Research, Development, and Technology

Turner-Fairbank Highway Research Center

6300 Georgetown Pike

McLean, VA 22101-2296

PUBLICATION NO. FHWA-HRT-23-005 DECEMBER 2022 

Magnetic Flux-Based Nondestructive 

Evaluation Technologies for Assessing 

Corrosion Damage in External and 

Internal Post-Tensioned Tendons: 

Development Efforts and 

Evaluation Results
FOREWORD

This report documents research efforts to develop and validate three types of magnetic 

flux-based nondestructive evaluation (NDE) technologies for external and internal 

post-tensioned (PT) tendons. 

This study provides bridge program personnel from Federal, State, and local transportation 

departments with information on the advantages and limitations of magnetic-based NDE 

technologies. Bridge design engineers, specification writers, consultants, and 

manufacturers/suppliers of PT tendon-related systems, products, and materials may find this 

report to be of interest. 

Jean A. Nehme, Ph.D., P.E.

Director, Office of Infrastructure

Research and Development

Notice

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

(USDOT) in the interest of information exchange. The U.S. Government assumes no liability for 

the use of the information contained in this document. 

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

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

objective of the document. 

Quality Assurance Statement

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

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

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

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

ensure continuous quality improvement. 

Recommended citation: Federal Highway Administration, Magnetic Flux-Based Nondestructive 

Evaluation Technologies for Assessing Corrosion Damage in External and Internal 

Post-Tensioned Tendons: Development Efforts and Evaluation Results (Washington, DC: 2022) 

https://doi.org/10.21949/1521948.
TECHNICAL REPORT DOCUMENTATION PAGE

1. Report No.

FHWA-HRT-23-005

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

4. Title and Subtitle

Magnetic Flux-Based Nondestructive Evaluation Technologies for Assessing Corrosion 

Damage in External and Internal Post-Tensioned Tendons: Development Efforts and 

Evaluation Results

5. Report Date

December 2022

6. Performing Organization 

Code:

7. Author(s)

Seung-Kyoung Lee (ORCID: 0000-0001-7367-5197), Toshiyuki Moriya, Hiroaki Itoi, 

Masamichi Sugawara, and Hironori Kanemaru

8. Performing Organization 

Report No.

9. Performing Organization Name and Address

SK Lee & Associates, Inc.

10813 Fieldwood Dr.

Fairfax, VA 22030

Tokyo Rope Mfg. Co., Ltd. 

3-6-2 Nihonbashi, Chuo-Ku, Tokyo 103-8306, Japan

10. Work Unit No.

11. Contract or Grant No.

DTFH61-14-D-00011

12. Sponsoring Agency Name and Address

Office of Infrastructure Research and Development

Turner-Fairbank Highway Research Center

Federal Highway Administration 

6300 Georgetown Pike

McLean, VA 22101-2296

13. Type of Report and 

Period Covered

Final report, September 

2008–December 2019

14. Sponsoring Agency Code

HRDI-20

15. Supplementary Notes

The Technical Contact was Hoda Azari (HRDI-20), and the Contracting Officer’s Representative was Cara Fitzgerald 

(HRDI-1).

16. Abstract

This report presents research work related to developing and evaluating magnetic flux-based nondestructive evaluation

(NDE) technologies for detecting corrosion damage in external and internal post-tensioned (PT) tendons. For the external 

tendons, two types of magnetic main flux method (MMFM) systems—a solenoid type and a permanent magnet type—

were developed and validated in the laboratory and in the field. The solenoid-type MMFM system was the most accurate 

NDE method. Its lowest damage detection limit would be 0.4-percent section loss for the point measurement method and 

1.0-percent section loss for the scan measurement method. The permanent-type MMFM system is suitable for locating 

potential problem areas containing more than 3.0-percent section loss in external PT tendons. It has more versatile 

applications in the field because of its simpler hardware and speedy operation. Ideally, both MMFM systems can be 

employed in sequence during the field investigations: the permanent magnet type identifies potentially problematic areas 

by quickly scanning all suspected tendons, followed by the point measurements in the suspicious areas with the solenoid 

type. A return flux method prototype was developed for the internal tendons. It reached the immediate goal of detecting 

relatively small section losses hidden in internal tendons in laboratory environments and proved its concept successfully. 

It could detect a section loss larger than 15.3-percent in internal mockup tendons surrounded by vertical rebars at 6.0-inch 

or wider spacing when the clear concrete cover was less than 7.5 inches for metal ducts and 6.5 inches for polyethylene

ducts. Without interference from the vertical rebars, as little as 9.0-percent section loss could be identified at the 6.0-inch 

cover. However, poor results were obtained in the anchorage zone in the box girder specimen. It is recommended that the 

present prototype be further improved to make it a field-deployable NDE system.

17. Key Words

Corrosion, post-tensioned tendon, nondestructive evaluation, grout 

void, seven-wire strand, magnetic main flux method, return flux 

method, magnetic flux, magnetic field, section loss, Hall effect sensor

18. Distribution Statement

No restrictions. 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

98

22. Price

N/A 

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

TABLE OF CONTENTS

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

CHAPTER 2. PRELIMINARY EVALUATION OF SIX NDE METHODS.......................... 7

TEST SPECIMENS ................................................................................................................ 7

PRELIMINARY LABORATORY TESTS .......................................................................... 9

Ultrasonic Flaw Detection and SE/IR Methods .................................................................. 9

Guided Long-Range Wave MsS Method .......................................................................... 10

RM Method ....................................................................................................................... 12

Thermography with Induction Heating Method ............................................................... 14

MMFM .............................................................................................................................. 14

CHAPTER 3. MMFM FOR EXTERNAL TENDONS ........................................................... 17

INTRODUCTION TO MMFM ........................................................................................... 17

Solenoid-Type MMFM System ........................................................................................ 18

Permanent Magnet-Type MMFM System ........................................................................ 22

EXPERIMENTAL PROCEDURE...................................................................................... 24

Laboratory Specimens ...................................................................................................... 24

Development of Solenoid-Type MMFM System ............................................................. 26

Development of Permanent Magnet-Type MMFM System ............................................. 27

Comparison of Two MMFM Systems .............................................................................. 27

Field Evaluations in the Varina-Enon Bridge ................................................................... 29

TEST RESULTS AND DISCUSSION ................................................................................ 31

Laboratory Evaluations ..................................................................................................... 31

Field Evaluations .............................................................................................................. 39

CHAPTER 4. RFM FOR INTERNAL TENDONS ................................................................. 47

PREVIOUS RESEARCH EFFORTS ................................................................................. 47

FIU Studies ....................................................................................................................... 47

The University of Toledo Study—Induced Magnetic Field Detection (IMFD) Method .. 53

PRINCIPLE OF RFM .......................................................................................................... 57

DEVELOPMENT OF RFM PROTOTYPES .................................................................... 58

EXPERIMENTAL PROCEDURE...................................................................................... 61

Test Specimens ................................................................................................................. 61

