Long-Term Pavement 

Performance Program

Manual for 

Falling Weight 

Deflectometer

M e a s u r e m e n t s

Research, Development, and Technology

Turner-Fairbank Highway Research Center

6300 Georgetown Pike

McLean, VA 22101-2296

Publication No. FHWA-HRT-06-132 December 2006

Version 4.1
Foreword 

This manual is intended for use in collection of Falling Weight Deflectometer (FWD) data for the 

Long-Term Pavement Performance (LTPP) program. As such, it contains background information 

on FWD equipment and the general role of FWD testing within LTPP as well as a set of field 

operations guidelines for the data collection process. 

This is the fourth major version of the LTPP Manual for Falling Weight Deflectometer 

Measurements. The first two were developed under the auspices of the Strategic Highway Research 

Program; the last two were developed under contract to the Federal Highway Administration. The 

revisions to this manual reflect the changing data needs of LTPP as well as changes in FWD 

equipment. 

The LTPP program is a continuing, active program. Current information and access to other 

technical references are available at the LTPP Web site at 

http://www.tfhrc.gov/pavement/ltpp/ltpp.htm. Submit LTPP data requests, technical questions, and 

data user feedback to LTPP customer service by e-mail at ltppinfo@fhwa.dot.gov. 

Gary L. Henderson 

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. This report does not constitute a standard, specification, or 

regulation. 

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-06-132 

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

5. Report Date 

December 2006 

4. Title and Subtitle 

LTPP Manual for Falling Weight Deflectometer 

Measurements, Version 4.1 6. Performing Organization Code 

7. Author(s) 

Peter N. Schmalzer 

8. Performing Organization Report No. 

9. Performing Organization Name and Address 10. Work Unit No. (TRAIS) 

 MACTEC Engineering and Consulting, Inc. 

 12104 Indian Creek Court, Suite A 

 Beltsville, MD 20705-1242

11. Contract or Grant No. 

DTFH61-02-C-00007 

13. Type of Report and Period Covered 

Final Report, April 2005–July 

2006 

12. Sponsoring Agency Name and Address 

 Federal Highway Administration 

 Office of Infrastructure Research and Development 

 6300 Georgetown Pike 

 McLean, VA 22101-2296 14. Sponsoring Agency Code 

 

15. Supplementary Notes 

Contracting Officer’s Technical Representative (COTR): Aramis Lopez, Jr., HRDI-13. 

16. Abstract 

This document provides background information and field operations guidelines for the 

collection of Falling Weight Deflectometer (FWD) data on Long Term Pavement Performance 

(LTPP) test sections. It includes equipment setup, equipment calibration, test locations, and test 

procedures. 

17. Key Words 

Deflection measurement, falling weight 

deflectometer, LTPP 

18. Distribution Statement 

No restriction. This document is available to the 

public 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 

79 

22. Price 

N/A 

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 oC 

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 th Internat e ional System of Units. Appropriate rounding should be made to comply with Section 4 of ASTM E380. 

(Revised March 2003)
iii 

TABLE OF CONTENTS 

1 BACKGROUND ........................................................................................................................ 1

2 FWD-RELATED TERMINOLOGY ....................................................................................... 3

3 TEST PROCEDURE................................................................................................................. 5

4 SETUP......................................................................................................................................... 9

4.1 Physical Setup....................................................................................................................... 9 

4.1.1 Geophone Locations ...................................................................................................... 9 

4.1.2 Weight Package ........................................................................................................... 11 

4.2 Load Levels......................................................................................................................... 11 

4.3 Software Setup.................................................................................................................... 11 

4.3.1 Units............................................................................................................................. 11 

4.3.2 File Format................................................................................................................... 12 

4.3.3 Filters ........................................................................................................................... 12 

4.3.4 Data Checks ................................................................................................................. 12 

4.3.5 Drop Sequences ........................................................................................................... 12 

4.3.6 File Naming ................................................................................................................. 13 

5 TEST PLANS ........................................................................................................................... 15

5.1 Selecting the Appropriate Test Plan ................................................................................... 15 

5.2 Description of Test Plans.................................................................................................... 16 

5.3 Transverse Locations .......................................................................................................... 17 

5.4 Slab-Referenced Locations ................................................................................................. 18 

5.4.1 Mid-Panel..................................................................................................................... 19 

5.4.2 Joint Approach............................................................................................................. 19 

5.4.3 Joint Leave................................................................................................................... 19 

5.4.4 Corner .......................................................................................................................... 20 

6 OTHER MEASUREMENTS ASSOCIATED WITH FWD TESTING ............................. 21

6.1 Temperature Gradient Measurements................................................................................. 21 

6.2 Joint/Crack Width Measurements....................................................................................... 22 

6.3 Distress Identification and Commenting ............................................................................ 22 

6.4 Other Comments ................................................................................................................. 24 

7 ERROR CONDITIONS .......................................................................................................... 25

7.1 Roll-Off............................................................................................................................... 25 

7.2 Nondecreasing Deflections ................................................................................................. 25 

7.3 Overflow ............................................................................................................................. 26 

7.4 Load Variation .................................................................................................................... 26 

7.5 Deflection Variation............................................................................................................ 26 

7.6 Dealing With Deflection Errors.......................................................................................... 27 

7.7 Dealing With Load Errors................................................................................................... 28 

8 CALIBRATION AND VERIFICATION .............................................................................. 29

8.1 Reference Calibration ......................................................................................................... 29 

8.2 Relative Calibration ............................................................................................................ 29 

8.3 DMI Calibration.................................................................................................................. 30 

8.4 Temperature Sensor Verification........................................................................................ 30 

8.5 Reporting Requirements ..................................................................................................... 32 

8.5.1 Reference Calibration .................................................................................................. 32 
iv 

8.5.2 Relative Calibration ..................................................................................................... 33 

8.5.3 DMI Calibration........................................................................................................... 33 

8.5.4 Temperature Sensor Verification................................................................................. 33 

9 EQUIPMENT CHECKS......................................................................................................... 35

9.1 Before-Transit Checks ........................................................................................................ 35 

9.2 Before-Operations Checks.................................................................................................. 36 

9.3 Post-Operations Checks...................................................................................................... 36 

10 MAINTENANCE AND REPAIR......................................................................................... 37

11 COLD WEATHER FWD TESTING................................................................................... 39

12 DATA HANDLING PROCEDURES................................................................................... 41

12.1 Field Data Handling Procedures ....................................................................................... 41 

12.2 Office Data Handling Procedures ..................................................................................... 41 

13 DOCUMENTATION............................................................................................................. 43

14 VERSION HISTORY............................................................................................................ 45

14.1 Version 1.0........................................................................................................................ 45 

14.2 SPS 3 and SPS 4 Supplement to Version 1.0.................................................................... 45 

14.3 Version 2.0........................................................................................................................ 45 

14.4 Version 3.0........................................................................................................................ 45 

14.5 Version 3.1........................................................................................................................ 46 

14.6 Version 4.0........................................................................................................................ 46 

14.7 Version 4.1........................................................................................................................ 47 

APPENDIX A: DIAGRAMS OF FWD TEST PLANS ........................................................... 49

APPENDIX B: STANDARD FWD FORMS AND COMMON UNIT 

CONVERSIONS .............................................................................................. 61

APPENDIX C: UNIT CONVERSIONS FOR COMMON FWD-RELATED 

MEASUREMENTS ......................................................................................... 73
v 

 LIST OF FIGURES 

Figure 1. Diagram. Joint approach testing........................................................................................ 19 

Figure 2. Diagram. Joint approach-leave testing. ............................................................................. 20 

Figure 3. Diagram. Test plan 1. ........................................................................................................ 50 

Figure 4. Diagram. Test plan 2. ........................................................................................................ 51 

Figure 5. Diagram. Test plan 3. ........................................................................................................ 52 

Figure 6. Diagram. Test plan 4. ........................................................................................................ 53 

Figure 7. Diagram. Test plan 5. ........................................................................................................ 54 

Figure 8. Diagram. Test plan 6. ........................................................................................................ 55 

Figure 9. Diagram. Test plan 7. ........................................................................................................ 56 

Figure 10. Diagram. Test plan 8. ...................................................................................................... 57 

Figure 11. Diagram. Test plan 9. ...................................................................................................... 58 

Figure 12. Diagram. Test plan 10. .................................................................................................... 59 

Figure 13. Diagram. Test plan 11. .................................................................................................... 60 

LIST OF TABLES 

Table 1. Deflection sensor offsets for nine sensor FWDs. ................................................................. 9 

Table 2. Deflection sensor offsets for seven sensor FWDs. ............................................................. 10 

Table 3. Target Loads and Acceptable Ranges................................................................................. 11 

Table 4. Test plans by LTPP experiment and surface type............................................................... 15 

Table 5. Details of test plans............................................................................................................. 16 

Table 6. Transverse locations relative to outside lane edge.............................................................. 18 

Table 7. Temperature hole depths..................................................................................................... 21 

Table 8. Standard abbreviation list—distress. .................................................................................. 23 

Table 9. Standard abbreviation list—equipment, lane geometry, miscellaneous............................. 24 

1 

1 BACKGROUND 

A falling weight deflectometer (FWD) is a device designed to simulate deflection of a pavement 

surface caused by a fast-moving truck. The FWD generates a load pulse by dropping a weight. 

This load pulse is transmitted to the pavement through a 300-millimeter (mm) diameter circular 

load plate. 

The load pulse generated by the FWD momentarily deforms the pavement under the load plate 

into a dish or bowl shape. Envisioned from a side view, the shape of the deformed pavement 

surface is a deflection basin. 

Based on the force imparted to the pavement and the shape of the deflection basin, it is possible 

to estimate the stiffness of the pavement by using various computational methods. If the 

thickness of the individual layers is also known, the stiffness of those layers can also be 

calculated. 

In addition, an FWD can be used to determine the degree of interlock between adjacent slabs of a 

portland cement concrete (PCC) pavement. This degree of interlock is generally known as “load 

transfer efficiency” or LTE. Measurement of the LTE is obtained by placing the FWD load plate 

tangent to one side of the joint to be evaluated. A load pulse is then generated, and the 

deflections at equal distances on either side of the joint are measured. In a perfectly efficient 

joint, these deflections are equal. For most joints, the deflection on the unloaded slab is less than 

the deflection on the loaded slab. 

An FWD has two types of primary measurement devices. The first is a load cell, located directly 

above the load plate, and it measures the force imparted to the pavement. The second is a 

deflection sensor, also known as a “deflector.” The FWDs operated by the Long-Term Pavement 

Performance (LTPP) Program use geophones as the deflection sensors, although there are other 

types of deflection sensors used in other FWD designs. The LTPP FWDs have nine deflection 

sensors placed at fixed distances from the load plate to measure the shape of the deflection basin. 

In addition to the primary measurement devices, the LTPP FWDs have two additional types of 

measurement devices. The first is the distance measurement instrument (DMI), a high-accuracy 

odometer that measures the distance the FWD has traveled along a roadway. The second type is 

a temperature sensor. The LTPP FWDs have two categories of temperature sensors, an air 

temperature sensor and an infrared surface-temperature sensor. The data from these two 

temperature sensors, combined with data from nearby weather stations, are useful for estimating 

the temperature of various materials in the pavement structure. 

Knowing the temperature of the materials in the pavement structure is critical. For example, 

asphalt is hard and brittle at very low temperatures and soft and ductile at very high 

temperatures; therefore, the stiffness calculated from FWD data for these materials must be 

corrected for these temperature effects. In addition, the LTE between two PCC slabs varies as the 

slabs expand and contract or warp resulting from a difference in temperature between their top 

and bottom surfaces. 
2 

LTPP FWD operators are also required to perform manual measurements during FWD testing. 

These measurements consist of subsurface temperature measurements taken with a hand-held 

probe and joint width measurements taken during load-transfer testing. 

