Hardware items required for BM-SCS seismometer calibration stand assembly.

Vendor Part 

Number Item Vendor Quantity

62385K65

Tapered handle, smooth phenolic, 3

/8-inch, 16 x 

1

/2-inch threaded stud, 

1-inch diameter McMaster-Carr® 2

2198A85

Glass surface-mount bull’s-eye level 

center circle and cross lines, 11/4-inch

base diameter, 7/16-inch height McMaster-Carr® 1

91737A072

18-8 stainless steel fillister head Phillips 

machine screw 4-40 thread, 1/4-inch 

length McMaster-Carr® 3

1 inch = 25.4 mm

Table 29. Vendor contact information for BM-SCS seismometer 

calibration stand assembly.

Vendor Web site Notes

McMaster-Carr®www.mcmaster.com

See Web site for specific contact 

information in the “About Us” section.

Incodema www.incodema.com

Incodema Inc.

407 Cliff Street, Ithaca, NY 14850

607-277-7070

Table 30. Hardware costs for BM-SCS seismometer calibration stand assembly.

Part Quantity Supplier Cost1

Materials and machining 1 Incodema $1425.00

Miscellaneous hardware 1 $30.00 1

Indicates the cost in February 2009.
Redevelopment of the FWD Calibration Protocol and Equipment

166

Figure 69. Illustration. CLRP-SCS01 seismometer stand assembly.
Appendix E. Specifications and Drawings

167

Figure 70. Illustration. CLRP-SCS02 seismometer stand side rail.
Redevelopment of the FWD Calibration Protocol and Equipment

168

Figure 71. Illustration. CLRP-SCS03 seismometer stand top shelf.
Appendix E. Specifications and Drawings

169

Figure 72. Illustration. CLRP-SCS04 seismometer stand sensor shelf.
Redevelopment of the FWD Calibration Protocol and Equipment

170

Figure 73. Illustration. CLRP-SCS05 seismometer stand standoff.
Appendix E. Specifications and Drawings

171

Figure 74. Illustration. CLRP-SCS06 seismometer stand bottom shelf.
Redevelopment of the FWD Calibration Protocol and Equipment

172

Figure 75. Illustration. CLRP-SCS07 seismometer stand handle holder.
Appendix E. Specifications and Drawings

173

Figure 76. Illustration. CLRP-SCS08 seismometer stand connector pin.
Redevelopment of the FWD Calibration Protocol and Equipment

174

Figure 77. Illustration. CLRP-SCS09 seismometer stand shelf subassembly.
Appendix E. Specifications and Drawings

175

Figure 78. Illustration. CLRP-SCS10 seismometer stand accelerometer shelf.
Redevelopment of the FWD Calibration Protocol and Equipment

176

Figure 79. Illustration. CLRP-SCS11 seismometer stand accelerometer shelf subassembly.
Appendix E. Specifications and Drawings

177

GEOPHONE ADAPTERS

Table 31. Fabricated parts for use as BM-GA geophone adapters.

Drawing Number Description Quantity

CLRP-GA02 Carl Bro adapter 10

CLRP-GA03 JILS adapter 10

CLRP-GA04 KUAB adapter 10

Table 32. Hardware items required for use of BM-GA geophone adapters.

Vendor Part 

Number Item Vendor Quantity

90126A036 Society of Automotive Engineers flat 

washer, zinc-plated steel, 3/4-inch size, 13/16-inch inside diameter, 11

/2-inch

outside diameter, 0.108-inch minimum

thickness McMaster-Carr® 10

94744A273 Flat washer, zinc-plated steel, 3/8-inch

screw size, 0.406-inch inside diameter, 

11/4-inch outside diameter, 0.090–

0.112 inches thick McMaster-Carr® 10

92865A212 Hex head cap screw, grade 5 zinc-plated 

steel, 3/8-inch 24 Thread, 5/8 inches long, 

fully threaded McMaster-Carr® 10

KK-GA02 Knurled knob, steel with black oxide 

finish, M8 x 1 mm threaded through hole

Morton Machine 

Works 10

KK-GA03 Knurled knob, steel with black oxide 

finish, 3/8-24 threaded through hole

Morton Machine 

Works 10

6063K19 Metric steel two-piece clamp on shaft 

collar 20-mm bore, 40-mm outside 

diameter, 15-mm width McMaster-Carr® 10

5685K26 Encased ceramic round magnet W/hole 

113/32-inch diameter, 9/32-inch thick, 

20 pull lb McMaster-Carr® 10

6121K93 Steel round knurled-rim knob solid hub, 

1.5-inch diameter McMaster-Carr® 10

92949A246 18-8 SS button head socket cap screw 

10-24 thread, 7/8-inch length McMaster-Carr® 10

90730A011 18-8 SS undersized machine screw hex 

nut 10-24 screw size, 5/16-inch width, 

7

/64-inch height McMaster-Carr® 10

1 inch = 25.4 mm
Redevelopment of the FWD Calibration Protocol and Equipment

178

Table 33. Vendor contadct information for BM-GA geophone adapters.

Vendor Web site Notes

McMaster-Carr® http://www.mcmaster.com

See Web site for area specific contact 

information located in the “About 

Us” section.

Morton Machine 

Works http://www.mortonmachine.com

125 Gearhart St, PO Box 97 

Millersburg, PA 17061

800-441-2751

Table 34. Hardware costs for BM-GA geophone adapters.

Part Quantity Cost1 Notes

Dynatest®/JILS adapter 10 $50.00

Price for all 10 adapters 

including bolts and washers.

Carl Bro adapter 10 $50.00 Price for all 10 adapters.

KUAB Geophone 

adapter 10 $150.00

Price for all 10 adapters 

including collars, magnets, 

knobs, washers, bolts, and nuts. 1

Indicates the cost in February 2009.
Appendix E. Specifications and Drawings

179

Figure 80. Illustration. CLRP-GA01 geophone adapter assemblies.
Redevelopment of the FWD Calibration Protocol and Equipment

180

Figure 81. Illustration. CLRP-GA02 Carl Bro adapter.
Appendix E. Specifications and Drawings

181

Figure 82. Illustration. CLRP-GA03 JILS adapter.
Redevelopment of the FWD Calibration Protocol and Equipment

182

Figure 83. Illustration. CLRP-GA04 KUAB adapter.
Appendix E. Specifications and Drawings

183

REFERENCE LOAD CELL

For a photo of the reference load cell, see figure 104.