Experimental Setup and Testing ....................................................................................... 67

TEST RESULTS AND DISCUSSION ................................................................................ 69

First Laboratory Mockup .................................................................................................. 69

Second Laboratory Mockup .............................................................................................. 78

Grouted Field Tendon Specimen ...................................................................................... 81

Precast Concrete Box Girder ............................................................................................ 82

CHAPTER 5. CONCLUSIONS ................................................................................................. 86

REFERENCES ............................................................................................................................ 88
iv

LIST OF FIGURES

Figure 1. Photos. Examples of corrosion-induced tendon failures. ................................................ 2

Figure 2. Photos. Introduction of artificial damage into field tendon specimens. .......................... 7

Figure 3. Photo. Fully assembled test slabs ready for concrete casting. ......................................... 8

Figure 4. Photos. Preparation of a 25-ft long mockup tendon. ....................................................... 8

Figure 5. Photos. Ultrasonic measurements. ................................................................................... 9

Figure 6. Photos. SE/IR measurements......................................................................................... 10

Figure 7. Photos. MsS measurements. .......................................................................................... 11

Figure 8. Graph. MsS data from the 25-ft mockup. ...................................................................... 11

Figure 9. Graph. MsS data from the grouted tendon specimen. ................................................... 12

Figure 10. Photos. RM measurements. ......................................................................................... 13

Figure 11. Graph. Example of RM measurement data. ................................................................ 13

Figure 12. Photos. Induction thermography. ................................................................................ 14

Figure 13. Illustration. Flow of magnetic flux in a metal object.(23) ............................................. 17

Figure 14. Illustration. Schematic of a magnetic flux measurement.(23) ....................................... 17

Figure 15. Illustration. Schematic of the solenoid-type MMFM system.(23) ................................ 18

Figure 16. Illustration. Schematic of scan measurement.(23) ......................................................... 19

Figure 17. Equation. Determination of magnetic flux change in scan measurement.(23) .............. 19

Figure 18. Illustration. Schematic of point measurement.(23)........................................................ 20

Figure 19. Illustration. Examples of the magnetic hysteresis loop.(23) .......................................... 20

Figure 20. Equation. Determination of magnetic flux change in point measurement.(23)............. 21

Figure 21. Graph. Magnetic hysteresis loops for wire ropes having different cross sections. ..... 21

Figure 22. Illustration. Schematic of scan measurement by the permanent magnet type. ............ 22

Figure 23. Illustration. Arrangement of 16 Hall effect sensors. ................................................... 23

Figure 24. Illustration. Schematic representation of MMFM data signals. .................................. 23

Figure 25. Photos. Examples of artificial damage. ....................................................................... 24

Figure 26. Photos. Damage conditions in tendon D specimen. .................................................... 25

Figure 27. Photos. Solenoid-type magnetizers. ............................................................................ 26

Figure 28. Photos. Permanent magnet-type magnetizer. .............................................................. 27

Figure 29. Photos. First MMFM testing in the Varina-Enon Bridge. ........................................... 30

Figure 30. Photos. Second MMFM testing in the Varina-Enon Bridge. ...................................... 31

Figure 31. Photos. Artificial damage conditions of tendon A specimen. ..................................... 32

Figure 32. Graph. Scan measurement data from tendon A specimen. .......................................... 32

Figure 33. Graphs. Magnetic flux versus defect length data. ....................................................... 33

Figure 34. Graph. Relationships between cut length and reduction of magnetic flux. ................. 34

Figure 35. Graph. Permanent magnet-type data from the tendon D specimen. ............................ 35

Figure 36. Graph. Linear correlation between magnetic flux data and estimated section 

losses of the laboratory data. ............................................................................................... 38

Figure 37. Graph. Linear correlation between actual section losses and estimated section 

losses based on the second point measurement dataset. ...................................................... 38

Figure 38. Photo. MMFM testing of the P12S12TW-3 tendon. ................................................... 39

Figure 39. Graph. Scan measurement data from the P12S12TW-3 tendon. ................................. 40

Figure 40. Photos. Closeup views of corroded strands. ................................................................ 40

Figure 41. Graph. Linear regression fits between magnetic flux and estimated section 

losses of the P12S12TW-3 tendon in relation to the laboratory datasets. ........................... 42
v 

Figure 42. Graph. Influence of stress on the reduction of magnetic flux. .................................... 43

Figure 43. Photo. Uncovered condition of a tendon exhibiting grout deficiencies. ..................... 44

Figure 44. Graphs. Permanent magnet-type MMFM data of a tendon. ........................................ 45

Figure 45. Photos. Laboratory setup for MFL testing.(16) ............................................................. 47

Figure 46. Graph. Laboratory test results obtained with the mockup.(16) ..................................... 48

Figure 47. Illustration. Details of the decommissioned box girder segment.(16) ........................... 49

Figure 48. Graph. MFL test results of a bridge segment.(16) ......................................................... 49

Figure 49. Photos. Laboratory setup for induction measurement method.(16) .............................. 50

Figure 50. Graph. Inductance measurement data from the laboratory mockup.(16) ...................... 51

Figure 51. Photo. Modified second prototype elliptical coil used for inductance testing on 

the top slab of a precast segment. ........................................................................................ 51

Figure 52. Graph. Inductance readings along the length of the segment for an increasing 

number of strands.(16) ........................................................................................................... 52

Figure 53. Illustration. Magnetic flux lines passing through the steel specimen induced by a 

yoke-shaped electromagnet.(26) ............................................................................................ 53

Figure 54. Graphs. Induced magnetic field versus cross-sectional area of steel specimens as 

a function of airgap.(26) ........................................................................................................ 54

Figure 55. Graphs. Induced magnetic field versus cross-sectional area of steel specimens as 

a function of concrete thickness.(26)..................................................................................... 55

Figure 56. Graph. Estimated cross-sectional areas of the corroded seven-wire strand versus 

actually measured cross-sectional areas.(27) ........................................................................ 55

Figure 57. Photos. Field trial of two NDE systems.(28) ................................................................. 56

Figure 58. Illustration. Principle of RFM. .................................................................................... 57

Figure 59. Graph. Relationship between magnetic flux and cross-sectional area. ....................... 58

Figure 60. Illustration. Schematic of the first two prototypes. ..................................................... 59

Figure 61. Illustration. Schematic of the fourth prototype and its functionality. .......................... 60

Figure 62. Photos. First laboratory mockup.................................................................................. 61

Figure 63. Illustration. Schematics of first laboratory mockup (not to scale). ............................. 62

Figure 64. Illustration. Schematic of the modified RFM magnetizer. .......................................... 63

Figure 65. Illustration. Schematic of the second laboratory testing setup. ................................... 64

Figure 66. Photos. Second laboratory mockup. ............................................................................ 65

Figure 67. Photos. Tendon C specimen installed inside the mockup. .......................................... 65