LTPP FWD operators also must comment on nonequipment-related conditions encountered 

during testing that might reasonably be expected to cause anomalous measurements. These 

conditions usually are cracks or other pavement surface distresses. 
3 

2 FWD-RELATED TERMINOLOGY 

Use of proper terminology greatly eases troubleshooting and problem reporting. This manual 

uses specific terminology to refer to specific parts of the Dynatest® model 8002 FWD, which is 

the type operated by LTPP. Some of these terms also apply to other types of FWDs. This list is 

complete only for this manual. Parts not included here may be found in the owner’s manual. 

Buffer—A rubber block attached to the underside of the weight package to control the shape of 

the generated load pulse. On LTPP FWDs, the buffers are roughly cylindrical, and there are four 

mounted on the weight package. 

Control Box—Contains connectors for the geophones, load cell, temperature sensor, and other 

sensors mounted on the FWD. Located on the FWD trailer, the control box sends these signals to 

the signal processor located in the tow vehicle through the multisignal cable. The control box 

also has buttons for manual control of the FWD hydraulics. 

Geophone—Device used to measure deflection. It is yellow, roughly cylindrical, and about 25 

mm (0.98 inch) in diameter and 50 mm (1.97 inches) high. Geophones are mounted in springloaded sensor support brackets suspended along the sensor bar. Each geophone has a unique 

serial number that is used to identify critical calibration information in the FWD data collection 

software. The LTPP FWDs use 2,000-micron (80-mil, or 0.8-inch) geophones. 

Load Cell—Measures the force imparted to the pavement by the FWD. The load cell is located 

directly above the load plate and below the swivel. The load cell has a serial number, which is 

visible from the rear. 

Load Plate—Directly contacts the pavement surface to transmit the load. The type used by LTPP 

is solid, and it has a 300-mm (11.81-inch) diameter. It consists of three layers: the topmost is 

steel, the middle is polyvinyl chloride (PVC), and the bottommost is a ribbed rubber sheet. 

Multisignal Cable—Carries electrical signals from the control box to the signal processor. These 

signals include the outputs from the transducers on the FWD and the command signals for the 

FWD hydraulics. 

Signal Processor—Connects the trailer control box through a multisignal cable to the data 

collection computer using an RS-232 serial cable. The LTPP FWDs use a Dynatest 9000 signal 

processor located in the tow vehicle. 

Strike Plate—A flat-surfaced column above the load plate. The weight package is dropped on the 

strike plate during operation of the FWD. Braces and a cylinder, housing the center geophone, 

and to which the swivel attaches, are welded to the underside of the strike plate. 

Sensor Bar—A long bar mounted above the load plate where the geophone holders are mounted. 

An extension of the sensor bar continues behind the load plate toward the trailer hitch to allow a 

geophone to be mounted behind the load plate. 
4 

Swivel—Connects the load cell to the strike plate. It allows the load cell and load plate to rotate 

to provide good contact with the pavement surface. 

Transducer—Any measurement device that converts a physical response into an electrical signal. 

Transducers on an FWD include the load cell, deflection sensors, and temperature sensors. 

Weight—Removable steel weights mounted to the weight package. Their mass is 20 kilograms 

(kg) (44.1 pounds (lb)) each. 

Weight Package—The entire assembly that is raised and then dropped to generate load. 
5 

3 TEST PROCEDURE 

The following list summarizes the general procedures FWD operators are to perform at each test 

site. The step summaries refer to sections in this manual that give the details of each step. 

3.1 Before arriving at the site, an FWD operator must have a filled out copy of form F05 

“FWD Operations Planning.” A separate copy of this form must be filled out for each test 

section. 

3.2 When an FWD operator arrives at the site, he or she should inspect the test section for 

evidence of maintenance activities. Any missing or defaced site markings should be replaced 

before continuing. If there is evidence of recent maintenance activities, contact the Regional 

Support Contractor (RSC) office. If quantitative evidence exists of the effect of the maintenance 

activities on the layer structure, reevaluate the temperature gradient hole section of form F05. 

3.3 Prepare the temperature gradient holes as directed in manual section 6.1. List the depth of 

the holes on form F05 in the information blanks that are not crossed out. Previously drilled holes 

may be used if the operator first blows them out and remeasures the depths, which should 

correspond to those listed on form F05. If the operator drills new holes, the depths should be 

measured and filled out on form F01. The operator then fills the holes with mineral oil and waits 

at least 20 minutes (min) before taking the first temperature measurement. 

3.4 Perform the before-operations activities listed in manual section 9.2. 

3.5 Perform the standard buffer warm-up sequence or the cold weather buffer-warm-up 

sequence, as appropriate, outside of the test section limits. The buffer warm-up sequences and 

their appropriate temperature ranges are given in section 4.3.5. 

After completing the buffer warm-up sequence, examine the data from the final four drops to 

ensure that the load levels are not all trending in the same direction (i.e., consistently increasing 

or consistently decreasing). If they are trending in the same direction, repeat the standard buffer 

warm-up sequence until the load levels stabilize. 

After the last buffer warm-up sequence, examine the data from the four drops before the final 

four drops to ensure that the load levels for each drop height are within the limits given in section 

4.2. If they are not, adjust the drop height targets accordingly and make an additional drop at 

each drop height to ensure that the targets are correctly set. If at any time during testing the load 

levels stray from the limits for their respective drop heights, stop testing, readjust the targets, and 

repeat the pass. 

If the FWD is idle for more than 15 min, again perform a standard buffer-warm-up sequence. 

3.6 Position the FWD so that the center of the load plate is on the section start limit (i.e., 

Station 0+000). Set the DMI to 0 centimeters (cm) (0 feet (ft)), ascending. The DMI must be set 

in units of “feet” for continuity with previous LTPP FWD testing. 
6 

3.7 Position the FWD at the first test location. Test locations for the appropriate test plan 

number (as given on form F05) are listed in manual section 5. 

3.8 In the FWD data collection software, set the “Test Setup” as listed on form F05. Then 

open a new data file. Use the file name as listed on form F05. 

3.9 Check the local time on the computer and evaluate whether the test pass can be 

completed before local midnight. If it is not probable that testing can be completed before 

midnight, do not start testing until after midnight. Any test pass that starts before midnight and 

ends after midnight cannot be uploaded to the LTPP database. 

3.10 Perform the first temperature gradient measurement as described in manual section 6.1. 

Perform subsequent temperature gradient measurements every 30 min, plus or minus 10 min. For 

GPS sections, take temperature gradient measurements at alternate ends, with a 60-min interval 

between measurements at a given end. 

3.11 Enter the lane specification appropriate to the test location in the field provided in the 

data collection software. 

3.12 Start the testing sequence. 

3.13 Exit the vehicle during testing and examine the pavement surface in the vicinity of the 

FWD for distresses and defects. Note any distresses in the area around the FWD as described in 

manual section 6.3. 

3.14 During load transfer testing (i.e., lane specifications J4, J5, C4, C5, F4 or F5) measure the 

joint/crack width as described in manual section 6.2. Joint/crack widths must be measured for at 

least 25 percent of load transfer tests. If time permits, measure joint/crack widths for all load 

transfer tests. 

3.15 If the data collection software generates errors during testing, follow the instructions in 

manual section 7. After testing, at the “Accept/Reject” prompt choose “Accept” only if no errors 

were generated, or after following the error resolution procedures in manual section 7. 

3.16 After testing is complete and the load plate is up, proceed to the next test point. 

3.17 If the next test point is at an even station (i.e., 1+00, 2+00) check that the DMI 

corresponds to the pavement markings. If it does not correspond, correct the DMI. 

3.18 Repeat steps 3.11 through 3.17 for each test point in the test pass. 

3.19 After completing each test point in the pass, close the data file. The data file must be 

closed and a new data file opened before beginning in a new test pass. 

3.20 Return the FWD to the section start limit (i.e., station 0+000), and return the DMI to zero. 
7 

3.21 Perform steps 3.7–3.9 and 3.113.19 for the new pass. The temperature gradient 

measurement process does not need to be restarted, but it must be continued as long as FWD 

testing is being performed at the section. 

3.22 Repeat steps 3.20–3.21 for each remaining pass at the test section. 

3.23 After completing FWD testing at the section, perform one more temperature gradient 

measurement, regardless of the time since the previous measurement. 

3.24 If traffic control conditions permit, perform data backup and preliminary data processing 

before leaving the site as described in manual section 12.1. Check that forms F01 and F02 are 

correctly filled out, and that form F05 is initialed and dated. 

3.25 Perform after-operations activities described in manual section 9.3. 

9 

4 SETUP 

4.1 PHYSICAL SETUP 

The physical setup described in this manual is mandatory for all LTPP-owned FWDs. Other 

FWDs collecting data on behalf of LTPP may not be capable of the setup described. In those 

cases, the setup should be followed as closely as possible. 

4.1.1 Geophone Locations 

For all LTPP testing, maintain the same placement of the geophones on the FWD. Measure 

geophone offsets from the center of the load plate to the center of the geophone holder. Measure 

the location of each geophone directly from the center of the load plate to avoid accumulated 

error. Offsets in front of the load plate (i.e., in the direction of the hitch) are considered positive. 

Offsets behind the load plate (i.e., in the direction of the rear bumper) are considered negative. 

The required offsets are shown in Table 1. 

Table 1. Deflection sensor offsets for nine sensor FWDs. 

Deflection Sensor Offset 

D1 0 mm 

D2 203 mm 

D3 305 mm 

D4 457 mm 

D5 610 mm 

D6 914 mm 

D7 1,219 mm 

D8 1,524 mm 

D9 −305 mm 

 1mm = 0.039 inch 

If testing is to be performed on behalf of LTPP by an FWD that mounts only seven deflection 

sensors, use the sensor spacings shown in Table 2. 
10 

Table 2. Deflection sensor offsets for seven sensor FWDs. 

Deflection Sensor 

Offset 

(Flexible Pavements) 

Offset 

(Rigid Pavements) 

D1 0 mm 0 mm 

D2 203 mm −305 mm 

D3 305 mm 305 mm 

D4 457 mm 457 mm 

D5 610 mm 610 mm 

D6 914 mm 914 mm 

D7 1,524 mm 1,524 mm 

1mm = 0.039 inch 

Non LTPP-operated FWDs and LTPP-operated FWDs that have undergone overhaul or 

replacement of the sensor bar or deflection sensor holders must have the deflection sensor offset 

measured and set accurately using the following procedure. 

1. Raise the FWD load plate and engage the transport locks and hitch pin in front sensor bar 

guide. Make sure the “Man Key” on the control panel is switched to “ON.” 

2. Check the springs and foam rubber guides on all deflector holders to ensure they are in good 

condition. Make sure that spring tensions are properly adjusted so that a force on the end 

feeler can move the holder and feeler upward until the feeler is at least 5 mm (0.2 inch) 

inside the bottom of the holder body, and that it returns easily when released. (If this does not 

occur, apply a few drops of silicone oil to the top of the guide rod). 

3. Use a steel tape measure with 1 mm (0.039 inch) graduations and 3 meters (m) (9.84 ft) or 

greater in length, and zero it on the center geophone feeler that projects through the bottom 

of the loading plate. Apply constant positive pressure on tape to eliminate any sag in tape 

throughout its length. Measure the location of every other defection sensor from this zero 

point to the center of the sensor’s feeler. It is not adequate to measure only from center to 

center between individual deflector sensors; this measurement alone will result in an 

accumulating error. By measuring from the rear of the contact screw, a more repeatable and 

accurate measurement can be taken. To do this it is necessary to compensate in the 

measurement for the distance from the outer edge of the contact screw to the center. Check 

position measurements at least twice.

4. For LTPP-operated FWDs, when the location of the deflection sensor holders is properly set, 

lock them in place. To do this, drill through the top of the deflection sensor holder where it 

passes over the sensor bar and through the sensor bar itself with a 6.7-mm (17/64-inch) bit. 