Table 35. Fabricated parts for the BM-LC reference load cell.

Drawing Number Description Quantity

CLRP-LC02 Reference load cell body 1

CLRP-LC03 Base plate 1

CLRP-LC04 Measuring link 3

CLRP-LC05 Cover plate 1

CLRP-LC06 Foot 3

CLRP-LC07 Guide finger 3

CLRP-LC08 Interior connector wiring 1

CLRP-LC09 Strain gauge layout 3

CLRP-LC10 Bridge wiring detail 1

Table 36. Equipment and hardware items required for the BM-LC reference load cell.

Vendor Part 

Number Item Vendor Quantity

92196A327

Socket head cap screw, 18-8 

SS, 1/4-inch 28 thread, 11/4-inch 

length McMaster-Carr® 6

92196A323

Socket head cap screw, 18-8 

SS, 1/4-inch 28 thread, 7/8-inch 

length McMaster-Carr® 6

92210A337

Flat head socket cap screw, 

18-8 SS, 1/4-inch 28 thread, 

11/4-inch length McMaster-Carr® 6

91737A072

Fillister head Phillips machine 

screw, 18-8 SS, #4-40 thread, 1

/4-inch length McMaster-Carr® 4

654-MS3102A14S-5P

Amphenol box mount 

receptacle, MS3102A-14S-5P Mouser Electronics 1

N2A-13-S054Y-350

Transducer-class strain gauges, 

general purpose 90-degree

rosette, 1/8-inch gauge length, 

350 Ω resistance

Vishay MicroMeasurements 12

1 inch = 25.4 mm
Redevelopment of the FWD Calibration Protocol and Equipment

184

Table 37. Vendor contact information for the BM-LC reference load cell.

Vendor Web site Notes

McMaster-Carr® http://www.mcmaster.com

See Web site for specific contact information 

located in the “About Us” section.

Mouser Electronics http://www.mouser.com

See Web site for specific contact information 

located in the “Contact Us” section.

Vishay MicroMeasurements http://www.vishay.comSee Web site for specific contact information 

located in the “Sales Representatives” section.

Note: A complete reference load cell assembly, cable, and case can be purchased from Half-Space Engineering, 

6055 Quaking Aspen Road, Reno NV, 89510 (pete@half-space.com). Cost depends on what additional hardware is 

needed (signal conditioner, data acquisition board, etc.).
Appendix E. Specifications and Drawings

185

Figure 84. Illustration. CLRP-LC01 load cell assembly.
Redevelopment of the FWD Calibration Protocol and Equipment

186

Figure 85. Illustration. CLRP-LC02 load cell body.
Appendix E. Specifications and Drawings

187

Figure 86. Illustration. CLRP-LC03 base plate.
Redevelopment of the FWD Calibration Protocol and Equipment

188

Figure 87. Illustration. CLRP-LC04 measuring link.
Appendix E. Specifications and Drawings

189

Figure 88. Illustration. CLRP-LC05 cover plate.
Redevelopment of the FWD Calibration Protocol and Equipment

190

Figure 89. Illustration. CLRP-LC06 foot.
Appendix E. Specifications and Drawings

191

Figure 90. Illustration. CLRP-LC07 guide fingers.
Redevelopment of the FWD Calibration Protocol and Equipment

192

Figure 91. Illustration. CLRP-LC08 interior connector wiring.
Appendix E. Specifications and Drawings

193

Figure 92. Illustration. CLRP-LC09 strain gauge layout.
Redevelopment of the FWD Calibration Protocol and Equipment

194

Figure 93. Illustration. CLRP-LC10 bridge wiring.
Appendix E. Specifications and Drawings

195

DATA ACQUISITION SYSTEM

Table 38. Cable diagrams required for BM-DAQ.

Drawing Number Description Quantity

CLRP-DAQ01 Vishay to KUSB DAQ cable 1

CLRP-DAQ01A Vishay 2310B BNC to DAQ cable 1

CLRP-DAQ01B

Vishay 2310B BNC to DAQ cable 

(stock parts) 1

CLRP-DAQ02 Vishay to load cell cable 1

CLRP-DAQ03 Accelerometer signal cable 1

CLRP-DAQ04 Pushbutton to KUSB DAQ cable 1
Redevelopment of the FWD Calibration Protocol and Equipment

196

Table 39. Equipment and hardware items required for BM-DAQ.

Vendor Part Number Item Vendor Quantity

Model 2310B Vishay 2310 signal conditioner

Vishay MicroMeasurements 1

2310-A20 Kit

Vishay stabilizer bar and line cord 

accessory package

Vishay MicroMeasurements 1

KUSB-3108 Keithley 16-bit USB DAQ Keithley 1

571-5-1634504-1

BNC panel mount radio frequency (RF)

connector Mouser Electronics 1

566-8723

Belden 8732, two-pair shielded cable, 

25-ft sections Mouser Electronics 4

654-PT06A-14-15P-SR

Amphenol PT06A-14-15P <SR> straight 

plug Mouser Electronics 2

654-PT06A-10-6S-SR

Amphenol PT06A-10-6S <SR> straight 

plug Mouser Electronics 1

654-MS3106F-14S-5S

Amphenol MS3106A 14S-5S straight 

plug Mouser Electronics 1

2062325

Resistors, 2.2 kΩ, 

1

/4 Watt, 5 percent 

carbon film Radio Shack® 2

2103233 Terminal strip, 5 position Radio Shack® 1

2062543 Pushbutton, momentary, 1.5 A Radio Shack® 1

278-0232 Enclosure, black plastic Allied Electronics 1

91772A123

Pan head Philips machine screw, 18-8 

stainless steel, #5-40 x 5/16-inch length McMaster-Carr® 1

91841A006

Nut, 18-8 stainless steel, #5-40 thread, 

7

/64-inch height McMaster-Carr® 1

510-0194 BNC to BNC cable, 10-ft length Allied Electronics 1

523-T3324-501 Amphenol circular DIN connector Mouser Electronics 1

546-1455T2202

Hammond enclosure, aluminum 

extruded, 8.66 x 6.3 x 2.1 inches Mouser Electronics 1

523-T3327-100

Amphenol circular DIN connector, 

panel mount Mouser Electronics 1

571-5-1634504-1 BNC panel mount RF connector Mouser Electronics 1

523-T3327-100

Amphenol circular DIN connector, 

panel mount Mouser Electronics 1

523-T3324-501 Amphenol circular DIN connector Mouser Electronics 1

546-1455T2202

Hammond enclosure, aluminum 

extruded, 8.66 x 6.3 x 2.1 inches Mouser Electronics 1

Velcro, hook and loop, adhesive backed, 

industrial strength Local hardware store

1 inch = 25.4 mm

1 ft = 0.305 m
Appendix E. Specifications and Drawings

197

Table 40. Vendor contact information for BM-DAQ.