Figure 68. Photo and illustration. Precast box girder specimen. .................................................. 66

Figure 69. Illustration. Schematic of the RFM system components. ............................................ 67

Figure 70. Photos. RFM testing on the first laboratory mockup. ................................................. 68

Figure 71. Photos. RFM testing on the box girder specimen........................................................ 68

Figure 72. Graphs. RFM data from the PE duct in free length zone with no vertical rebars: 

27.6-percent at 14.5-ft was detected. ................................................................................... 70

Figure 73. Graphs. RFM data from the metal duct in the free length zone with no vertical 

rebars: 27.6-percent at 14.5-ft was detected. ....................................................................... 71

Figure 74. Graphs. RFM data from PE duct in the free length zone with no vertical rebars: 

15.3-percent at 14.5-ft and 12.3-percent at 17.5-ft were detected....................................... 72

Figure 75. Graphs. RFM data from the metal duct in the free length zone with no vertical 

rebars: 15.3-percent at 14.5-ft and 12.3-percent at 17.5-ft were detected. .......................... 73

Figure 76. Graphs. RFM data from the PE duct in free length zone with vertical rebars at 

6-inch spacing: 27.6-percent damage was detected at 14.5-ft............................................. 74
vi 

Figure 77. Graphs. RFM data from the metal duct in the free length zone with vertical 

rebars at 6-inch spacing: 27.6-percent damage was detected at 14.5-ft. ............................. 75

Figure 78. Graphs. RFM data from the PE duct in the free length zone with vertical rebars 

at 6-inch spacing: 15.3-percent at 14.5-ft was detected. ..................................................... 76

Figure 79. Graphs. RFM data from the metal duct in the free length zone with vertical 

rebars at 6-inch spacing: 15.3-percent at 14.5-ft was detected. .......................................... 77

Figure 80. Graphs. RFM data of the metal duct in the anchorage zone with vertical rebars 

at 6-inch spacing.................................................................................................................. 78

Figure 81. Graph. HGx graph for the metal duct without any strands under two gaps. ................. 79

Figure 82. Graph. HGx graph for the five-strand bundle containing three 20-percent section 

losses under different rebar covers. ..................................................................................... 79

Figure 83. Graph. Polynomial regression analysis results for HGx. .............................................. 80

Figure 84. Graphs. RFM data from the Varina-Enon Bridge tendon specimen with no 

rebars. .................................................................................................................................. 81

Figure 85. Graphs. RFM data from duct number 4 in the box girder. .......................................... 82

Figure 86. Graphs. Relationship between RFM parameters and number of strands as a 

function of the airgap. ......................................................................................................... 83
vii 

LIST OF TABLES

Table 1. Summary of initial damage introduced in the laboratory specimens. ............................. 25

Table 2. Comparison of solenoid-type and permanent magnet-type MMFM systems................. 27

Table 3. Summary of the first point measurement dataset collected with the first-generation 

solenoid-type magnetizer. ................................................................................................... 36

Table 4. Summary of the second point measurement dataset collected with the 

second-generation solenoid-type magnetizer. ..................................................................... 37

Table 5. Summary of point measurement data from the Varina-Enon Bridge tendons................ 41

Table 6. Summary of detection capabilities for the first laboratory mockup. .............................. 84

Table 7. Summary of detection capability for the Varina-Enon Bridge tendon specimen. .......... 84
viii

LIST OF ABBREVIATIONS

Abbreviations 

AC alternating current

AE acoustic emission

AT ampere-turn

DC direct current

EIS electrochemical impedance spectroscopy

FDOT Florida Department of Transportation

FHWA Federal Highway Administration

FIU Florida International University

GPR ground penetrating radar 

HCP half-cell potential 

HDPE high-density polyethylene 

IE impact echo

IMFD induced magnetic field detection

IR impulse response 

IRT infrared thermography

MFL magnetic flux leakage

MsS magnetostrictive sensor

MMFM magnetic main flux method 

NDE nondestructive evaluation 

NDT nondestructive test 

PE polyethylene 

PSC prestressed concrete

PT post-tensioned
ix

PVC polyvinyl chloride

RFM return flux method

RM remanent magnetism 

TFHRC Turner-Fairbank Highway Research Center

TRM Tokyo Rope Manufacturing 

SE sonic echo

USE ultrasonic echo

VDOT Virginia Department of Transportation

Measurements

φ magnetic flux

Δφ difference in magnetic flux

H magnetic field or magnetic flux density 

Bs saturated magnetic field

HGx axial Hall effect sensor

HGy radial Hall effect sensor

1 

CHAPTER 1. INTRODUCTION

The prestressed concrete (PSC) bridge structures in the United States have incorporated 

high-strength steel as economic and efficient structural components for more than five decades. 

The PSC bridge’s structural integrity relies on the high-strength prestressing steel, for example, 

wires, seven-wire strands, and bars that have a smaller cross-sectional area than ordinary 

reinforcing steel. Post-tensioned (PT) tendons are a good example of their use. 

The corrosion-induced section loss of prestressing steel is usually localized and, thus, more 

critical than the equivalent loss of reinforcing steel. Compounding the situation, the prestressing 

steel is embedded in concrete and enclosures such as ducts and anchorages. No apparent external 

signs of structural deficiencies appear until it is too late to intervene, or, at best, the damage level 

of the highly stressed steel becomes critical.

(1) Since 1999, corrosion-induced tendon failures 

have been reported in the United States. Figure 1 shows photographs of four tendon failure cases. 

Figure 1-A shows a corroded anchor head recovered from the Niles Channel Bridge in Florida in 

1999. It failed after 13-yr of service. Figure 1-B shows a failed tendon section on the 

Varina-Enon Bridge in Virginia in 2007. The tendon in question failed prematurely after 17-yr of 

service due to severe corrosion of 19 0.6-inch seven-wire strands. As a result, the Virginia 

Department of Transportation (VDOT) replaced the fractured tendon and another one exhibiting 

severe corrosion. Figure 1-C shows the first failed tendon in the Ringling Causeway Bridge in 

Florida in January 2011 after fewer than 8-yr of service. Another tendon also failed in the same 

year (July 2011). Figure 1-D shows the Roosevelt Bridge in Florida, which was temporarily 

braced during the emergency repair of the corroded internal tendons in 2020. Similar PT 

corrosion problems have been observed in other countries, including the United Kingdom, 

France, Italy, South Korea, and Hong Kong. Two recent synthesis reports compiled many more 

corrosion problems, including the cases cited in this paragraph.(3,6)
2 

© 2002 FDOT. 

A. Niles Channel Bridge.(2)

Source: FHWA. 

B. Varina-Enon Bridge.(3)

© 2011 FDOT. 

C. Ringling Bridge.

(4)

© 2020 FDOT. 

D. Roosevelt Bridge.(5)

Figure 1. Photos. Examples of corrosion-induced tendon failures. 