Tap the sensor bar itself to accommodate a 6M x 1.0 mm (thread pitch) screw. The deflection 

sensor holder can now be secured with a 6M x 1.0 mm to 20mm stainless steel hex-head 

screw. Retain the screw with a flat metal washer and medium thread-locking compound.
11 

4.1.2 Weight Package 

Configure the weight package the same for all LTPP testing. For Dynatest model 8002 FWDs, 

use three standard weights per side. Use two buffers per side. When new buffers are installed on 

the FWD, fill out form F04 and submit it to the FHWA LTPP FWD task leader. 

If testing is to be performed on behalf of LTPP by an FWD other than a Dynatest model 8002 

FWD, select a combination of buffers and weights that achieves the load requirements described 

in manual section 4.2 and comes as close as possible to a 13-millisecond (ms) pulse rise time. 

4.2 LOAD LEVELS 

Four load levels are defined here for LTPP testing. The acceptable load range for each drop 

height is between 90 percent and 110 percent of the target value. Experience has shown that drop 

loads for a given drop height tend to decrease slightly over the course of a day of testing. Setting 

the drop load at 103 percent of the target load at the beginning of the day will minimize the 

deviation over the course of the day for most cases. 

Each drop height must be within the acceptable range shown in Table 3 for all testing. Table 

measurements are in kilonewtons (kN) and kips (1 kip = 1 × 103 lb). Drop heights may not be 

adjusted during a test pass. 

Table 3. Target loads and acceptable ranges. 

Drop Height Target Load, kN (kips) Acceptable Range, kN (kips) 

1 26.7 (6.0) 24.0 to 29.4 (5.4 to 6.6) 

2 40.0 (9.0) 36.0 to 44.0 (8.1 to 9.9) 

3 53.4 (12.0) 48.1 to 58.7 (10.8 to 13.2) 

4 71.2 (16.0) 64.1 to 78.3 (14.4 to 17.6) 

Note: Drop height 1 is not used for testing on rigid pavements. 

4.3 SOFTWARE SETUP 

This section includes specific software settings required for LTPP testing. Instructions on how to 

enter these settings into the data collection software are given in the LTPP FWD Data Collection 

Software Manual.

4.3.1 Units 

All FWD data collected for LTPP should be in International System (SI) units, with the 

exception of station units, which should be in feet. Specifically, temperature should be recorded 

and displayed in Celsius (C), load in kilonewtons (kN), deflection in micrometers (μm) 

(microns), and deflection sensor offsets in millimeters (mm). 
12 

4.3.2 File Format 

Data collected with LTPP FWDs should be in the FWDWin MDB format. Data collected with 

non-LTPP seven -sensor FWDs should be in the R80-20 format, where possible. For FWDs not 

supporting either of these formats, contact the FHWA LTPP FWD task leader for instructions 

with a copy to the Technical Support Services Contractor (TSSC) before testing begins. 

4.3.3 Filters 

Collect data with all filters and smoothing turned off. 

4.3.4 Data Checks 

Enable the following checks: 

• Roll-off 

• Nondecreasing deflections 

• Overflow 

• Load variation—set to +/− (0.18 kN + 0.02X) 

• Deflection variation—set to +/− (2 μm + 0.01X) 

Further information on what these checks are and what to do if they fail appears in manual 

section 6. 

4.3.5 Drop Sequences 

Two different drop sequences are used for LTPP testing: 

• Flexible testing: C,C,C,1,1,1,1H,2,2,2,2H,3,3,3,3H,4,4,4,4H 

• Rigid testing of both jointed concrete pavements (JCP) and continuously reinforced concrete 

pavements (CRCP): C,C,C,2,2,2,2H,3,3,3,3H,4,4,4,4H 

Where C is a seating drop (no data saved) from drop height 3, 

1 is a drop from drop height 1, 

2 is a drop from drop height 2, 

3 is a drop from drop height 3, 

4 is a drop from drop height 4, 

H indicates that the full-time history for that drop is to be saved. 

In addition, one of the following drop sequences is used to warm up the buffers before testing: 

• Standard buffer warm-up sequence (in ambient temperatures above 10 degrees Celsius (°C) 

(50 degrees Fahrenheit (°F)): 

(1,2,3,4,4,4,4) repeated 8 times. 
13 

• Cold-weather buffer warm-up sequence (in ambient temperatures below 10 °C (50 °F): drop 

height (1) repeated 32 times, followed by the standard buffer warm-up sequence. 

Furthermore, the reference calibration and the relative calibration drop sequences from the 

“SHRP-LTPP FWD Calibration Protocol” should be preprogrammed into the LTPP-owned 

FWDs. 

4.3.6 File Naming 

The file name for an FWD data file should be eight characters in the following format: 

XXYYYYZN 

Where XX is the state code for section location, 

YYYY is the LTPP section ID for the section, 

Z represents the site visit (A for the first visit, B for the second visit, and so on), and 

N represents the pass number. 

For example, for the first pass of the first visit to section 3807 in North Carolina, the file name 

should be 373807A1. Test passes are described in detail in manual section 5.2. 

15 

5 TEST PLANS 

5.1 SELECTING THE APPROPRIATE TEST PLAN 

Test plans specify the location of test points at an LTPP section. There are several different test 

plans. The appropriate test plan for an LTPP section is determined by the experiment designation 

of the section, and in some cases the surface type or other properties of the section. The test plan 

applicable to a given LTPP section should be determined according to Table 4. 

Table 4. Test plans by LTPP experiment and surface type. 

Experiment Surface Type Test Plan Note 

GPS 1 Flexible 1 

GPS 2 Flexible 1 

GPS 3 JPCP* 2 

GPS 4 JRCP** 2 

GPS 5 CRCP*** 3 

GPS 6 Flexible 1 

GPS 7 Flexible 1 

GPS 9 JPCP 2 

SPS 1 Flexible 4 

SPS 2 JPCP 5 

JPCP 7 4.3 m lane width sections 

SPS 3 Flexible 8 

SPS 4 JPCP 9 

SPS 5 Flexible 4 

JPCP 5 

Flexible over JPCP 10 Sections 3, 6, 7, 8 

SPS 6 

Flexible over JPCP 11 Section 4 

SPS 7 JPCP 5 

CRCP 6 

SPS 8 Flexible 4 

SPS 8 JPCP 5 

SPS 9 Flexible 4 

Flexible over PCC 10 

 *JPCP—jointed plain concrete pavement. 

 **JRCP—jointed reinforced concrete pavement. 

 ***CRCP—continuously reinforced concrete pavement. 
16 

5.2 DESCRIPTION OF TEST PLANS 

Each test plan consists of one or more test passes. Data from each test pass should be stored in a 

separate data file. Each test pass should be performed in the direction of the flow of traffic, from 

the section approach limit (0+000) to the section leave limit (0+152.4 (5+00) for most test 

sections). 

Each test pass consists of testing according to one or more lane specification. It is critical that the 

operator enter the lane specification appropriate to the test location in the field provided in the 

data collection software. The longitudinal offset between test locations vary according to the test 

plan and test pass. Table 5 includes detailed information for each test plan and test pass. 

Table 5. Details of test plans. 

Test Plan Pass # Lane 

Spec. 

Lane 

Description 

Test 

Type Test Interval Number of Testsa

1 F1 ML Basin 7.6 m (25 ft) 21 1 3 F3 OWP Basin 7.6 m (25 ft) 21 

1 J1 ML–MP Basin 20 slabs 20 

J2 PE–CR Basin 20 slabs 20 2 J3 PE–MP Basin 20 slabs 20 

J4 OWP–JA LT 20 slabs 20 

2 

3 J5 OWP–JL LT 20 slabs 20 

1 C1 ML–MP Basin 20 slabs 20 

C2 PE–CR Basin 20 slabs 20 2 C3 PE–MP Basin 20 slabs 20 

C4 OWP–JA LT 20 slabs 20 

3 

3 C5 OWP–JL LT 20 slabs 20 

1 F1 ML Basin 15.2 m (50 ft) 11 4 3 F3 OWP Basin 15.2 m (50 ft) 11 

1 J1 ML–MP Basin 10 slabs b 10 c

J2 PE–CR Basin 10 slabs b 10 c

2 J3 PE–MP Basin 10 slabs b

 10 c

J4 OWP–JA LT 10 slabs b 10 c

5 

3 J5 OWP–JL LT 10 slabs b

 10 c

1 C1 ML–MP Basin 10 slabs 10 

C2 PE–CR Basin 10 slabs 10 2 C3 PE–MP Basin 10 slabs 10 

C4 OWP–JA LT 10 slabs 10 

6 

3 C5 OWP–JL LT 10 slabs 10 
17 

Table 5. Details of test plans—Continued.

Test Plan Pass # Lane 

Spec. 

Lane 

Description 

Test 

Type Test Interval Number of Testsa

1 J1 ML–MP Basin 10 slabs 10 

J2 PE–CR Basin 10 slabs 10 2 J3 PE–MP Basin 10 slabs 10 

J4 OWP–JA LT 10 slabs 10 3 J5 OWP–JL LT 10 slabs 10 

J7 WLE–CR Basin 10 slabs 10 

7 

4 J8 WLE–MP Basin 10 slabs 10 

1 F1 ML Basin 30.4 m (100 ft) 6 8 3 F3 OWP Basin 30.4 m (100 ft) 6 

J4 OWP–JA LT varies 

J5 OWP–JL LT varies 9 1 J6 OWP–MP Basin 

Every third 

joint/slab 

starting with 

first joint/slab 

varies 

1 F1 ML Basin 6 10 3 F3 OWP Basin 

Same as J1/J3 

before overlay 6 

1 F1 ML Basin 10 

F3 OWP Basin 

Same as J1/J3 

before overlay 10 

F4 OWP–JA LT 10 11 3 

F5 OWP–JL LT 

Same as J4/J5 

before overlay 10 a

 For PCC testing, if fewer effective slabs exist at a section than the number listed in the “Test interval” column, the number 

of slabs to be tested should equal the number of slabs that exist. b

 Test 20 slabs for 304.8 m (1,000 ft) test sections. c

 20 tests for 304.8 m (1,000 ft) test sections. 

Abbreviations: ML–midlane; OWP–outer wheel path; PE–pavement edge; WLE–widened lane edge; MP–mid-panel; CR–

corner; LT–load transfer; JA–joint approach (load plate on approach slab); JL–joint leave (load plate on leave slab)

Appendix A contains diagrams of these test plans. 

5.3 TRANSVERSE LOCATIONS 

The transverse location of a test pass is given as a relative position in the lane containing the 

LTPP test section. These relative positions are midlane (ML), outer wheel path (OWP), 

pavement edge (PE) and widened lane edge (WLE). As shown in table 6, these positions are to 

be measured from the outside lane edge (OLE) to the center of the FWD load plate. 
18 

Table 6. Transverse locations relative to outside lane edge. 

Offset from OLE 

Relative Position 3.66 m (12 ft) 

Nominal Lane Width 

4.27 m (14 ft) 

Nominal Lane Width 

ML 1800 mm ± 150 mm 1800 mm ± 150 mm 

OWP 760 mm ± 75 mm 760 mm ± 75 mm 

PE 150 mma ± 75 mm 150 mm ± 75 mm 

WLE NA 150 mmb ± 75 mm a

 If proper seating of the load plate at this offset is impossible due to pavement features, the offset may be 

increased. 

b Testing in the widened lane edge is referenced to the outer edge of the pavement slab, not the OLE. 

For a normal paving lane (nominally 3.7 m (12 ft) wide), the OLE is defined as the lane shoulder 

interface unless the outside edge of the painted shoulder stripe is more than 150 mm (6 inches) 

inside the lane shoulder interface, in which case the OLE is defined as the outside edge of the 

painted shoulder stripe. For a wide paving lane (nominally 4.0 m (13 ft) or wider), the OLE is 

defined as the outside edge of the painted shoulder stripe. 

This transverse location must be maintained within the error band listed for all test locations in 

the test pass. 