Vendor Web site Notes

McMaster-Carr® http://www.mcmaster.com

See Web site for specific contact information 

located in the “About Us” section.

Mouser Electronics http://www.mouser.com

See Web site for specific contact information 

located in the “Contact Us” section.

Allied Electronics http://www.alliedelec.com

See Web site for specific contact information 

located in the “Customer Service” section.

Radio Shack® http://www.radioshack.com

See Web site for specific contact information 

located in the “Contact Us” section.

Keithley Instruments http://www.keithley.com

See Web site for specific contact information 

located in the “Contact Us” section.

Vishay MicroMeasurements http://www.vishay.comSee Web site for specific contact information 

located in “Sales Representatives” section.
Redevelopment of the FWD Calibration Protocol and Equipment

198

Figure 94. Illustration. CLRP-DAQ01 Vishay 2310 to DAQ cable.
Appendix E. Specifications and Drawings

199

Figure 95. Illustration. CLRP-DAQ01A Vishay 2310B BNC to DAQ cable.
Redevelopment of the FWD Calibration Protocol and Equipment

200

Figure 96. Illustration. CLRP-DAQ01B Vishay 2310B BNC to DAQ cable (stock parts).
Appendix E. Specifications and Drawings

201

Figure 97. Illustration. CLRP-DAQ02 Vishay to load cell cable.
Redevelopment of the FWD Calibration Protocol and Equipment

202

Figure 98. Illustration. CLRP-DAQ03 accelerometer signal cable.
Appendix E. Specifications and Drawings

203

Figure 99. Illustration. CLRP-DAQ04 pushbutton to KUSB DAQ cable and cable wiring.
Redevelopment of the FWD Calibration Protocol and Equipment

204

Figure 100. Illustration. CLRP-DAQ06 KUSB protection wiring.
Appendix E. Specifications and Drawings

205

Figure 101. Illustration. CLRP-DAQ07 KUSB enclosure rear panel.
Redevelopment of the FWD Calibration Protocol and Equipment

206

Figure 102. Illustration. CLRP-DAQ08 KUSB enclosure front panel.
207

APPENDIX F. HARDWARE USE AND INSTALLATION GUIDE

OVERVIEW/SCOPE

The purpose of this guide is to describe the installation and proper use of the following 

components of the FWD calibration system (see figure 103):

• Calibration facilities.

• Concrete test pad.

• Reference load cell.

• Concrete anchors.

• Ball-joint anchor.

• Accelerometer box.

• Geophone calibration stand.

• Seismometer calibration stand.

• Geophone adapters.

• Data acquisition components (Keithley DAQ and Vishay signal conditioner).

• Portable computer.

The instructions provided here assume the use of the WinFWDCal software package.
Redevelopment of the FWD Calibration Protocol and Equipment

208

Figure 103. Photo. Calibration system components.

FACILITIES

Calibration center facilities require the following characteristics:

• Easy access for the FWD trailer and the tow vehicle.

• A large level floor that can accommodate both the FWD trailer and the tow 

vehicle indoors.

• A stable indoor temperature between 50 and 100 °F (10 and 38 °C).

• Good security cabinets for calibration equipment.

TEST PAD

While a specially built test area for FWD calibration is not required by AASHTO R32-09, some 

agencies either need or prefer to build a concrete test pad.(1) If so, the test pad should have the 

following specifications:

• A 12- by 15-ft (4- by 4.5-m) height and an 8-ft (2.5-m)-wide (5-ft (1.75-m) minimum 

allowable) clear zone around the perimeter to allow for maneuvering of FWD and the 

calibration equipment.

• A level, smooth, crack-free Portland cement concrete surface. A modest amount of 

hairline cracking is permissible. Should the test pad develop cracks that are visibly open 

0.0585 inches (1.5 mm) or more, it should be replaced.
Appendix F. Hardware Use and Installation Guide

209

• It should be isolated from the surrounding floor by impregnated felt bond breaker or 

sawed and caulked joint.

• It should be at a distance of 20 to 24 inches (500 to 600 mm) behind the FWD load 

plate, the test pad should deflect 20 ±4 mil (500 ±100 μm) at a peak dynamic load of 

16,000 lbf (71 kN).

REFERENCE LOAD CELL

Parts and Tools

Table 41. Parts and tools for load cell calibration.

Tool/Equipment Quantity Notes

Reference load cell 

assembly 1 Drawing CLRP-LC01

1

/4 28 x 1-inch socket head 

cap screw 6

McMaster-Carr® part 

number (p/n) 92196A325

1

/4 28 x 3/4-inch socket head 

cap screw 6

McMaster-Carr® p/n 

92196A321

Vishay signal conditioner 1

Load cell signal cable 1 Drawing CLRP-DAQ02

3

/16-inch hex wrench 1

Torque wrench 1 Capable of 100 inch-lbf

1 inch = 25.4 mm

1 lbf = 4.45 N
Redevelopment of the FWD Calibration Protocol and Equipment

210

Figure 104. Photo. Reference load cell assembly.

Calibration

The reference load cell requires annual calibration to ensure accuracy. To calibrate the reference 

load cell, a universal testing machine with a load capacity of 120,000 lbf (500 kN) or more is 

needed. Although the load cell is calibrated to only 24,000 lbf (107 kN), the higher capacity of 

the testing machine assures that the test frame will be adequately rigid.

The Vishay 2310 signal conditioner, Keithley KUSB-3108 DAQ, the reference load cell, and the 

load cell signal cable are considered one system and must be calibrated together. Once the 

calibration is complete, it only applies to the entire system that was used during the calibration. 

If any component of the system changes, then the load cell needs a new calibration.