Assessing the tendon conditions at randomly selected locations can be unreliable, destructive, 

and even impossible in many cases. Moreover, a full-scale inspection of any large PT bridge can 

be time consuming and cost prohibitive in reality. Therefore, bridge owners tend to rely on 

limited inspection data to determine the current state of the bridge. This kind of maintenance 

practice can lead to a false sense of security and run the risk of underestimating corrosion 

damage that may be occurring somewhere in the bridge.(1) Once a PT bridge has corrosion issues, 

it is very difficult or even impossible to replace the affected tendons, and corrective actions can 

be expensive. For instance, the Florida Department of Transportation (FDOT) spent an estimated 

$55 million on repairing 11 PT bridges.(7)

A web document published in 1998 recommended additional research and field trials on 

nondestructive evaluation (NDE) and nondestructive testing (NDT) techniques designed for 

grouted PT tendons.(8) In 2007, the Federal Highway Administration (FHWA), partnering with the 

American Association of State Highway and Transportation Officials and Transportation System 

Preservation Technical Services Program, organized a national workshop to identify bridge 

preservation and corrosion mitigation research needs. This preservation workshop produced a 

comprehensive report entitled Transportation System Preservation Research, Development, and 

Implementation Roadmap.(9) The workshop participants chose “improved inspection techniques 

for steel prestressing strands, cables, and ropes” as the most urgent and important issue for bridge 
3 

preservation. Their recommendation can be justified because corroding high-strength materials 

can be more critical than the corrosion of ordinary reinforcing steel and steel members. 

When the PT bridges need to be inspected, visual inspections are still primarily depended on to 

observe external signs of concrete deterioration, such as cracks, delaminations, spalling, rust 

stains, deflections, and so on. In certain situations, limited duct drilling and borescope inspection 

can be implemented. The ineffectiveness of the current inspection methodologies can be 

recognized from recent PT tendon failures observed in two U.S. PT bridges. The Wando River 

Bridge had been rated satisfactory in three biannual inspections performed in 2012, 2014, and 

2016, despite the ongoing hidden corrosion problems. The inspectors could not identify the 

insidious corrosion problems due to the inability to inspect inside the ducts, diaphragms, and 

anchors. Even after a thorough condition assessment after the first tendon failed in 2016, the 

second tendon failed in 2018, although both failed tendons were situated very close to each 

other. The Mid-Bay Bridge also experienced tendon failures in 2000 and 2018. These cases 

highlight how critical it is to develop reliable NDE technologies for detecting corrosion damage 

in PT tendons before it is too late.

(3)

Several published reports included the literature review of the NDT/NDE technologies 

applicable to PT tendons. The most comprehensive literature review can be found in a 2015 

research report entitled Designing and Detailing Post Tensioned Bridges to Accommodate 

Nondestructive Evaluation and its summary version in an FHWA TechBrief in 2018.(10,11) Each 

of the following methods was discussed in terms of applications, methodology, limitations, the 

viability in PT applications, and references: 

1. Acoustic emission (AE). 

2. Electrically isolated tendon. 

3. Ground penetrating radar (GPR). 

4. Half-cell potential (HCP). 

5. Impact echo (IE). 

6. Infrared thermography (IRT). 

7. Magnetic flux leakage (MFL). 

8. Radiography. 

9. Time domain reflectometry. 

10. Ultrasonic testing. 

11. Visual inspection. 

The information provided in the reports should be helpful for ordinary engineers to grasp the 

fundamentals of individual methods. 

A National Cooperative Highway Research Program research report published in 2014 also 

briefly discussed the capabilities and limitations of the following 14 NDE methods for PT tendons 

and stay cables in terms of accuracy, precision, ease of use, inspection requirements, and cost:

(12)

1. Electromagnetic methods: GPR, IRT, and electrical capacitance tomography.

2. Magnetic methods: MFL, magnetic main flux method (MMFM)-permanent magnet type, 

and MMFM-solenoid type. 
4 

3. Mechanical wave and vibration methods: IE, ultrasonic tomography, ultrasonic echo 

(USE), sonic/ultrasonic pulse velocity, low-frequency ultrasound, and sounding. 

4. Electrochemical method: electrochemical impedance spectroscopy (EIS).

5. Combinations of methods: GPR/USE, GPR/IE, MFL/sounding, MFL/IE, and IRT/USE. 

6. Visual inspection.

The researchers’ short literature review may not be sufficient to understand the state of the 

practice fully, but the researchers’ own extensive laboratory evaluations using realistic mockup 

specimens led to useful conclusions: None of the NDE technologies they investigated was 

capable of identifying grout defects (compromised grout/void/water infiltration) and strand 

defects with high accuracy for the ducts buried in the concrete (webs, flanges, deviators, and 

anchorage regions of the PT system) and the deck and pylon anchorages of the stay cable system. 

However, it was noted that two NDE methods, namely, USE and IE, could identify grout defects 

in internal ducts with low to medium accuracy.

FDOT sponsored two NDE/NDT studies focused on PT tendons and published two reports in 

2003.(13,14) The first report contained the assessment results of four NDT methods: impulse radar, 

IE, MFL, and high-powered x-ray imagining. These methods were applied onto internal PT 

tendons installed in Ramp D, which was located in the Fort Lauderdale-Hollywood International 

Airport interchange before it was demolished.

(13) The second report was related to developing 

and validating an NDE method used in the field to detect voids and cracks in the grouted internal 

tendons among three techniques: scanning IE, spectral analysis of surface waves, and ultrasonic 

imaging tomography.(14) 

Florida International University (FIU) performed additional laboratory and field investigations 

sponsored by FDOT. As a result, two reports were published in 2012 and 2017.(15,16) The 2012 

report contained a general description and some exemplary data of the following NDE methods 

classified in seven groups:

1. Visual methods: direct visual inspection, long-term video monitoring.

2. Magnetic method: MFL.

3. Mechanical wave and vibration methods: sounding, AE, IE, impulse response (IR), 

ultrasonic imaging tomography, ultrasonic guided wave testing, global vibration 

response.

4. Electromagnetic wave propagation methods: IRT, impulse radar/GPR.

5. Electrochemical methods: HCP, linear polarization resistance, EIS, electrochemical 

noise.

6. Penetrating radiation methods: radiography, x-ray diffraction.

7. Direct prestress measurement technique.

As part of the 2012 study, a survey of State DOTs was also conducted to collect information 

regarding the need for major corrosion-related problems and challenges for the States. According 

to State DOT responses, 23 of 27 State DOTs needed corrosion inspection of steel strands or stay 

cables: Ten DOTs had major corrosion problems and challenges; seven DOTs had carried out 

research projects to develop the corrosion detection methodology; and five DOTs had used NDE

tools for inspecting pretensioned strands, PT strands, or stay cables.
5 

The 2017 report identified practical and effective NDT technologies that could be used for the in

situ condition assessment of internal PT tendons before, during, and after the demolition of 

Ramp D at the Fort Lauderdale-Hollywood International Airport and laboratory testing: IR, IRT, 

AE, MFL, interferometric phase radar, and inductance measurement. Some results included in the 

2017 report will be discussed further in chapter 4.