5.4 SLAB-REFERENCED LOCATIONS 

FWD testing on PCC surfaces is referenced to effective slabs, not absolute longitudinal position. 

When testing has been performed at a PCC-surfaced section previously, then the operator should 

test the same slabs again as those tested previously. The operator should use form F10 to identify 

slabs tested previously. On form F10, slabs are referenced by the station of the joint or crack that 

defines the approach end of the slab. If a previously tested slab is subdivided by a new transverse 

crack, then that portion of the slab which is bounded by the original approach joint or crack and 

the new transverse crack should be tested. 

If testing has not been performed previously at a PCC-surfaced section, then the operator should 

determine the number of slabs to be tested according to the test plan. Then the operator should 

determine the number of effective slabs that are wholly within the section limits (i.e., only count 

slabs that begin and end in the test section). Slabs can be bounded by either joints or full-width 

transverse cracks that are working. 

If the number of effective slabs is less than or equal to the number of slabs to be tested, then all 

effective slabs should be tested. If the number of effective slabs is greater than the number of 

slabs to be tested, then a subset of the effective slabs equal to the number of slabs to be tested 

should be selected. The operator should take care to evenly space the slabs to be tested along the 

test section. Testing in all test passes at the section should be done in reference to the same slabs. 

The test plans specify test locations relative to the slab to be tested as mid-panel, joint approach, 

joint leave, and corner. 
19 

5.4.1 Mid-Panel 

Mid-panel testing should be performed with the load plate located as close to the center of the 

effective slab as possible. The load plate should be within 0.3 m (1 ft) or 10 percent of the 

effective slab length of the center (as measured along the test pass), whichever is smaller. 

5.4.2 Joint Approach 

Joint approach testing should be performed with the load plate tangent to the joint or crack 

defining the approach end of the slab to be tested. The load plate should be located on the slab 

immediately before the selected slab. The edge of the load plate should be within 50 mm (2 

inches) of the joint, but under no circumstances should it bridge the joint. Figure 1 shows a 

diagram of joint approach testing. 

Direction of Traffic 

APPROACH SLAB SELECTED SLAB 

D

9 

D

2 

D

3 

Figure 1. Diagram. Joint approach testing. 

5.4.3 Joint Leave 

Joint leave testing should be performed with the load plate tangent to the joint or crack defining 

the approach end of the slab to be tested. The load plate should be located on the selected slab 

and the edge of the load plate should be within 50 mm (2 inches) of the joint, but under no 

circumstances should it bridge the joint. A diagram of joint approach leave testing is shown in 

Figure 2. 
20 

Direction of Traffic 

APPROACH SLAB SELECTED SLAB 

D

9 

D

2 

D

3 

Figure 2. Diagram. Joint approach-leave testing. 

5.4.4 Corner 

For JCP pavements (test plans 2, 5, and 7), corner testing should be performed the same as joint 

leave testing except that the load plate should be tangent to both the joint or crack defining the 

approach end of the slab and the longitudinal joint defining the outside edge of the slab. The 

edge of the load plate should be no more than 75 mm (3 inches) from the joint or crack defining 

the approach end of the slab and no more than 75 mm (3 inches) from the longitudinal joint. 

For CRCP pavements (test plans 3 and 6) the corner test should be performed with the load plate 

centered on the transverse crack defining the approach end of the effective slab and tangent to 

the longitudinal crack defining the outside edge of the slab. The edge of the load plate should be 

no more than 75 mm (3 inches) from the longitudinal joint. 
21 

6 OTHER MEASUREMENTS ASSOCIATED 

WITH FWD TESTING 

6.1 TEMPERATURE GRADIENT MEASUREMENTS 

Up to five holes should be drilled in the bound layers of the pavement for measuring sub-surface 

temperatures. Only holes that terminate in an unbound layer may be eliminated. Table 7 shows 

the depths to which the holes should be drilled. 

Table 7. Temperature hole depths. 

1 mm = 0.039 inch 

All holes should be drilled as close as possible to the depths listed in table 7. The tolerance for 

hole number 1 is ±5 mm (±0.2 inch). The tolerance for holes 2 through 5 is ±10 mm (±0.39 

inch). The holes should be drilled in decreasing order of depth to allow extra time for the heat 

generated during drilling to dissipate. 

If a hole is within ±25 mm (±0.98 inch) of the reported bottom of the bound layers, its depth 

should be decreased to 25 mm above the reported bottom of the bound layers. 

The holes should be drilled in the center of the OWP, but they should be offset at least 0.5 m 

(1.64 ft) from each other in the longitudinal direction. For GPS sections, the holes should be 

located on both ends of the test section, just outside the monitoring area. Typically the holes 

should be located near station 0-001 (0-03) and 0+153.4 (5+03). For SPS sections, holes should 

be located at only one end of the section, just outside the monitoring area. The operator should 

choose the end of the section that is most representative of the monitoring area. 

Holes should be drilled with a portable hammer drill using a 13-mm (0.51-inch) diameter bit. 

After the hole is drilled to the required depth, it should be cleared of debris and dust by blowing 

through a short piece of plastic tubing or other suitable device. The actual depth of the hole 

should be measured after it is cleared of debris. Then it should be filled with 15 to 25 mm (0.59 

to 0.98 inch) of mineral oil to provide thermal conductivity between the pavement and the 

temperature probe. The hole should be covered with tape (such as duct tape) and the tape slit to 

allow the probe to be inserted. 

In addition to subsurface temperature measurements, an infrared (IR) surface temperature 

measurement should be obtained at the same time that subsurface temperature readings are 

Hole Number Hole Depth (mm) 

1 25 

2 50 

3 100 

4 200 

5 300 
22 

obtained. The location of the measurement should also be in the OWP and at least 0.5 m (1.64 ft) 

from the nearest hole in the longitudinal direction. The operator should take care that the area 

where the IR surface temperature is taken is free of oil, dirt, and other foreign debris. During 

temperature measurements the hand-held IR device should be held at a height consistent with the 

height of the FWD-mounted IR measurement device. IR temperate measurements should be 

taken three times in quick succession. The results should be averaged and entered on form F01. 

6.2 JOINT/CRACK WIDTH MEASUREMENTS 

Joint/crack width measurements should be taken only on PCC surfaced pavements. An exception 

is testing done in accordance with test plan 11. On flexible surfaced pavements, cracks should be 

noted according to the instructions in manual section 6.3. 

On PCC surfaced pavements, joint/crack width measurements should be taken for at least 

25 percent of the load transfer tests; however, operators are encouraged to perform such 

measurements for all load transfer tests if time permits. 

Joint/crack width measurements should be performed using calipers with tapered jaws for 

measuring inside dimensions. The resolution of the calipers should be at least 0.3 mm (0.01 

inch). 

On transverse cracks, the goal is to measure the minimum width of the opening that extends 

through the pavement. If the cracks are spalled, the width of the opening may need to be 

estimated. 

On sawed joints, the goal is the measure the sawn width (as opposed to the actual opening). It 

may be necessary to depress the joint sealant to measure the opening, especially if the joints are 

spalled. 

Joint/crack width openings should be measured at several points in the OWP, and the 

measurements averaged and rounded to the nearest 1 mm (0.039 inch). This average should be 

entered into the FWD data collection software in the appropriate field. 

Measurements less than 1 mm are hard to make with calipers because the jaws are too wide to 

enter the opening. In these situations, the measurement should be recorded as 1 mm (0.039 inch). 

Measurements in excess of 25 mm (0.98 inch) should be recorded as 25 mm (0.98 inch). 

6.3 DISTRESS IDENTIFICATION AND COMMENTING 

The type and severity of pavement distress may influence the deflection response of a pavement; 

therefore, FWD operators should record any distress within the lane tested from about 0.3 m 

(11.81 inches) in front of the forward most deflection sensor to 0.9 m (35.4 inches) behind the 

load plate and from 0.3 m (11.81 inches) on the left of the load plate to 0.3 m (11.81 inches) on 

the right of the load plate. The operator should enter this information at the “Comments” prompt 

immediately following the test sequence. Where possible, operators should use standard 

abbreviations listed in tables 8 and 9. Operators should avoid other abbreviations unless they are 

necessary because of space limitations. 
23 

Table 8. Standard abbreviation list—distress. 

Category Full Word or Phrase Standard Abbreviation 

Bleeding BLD 

Block cracking BCR 

Blowups BLWUP 

Corner break CBRK 

Durability cracking DCR 

Edge crack ECR 

Fatigue crack FCR 

Faulting FLT 

Joint seal damage JTSD 

Lane shoulder dropoff LSDROP 

Longitudinal crack LCR 

Map cracking MAP 

Patch PATCH 

AC patch ACPATCH 

PCC patch PCPATCH 

Polished aggregate POLAG 

Popouts POUT 

Pothole PTHL 

Pumping PUMP 

Ravelling RVL 

Rough surface RSURF 

Rutting RUT 

Scaling SCL 

Shoving SHOV 

Spalling SPL 

Transverse crack TCR 

Transverse construction joint damage TCJTD 

Low severity LSEV 

Moderate severity MSEV 

Distress 

High severity HSEV 
24 

Table 9. Standard abbreviation list—equipment, lane geometry, miscellaneous. 

Category Full Word or Phrase Standard Abbreviation

Load plate LP 

Deflection sensor DS (or DS1, DS2 ...) 

Excess variation EXVAR 

Nondecreasing deflections NDD 

Roll-off ROFF 

Rejection RJCT 

Accept ACPT 

Sensor bar SBAR 

Air temperature ATMP 

Surface temperature STMP 

Deflection DEFL 

Equipment 

Load LD 

Inner wheel path IWP 

Outer wheel path OWP 

Both wheel paths BWP 

Mid-lane ML 

Outer lane edge OLE 

Inner lane edge ILE 

Lane stripe LSTR 

Shoulder stripe SHSTR 

Joint JT 

Longitudinal joint LJT 

Lane Geometry 

Transverse joint TJT 

Between BTWN 

Core hole CHOLE 

Due to time limitations DTTL 

Due to weather conditions DTWC 

Moved forward MVFRD 

Test pit TPIT 

Miscellaneous 

Truck traffic TRKTRF 

6.4 OTHER COMMENTS 

In addition to distress comments, other unusual conditions or events deserve comments. 

Operators should comment on data with nondecreasing deflections, excess variation, or other 

software-generated errors that could not be cleared by following the instructions in manual 

section 6. Operators should also comment on other events such as delays in testing because of 

breakdowns or weather, pavement changes within the section, moisture seeping out of cracks, or 

other conditions that could affect deflection measurements. 
25 

7 ERROR CONDITIONS 

As described in manual section 4.3.4, several data checks should be enabled in the FWD data 

collection software. They are described in the following paragraphs. 

7.1 ROLL-OFF 

“Roll-off” is an error condition that results when the deflection of the pavement surface as 

recorded by a deflection sensor does not return to near zero within 60 ms of the trigger 

activation. Unless the pavement structure is weak enough to be permanently deformed by the 

load pulse, this is not a believable measurement. All LTPP test sections have sufficient strength 

that roll-off can only be to the result of measurement error. 

Roll-off can be caused by poor contact between the deflection sensor and the pavement surface. 

This error can be incorrectly triggered when magnitude of the deflection approaches the 

resolution of the geophone. Thus, roll-off when the peak deflection is less than 25 μm (0.83 inch) 

is not necessarily indicative of error. 

If the roll-off error is triggered on a deflection that is greater than 25 μm (0.83 inch), then the 

operator should follow the error resolution instructions in manual section 7.6. Otherwise the 

measurement should be accepted and testing should continue. 

7.2 NONDECREASING DEFLECTIONS 

“Nondecreasing deflections” is an error condition that results when the deflections measured by 

the deflection sensors do not decrease with increasing distance from the load plate. Deflections 

should always decrease with increasing distance from the load plate. 