The bolts that hold the cover plate and feet onto the load cell (see figure 105) should not be 

removed under any circumstances. If any of these bolts are removed, the load cell calibration 

becomes invalid and must be returned to an approved calibration center for recalibration.
Appendix F. Hardware Use and Installation Guide

211

Figure 105. Photo. Bottom view of the reference load cell.

Use

1. Ensure that the guide fingers are tightened to the load cell body during FWD load 

cell calibration.

2. Attach the signal cable form the load cell to the signal conditioner and allow the electronics 

to warm up for at least 1 h.

3. Carefully align the reference load cell under the FWD load plate, ensuring that the guide 

fingers and the load plate do not interact (see figure 106).

4. Follow the on-screen instructions provided by WinFWDCal for a complete load 

cell calibration.
Redevelopment of the FWD Calibration Protocol and Equipment

212

Figure 106. Photo. Reference load cell positioned under the FWD load plate.

CONCRETE ANCHOR INSTALLATION

The concrete anchors suggested for use with the FWD calibration hardware are of the 

drop-in variety (see figure 107). These instructions are for anchors from McMaster-Carr®

p/n 97082A031 only. For any other equivalent anchor, refer to the manufacturer’s 

installation instructions.

Parts and Tools

Table 42. Parts and equipment for concrete anchor install.

Tool/Equipment Quantity Notes

Concrete anchors 2

McMaster-Carr®, p/n 

97082A031

Anchor setting tool 1

McMaster-Carr® p/n 

97077A120

Hammer 1

Drill 1 Optionally a hammer drill

1

/4-inch masonry drill bit 1

1

/2-inch masonry drill bit 1

3

/8-inch inside diameter 

washers 2

McMaster-Carr® p/n 

94744A273

1 inch = 25.4 mm
Appendix F. Hardware Use and Installation Guide

213

Figure 107. Photo. Concrete anchor.

Installation

1. Determine the appropriate location for the ball-joint anchor on the test pad. The FWD needs 

to impart a deflection of 16–24 mil (400–600 μm) at a 16,000-lbf (71-kN) load where the 

ball-joint will be located.

2. Using the base bar from the ball-joint anchor (B04, drawing CLRP-BJ03) as a guide, mark 

where the two anchors will be installed. The holes should be 6 inches (152.4 mm) apart from 

center to center.

3. Drill pilot holes at the anchor locations using a 1

/4-inch (6.35-mm) masonry bit and, if 

available, a hammer drill. The holes should be slightly greater than 1.5 inches (38.1 mm) 

deep to ensure that the anchor rests below the surface of the concrete. Ideally, the holes will 

be drilled about 1

/16 inches (1.59 mm) deeper than the height of the anchor (see figure 108).
Redevelopment of the FWD Calibration Protocol and Equipment

214

Figure 108. Photo. Concrete anchor installation step 3.

4. Drill the final holes for the anchors with a 1/2-inch (12.7-mm) bit. Do not use a hammer drill 

for these holes.

5. Blow out the debris from holes with compressed air.

6. Drop the anchors into the holes and gently tap on the top of the anchor with a hammer to 

insert it into the hole. Be careful not to damage the anchor. If the anchor does not sit flush in 

the hole, do not attempt to drive it in by striking the anchor itself directly because this will 

damage the anchor. If necessary, remove the anchor and drill a little deeper (see figure 109).
Appendix F. Hardware Use and Installation Guide

215

Figure 109. Photo. Concrete anchor installation step 6.

7. Once the anchors are at the correct depth in the hole, use the setting tool to expand the anchor 

by placing the setting tool inside the anchor and striking it with a hammer (see figure 110).

Figure 110. Photo. Concrete anchor installation step 7.
Redevelopment of the FWD Calibration Protocol and Equipment

216

BALL-JOINT ANCHOR

Parts and Tools

Table 43. Tools and equipment for ball-joint assembly.

No. Part/Equipment Quantity Notes

B01 Ball-joint 1 Techno/Sommer KG-60

B01a Ball 1 Techno/Sommer KG-60

B01b Socket 1 Techno/Sommer KG-60

B01c Clamp 1 Techno/Sommer KG-60

B01d Screw 2 Techno/Sommer KG-60

B02 Clamp 1 Drawing CLRP-BJ01

B03 Clamp base 1 Drawing CLRP-BJ02

B04 Base bar 1 Drawing CLRP-BJ03

B05 Rest stop 1 Drawing CLRP-BJ04

B06

M6 x 16-mm socket head 

cap screw 4

McMaster-Carr®

p/n 91292A135

B07

M8 x 16 mm socket head 

cap screw 6

McMaster-Carr®

p/n 91292A145

B08

M8 x 25 mm socket head 

cap screw 2

McMaster-Carr®

p/n 91292A148

B09

Loctite® #242 

threadlocker 1

B10 5-mm hex wrench 1

B11 6-mm hex wrench 1

B12

Dow Corning Molykote 

G-4500 alum thickened 

grease, 14.1-oz, Nlgi #2

McMaster-Carr®

p/n 4328T24

1 inch = 25.4 mm

1 oz = 28.35 g
Appendix F. Hardware Use and Installation Guide

217

Figure 111. Photo. Parts and tools for ball-joint assembly.
Redevelopment of the FWD Calibration Protocol and Equipment

218

Assembly

1. Attach the clamp (B02) to the clamp base (B03) with two M8 x 0.975-inch (25-mm) socket 

head cap screws (B08). 

Note: Do not apply Loctite® to the two screws.

2. Completely disassemble the ball-joint (B01) by removing its two screws (B01d). Thoroughly 

clean all old lubricant off the ball and socket and apply a thin layer of Molykote® type G 

lubricant (B12) on the mating surfaces.

3. Use Loctite® and three M8 x 0.624-inch (16-mm) socket head cap screws (B07) and mate 

the ball-joint’s ball (B01a) and the clamp components assembled in step 1 through the 

counterbored holes in the ball.

4. Reassemble the ball-joint (B01). 

Note: Do not apply Loctite® when installing the screws (B01d).

5. Attach the base bar (B04) to the completed ball-joint assembly with Loctite® (B09) and three 

M8 x 0.624-inch (16-mm) socket head cap screws (B07) through the counterbored holes in 

the bottom of the base bar. The ball-joint screws (B01d) must be aligned along the length of 

the base bar for access as shown in figure 112.