Although some bridge deck evaluations have used GPR, IE, electrical resistivity, and other 

NDE/NDT techniques, applying them to PSC bridges has been remained largely uncommon. 

After witnessing the Varina-Enon Bridge tendon failure in 2007, FHWA launched an in-house 

study in the following year to investigate the effectiveness of several NDE technologies for 

assessing insidious corrosion damage in external PT tendons and stay cables. As a result, the 

MMFM—a magnetic-based NDE technology—was identified as the most accurate NDE system, 

and it was chosen for further research. In 2016, FHWA initiated another research study to 

develop a proof-of-concept prototype for internal PT tendons using a similar magnetic flux-based 

system called the return flux method (RFM). 

This report presents the experimental details and outcomes of two in-house research studies 

related to developing and evaluating magnetic flux-based NDE technologies for locating hidden 

corrosion damage in external and internal PT tendons.

7 

CHAPTER 2. PRELIMINARY EVALUATION OF SIX NDE METHODS 

This chapter describes how preliminary evaluations of six NDE methods were conducted to 

identify the promising method(s) before the main research study was launched.

TEST SPECIMENS

For the first in-house study, VDOT donated seven grouted external PT tendon segments retrieved 

from the Varina-Enon Bridge. They were 4.0 inches in diameter and 7–10-ft in length. Each 

tendon segment contained 19 0.6-inch, 270-ksi seven-wire strands. Among them, three segments 

were selected for laboratory testing and labeled as tendons A, B, and C. Different levels of 

artificial damage were introduced to them at Turner-Fairbank Highway Research Center 

(TFHRC) of FHWA. A saw equipped with a diamond blade (and an impressed current method 

(figure 2-A) and an impressed current method (figure 2-B) were employed to make the 

predetermined damage in the tendon specimens.

Source: FHWA.

A. Saw-cut method. 

Source: FHWA. 

B. Impressed current method. 

Figure 2. Photos. Introduction of artificial damage into field tendon specimens.

The latter employed a potentiostat as a direct current (DC) power source, one of the strands in 

tendon A as a working electrode, a 3/8-inch diameter graphite rod as a counter electrode, and a 

3.5-percent sodium chloride solution (by weight) as an electrolyte. First, a 0.5-inch hole was 

made through the high-density polyethylene (HDPE) duct to penetrate 1/4-inch deep into the 

grout (no bare strand exposed). Next, a small piece of sponge and the graphite rod were inserted 

into the hole. Finally, approximately 1 mA current was impressed from the strand (anode) to the 

graphite rod (cathode) while dispensing the saltwater into the sponge/grout periodically. The 

impressed current was maintained for 7 d without interruptions. This method was intended to 

introduce more realistic corrosion damage to a strand than the mechanical saw cuts. 

In addition to the field specimens, two types of laboratory mockups were fabricated. The first 

type used eight concrete test slabs, to simulate a reinforced concrete bridge deck containing 

internal PT tendons, and longitudinal and transverse rebars. Each slab had overall dimensions of 
8 

7-ft long by 1.5-ft wide by 8-inches thick. These slabs were intended for two concurrent studies: 

corrosion resistance of next-generation reinforcing bars and evaluation of existing NDE methods 

for internal PT tendons.(17) For the latter, each slab contained a 1.5-inch galvanized metal duct 

and a 2.0-inch plastic duct in the middepth. The ducts contained unstressed seven-wire strands 

having different damage conditions without grout. Figure 3 shows some of the assembled molds 

before concrete pouring. 

Source: FHWA.

Figure 3. Photo. Fully assembled test slabs ready for concrete casting.

The second type was a bundle of three 25-ft-long, 0.6-inch, seven-wire strands containing 

various levels of artificial damage along the strand length. After the strands were bundled on the 

ground (figure 4-A), the strands were inserted into a 2-inch polyvinyl chloride (PVC) pipe. As 

the final step, the pipe was filled with a commercially available prepackaged grout (figure 4-B).

Source: FHWA. 

A. A bundle of seven-wire strands. 

Source: FHWA. 

B. Grout filling in the mockup tendon. 

Figure 4. Photos. Preparation of a 25-ft long mockup tendon.
9 

PRELIMINARY LABORATORY TESTS 

At the beginning of this study, six NDE methods were identified as potentially viable techniques 

to detect insidious corrosion in the external PT tendons. Individual technology owners 

voluntarily participated in trying their systems on identical specimens. The participants tested the 

following NDE methods: 

1. Ultrasonic flaw detection. 

2. Sonic echo (SE)/ IR methods. 

3. Magnetostrictive sensor (MsS) method. 

4. Remanent magnetism (RM) method. 

5. Thermography with induction heating method. 

6. MMFM. 

The following section briefly introduces each of the NDE methods and summarizes their test 

results, except for the MMFM, which will be fully discussed in the next chapter.

Ultrasonic Flaw Detection and SE/IR Methods

The ultrasonic flaw detection method uses the sound wave that travels through the interior of a 

material at a certain velocity until the wave is reflected at a boundary created by a different 

material or different physical conditions, such as internal cracks or voids. Travel speed depends 

on the material’s properties, such as density and elastic properties. 

Figure 5 shows ultrasonic measurements on one end of a seven-wire strand placed in an empty 

plastic internal duct (figure 5-A) and one end of a seven-wire strand in a grouted tendon 

specimen retrieved from the Varina-Enon Bridge (figure 5-B).

Source: FHWA. 

A. An ungrouted plastic duct. 

Source: FHWA. 

B. A grouted external tendon. 

Figure 5. Photos. Ultrasonic measurements. 
10 

The SE method is usually conducted with the IR as the SE/IR method to check foundation 

structures. The generated wave from an impulse hammer travels down in a shaft or a pile until an 

impedance change is encountered by a defect. Then, the reflected wave is measured by a receiver 

placed next to the impact point. The SE method analyzes the data in the time domain, whereas 

the IR method analyzes the data in the frequency domain. The two analysis methods complement 

each other to allow the most accurate foundation length and defect analysis possible.(18) The

SE/IR method was tried on one end of an ungrouted seven-wire strand that had saw cuts inside 

an internal plastic duct (figure 6-A) and one end of another Varina-Enon Bridge tendon specimen

(figure 6-B). 

Source: FHWA. 

A. An ungrouted plastic duct. 

Source: FHWA. 

B. A grouted external tendon. 

Figure 6. Photos. SE/IR measurements. 

At the end of the laboratory tests, it was clear that none of these NDE methods would work for 

the grouted PT tendons due to significant signal attenuation in the grout. Therefore, no further 

testing was pursued.