This condition can sometimes occur legitimately if there is a transverse crack or other 

discontinuity between the two sensors that exhibit the nondecreasing deflections. If the operator 

observes such a crack or discontinuity between the two flagged sensors, then the test should be 

accepted and the observation should be recorded at the “Comments” prompt. 

Nondecreasing deflections can sometimes be triggered incorrectly when the magnitude of the 

deflections approaches the resolution of the deflection sensor. If the larger of the two deflections 

is less than 10 μm (0.39 inch), then the test should be accepted, and testing should continue. 

Nondecreasing deflection may sometimes occur between deflection sensors 1 and 2 on extremely 

weak pavements because of permanent deformation of the pavement by the FWD. 

Nondecreasing deflections between deflection sensors 1 and 2 have also been observed on very 

stiff PCC pavements where the deflection basin is fairly uniform near the load plate and the 

difference in deflections in that area is less than the random error inherent to the deflection 

sensors. 
26 

If the deflections are not small and there are no cracks or discontinuities, or the nondecreasing 

deflections are not between sensors 1 and 2 and the pavement is not very weak or very stiff, then 

this error usually indicates poor seating between one or both of the flagged sensors and the 

pavement surface. The operator should follow the error resolution instructions in manual 

section 7.6. 

7.3 OVERFLOW 

“Overflow” is an error condition that results when a measured deflection exceeds the range of 

the deflection sensor. For the FWDs operated by LTPP, this range is 2,000 microns (80 mils, or 

0.08 inch). Deflections of that magnitude are only expected on extremely weak pavements or 

when testing using extremely high load levels. For testing at an LTPP test section using the load 

levels specified in this document, it is not reasonable to expect that the deflections will exceed 

2,000 microns (80 mils, or 0.08 inch). 

If this error is encountered during testing, it is likely that the flagged sensor does not have good 

contact with the pavement surface. The operator should follow the error resolution instructions in 

manual section 7.6. 

7.4 LOAD VARIATION 

“Load variation” is an error condition that results when the peak load for repeat drops at the 

same drop height varies by more than the amount specified in manual section 4.3.4. This 

condition can occur legitimately on weak pavements where the structure is damaged by FWD 

testing, or on pavements with a saturated base or subgrade layer (such as during the spring thaw 

in wet-freeze zones). It may also occur if the load plate is not seated properly on the pavement 

surface, either because of loose debris or irregularities in the pavement surface. 

If this error occurs during testing, the operator should follow the error resolution instructions in 

manual section 7.7. 

7.5 DEFLECTION VARIATION 

“Deflection variation” is an error condition that results when the load-normalized peak 

deflections for repeat drops vary by more than the amount specified in manual section 4.3.4. This 

condition can occur legitimately only if the stiffness of the pavement is changed by the FWD 

testing itself. This generally occurs only if the pavement is extremely weak or the unbound layers 

are saturated. 

Deflection variations can occur as a result of uneven pavement surface conditions causing poor 

seating of the load plate or deflection sensors or vibrations generated by heavy equipment 

operating nearby, especially trucks traveling in adjacent lanes. 

If this error occurs during testing, the operator should follow the error resolution instructions in 

manual section 7.6. 
27 

7.6 DEALING WITH DEFLECTION ERRORS 

If deflection errors occur, the operator must attempt to identify the source of those errors. If the 

errors caused by the FWD equipment, then those problems must be fixed before testing 

continues. If the errors result from localized pavement conditions, the operator should reposition 

the FWD and comment on the condition. If the errors are the result of pavement conditions that 

are representative of the test section as a whole, or due to factors beyond the operator’s control 

such as truck traffic, the operator should accept the error and comment on the condition. Manual 

sections 7.1, 7.2, 7.3, and 7.5 detail possible causes of the various types of deflection errors. 

If the deflection errors appear to be to the result of truck traffic in an adjacent lane, the operator 

should attempt to pause the test sequence to allow the trucks to pass, and then continue the 

sequence during lulls in the traffic. If there are too many trucks for this method to be practical, 

the operator should provide a comment for drop sets that were potentially affected by the truck 

traffic. 

Operators must be very careful to not to overlook the FWD equipment itself as the source of the 

error. The following list is a summary of the troubleshooting process that is recommended in all 

cases, and the operator should deviate from the process only when confident of the source of the 

error, and the error source is not well suited for resolution using this troubleshooting process. 

7.6.1 For the first such error at a test location, it is recommended that the operator get out of 

the tow vehicle and check the flagged deflection sensor (or sensors). Check that the 

deflection sensor is seated securely in the sensor holder, that the screws retaining the 

sensor magnet and sensor holder are tight, that the deflection holder is not resting on a 

loose stone or crack, and that the holder springs and foam bushing are in good shape. 

7.6.2 If several sensors are flagged, it is recommended that the operator check all analog 

connections: sensor to control box, control box to multisignal cable and multisignal cable 

to signal processor. 

7.6.3 The data should be rejected and the test repeated without repositioning the FWD. 

7.6.4 If the errors persist, the operator must reject the test and perform the optional activities in 

manual section 7.6.1 and 7.6.2. If problems with the equipment are discovered and 

corrected, all data collected in this test pass should be discarded, and the test pass should 

be restarted. 

7.6.5 If errors persist and the test being performed is a load transfer test, or if the FWD cannot 

be moved forward because of a joint or transverse crack, then the results should be 

accepted whether or not they contain errors. The operator should enter a comment stating 

“Error could not be resolved.” 

7.6.6 If the errors persist and the FWD can be repositioned, the operator should reject the data 

and move the FWD forward .6 meters (2 ft). The operator should repeat the test a fourth 
28 

time. If the errors persist the operator should accept the results and at the prompt enter a 

comment stating “Error could not be resolved.” 

7.6.7 If the error could not be cleared, then for all subsequent errors of the same type in the 

same test pass the activities listed in manual sections 7.6.1 and 7.6.2 need not be 

repeated. 

7.7 DEALING WITH LOAD ERRORS 

If load errors occur, the operator must attempt to identify the source of those errors. If the errors 

result from problems with the FWD equipment, then those problems must be fixed before testing 

continues. If the errors are caused by localized pavement conditions, the operator should 

reposition the FWD and comment on the condition. If the errors are caused by pavement 

conditions that are representative of the test section as a whole, the operator should accept the 

error and comment on the condition. Manual section 7.4 gives details of the possible causes of 

load errors. 

Operators must be very careful to not to overlook the FWD equipment itself as the source of the 

error. The following summary of the troubleshooting process is recommended in all cases, and 

the operator should deviate from it only if confident of the source of the error and the error 

source is not well suited for resolution using the troubleshooting process. 

7.7.1 The data should be rejected and the test repeated without repositioning the FWD. 

7.7.2 If the error persists, the operator should get out of the tow vehicle and check the 

equipment. All analog connections should be checked: load cell to load cell cable, load 

cell cable to control box, control box to multisignal cable and multisignal cable to signal 

processor. The weight height targets should be checked to ensure that they are tight. The 

load plate should be raised and the swivel checked to ensure that it moves easily. Check 

the rubber sheet and pavement surface under the load plate for debris, and remove any . 

7.7.3 Reject the data and repeat the test without repositioning the FWD. 

7.7.4 If errors persist and the test being performed is a load transfer test, or if the FWD cannot 

be moved forward because of a joint or transverse crack, then accept the results whether 

or not they contain errors. Enter a comment stating “Error could not be resolved.” 

7.7.5 If the errors persist and the FWD can be repositioned, reject the data and the move the 

FWD moved forward .6 m (2 ft). Repeat the test a third time. If the errors persist then 

accept the results, and enter a comment stating “Error could not be resolved.” 

7.7.6 If the error could not be cleared, then for all subsequent errors of the same type in the 

same test pass, the operator need not repeat the activities in step 7.7.2. 
29 

8 CALIBRATION AND VERIFICATION 

Highly accurate load, deflection, and associated data are necessary to meet the requirements of 

the LTPP program. This section includes several calibration and verification requirements to 

ensure the accuracy of FWD measurements. If the FWD cannot meet the requirements of any of 

these procedures, it may not be used to collect data for LTPP. In such a situation the operator 

should fill out and submit an FWD Problem Report (FWDPR) (see appendix B) to the FHWA 

LTPP FWD task leader with copy to the TSSC. 

8.1 REFERENCE CALIBRATION 

Every FWD performing data collection on behalf of LTPP should undergo yearly reference 

calibration. An exception is units testing in Hawaii, Alaska, and Puerto Rico. This calibration 

must be performed at one of the four calibration centers established by the Strategic Highway 

Research Program (SHRP), or at an equivalent center as determined by the FHWA LTPP FWD 

task leader. The nominal yearly interval should not exceed 400 days. 

If the FWD load cell or signal processor is replaced, the FWD should undergo reference 

calibration before performing testing on behalf of LTPP regardless of the interval since the 

previous reference calibration. 

If a major component such as a deflection sensor, multisignal cable, or trailer printed circuit 

board (PCB) board is replaced, the FWD should undergo reference calibration as soon as 

practical, but in the meantime it can be used to collect data on behalf of LTPP. 

The most recent version of the FHWA LTPP FWD Reference and Relative Calibration Manual

should be followed during reference calibration. 

8.2 RELATIVE CALIBRATION 

Data should not be collected by an FWD on behalf of LTPP unless the FWD has undergone 

relative calibration within the previous 42 days. If the FWD is in regular use, relative calibration 

should be performed on a nominal monthly interval. Relative calibration should not be 

performed during periods when the FWD is idle. 

If a major component such as a deflection sensor, multisignal cable, or trailer PCB board is 

replaced, the FWD should undergo relative calibration before continuing to collect data on behalf 

of LTPP, regardless of the interval since the previous relative calibration. 

The most recent version of the FHWA LTPP FWD Reference and Relative Calibration Manual

should be followed during relative calibration. 
30 

8.3 DMI CALIBRATION 

Every FWD performing data collection on behalf of LTPP should undergo DMI calibration 

monthly while in service. Data should not be collected unless the FWD has undergone DMI 

calibration within the previous 42 days. 

If the tow vehicle undergoes maintenance, including replacement of tires, the DMI should be 

recalibrated before continuing to collect data on behalf of LTPP, regardless of the interval since 

the previous DMI calibration. 

The FWD DMI is calibrated by driving the vehicle over a known distance. The FWD data 

collection software can then calculate an appropriate calibration factor. 

The section used for DMI calibration must be straight, at least 150 m (492.13 ft) in length, and 

reasonably level. Because the FWD must be stopped at each end of the section, it cannot be 

performed on an active highway without traffic control. The section must be surveyed before the 

DMI calibration. The section should be measured using a surveyor’s tape (a measuring wheel is 

not acceptable). Ensure that the proper tension and alignment is applied to the tape. The operator 

should fill out form F07 immediately after each DMI calibration. 

Immediately before DMI calibration, the tire pressure for all of the tow vehicle tires should be 

set to the manufacturer’s specification, and then the operator should drive the vehicle at least 15 

min at highway speeds. The LTPP FWD Data Collection Software Manual has instructions on 

how to operate the FWD data collection software during DMI calibration. 

8.4 TEMPERATURE SENSOR VERIFICATION 

Every temperature sensor collecting data associated with LTPP FWD measurements should have 

its accuracy verified monthly, and unless the temperature sensor has been verified during the 

previous 42 days, data should not be collected. 

 

Following is a summary of a verification procedure; it should not be used as a calibration 

procedure. If a temperature sensor fails this procedure, it should be returned to the manufacturer 

for repair or recalibration, or it should be replaced with a new sensor. 

8.4.1 The following equipment and supplies are required for this procedure: 

• National Institute of Standards and Technology (NIST) traceable mercury thermometer 

(reference thermometer). 

• 4-liter (L) (1-gallon (gal)) bucket. 

• Hot plate. 

• Large wooden spoon or paint stirrer. 

• Medium-sized cooking pot, approximately 125 mm (5 inches) in diameter. 

• Leather heat-resistant gloves. 

• Cooking oil (about 0.5 L (1 pint)). 