6. Slide the rest stop (B05) over the ball-joint assembly and secure it to the base bar (B04) 

with four M6 x 0.624-inch (16-mm) socket head cap screws (B06). See figure 113 for a 

photograph of the assembly of the ball joint anchor.

Figure 112. Photo. Proper alignment of ball-joint screws.
Appendix F. Hardware Use and Installation Guide

219

Figure 113. Photo. Assembly of the ball-joint anchor.
Redevelopment of the FWD Calibration Protocol and Equipment

220

ACCELEROMETER BOX

Parts and Tools

Table 44. Parts and tools for accelerometer box assembly.

No. Part/Equipment Quantity Notes

A01a Accelerometer assembly 1

A01a Accelerometer 1 Silicon Designs model 2220-005

A01b

Amphenol PT02A-10-6P box 

mounting receptacle 1 Mouser p/n 654-PT02A106P

A01c Accelerometer wiring 1 Drawing CLRP-AB05

A02 Box bottom 1 Drawing CLRP-AB02

A03 Box top 1 Drawing CLRP-AB03

A04 Calibration platter 1 Drawing CLRP-AB04

A05

#4-40 x 3/8-inch flat head 

Phillips machine screw 4 McMaster-Carr® p/n 96877A209

A06

#4-40 x 1/2-inch pan head 

Phillips machine screw 2 McMaster-Carr® p/n 91400A110

A07 #4 retaining washer 2 McMaster-Carr® p/n 91755A205

A08

#4-40 x 1/4-inch fillister head 

Phillips machine screw 7 McMaster-Carr® p/n 91737A072

A09

#10-24 x 1/2-inch knurled head 

thumbscrew 2 McMaster-Carr® p/n 91746A876

A10

Bubble level, glass surface 

mount 1 McMaster-Carr® p/n 2198A85

A11

Leveling mount with

polyethylene base, 3/8-inch

16 x 1-inch stud 3 McMaster-Carr® p/n 23015T64

A12 3/8-inch 16 locking wig nut 3 McMaster-Carr® p/n 98520A145

A13 #1 Phillips head screwdriver 1

A14 Loctite® #242 threadlocker 1

1 inch = 25.4 mm

Note: Blank cells indicate that no notes are available.
Appendix F. Hardware Use and Installation Guide

221

Assembly

1. The accelerometer is attached to the box top (A03) using two #4-40 x 1/2-inch (12.7-mm) pan 

head machine screws (A06) with #4 nylon retaining washers (A07). The sensor element of 

the accelerometer must be oriented in the correct alignment. The element is marked on the 

casing of the accelerometer as shown in figure 114. This element must be centered on and 

mated to the inside face of the box top.

2. Coil the accelerometer wiring (A01c) into the box top and slide the wire through the channel 

in the front of the box top so that Amphenol receptacle (A01b) sits flush against the front of 

the box.

3. The Amphenol receptacle (A01b) is attached to the face of the box top with four 

#4-40 x 1/4-inch (6.35-mm) fillister head machine screws (A08) and medium strength 

Loctite® #242 (A14).

4. The box top and box bottom (A02) are mated together with four #4-40 x 3

/8-inch (9.52-mm)

flat head machine screws (A05) and medium strength Loctite® #242 (A14). The final 

assembly is shown in figure 115.
Redevelopment of the FWD Calibration Protocol and Equipment

222

Figure 114. Photo. Parts for accelerometer box assembly.
Appendix F. Hardware Use and Installation Guide

223

Figure 115. Photo. Accelerometer box assembly.
Redevelopment of the FWD Calibration Protocol and Equipment

224

Calibration Platter Assembly

1. Attach the bubble level (A10) to the calibration platter (A04) with three #4-40 x 1/4-inch 

(6.35-mm) fillister head machine screws (A08). Note that the threaded holes for the 

thumbscrews are not centered on the side of the calibration platter. Check for proper 

alignment of the accelerometer box on the platter before mounting the bubble level.

2. The three leveling mounts are threaded into the calibration platter and topped with three

3

/8-inch (9.52-mm) 16 locking wing nuts (A12).

3. The accelerometer box is attached to the calibration platter with two #10-24 x1

/2-inch

(12.7-mm) knurled head thumbscrews (A09) as shown on the left in figure 116.

Figure 116. Photo. Accelerometer box attached to calibration platter.

Accelerometer Storage

1. When not in use, attach the accelerometer box to the calibration platter with the two #10-24 

knurled head thumbscrews (A09) provided.

2. The accelerometer needs to be stored on the platter and be level for a minimum of 24 h prior 

to use.

Accelerometer Calibration

1. Place the calibration platter, with the accelerometer box attached, on a flat surface and 

carefully level the platter.

2. Remove the thumbscrews and press the accelerometer box on to the calibration platter with a 

finger. Otherwise, the weight of the cable may pull the accelerometer box onto the floor.
Appendix F. Hardware Use and Installation Guide

225

3. While following the on screen instructions from WinFWDCal, continue to press the 

accelerometer box to the calibration platter. 

4. Flip the accelerometer box over when instructed.

CALIBRATION STANDS

Parts and Tools

Table 45. Equipment for calibration stand assembly.

Number Tool/Equipment Quantity Notes

S01 Geophone calibration stand assembly 1 Drawing CLRP-GCS01

S02

Seismometer calibration stand 

assembly 1 Drawing CLRP-SCS01

S03

Phenolic handle,

3

/8-inch 

16 x 1/2-inch stud 4

McMaster-Carr® p/n 

62385K65

S04 Bubble level, glass surface mount 2

McMaster-Carr® p/n 2198A85; 

Also No. A10 in this guide.

S05

#4-40 x 1/4-inch fillister head Phillips

machine screw 6

McMaster-Carr® p/n 

91737A072; Also No. A08 in 

this guide.

S06

Carl Bro geophone adapters with

M8 1 mm threaded through hole 10

Morton Machine Works p/n 

KK-GA02

S07

JILS geophone adapter with 3/8-inch 

24 threaded through hole 10

Morton Machine Works p/n 

KK-GA03

S08 3/4-inch inside diameter washer 10

McMaster-Carr® p/n 

90126A036

S09 3/8-inch inside diameter washer 10

McMaster-Carr® p/n 

94744A273

Also No. B11 in this guide.