Guided Long-Range Wave MsS Method 

The MsS is a type of transducer that can generate and detect time-varying stresses or strains in 

ferromagnetic materials. Magnetostriction is a property of ferromagnetic materials that causes 

them to change their shape or dimensions during magnetization. The operating frequency of the 

MsS ranges from a few hertz to several hundred kilohertz (kHz). The sensor has a broad 

frequency response and can be used over the entire operating frequency range. The sensor can 

transmit and detect elastic waves in a ferromagnetic material and thus perform the functions of a 

piezoelectric-type ultrasonic transducer. Since the signal in the receiving sensor is caused by 

time-varying strain or stress, the MsS can also perform the functions of a strain gauge, vibration 

sensor, accelerometer, and piezoelectric AE sensor.(19) For the laboratory tests, the longitudinal 

wave mode with a 10–30 kHz frequency was employed. The wave velocity was approximately 

1.8 × 105 inches/s. 
11 

Figure 7 shows close-up views of MsS setup on a grouted 2-inch diameter and 25-ft long

mockup (figure 7-A) and a grouted Varina-Enon Bridge tendon specimen (figure 7-B). 

Source: FHWA. 

A. A grouted 25-ft-long mockup. 

Source: FHWA. 

B. A grouted external tendon. 

Figure 7. Photos. MsS measurements. 

Figure 8 and Figure 9 show examples of the MsS data from the 25-ft-long mockup and the field 

tendon sample, respectively. 

Source: FHWA.

Figure 8. Graph. MsS data from the 25-ft mockup.
12 

Source: FHWA.

Figure 9. Graph. MsS data from the grouted tendon specimen. 

The MsS could not identify the locations of the artificial damage in the specimens. It was 

concluded that cementitious grout and no tension in the specimens contributed to very high wave 

attenuation. Consequently, no further testing was performed.

RM Method

The RM Method can detect potentially unsafe conditions in pretensioned and PT concrete 

structures by locating wire fractures in the prestressing steel. The residual magnetic field of the 

tendons is measured at the concrete surface once the tendons have been premagnetized with an 

electromagnet. Wire fractures produce characteristic magnetic leakage fields that can be 

measured with appropriate sensors at the concrete surface. The parameters associated with 

fractured wires have been quantitatively identified in the laboratory and confirmed in the field. 

The knowledge of these parameters can estimate the reduction of the cross-sectional area or the 

number of fractured wires in a tendon. After the multilevel process of measuring data related to

certain concrete units’ magnetic states, clear signals can be seen at the fracture locations.(20)

The RM method was tried on several Varina-Enon Bridge tendon specimens containing various 

artificial saw-cut damages and ungrouted internal tendon mockups. Figure 10 shows the 

electromagnet carriage of the RM method on a Varina-Enon Bridge tendon specimen

(figure 10-A) and an internal tendon mockup (figure 10-B). 
13 

Source: FHWA. 

A. Grouted external tendon. 

Source: FHWA. 

B. Internal tendon mockup. 

Figure 10. Photos. RM measurements. 

Figure 11 shows an example of RM method data collected from a Varina-Enon Bridge tendon 

specimen containing multiple saw cuts. The arrows in the graph indicate possible damage 

locations determined by using the RM method. 

Source: FHWA.

Figure 11. Graph. Example of RM measurement data. 

After comparing actual damage locations in the specimens and the RM method data, it was 

determined that the RM method identified only some of the damage locations, and section losses 

at the identified damage locations could not be quantitatively estimated. It also failed to detect 

artificial damages in the internal tendon mockups. Therefore, no further testing was performed.
14 

Thermography With Induction Heating Method

Thermography for detecting corrosion of reinforcement in concrete was developed based on the 

principle that a corrosion layer surrounding a rebar serves as a thermal insulator. Since corroded 

rebars cool more slowly than uncorroded ones upon heating, the corrosion can be detected by 

measuring the difference in cooling rate. This method does not rely on direct solar heating of flat 

surfaces.(21) 

The employed thermography system consisted of an infrared camera, an induction heater, and a 

power generator. The induction heater raised the temperature of the specimens to about 110 °F.

Figure 12 shows an experimental setup being tested on three Varina-Enon Bridge tendon 

specimens (figure 12-A) and a thermal image of a specimen with an infrared camera 

(figure 12-B).

Source: FHWA. 

A. Measurement setup. 

Source: FHWA. 

B. Thermal image. 

Figure 12. Photos. Induction thermography. 

Although thermography could generate differential thermal images of the tendons, it was 

concluded that this technology was not useful to locate hidden corrosion damage in the grouted 

tendons, especially in the field. Therefore, no further testing was performed.

MMFM

The MMFM relies on a fundamental principle that when a ferromagnetic material, such as a steel 

strand, is magnetized to saturation, the magnitude of the magnetic flux flowing in the material is 

proportional to its cross-sectional area. Therefore, if there is a reduction of the cross-sectional 

area (i.e., a corrosion-induced section loss), a change of magnetic flux occurs accordingly. 

FHWA learned about the MMFM system being tested at the Carlton Laboratory of Columbia 

University by a research team from the Tokyo Rope Manufacturing (TRM) Company in Japan. 

As part of another FHWA study, they were testing a full-scale mockup of the Manhattan 

Bridge’s main suspension cable.(22) After exchanging several emails, FHWA and TRM agreed to 

conduct a laboratory trial of the MMFM system. In preparation for the trial, the TRM designed 
15 

and manufactured a solenoid-type magnetizer for the 4-inch-diameter external tendons, and 

FHWA brought three Varina-Enon Bridge tendon specimens to the Carlton Laboratory in 

September 2008. After observing the promising performance of the MMFM system, the second 

round of laboratory testing was carried out with an improved (the second-generation) 

solenoid-type MMFM system at TFHRC in June 2009. 

The preliminary laboratory tests determined that MMFM was the most accurate among the NDE 

methods evaluated during the voluntary participation. Therefore, the subsequent research effort 

was focused only on the MMFM systems. The next chapter describes the details of multiple-phase 

testing and improvements of MMFM systems designed for the external PT tendons.

17 

CHAPTER 3. MMFM FOR EXTERNAL TENDONS

Based on the preliminary laboratory test results of six NDE methods, MMFM was chosen for the 

full laboratory study as a potentially viable method for detecting insidious corrosion damage in

the external PT tendons. This chapter presents details of the MMFM systems.

INTRODUCTION TO MMFM 

This NDE technology relies on a fundamental principle that when a ferromagnetic material, such 

as a steel strand, is magnetized to saturation, the magnitude of the magnetic flux going into the 

material is proportional to its cross-sectional area. Therefore, if corrosion damage reduces the 

cross-sectional area, then the magnetic flux decreases accordingly. Figure 13 depicts this 

phenomenon. 

© 2010 Tokyo Rope Manufacturing Co.

Figure 13. Illustration. Flow of magnetic flux in a metal object.