• Ice. 
31 

• Water. 

• Copies of forms F08 and F09 (see appendix B). 

8.4.2 Park the FWD and tow vehicle on a smooth surface in an area with good ventilation that 

is not exposed to direct sunlight. 

8.4.3 Start the FWD data collection software and enter a screen from which the air and IR 

surface temperature measurements can be read. Further instructions are given in the 

LTPP FWD Data Collection Software Manual. 

8.4.4 Unclip the FWD air temperature mounted sensor so that it hangs freely. 

8.4.5 Prepare an ice water bath. Place ice and water in the 4 liter (1 gal) bucket and stir with the 

wooden spoon or paint stirrer. Stir until the reference temperature records a temperature 

that is less than or equal to 2 °C (35.6 °F). 

8.4.6 Place the bucket under the FWD air temperature mounted sensor. When the reading from 

the IR temperature stabilizes, record the temperature from both the IR sensor and the 

reference thermometer on form F08. Remove the reference thermometer, then stir the ice 

bath for 1 min, and record the measurements from both sensors again. If the recorded 

temperatures vary by more than 2 °C (3.6 °F) for either data set, stir for another minute, 

and then record both temperatures again. If the IR temperature sensor varies by more than 

2 °C (3.6 °F) from the reference thermometer for two or more data sets, then the IR 

temperature sensor is unacceptable. 

8.4.7 Repeat the procedure in 8.4.6 for the hand-held IR temperature sensor. Hold the handheld IR temperature sensor at a height consistent with the FWD-mounted IR temperature 

sensor. 

8.4.8 Stir the ice bath for another minute. Place the FWD air temperature mounted sensor and 

the reference thermometer in the bath. When the reading from the air temperature sensor 

stabilizes, record the readings of both sensors. Stir for another minute and record both 

temperatures again. If the recorded temperatures vary by more than 2 °C (3.6 °F) for 

either data set, stir for another minute, and then record both temperatures again. If the air 

temperature sensor varies by more than 2 °C (3.6 °F) from the reference thermometer for 

two or more data sets, then the air temperature sensor is unacceptable. 

8.4.9 Repeat the procedure in 8.4.6 for the hand-held temperature probe. 

8.4.10 Prepare the room temperature water bath. Empty the bucket and fill it with warm tap 

water. Allow it to sit for 10 min and then stir for 1 min. 

8.4.11 Place the bucket under the FWD air temperature mounted sensor. When the reading from 

the IR temperature sensor stabilizes, record the temperature from both the IR sensor and 

the reference thermometer on form F08. Stir the water for 1 min and then record the 

measurements from both sensors again. If the recorded temperatures vary by more than 
32 

2 °C (3.6 °F) for either data set, stir for another minute and then record both temperatures 

again. If the IR temperature sensor varies by more than 2 °C (3.6 °F) from the reference 

thermometer for two or more data sets, then the IR temperature sensor is unacceptable. 

8.4.12 Repeat the procedure in 8.4.11 for the hand-held IR temperature sensor. Hold the handheld IR temperature sensor at a height consistent with the FWD-mounted IR temperature 

sensor. 

8.4.13 Stir the water for another minute. Place the FWD air temperature mounted sensor and the 

reference thermometer in the water. When the reading from the air temperature sensor 

stabilizes, record the readings of both sensors. Stir for another minute and record both 

temperatures again. If the recorded temperatures vary by more than 2 °C (3.6 °F) for 

either data set, stir for another minute and then record both temperatures again. If the air 

temperature sensor varies by more than 2 °C (3.6 °F) from the reference thermometer for 

two or more data sets, then the air temperature sensor is unacceptable. 

8.4.14 Repeat the procedure in 8.4.13 for the hand-held temperature probe. 

8.4.15 If any IR temperature sensor was determined to be unacceptable in the low temperature 

or ambient temperature check, then it need not be checked at the high temperature. The 

high temperature check is optional if it is being performed in the field. The high 

temperature check is to be performed only for IR temperature sensors. 

8.4.16 Prepare the high-temperature oil bath. Pour cooking oil into the cooking pot to a depth of 

approximately 50 mm. Place the cooking pot on the hot plate and under the FWD air 

temperature mounted sensor. Stir the oil while it is warming on the hot plate. The 

operator who is stirring the oil must wear gloves. Heat until the oil temperature stabilizes 

at 60 °C (140 °F) (± 5 °C) (±3.6 °F), as determined using the FWD IR temperature 

sensor. 

8.4.17 Record the FWD IR temperature sensor and reference thermometer readings. Wait 5 min, 

and then record both sensors again. If the recorded temperatures vary by more than 2 °C 

(3.6 °F) for either data set, then wait 5 min and record both temperatures again. If the IR 

temperature sensor varies by more than 2 °C (3.6 °F) from the reference thermometer for 

two or more data sets, then the IR temperature sensor is unacceptable. 

8.4.18 Repeat the procedure in 8.4.17 for the hand-held IR temperature sensor. Hold the handheld IR temperature sensor at a height consistent with the FWD-mounted IR temperature 

sensor. 

8.5 REPORTING REQUIREMENTS 

8.5.1 Reference Calibration 

For LTPP-operated FWD equipment, the operator should submit the reference calibration results 

to the FHWA LTPP FWD task leader within 7 days after the calibration is completed. For non 

LTPP-operated FWD equipment collecting data on behalf of LTPP, the reference calibration 
33 

results should be submitted to the FHWA LTPP FWD task leader within 30 days of the time it 

first collects data for LTPP with those calibration factors. 

A copy of all reference calibration results should be kept in the RSC office. A paper copy of the 

most recent reference calibration results should be kept in the FWD tow vehicle. 

8.5.2 Relative Calibration 

A copy of all relative calibration results for LTPP-operated FWD equipment should be kept at 

the RSC office. A paper copy of the results of the most recent relative calibration should be kept 

in the FWD tow vehicle. It is not necessary to submit copies to the FHWA LTPP FWD task 

leader. 

8.5.3 DMI Calibration 

The operator should fill out form F07 each time the DMI is calibrated. The RSC office should 

keep a copy of all completed form F07s for LTPP-operated FWD equipment. A paper copy of 

the most recently completed form F07 should be kept in the FWD tow vehicle. It is not necessary 

to submit copies to the FHWA LTPP FWD task leader. 

8.5.4 Temperature Sensor Verification 

A copy of all completed forms F08 and F09 for LTPP-operated FWD equipment should be kept 

at the RSC office. A paper copy of the most recently completed forms F08 and F09 should be 

kept in the FWD tow vehicle. It is not necessary to submit copies to the FHWA LTPP FWD task 

leader. 

35 

9 EQUIPMENT CHECKS 

Because of the extensive use of FWDs in LTPP, routine equipment checks are extremely 

important. The FWD operator should perform the checks in this section each day that the FWD 

is traveling or in operation. These checks are a minimum. Operators are expected to keep an eye 

out for other anomalous conditions while performing these checks. These checks are not to 

supersede the manufacturer’s minimum requirements for warranty compliance. 

An operator must address any deficiencies noted while performing these checks before any 

further transit or testing. 

9.1 BEFORE-TRANSIT CHECKS 

The operator should perform the following checks on the tow vehicle before the start of travel for 

the day: 

• Fluid levels: 

− Engine oil. 

− Brake fluid. 

− Power steering. 

− Wiper fluid. 

− Coolant. 

− Transmission fluid. 

• Battery connections. 

• Hose conditions. 

• Tires inflated properly and in good condition. 

• Lights operational: 

− Headlights. 

− Taillights. 

− Turn signals. 

− Brake lights. 

− Strobe lights. 

• Interior uncluttered, equipment stowed well. 

The operator should perform the following checks on the FWD trailer before the start of travel 

for the day: 

• Ball tight. 

• Safety chains in place. 

• Breakaway cable connected. 

• Break fluid level good (for trailers with hydraulic brakes). 

• Trailer battery connections good. 

• Lights operational: 

− Taillights. 
36 

− Turn signals. 

− Brake lights. 

• Tires inflated properly and in good condition. 

• Transport locks engaged. 

• Hitch pin in front sensor bar guide. 

• Covers and latches secure. 

9.2 BEFORE-OPERATIONS CHECKS 

At a minimum, the FWD operator should perform the following checks after the FWD arrives at 

the test site and before testing begins: 

• Trays removed. 

• Transport locks unlocked. 

• Hitch pin removed from front sensor bar guide. 

• Hydraulic oil level good. 

• Raise/lower bar cable not frayed and well adjusted. 

• Geophones well-seated in geophone holders. 

• Geophone holder springs and foam guides in good condition. 

• Ribbed rubber sheet on load plate in good condition. 

• Load plate swivel free-moving. 

• Pressure switch boots in good condition. 

• Trailer control box electrical connections tight. 

• Buffers have no cracks or slits, and are level and tight. 

In addition, the operator should observe and listen to the hydraulics during the buffer warm-up 

sequence to ensure that they are free of air and operating correctly. 

9.3 POST-OPERATIONS CHECKS 

At a minimum, the FWD operator should perform the following checks before leaving the test 

site: 

• Transport locks engaged. 

• Hitch pin replaced in front sensor bar guide. 

• Pan replaced and secure. 

• Trailer access doors locked. 

• All supplemental testing equipment properly stowed. 

• Paper forms filed out, dated, signed, and filed. 
37 

10 MAINTENANCE AND REPAIR 

The RSCs are responsible for the maintenance and repair of the FWDs that they operate. This 

includes scheduled maintenance and reactive maintenance. FWDs should be maintained in 

accordance with the manufacturer’s recommendations and the LTPP FWD Maintenance Manual, 

publication number FHWA-LTPP-05-153. All maintenance and repair activities performed on 

LTPP-operated FWDs should be documented using form F03. 

39 

11 COLD WEATHER FWD TESTING 

The FWD tow vehicle should be warmed to achieve an interior temperature of at least 5 °C 

(40 °F) before turning on the signal processor or data collection computer. 

For testing in ambient temperatures at or below −10 °C (15 °F), the standard hydraulic fluid 

should be replaced with a lighter-weight synthetic fluid or other oils as recommended by the 

manufacturer. Some oils may not be appropriate for warm-weather operations. 

41 

12 DATA HANDLING PROCEDURES 

12.1 FIELD DATA HANDLING PROCEDURES 

If conditions permit, the operator should process all FWD data collected at a site using 

FWDConvert and FWDScan before leaving the site. The operator should resolve errors 

generated by FWDScan and note any necessary changes to the file for correction by office 

personnel. A backup of the Pavement Deflection Data Exchange (PDDX) file should be made on 

removable media. This backup, along with the original on the FWD data collection computer, 

should be kept in the FWD until the office acknowledges receipt. If conditions do not allow the 

operator to do this work before leaving the site, the operator should do the work as soon as 

convenient, but no more than 24 hours after data collection. 

The operator should submit completed copies of forms F01, F02, and F05 to the office along 

with the electronic FWD data. 

12.2 OFFICE DATA HANDLING PROCEDURES 

Electronic data should be received from the field along with completed copies of forms F01, 

F02, and F05. RSC office personnel should verify that the information on these forms 

corresponds to the electronic data received. Office personnel should resolve discrepancies before 

processing further data. 

Even though the operator processed the data with FWDScan in the field, office personnel should 

process it through FWDScan in the office. The Office personnel should review FWDScan output 

and investigate warnings or errors present in the output file. If office personnel make 

modifications to the data file, the information should be reprocessed through FWDScan. Upload 

of a file in which FWDScan found errors may cause referential problems in the LTPP Pavement 

Performance Database, and uploading such a file should be done only with the approval of the 

regional database manager. 

FWD data files should be uploaded to the Regional Information Management System (RIMS) 

using the most recent version of the upload filter provided for that purpose. Manual temperature 

measurements should be entered using the RIMS form provided for that purpose. 

Office personnel should archive the PDDX data file and paper forms according to the latest 

version of the applicable LTPP General Operations (GO) directives. 