S10

3

/8-inch -24 x 5/8-inch hex head cap 

screw 10

McMaster-Carr® p/n 

92865A212

S11

#10-24 x 1/2-inch knurled head 

thumbscrew 2

McMaster-Carr® p/n 

91746A876; Also No. A09 in 

this manual

S12

Two-piece clamp-on shaft collar 

20-mm bore, 40-mm outside diameter 10 McMaster-Carr® p/n 6063K19

S13

Magnet assembly with #10-24 button 

head cap screw and nut 10

McMaster-Carr® p/n 5685K26, 

p/n 92949A246, and p/n 

90730A011

S14 Modified knurled rim knob 10 McMaster-Carr® p/n 6121K93

1 inch = 25.4 mm
Redevelopment of the FWD Calibration Protocol and Equipment

226

Assembly

See figure 117 for a photograph of the equipment necessary for calibration stand assembly.

1. The bubble level (S04) is fastened to the handle holder of either stand (S01 or S02) with 

three #4-40 x 1/4-inch (6.35-mm) fillister head machine screws (S05). Since there are four 

potential places to attach the level, the calibration center operator should determine which 

location is appropriate for visibility and comfort during use.

2. Two phenolic handles (S03) are screwed in place at one of the two available positions 

depending on the preference of the operator.

3. The pushbutton assembly (described elsewhere) should be attached with Velcro® to the stand 

at the same level as the handles and in a spot where it can be easily reached during use. See 

figure 118 for an example.

4. For both the geophone calibration stand and the seismometer calibration stand, the connector 

pin is fastened to the stand with medium strength Loctite® #242. Figure 125 shows the

geophone calibration stand with a connector pin attached.

Figure 117. Photo. Equipment for calibration stand assembly.
Appendix F. Hardware Use and Installation Guide

227

Figure 118. Photo. Calibration stand with handles, bubble level, and pushbutton.

Location of Accelerometer Box

On the geophone calibration stand, the accelerometer box is fastened to a shelf half way up the 

stand with two #10-24 x 1

/2-inch (12.7-mm) knurled head thumbscrews (S11).

On the seismometer calibration stand, the accelerometer box sits in the middle of the third shelf 

up on the stand and is held on with two thumbscrews (S11). The accelerometer box needs to be 

attached before the seismometers to ensure ease of use.

Deflection Sensors

For the geophone calibration stand, there are four different types of geophones that can be 

calibrated: Carl Bro, Dynatest®, KUAB, and JILS.

The Carl Bro and the JILS geophones are fastened to the stand in similar manners, though they 

each require their own adapters. The geophone and its respective adapter (S06 and S07) screw 

together with a 3

/4-inch (19.05-mm) inside diameter washer (S08) placed between the adapter 

and shelf of the stand and a 3/8-inch (9.53-mm) inside diameter washer (S09) placed between the 

geophone and the shelf. Figure 119 shows a JILS geophone and adapter fastened to the stand. 

Carl Bro geophone adapters (S06) have different size threads and are painted red.

The Dynatest® geophones use a magnet to couple to the stand (see figure 120). To accomplish 

this, the JILS adapter (S07) is turned upside down and fastened to the stand with a 3/8-inch 
Redevelopment of the FWD Calibration Protocol and Equipment

228

(9.53-mm) to 24 x 5/8-inch (609.6 x 15.88-mm) hex head cap screw (S10) as shown in 

Figure 121. The Dynatest® geophone then sits on top of the adapter.

The KUAB geophones use a collar (S12), magnet (S13), and modified knob (S14) to attach to 

the stand, as shown in figure 122 and figure 123. The knobs are painted gold. The modifications 

to the knob are to drill and tap for a #10-24 machine screw thru the length of the knob, and to 

mill out a 1/8-inch (3.18 mm) deep by 1-inch (25.4-mm) diameter recess at the top of the shank to 

allow the magnet assembly to fit inside.

Figure 119. Photo. Attachment of a JILS or Carl Bro geophone to the stand.
Appendix F. Hardware Use and Installation Guide

229

Figure 120. Photo. Placement of the accelerometer box on the geophone stand.

Figure 121. Photo. Attachment of a Dynatest® geophone to the stand.
Redevelopment of the FWD Calibration Protocol and Equipment

230

Figure 122. Photo. KUAB geophone adapter equipment.

Figure 123. Photo. Attachment of a KUAB geophone adapter.

For the KUAB seismometer calibration stand, the seismometers are aligned in a two-column 

configuration as shown in figure 124. Users should tighten the setscrew at the bottom of the 

seismometer onto the standoffs on the stand.
Appendix F. Hardware Use and Installation Guide

231

Figure 124. Photo. Seismometer stand with sensors attached.

Using the Stand

During deflection sensor calibration, the calibration stand should be set up in the 

following order:

1. Attach the stand to the ball-joint anchor.

2. Fasten the accelerometer box to the stand after the accelerometer has been calibrated.

3. Place the deflection sensors in the correct configuration as shown on screen in the 

WinFWDCal program.

4. Use Velcro® to attach the pushbutton to the stand.

The following procedure is used when going from one trial to the next in relative and 

reference calibration:

• For Dynatest® geophones, remove the sensor by tilting its casing until the magnetic 

force is reduced enough to pull it out of the stand. Never pull on the sensor’s wiring to 

remove it.

• For the Carl Bro and JILS geophones, loosen the adapter by unscrewing it to a point 

where the adapter/sensor combination can be slid out of the stand as a unit.
Redevelopment of the FWD Calibration Protocol and Equipment

232

• For KUAB geophones, remove the sensor and collar by tilting the casing until the 

magnetic force is reduced enough to pull it out of the stand. Never pull on the sensor’s 

wiring to remove it.

• For KUAB seismometers, do not move the sensors in the stand. Simply rotate the stand 

horizontally about the connecting pin 180 degrees.

To couple the stand to the ball-joint, use the following:

• For both calibration stands, the connector pin slides into the clamp as shown in

figure 125. The clamp is then tightened so that the stand cannot be removed.

• It is very important that the pin extending from the calibration stand is tightly fixed in the 

stand. Use several drops of Loctite® on the threads to assure that the pin does not come 

loose. Doing so will reduce the variability of the deflection data that is collected.