(23)

When a magnetizer and a search coil are installed on a metal object and current flows through 

the wrapped electrical cable around the magnetizer, magnetic flux (φ) flows inside the object. 

The flux can be measured by a flux meter connected to the search coil, as shown in figure 14. 

© 2010 Tokyo Rope Manufacturing Co.

Figure 14. Illustration. Schematic of a magnetic flux measurement.(23)
18 

The induced voltage in the search coil is proportional to the rate at which the magnetic flux 

changes. Therefore, the total change in magnetic flux (Δφ) is calculated by integrating the 

induced voltage over time, according to the equation embedded in figure 14. 

There are two types of MMFM systems specifically designed for external PT tendons (and stay 

cables): the solenoid type, using alternating current (AC), and the permanent magnet type, using 

permanent magnets. Both are equipped with unique features to accommodate PT tendons in the 

field, as discussed in the following section. 

Solenoid-Type MMFM System

Figure 15 shows a schematic of a solenoid-type MMFM system, which consists of the 

magnetizing unit, measuring unit, and computing unit. 

© 2010 Tokyo Rope Manufacturing Co.

PC = personal computer.

Figure 15. Illustration. Schematic of the solenoid-type MMFM system.(23) 

This MMFM system can function only when the following units work in sequence: 

1. The magnetizing unit magnetizes the object: a magnetizer (an assembly of an 

aluminum bobbin and electrical wire wrapped around the bobbin), rolling wheels, 

electrical cable, polarity switch, and DC supply. 

2. The measuring unit measures the magnetic field and the magnetic flux: a search coil, 

a flux meter, Hall effect sensor(s), and a gauss meter. A Hall effect sensor produces 

the voltage proportionally to the intensity of the magnetic field that passes through 

the sensor, and a gauss meter measures the changes in the magnetic field.

3. The computing unit stores the experimental data and performs data analysis by using 

proprietary software: a data recorder and a laptop computer.

The magnetizer can magnetize the metal object up to saturation level, depending on the 

magnitude of the electrical current. The solenoid-type MMFM system operates in two 
19 

measurement methods: scan measurement and point measurement. Each method has unique 

application features, and, depending on the objective of a particular testing program, any one or 

both methods can be chosen. Each measurement method is described in the following sections. 

Scan Measurement

Scan measurements are made by moving the magnetizer and search coil at a constant speed 

along the object’s length. When a constant current maintains a constant magnetic field, constant 

magnetic flux flows into the cross-sectional area of the object. As previously discussed in 

conjunction with figure 13, magnetic flux decreases where the cross-sectional area decreases (the 

magnetic flux also depends on the magnetic history of the material). Figure 16 illustrates the scan 

measurement method schematically. 

© 2010 Tokyo Rope Manufacturing Co.

Figure 16. Illustration. Schematic of scan measurement.(23)

The schematic depicts the magnetizer scanning three locations while it is being moved from left 

to right. The reduced magnetic flux data are also illustrated at two locations having corrosion 

damage. The difference in magnetic flux (Δφ) measured between two arbitrary points at 0 and x

along the object can be calculated as a function of time (t) using induced voltage, according to 

figure 17. 

Figure 17. Equation. Determination of magnetic flux change in scan measurement.

(23)

Δφ serves as a proxy for the relative section loss between the two points. This qualitative 

information is useful for quickly scanning the physical condition of the entire metal object and 

allows the user to identify suspected areas for performing an indepth evaluation with the point 

measurement method.
20 

Point Measurement

The point measurement is performed by parking the magnetizer at a target point. Figure 18

illustrates a point measurement method schematically. 

© 2010 Tokyo Rope Manufacturing Co.

Figure 18. Illustration. Schematic of point measurement.(23)

By varying bipolar magnetization electrical current, a gauss meter can measure the changes in 

the magnetic field (H, or magnetic flux density) via the Hall effect sensor. The direction of the 

electrical current can be reversed by using a polarity switch. When the magnetic field changes, a 

flux meter connected to the search coil can measure the corresponding change in magnetic flux. 

A point measurement produces a unique magnetic hysteresis loop, which shows the relationship 

between H and φ in the particular experimental setup. To produce any hysteresis loop, an 

electrical current needs to energize the magnetizer to full saturation level. Then, the current 

direction is reversed, which energizes the magnetizer again to the other saturation level in the 

opposite direction. Figure 19 shows two examples of hysteresis loop: One is generated under 

higher stress than the other. 

© 2010 Tokyo Rope Manufacturing Co.

Figure 19. Illustration. Examples of the magnetic hysteresis loop.(23)

Flux meter

Gauss meter
21 

This figure illustrates that metallic objects under higher stress tend to exhibit a more gradual 

bending toward the saturation level than those under lower stress do. An operator can determine 

Bs (the magnetic field at the saturation level) and 2Δφ (the total change in magnetic flux) under a 

very strong bipolar magnetic field by reviewing the hysteresis loop. The equation in figure 20

shows a proportional relationship among 2Δφ, Bs, and the cross-sectional area (A) of the metal 

object, which can estimate the absolute cross-sectional area at the measurement spot.

Figure 20. Equation. Determination of magnetic flux change in point measurement.(23) 

Where:

H = magnetic field. 

m = arbitrary measurement spot. 

Bs is the saturated magnetic field.

A is the cross-section area at the measurement spot.

Figure 21 shows four actual magnetic hysteresis loops of wire ropes having different 

cross-sectional areas.

© 2016 Tokyo Rope Manufacturing Co.

1 mm2 = 0.00155 cm2

; kA/m = kiloamperes per meter.

Figure 21. Graph. Magnetic hysteresis loops for wire ropes having different cross sections. 

The preceding examples are useful for visualizing the effect of cross-sectional area on magnetic 

flux: Wire ropes with larger cross-sectional areas require more magnetic flux to reach saturation 

level than ones with smaller cross-sectional areas.

These magnetic characteristics suggest that magnetic flux is sensitive to subtle changes in stress 

and cross-sectional area. Therefore, periodic point measurements at fixed locations can monitor 

changes in stress in the tendons and stay cables, or the development of new corrosion damage in 

the monitoring locations. 
22 

Permanent Magnet-Type MMFM System

This system was developed to eliminate an AC power requirement and tedious wire winding 

work in the field. Thus, it improves the field applicability with significantly reduced field 

preparation time, but it also reduces detection accuracy due to insufficient magnetism compared 

with the solenoid type. It works only with the scan measurement method over a metal object.

Several upgrades have been made over 6 yr to improve its performance in the field. 

As shown in figure 22, the permanent magnet-type magnetizer consists of a half-split bobbin, 

eight permanent magnets, a sensor unit, rolling wheels, and two spring-loaded clamps to lock the 

bobbin. The sensor unit contains one or two search coils, axial Hall effect sensors (HGx), and 

radial Hall effect sensors (HGy). 

Source: FHWA. 

N = north pole; S = south pole.