43 

13 DOCUMENTATION 

The following documentation should be kept in the FWD tow vehicle when it is collecting data 

on behalf of LTPP: 

• LTPP Manual for Falling Weight Deflectometer Measurements. 

• LTPP FWD Data Collection Software Manual. 

• Distress Identification Manual for the Long-Term Pavement Performance Program. 

• SHRP-LTPP FWD Calibration Protocol. 

• FWDConvert Software User’s Manual. 

• FWDScan Software User’s Manual. 

• FWDCal Software User’s Manual. 

Only the most recent version of each document should be kept. 

In addition, all active LTPP FWD and applicable GO directives should be kept in the tow 

vehicle. Inactive directives should not be kept in the tow vehicle. 

The operator should keep at least five blank copies of all forms in appendix B of this manual in 

the tow vehicle. 

45 

14 VERSION HISTORY 

14.1 VERSION 1.0 

The initial version of this manual , the SHRP-LTPP Manual for FWD Testing and Operational 

Field Guidelines, was released January 1989 (SHRP reference “SHRP-LTPP-OG-002”). This 

guide included instructions for testing with edition 10 of the Dynatest FWD data collection 

software only. This guide included only three test plans: flexible, JCP, and CRCP sections, and it 

was applicable to only GPS testing. 

14.2 SPS 3 AND SPS 4 SUPPLEMENT TO VERSION 1.0 

The Pavement Maintenance Effectiveness Program of SHRP published modifications to version 

1.0 of the FWD guide for testing at SPS 3 and SPS 4 test sections. The following modifications 

were published in SHRP report SHRP-H-358 dated November 1993 (pages 134–137); however, 

the modifications were in effect before that date: 

• SPS 3 sections, testing interval set to 30 m (100 ft) instead of 8 m (25 ft). 

• For SPS 3 sections, only three drops at each drop height are to be recorded, instead of four. 

• For SPS 4 sections, testing is to be performed at every third panel. 

• For SPS 4 sections, all testing is to be performed in the OWP. This includes JA, JL, and MP 

testing. (For MP testing on SPS 4 sections, lane specification ‘J6’ is now used; however, this 

was not specified in the supplement.) 

• For SPS 4 sections, only three drops at each drop height are to be recorded for loss of support 

testing instead of four. 

14.3 VERSION 2.0 

 Major update released May 7, 1993. 

• Appendix B includes specific test plans for all SPS experiments. 

• Relative calibration procedure updated. 

• Reference calibration procedure added in appendix A. 

14.4 VERSION 3.0 

Major update released January 21, 2000. 

• Updated for use with edition 25 of the Dynatest FWD data collection software. 

• Deflection sensor spacing updated for nine active sensors. 

• Added verification procedure for FWD-mounted temperature sensors and Temperature Sensor 

Checks (TSCs) form. 

• Removed relative calibration procedure from main body of document. 
46 

• Replaced reference calibration procedure with “SHRP/LTPP FWD Calibration Protocol” 

(March 1994) in appendix F. (This protocol includes a relative calibration procedure.) 

14.5 VERSION 3.1 

Minor update released October 31, 2000. 

• Fixed some figures, typos, and formatting. 

• Updated to reflect further changes to the Dynatest edition 25 software. 

14.6 VERSION 4.0 

Major update released April 1, 2005. This is the first complete rewrite of the LTPP Manual for 

Falling Weight Deflectometer Measurements since version 1.0. 

• Removed FWD data collection software-specific information. (This information was placed in 

the LTPP FWD Data Collection Software Manual.)

• Integrated test plans in appendices A and B into main body of document to reduce redundancy 

(sections 5.1 and 5.2). 

• Removed appendix C because Seasonal Monitoring Program (SMP) testing was no longer 

being performed. 

• Removed appendix D because P59 (subsurface testing) was no longer being performed. 

• Removed appendix E because it was a copy of LTPP Directive GO-21 that may be superseded 

in the future. 

• Removed appendix F because it was a copy of a separate document (“SHRP-LTPP FWD 

Calibration Protocol”) that may be updated. 

• Removed appendices G and H because they were specific to the FWD data collection 

software. 

• Integrated LTPP Directive FWD-24, “Modifications to LTPP FWD Geophone Holders,” in 

body of document (section 4.1.1). 

• Integrated LTPP Directive FWD-27, “Thermal Gradient Temperature Measurements During 

Deflection Testing,” in body of document (section 6.1). 

• Integrated LTPP Directive FWD-28, “Storage of LTPP FWD Raw (Peak and Time History) 

Data,” in body of document (section 12.2). 

• Required all testing to be performed with drop load within 10 percent of target value (section 

4.2). 

• Required monthly verification of hand-held temperature instruments as well as FWDmounted temperature instruments (section 8.4). 

• Added forms F05, F06, F07, F08, F09, and F10. (Forms F08 and F09 replace the TSC form.) 

• Modified drop sequence for SPS 3 and SPS 4 sections to match standard LTPP drop 

sequence. 

• Added “Unit Conversions for common FWD-Related Measurements” (appendix B). 
47 

14.7 VERSION 4.1 

Minor update released July 1, 2006. 

• Clarified relative calibration interval in section 8.2 to indicate that the relative calibration is 

not required during extended periods when the FWD is not in service. 

• Made various minor formatting changes required for official publication by FHWA. 

• Added reference to LTPP FWD Maintenance Manual, FHWA-LTPP-05-153, in section 10. 

49 

Appendix A: Diagrams of FWD Test Plans 

This appendix contains the 11 diagrams of the FWD test plans. 
50 

TP

F

0

F

1

F

3

F

F3

1

7.6 m

TP

15.2 m

0 - 015.2 0 + 000 0 + 152.4 0 +170.7

152.4 m Test Section 18.3 m

CL

Mid - Lane

Outer Wheel Path

Shoulder

Direction of Travel

.76 m

NOTES: 

1. FWD tests to be conducted at test pit locations (TP) on P0 (First set of tests).

2. See Table A-1 for further details.

3. Lateral offsets shown represent nominal distances.

P &0 P3

P 1

1.8 m

1 m = 3.28 ft 

Figure 3. Diagram. Test plan 1. 
51 

TP

J0 P &0 P

3 J 5

TP

15.2 m

0 - 015.2 0 + 000 0 + 152.4 0 + 170.7

152.4 m Test Section 18.3 m

CL

Mid - Lane

Outer Wheel Path

Shoulder

Direction of Travel

1.8 m

.76 m

NOTES: 

1. FWD tests (J0) conducted at test pit locations (TP) on P0 (First set of tests). 

 Stationing will vary to locate TP at midpanel.

2. Number of panels and panel length (X) will vary depending upon specific joint spacing, transverse

 crack pattern and pavement type. Operator should refer to test and Table 1 for further details. A

 maximum of 20 effective slabs (panels) should be tested.

3. Lateral offsets shown represent nominal distances.

P 1

P 2

J 4

J 2 J

3

"X" PANEL

Pavement Edge

.15 m

J

1

J

0

1 m = 3.28 ft 

Figure 4. Diagram. Test plan 2. 
52 

TP

C0 P &0 P3

TP

15.2 m

0 - 015.2 0 + 000 0 + 152.4 0 + 170.7

152.4 m Test Section 18.3m

CL

Mid - Lane

Outer Wheel Path

Shoulder

Direction of Travel

1.8 m

.76 m

NOTES: 

1. FWD tests conducted at test pit locations (TP) on P0 (First set of tests).

 

2. See Table 1 for further details.

 

3. Lateral offsets shown represent nominal distances.

P 1

P 2 C2 C3 Pavement Edge

.15 m

C

0

C1

C4 C5 C4 C5

C3

C1

C2

Approx. 7.6 m

1 m = 3.28 ft 

Figure 5. Diagram. Test plan 3. 
53 

F

3

0 + 000 0 + 152.4

152.4 m Test Section

L

Mid - Lane

Outer Wheel Path

Shoulder

Direction of Travel

1.8 m

0.76 m

P1

P

3

15.2 m

F3

F

1

15.2 m 15.2 m

F

1

F

1

F

1

F 3 F3

1 m = 3.28 ft 

Figure 6. Diagram. Test plan 4. 
54 

0 + 000 0 + 152.4

152.4 m Test Section

Mid - Lane

Outer Wheel Path

Shoulder

Direction of Travel

1.8 m

NOTES:

1. Five test points per panel done in three passes.

2. Ten panels should be tested on all sections.

P

1

P

3

J

4 J

5

J 2 3 J

1 J

.15 m

.76 m P

2

"X" Panel

1 m = 3.28 ft 

Figure 7. Diagram. Test plan 5. 
55 

TP

C 0 P &

0 P3

TP

15.2 m

0 - 015.2 0 + 000 0 + 152.4 0 + 170.7

152.4 m Test Section 18.3 m

Mid - Lane

Shoulder

Direction of Travel

1.8 m

0.76 m

P 1

P 2

C 2

C 3 Pavement Edge

.15 m

C

0

C 1

C 4 C 5 C4 C 5

C 3

C 1

C2

Outer Wheel Path

Approx. 15.2 m

L

1 m = 3.28 ft 

Figure 8. Diagram. Test plan 6. 
56 

1 m = 3.28 ft 

Figure 9. Diagram. Test plan 7. 

0 + 000 0 + 152.4

152.4 m Test Section

L

Mid - Lane

Shoulder

Direction of Travel

2.4 m

NOTES:

1. Ten panels should be tested on all sections.

2. Seven test points per panel done in four passes.

3. Lane specifications J7 and J8 are on shoulder.

4. Lateral offsets shown represent nominal distances.

P1

0.15 m

0.61 m

"X" Panel

Outer Wheel Path 0.15 m

0.76 m

J 4

J 5

J

2

J 7

J 3

J 8

J

1

Pavement Edge

Widened Lane Edge

P2

P4

P3
57 

0 + 000 0 + 152.4

152.4 m Test Section

Mid - Lane

Outer Wheel Path

Shoulder

Direction of Travel

1.8 m

0.76 m

P

3

P

1

30.5 m 30.5 m

F

1

F

3

F

1

F

3

F

1

F

3

L

1 m = 3.28 ft 

Figure 10. Diagram. Test plan 8. 
58 

0 + 000 0 + 152.4

152.4 m Test Section

Outer Wheel Path

Shoulder

Direction of Travel

NOTES:

1. Panel Length "X" will be variable depending upon specific joint

 spacing, transverse crack pattern and pavement type.

2. Test all panels on under seal sections and every third panel on

 remaining sections.

3. Lateral Offsets shown represent Nominal Distances.

P

3

J 4 J

5

0.76 m

"X" Panel

J

6

Joint

L

1 m = 3.28 ft 

Figure 11. Diagram. Test plan 9. 
59 

0 + 000 0 + 152.4

152.4 m Test Section

L

Mid - Lane

Outer Wheel Path

Shoulder

Direction of Travel

1.8 m

0.76 m

P3

P

1

F

1 F

1

F

3

F

1 F1

F

3 F3 F3

X YZ

1 m = 3.28 ft 

Figure 12. Diagram. Test plan 10. 
60 

P &

3 P5

F

5

0 + 000 0 + 152.4

152.4 m Test Section

L

Mid - Lane

Outer Wheel Path

Shoulder

Direction of Travel

1.8 m

.76 m

P 1

F 4 F

3

"X" PANEL

F

1

1 m = 3.28 ft 

Figure 13. Diagram. Test plan 11. 
61 

Appendix B: Standard FWD Forms and Common Unit 

Conversions 

This appendix is a collection of forms applicable to the operation and recordkeeping for LTPP 

FWDs. 

Following is a list of the forms included: 

• Form F01, LTPP FWD Monitoring Temperature Measurements. 

• Form F02, LTPP FWD Monitoring Field Activity Report. 

• Form F03, LTPP Monitoring Maintenance and Repair Summary. 

• Form F04, LTPP FWD Monitoring FWD Buffer Shape. 