• The screws holding the pin in the clamp must be tightened firmly. The two bolts that hold 

down the ball-joint anchor to the floor must also be tight. However, do not use Loctite®

on these connections.

• Finally, the two screws that adjust the ball-joint friction must be set so the calibration 

stand will stay upright when it vertical and not held, but a small tap horizontally will 

cause the stand to fall over easily to the rest stop. 

Figure 125. Photo. Coupling the calibration stand and ball-joint anchor.
Appendix F. Hardware Use and Installation Guide

233

DATA ACQUISITION SYSTEM

Parts and Tools

Table 46. Parts and equipment for data acquisition.

No. Tool/Equipment Quantity Notes

USB Standard USB cable 1

D01 Vishay to KUSB DAQ cable 1 Drawing CLRP-DAQ01 

D01A

Vishay 2310B to KUSB DAQ 

cable 1 Drawing CLRP-DAQ01A

D01B

Vishay 2310B to DAQ cable 

(stock parts) 1 Drawing CLRP-DAQ01B

D02 Vishay to load cell cable 1 Drawing CLRP-DAQ02 

D03 Accelerometer signal cable 1 Drawing CLRP-DAQ03 

D04

Pushbutton to KUSB DAQ 

cable 1 Drawing CLRP-DAQ04 

D05

Keithley KUSB-3108 16-bit 

DAQ 1

D06

Vishay 2310 signal 

conditioner 1

Note: Blank cells indicate no notes area available.

Cable Assembly

All cables should be fabricated according to their respective drawings. A full set of calibration 

cables includes an accelerometer signal cable (D05), a load cell signal cable (D04), a Vishay to 

KUSB DAQ cable (D03), and a pushbutton assembly (D06).

Connection Breakdown

Figure 126 illustrates FWD data acquisition system components and connections. The following 

list explains the connection breakdown:

• Calibration center personal computer to Keithley KUSB-3108 DAQ USB: Use a 

standard USB cable and plug into an available USB 2.0 port on the calibration computer.

• Vishay to KUSB DAQ cable (D01): Refer to drawing CLRP-DAQ01 for information on

how to connect the signal cable to DAQ. The Molex® connector (serial number (s/n)

38331-5608) plugs into the output receptacle in the back of the signal conditioner.

• Vishay to load cell cable (D02): The Amphenol 15-pin plug (s/n PT06A-14-15P <SR>) 

end of the load cell signal cable (drawing CLRP-DAQ02) connects to the input receptacle 

on the back of signal conditioner. The Amphenol five-pin plug (s/n MS3106A 14S-5S) 

end of the cable connects into the receptacle on the load cell.

• Vishay to accelerometer cable (D03): The Amphenol 15-pin plug (s/n PT06A-14-15P 

<SR>) end of the accelerometer signal cable (drawing CLRP-DAQ03) connects to the 
Redevelopment of the FWD Calibration Protocol and Equipment

234

input receptacle on the back of signal conditioner. The Amphenol six-socket plug (s/n 

PT06A-10-6S <SR>) end of the cable connects into the receptacle on the 

accelerometer box.

• Pushbutton to KUSB DAQ cable (D04): Refer to drawing CLRP-DAQ04 for 

information on how to connect the pushbutton to DAQ.

Figure 126. Illustration. FWD data acquisition system components and connections.

Vishay 2310 Signal Conditioner Settings

Table 47. Settings for the Vishay 2310 signal conditioner.

Vishay 2310 Setting Load Cell Accelerometer

Excitation 10 V 10 V

Filter 1 kHz 1 kHz

Gain Load cell dependant1 2.0 x 1

Auto balance Procedure dependant2 Always off

AC in Fully extended Fully extended 1

Indicates that each calibration center is provided with the correct gain for its reference load 

cell from the annual calibration. 2

Indicates that during the FWD load cell calibration, WinFWDCal provides instruction for 

when to use the auto balance switch on the Vishay signal conditioner.
Appendix F. Hardware Use and Installation Guide

235

COMPUTER

Table 48 presents the minimum requirements for a calibration computer. A laptop is 

recommended for portability.

Table 48. Minimum requirements for computer hardware.

Item Requirement

Operating system

Microsoft Windows XP® SP3 or 

later (not including Windows 7®)

Physical memory (RAM) 1 GB

Hard disk space At least 25 GB free space

Video adapter DVI/VGA

Display

17-inch external color monitor 

(15-inch liquid crystal display on 

laptop)

Optical drive CD-RW (DVD ±RW preferred)

Removable storage

3.5-inch floppy disk drive and

256 MB USB flash drive

USB 2.0 ports At least four

Printer Inkjet or laser

1 inch = 25.4 mm

Note: The USB flash drive, 3.5-inch floppy drive, and CD writer are used for 

transferring data between the FWD computer and the calibration computer.

237

APPENDIX G. SOFTWARE FLOW CHARTS

The following flow charts show the general outline of WinFWDCal Version 2.0.

(11) The 

Microsoft Visual Basic® source code is available upon request from FHWA or CLRP. 

Figure 127. Illustration. WinFWDCal startup flowchart.
Redevelopment of the FWD Calibration Protocol and Equipment

238

Figure 128. Illustration. WinFWDCal start new calibration flowchart.
Appendix G. Software Flow Charts

239

Figure 129. Illustration. WinFWDCal open and delete interim calibrations flowchart.
Redevelopment of the FWD Calibration Protocol and Equipment

240

Figure 130. Illustration. WinFWDCal main menu flowchart.
Appendix G. Software Flow Charts

241

Figure 131. Illustration. WinFWDCal calibrate accelerometer flowchart.
Redevelopment of the FWD Calibration Protocol and Equipment

242

Figure 132. Illustration. WinFWDCal edit setup data flowchart.
Appendix G. Software Flow Charts

243

Figure 133. Illustration. WinFWDCal determine trigger and number of drops flowchart.
Redevelopment of the FWD Calibration Protocol and Equipment

244

Figure 134. Illustration. WinFWDCal calibrate FWD load cell flowchart.
Appendix G. Software Flow Charts

245

Figure 135. Illustration. WinFWDCal calibrate FWD sensors flowchart.
Redevelopment of the FWD Calibration Protocol and Equipment

246

Figure 136. Illustration. WinFWDCal center documentation flowchart.
Appendix G. Software Flow Charts

247

Figure 137. Illustration. WinFWDCal monthly calibration flowchart.
Redevelopment of the FWD Calibration Protocol and Equipment

248

Figure 138. Illustration. WinFWDCal search for file and convert to PDDX input flowchart.
Appendix G. Software Flow Charts

249

Figure 139. Illustration. WinFWDCal review time history by clicking on drop cell 

flowchart.
Redevelopment of the FWD Calibration Protocol and Equipment

250

Figure 140. Illustration. Quit WinFWDCal flowchart.
251

ACKNOWLEDGEMENTS

The following calibration center operators provided helpful assistance, advice, and permission to 

use information during the development of the calibration protocol:

• Edward Trujillo, CDOT.