• Form F05, LTPP FWD MonitoringFWD Operations Planning. 

• Form F06, LTPP FWD Monitoring FWD Test Comments. 

• Form F07, LTPP FWD MonitoringFWD DMI Calibration. 

• Form F08, LTPP FWD Monitoring IR Temperature Sensor Checks. 

• Form F09, LTPP FWD Monitoring Air/Manual Temperature Sensor Checks. 

• Form F10, LTPP FWD Monitoring FWD Test Slab Locations. 

• Long-Term Pavement Performance (LTPP) Falling Weight Deflectometer (FWD) Testing 

FWD Problem Report (FWDPR). 
62 

LTPP FWD Monitoring 

Temperature Measurements—Form F01 

Region 

State Code 

LTPP Section ID 

[ ___ ___ ]

[ ___ ___ ]

[ ___ ___ ___ ___ ]

AGENCY ______________________ TESTING __________________ 

LTPP EXPERIMENT CODE ___________ ROUTE / HIGHWAY NUMBER _____________ 

TESTING DATE ____________ SHEET NUMBER _________ FIELD SET NO. __________ 

LOCATION STATION _______________ 

NOMINAL 

DEPTH 

IR 

Surface 

25 mm 50 mm 100 mm 200 mm 300 mm 

ACTUAL 

DEPTH 

 

TIME T0 °C T25 °C T50 °C T100 °C T200 °C T300 °C 

OPERATOR COMMENT 

 

LOCATION STATION _______________ 

NOMINAL 

DEPTH 

IR 

Surface 

25 mm 50 mm 100 mm 200 mm 300 mm 

ACTUAL 

DEPTH 

 

TIME T0 °C T25 °C T50 °C T100 °C T200 °C T300 °C 

OPERATOR COMMENT 

 

NOTES: 1) Ti = Temperature in Depth i, °C 

Operator comments should include weather related comments, and any other comments necessary to explain unusual 

temperature readings. Additional comments may be included on the back, and must be referenced to station and time, and a 

note to this effect must be added to the “Operator Comments” field. 

2) 

Test Completed 

____________________ ____- _____ - _____ __________________________ 

 FWD Operator DD - MMM - YYYY Affiliation 

1 m = 3.28 ft; °C = (°F − 32) * 5/9 
63 

LTPP FWD Monitoring 

Field Activity Report—Form F02 

Region 

State Code 

LTPP Section ID 

[ ___ ___ ]

[ ___ ___ ]

[ ___ ___ ___ ___ ]

AGENCY TESTING 

LTPP EXPERIMENT CODE ROUTE/HIGHWAY NUMBER 

TESTING DATE SHEET NUMBER FIELD SET NO. 

FWD AND TOW VEHICLE BEFORE OPERATION CHECKS (initial) 

TIME ODOMETER 

START TRAVEL 

END TRAVEL 

READY TO TEST 

TRAFFIC CONTROL READY 

TEMP. HOLES DRILLED 

BEGIN TESTING 

END TESTING 

START TRAVEL 

END TRAVEL 

DOWN TIME HOURS REASON(S) 

NUMBER OF TESTS: BASIN JT/CRACK 

TP 

OWP 

PE 

ML 

ADDITIONAL REMARKS REGARDING TESTING 

FIELD SAMPLING AND TESTING CREW TRAFFIC CONTROL CREW 

NAMES: AGENCY: 

NAMES: 

COPIES: RSC
64

LTPP Monitoring 

Maintenance and Repair Summary—Form 

F03 

Region 

FWD Serial Number

 [ ___ ___ ] 

8002- [ ___ ___ ___ 

]

Date Odometer Problem Cost 

Description 1

Description of 

Maintenance 

Performed 

By Labor Parts Total 

 

1

 Enter “Routine” for routine maintenance 
65 

LTPP FWD Monitoring 

FWD Buffer Shape—Form F04 Region [ ___ ___ ]

Deflection Unit ID: 8002 – [ ___ ___ ___ ] 

Buffer Shape: [ ___ ] (see following code descriptions) 

Assign Date: [ ___ ___ - ___ ___ ___ - ___ ___ ___ ___ ] 

De-assign Date: [ ___ ___ - ___ ___ ___ - ___ ___ ___ ___ ] 

Code Description 

1 Flat—100-mm (3.94-inch) diameter, flat (90o) buffers. 

2 Fully Rounded—100-mm (3.94-inch) diameter, “knife cut” variable cone shaped 

(45o) buffers. 

3 Semi-Rounded—110-mm (4.33-inch) diameter, tapered (60o) buffers. 

9 Unknown—buffer shape is unknown. 
66 

LTPP FWD Monitoring 

FWD Operations Planning—Form F05 

Region 

State Code 

LTPP Section ID 

[ ___ ___ ]

[ ___ ___ ]

[ ___ ___ ___ ___ ]

Experiment Designation: __ PS __ 

Test Setup: Flexible / Rigid (circle one) 

Total thickness of bound layers: ___ ___ ___ ___ mm 

Test Plan Number: ___ 

Temperature Holes (cross out holes that are not to be used) 

Hole Number Nominal Hole Depth Adjusted Hole Depth1

1 25 mm (0.98 inch) 

2 50 mm (1.97 inches) 

3 100 mm (3.94 inches) 

4 200 mm (7.87 inches) 

5 300 mm (11.81 inches) 

Note 1 – Only fill out if hole is within ±25 mm (±0.98 inch) of bottom of bound layers 

File Names (cross out passes that are not to be used) 

Pass Number Filename 

1 ___ ___ ___ ___ ___ ___ ___ ___ 

2 ___ ___ ___ ___ ___ ___ ___ ___ 

3 ___ ___ ___ ___ ___ ___ ___ ___ 

4 ___ ___ ___ ___ ___ ___ ___ ___ 

5 ___ ___ ___ ___ ___ ___ ___ ___ 

Prepared By: Date Prepared: 

Tested By: Date Tested: 
67 

LTPP FWD Monitoring 

FWD Test Comments—Form F06 

Region 

State Code 

LTPP Section ID 

[ ___ ___ ]

[ ___ ___ ]

[ ___ ___ ___ ___ ]

Date ___ ___ - ___ ___ ___ - ___ ___ ___ ___ 

Test Pass ____ 

Filename ___ ___ ___ ___ ___ ___ ___ ___ 

Time Station Lane Spec Comment 

 

Note: Time, Station, and Lane Spec should match the stored data. Necessary modifications to those values should be noted in 

the “Comments” column. 

Tested By: 
68 

LTPP FWD Monitoring 

FWD DMI Calibration—Form F07 

Region [ ___ ___ ] 

Date (DD-MMM-YYYY) ___ ___ - ___ ___ ___ - ___ ___ ___ ___ 

Deflection Unit ID: 8002 – [ ___ ___ ___ ] 

Section Length (feet) ___ ___ ___ ___ 

New Calibration Factor 

(counts per km) 

___ ___ ___ , ___ ___ ___ 

Performed By: 
69 

LTPP FWD Monitoring 

IR Temperature Sensor Checks—Form F08 Region [ ___ ___ ]

Date (DD-MMM-YYYY) ___ ___ - ___ ___ ___ - ___ ___ ___ ___ 

Deflection Unit ID: 8002 – [ ___ ___ ___ ] 

Location Performed _______________________________________________ 

FWD-Mounted IR Sensor Serial No. _______________________________________________ 

Hand-Held IR Sensor Serial No. _______________________________________________ 

FWD-Mounted IR Sensor Hand-held IR Sensor 

Check Reading 

Reference 

Therm. (°C) Reading 

(°C) 

Error Pass? Reading 

(°C) 

Error Pass? 

1 Y / N Y / N 

Cold 2 Y / N Y / N 

3 (opt.) Y / N Y / N 

1 Y / N Y / N 

2 Y / N Y / N Room 

Temp. 3 (opt.) Y / N Y / N 

1 Y / N Y / N 

Hot 2 Y / N Y / N 

3 (opt.) Y / N Y / N 

Acceptable? YES / NO YES / NO 

Performed By: 
70

LTPP FWD Monitoring 

Air/Manual Temperature Sensor Checks—Form F09 Region [ ___ 

Date (DD-MMM-YYYY) ___ ___ - ___ ___ ___ - ___ ___ ___ ___ Deflection Unit ID: 8002 – [ ___ ___ ___ ] Location Performed _______________________________________________ 

Serial Numbers 

FWD Air Temp Hand-held Sensor 1 Hand-held Sensor 2 Han

FWD Air Sensor Hand-held Sensor 1 Hand-held Sensor 2 

Check Reading 

Ref. 

Therm 

(°C) 

Reading 

(°C) 

Err. Pass? Reading 

(°C) 

Err. Pass? Reading 

(°C) 

Err. Pass? 

1 Y / N Y / N Y / N 

Cold 2 Y / N Y / N Y / N 

3 (opt.) Y / N Y / N Y / N 

1 Y / N Y / N Y / N 2 Y / N Y / N Y / N 

Room Temp. 3 (opt.) Y / N Y / N Y / N 

Acceptable? YES/NO YES/NO YES/NO 

Performed By: 
71 

LTPP FWD Monitoring 

FWD Test Slab Locations—Form F10 

Region 

State Code 

LTPP Section ID 

[ ___ ___ ]

[ ___ ___ ]

[ ___ ___ ___ ___ ]

Number of Slabs to be Tested 

(circle one) 

10/20 

Test Slab Location of joint/crack on approach end of slab (ft) 

1 

2 

3 

4 

5 

6 

7 

8 

9 

10 

11 

12 

13 

14 

15 

16 

17 

18 

19 

20 

Date Prepared ___ ___ - ___ ___ ___ - ___ ___ ___ ___ 

Prepared By ______________________________________ 
72 

FWDPR #:______ 

LONG-TERM PAVEMENT PERFORMANCE (LTPP) 

FALLING WEIGHT DEFLECTOMETER (FWD) TESTING 

FWD PROBLEM REPORT (FWDPR)

Attention: Eric Weaver FAX: (703) 285-2767 

Gonzalo R. Rada FAX: (301) 210-5032 

Type of Problem: 

 Guidelines 

 Equipment 

 Software 

 Name: 

 Version: 

 Other: 

___ 

___ 

___ 

____________ 

____________ 

____________ 

Reported by: ________________________ 

Agency: ________________________ 

Date: ________________________ 

Urgent?(Y/N) ___ Page ____ of ____ 

Description: 

 THIS SECTION FOR USE BY FHWA AND TSSC 

Received by: ________________ Date Received: __________________ 

Referred to: ________________ 

Date Referred: ________________ 

Approved by: __________________ 

Date Approved: __________________ 

Resolution: 

Notes: 
73 

Appendix C: Unit Conversions for Common 

FWD-Related Measurements 

Deflection 

mils (inch * 10-3) microns (meter * 10-6) 

16 406 

24 610 

80 2032 

microns = mils * 25.4 

Load 

kips (pound * 103) kN (Newton * 103) kPa (Pascal * 103

) 

(for 300 mm diameter plate) 

6 (5.4 to 6.6) 26.7 (24.0 to 29.4) 378 (340 to 416) 

9 (8.1 to 9.9) 40.0 (36.0 to 44.0) 566 (509 to 662) 

12 (10.8 to 13.2) 53.4 (48.1 to 58.7) 755 (680 to 830) 

16 (14.4 to 17.6) 71.2 (64.1 to 78.3) 1007 (907 to 1108) 

kN = kip * 4.448 

 kPa = kN * 14.15 

(for 300 mm-diameter (11.81-inch) plate only) 

Temperature 

Fahrenheit Celsius 

0 -17.8 

32 0 

70 21.1 

100 37.8 

°C = (°F − 32) * 5/9 

Distance 

feet meters 

12 3.66 

14 4.27 

25 7.62 

50 15.2 

100 30.4 

500 152.4 

1000 304.8 

meters = feet * 0.3048 
Recycled

Recyclable HRDI-13/12-06(1M)E