• Dave Wassel, PennDOT.

• John Ragsdale, Texas A&M University on behalf of TxDOT.

• Richard Stubstad, Applied Research Associates.

• Anders Sorensen, Dynatest® A/S.

• Gary Sanati, Foundation Mechanics Inc. (JILS).

• Jørgen Mikkelsen, Grontmij | Carl Bro A/S.

We would also like to thank the following State transportation departments for providing 

technical support and for their financial participation in the pooled fund TPF-5(039):

• Caltrans.

• CDOT.

• Georgia Department of Transportation.

• Iowa Department of Transportation.

• INDOT.

• Kansas Department of Transportation.

• Mn/DOT.

• Mississippi Department of Transportation.

• MDT.

• New Jersey Department of Transportation.

• New York Department of Transportation.

• North Dakota Department of Transportation.

• PennDOT.
Redevelopment of the FWD Calibration Protocol and Equipment

252

• South Carolina Department of Transportation.

• South Dakota Department of Transportation.

• TxDOT.

• Washington Department of Transportation.
253

REFERENCES

1. AASHTO R32-09. (2010). “Calibrating the Load Cell and Deflection Sensors for a Falling 

Weight Deflectometer,” Standard Specifications for Transportation Materials and Methods 

of Sampling and Testing, Part 1B: Specifications, Association of State Highway and 

Transportation Officials, Washington, DC.

2. Irwin, L.H., Yang, W.S., and Stubstad, R.N. (1988). “Deflection Reading Accuracy and 

Layer Thickness Accuracy in Backcalculation of Pavement Layer Moduli,” Nondestructive 

Testing of Pavements and Backcalculation of Moduli, American Society for Testing and 

Materials, Philadelphia, PA.

3. Strategic Highway Research Program. (1994). SHRP/LTPP FWD Calibration Protocol, 

Transportation Research Board, Washington, DC.

4. Orr, D.P. and Wallace, J.M.S. (2002) “Falling Weight Deflectometer Calibration: Looking 

Back to the Future,” Pavement Evaluation, Virginia Tech University, Roanoke, VA.

5. FHWA. (2010). Resolution on Falling Weight Deflectometer (FWD) Calibration Centers, 

Federal Highway Administration, Washington, DC. Obtained from http://www.

pooledfund.org/documents/TPF-5_039/resolution.pdf. Site last accessed October 21, 2010.

6. Cornell Local Roads Program. (1994). Reference Calibration of Falling Weight 

Deflectometers: Software Users Guide and Instruction Manual, FWDREFCL. Version 3.72, 

Cornell Local Roads Program, Ithaca, NY.

7. AASHTO. (1998). Pavement Deflection Data Exchange: Technical Data Guide, Version 1.0,

American Association of State Highway and Transportation Officials, Washington, DC.

8. Cornell Local Roads Program. (2010). Software Users Manual: PDDXconvert, CLRP Report 

No. 2010-05, Cornell Local Roads Program, Ithaca, NY.

9. AASHTO R32-03. (2006). “Calibrating the Load Cell and Deflection Sensors for a Falling 

Weight Deflectometer,” Standard Specifications for Transportation Materials and Methods 

of Sampling and Testing, Part 1B: Specifications, Association of State Highway and 

Transportation Officials, Washington, DC.

10. AASHTO R33-03. (2010). “Calibrating the Reference Load Cell Used for Reference 

Calibrations for Falling Weight Deflectometer,” Standard Specifications for Transportation 

Materials and Methods of Sampling and Testing, Part 1B: Specifications, Association of 

State Highway and Transportation Officials, Washington, DC.

11. Cornell Local Roads Program. (2010). Software Users Manual: WinFWDCal, CLRP Report 

No. 2010-06, Cornell Local Roads Program, Ithaca, NY.
Redevelopment of the FWD Calibration Protocol and Equipment

254

12. National Geodetic Survey. (2009). Surface Gravity Prediction in the Conterminous USA, 

National Geodetic Survey, Silver Spring, MD. Obtained from: http://www.ngs.noaa.gov/

TOOLS/Gravity/gravcon.html. Site last accessed October 21, 2010.

13. Dynatest® International A/S. (2010). The Dynatest Group of Companies, Glostrup, Denmark. 

Obtained from: http://www.Dynatest.com. Site last accessed October 21, 2010.

14. Engineering and Research International, Inc. (2010). KUAB Falling Weight Deflectometer, 

Engineering and Research International, Inc., Savoy, IL. Obtained from: http://www.

erikuab.com/kuab.htm. Site last accessed October 21, 2010.

15. JILS. (2009). The JILS, Foundation Mechanics, Falling Weight Deflectometer, JILS, El 

Segundo, CA. Obtained from: http://www.jilsfwd.com. Site last accessed October 21, 2010.

16. Minitab. (2005). Minitab Statistical Software, Release 14, Minitab, Inc., State College, PA.

17. ASTM E177-06b. (2006). “Standard Practice for Use of Terms Precision and 

Bias in ASTM Test Methods,” Annual Book of ASTM Standards, 14.02, ASTM International, 

West Conshohocken, PA.

18. Cornell Local Roads Program. (2010). Software Users Manual: RefLCCal, CLRP Report No. 

2010-07, Cornell Local Roads Program, Ithaca, NY.

19. ASTM E-74-06. (2005). “Standard Practice of Calibration of Force-Measuring Instruments 

for Verifying the Force Indication of Testing Machines,” Annual Book of ASTM Standards, 

Volume 03.01, ASTM International, West Conshohocken, PA.

HRDI-30/10-11(500)E Recycled

Recyclable