Figure 22. Illustration. Schematic of scan measurement by the permanent magnet type. 

To ensure detecting a change of the magnetic flux wherever a defect exists in the metal object, 

the latest permanent magnet type has 16 Hall effect sensors around the metal object: eight radial 

sensors facing the center of the object and eight axial sensors in the longitudinal direction. 

Figure 23 shows the Hall effect sensor arrangement. 

S N S N S N 
23 

© 2016 Tokyo Rope Manufacturing Co.

IC = integrated circuit.

Figure 23. Illustration. Arrangement of 16 Hall effect sensors.

Figure 24, in conjunction with an imaginary defect in figure 22, describes how individual sensors 

respond when the magnetizer moves through defect-free locations (point A and point C in 

figure 22) and a defect containing the location (point B in figure 22) during a scan measurement. 

Source: FHWA. 

Figure 24. Illustration. Schematic representation of MMFM data signals. 

As magnetic flux leaks at the defect (point B in figure 22), the leaked magnetic flux is divided 

into a vertical component (radial) and a horizontal component (axial). During this transition from 

defect-free to defect to defect-free conditions, change of magnetic flux influences the sensors in 

the following ways: 

• Search coils: Direct voltage signals from two search coils are automatically integrated by 

the change of magnetic flux. These integrated voltages begin to decrease beyond point A, 

become the lowest at point B, and return to the normal value at point C and onward. 

 
24 

• HGy in the upper region: After passing point A, magnet flux starts leaking. The vertical 

(i.e., perpendicular to the metal object) component of the leaked magnetic flux increases 

and then decreases until it becomes the normal value at point B. After passing point B, 

the vertical component of the leaked magnetic flux increases again in the opposite 

direction and then decreases until it becomes the normal value again at point C and 

onward. 

• HGx in the upper region: After passing point A, magnet flux starts leaking. The horizontal 

(i.e., parallel to the metal object) component of the leaked magnetic flux increases until it 

becomes the highest at point B. After passing point B, the horizontal component of the 

leaked magnetic flux decreases until it becomes the normal value at point C and onward.

• HGy and HGx sensors in the bottom region: Because these Hall effect sensors are situated 

far away from the defect located in the upper region, the effect of the magnetic flux 

fluctuations is minimal.

EXPERIMENTAL PROCEDURE

Laboratory Specimens 

As described in chapter 2, three tendon specimens retrieved from the Varina-Enon Bridge, 

labeled tendons A, B, and C, were employed to evaluate the solenoid-type MMFM system. The 

specimens contained different levels of artificial damage: saw cuts for large physical damage and

impressed current damage for realistically small pit damage. 

Figure 25 shows examples of damage from three saw cuts (figure 25-A) and minor impressed 

current damage of approximately 0.2-percent section loss (figure 25-B). 

Source: FHWA. 

A. Damage from three saw cuts. 

Source: FHWA. 

B. Impressed current corrosion damage. 

Figure 25. Photos. Examples of artificial damage. 
25 

Table 1 summarizes the initial damage conditions of the specimens introduced by saw cuts and 

the impressed current method. 

Table 1. Summary of initial damage introduced in the laboratory specimens.

Specimen ID Damage ID (degree of damage) 

Tendon A R1 (minor corrosion by impressed current) 

Tendon A R2 (3 saw cuts)

Tendon A R3 (minor corrosion by impressed current) 

Tendon B R1 (1 saw cut) 

Tendon B R2 (2 saw cuts)

Tendon B R3 (1 saw cut) 

Tendon C R1 (1 saw cut) 

Tendon C R2 (1 saw cut) 

Tendon C R3 (2 saw cuts)

Tendon C R3' (5 wires removed from R3)

ID = identifier; R = damage identifier. 

In order to evaluate the accuracy of the permanent magnet-type MMFM system, a blind test was 

performed using a tendon D specimen from the Varina-Enon Bridge. Figure 26 shows two 

artificial section losses of 0.4-percent (figure 26-A) and 1.4-percent (figure 26-B).

Source: FHWA.

A. Damage number 1 

(0.4-percent section loss). 

Source: FHWA. 

B. Damage number 2 

(1.4-percent section loss). 

Figure 26. Photos. Damage conditions in tendon D specimen. 
26 

Development of Solenoid-Type MMFM System

During the study’s early stage, researchers designed and fabricated two solenoid-type 

magnetizers, labeled the first generation and the second generation. 

Figure 27 shows the first-generation solenoid-type testing at Columbia University’s Carleton 

Laboratory (figure 27-A) and the second-generation solenoid-type testing at TFHRC 

(figure 27-B).

Source: FHWA. 

A. First-generation magnetizer. 

Source: FHWA. 

B. Second-generation magnetizer. 

Figure 27. Photos. Solenoid-type magnetizers.

The first-generation solenoid-type magnetizer was specifically designed for the external PT

tendon specimens. A 6-inch diameter plastic casing (sensor holder) was the inner part of the 

magnetizer, and the casing grooves housed a Hall effect sensor and a search coil. The sensor 

holder was inserted into an aluminum alloy bobbin to form the core of the magnetizer. Then, the 

electrical cable was wrapped around the bobbin in several layers to complete the magnetizer 

assembly. The magnetizer was pushed and pulled along the tendon length to perform scan 

measurements. An overhead crane suspended the magnetizer due to its heavy weight. During the 

first trial, the operators realized that the first-generation solenoid-type magnetizer was difficult to 

maneuver. The inconsistent moving speed and frequent jerky motions during scanning also 

reduced the quality of the MMFM data. Despite the unexpected obstacles, the MMFM system 

was able to detect many damage locations (not section loss). 

Based on experience gained from the first MMFM testing, the second-generation solenoid-type 

magnetizer was developed for the 4-inch tendons. It had improved features regarding size, 

weight, magnetomotive force, and ease of operation. For example, the second-generation 

magnetizer was 65-percent lighter (from 220 to 77 lb) and 37-percent smaller in overall diameter 

(from 15.7 to 10 inches) than the first-generation magnetizer. The magnetomotive force was also 

31-percent reduced (from 40,000 to 27,500 ampere-turn (AT)). The improved magnetizer was 

equipped with plastic wheels to slide the magnetizer over an external PT tendon without a crane. 

As a result, the magnetizer’s mobility was significantly improved while providing smooth and 

continuous manual operation. Other components, such as power supplies and switch boxes, 

became more compact as well. For both solenoid types, the minimum power requirements 

remained the same at three-phase AC, 240-V and 30 A. 
27 

Development of Permanent Magnet-Type MMFM System

The permanent magnet-type magnetizer can be separated into two halves so that operators can 

mount the partially opened magnetizer onto external PT tendons and stay cables. 

Figure 28-A shows the opened permanent magnet-type magnetizer: half-split inner casing, a 

sensor unit containing a search coil and Hall effect sensors, four sets of half-split permanent 

magnets, half-split outer body shell, and four rollers on each end of the magnetizer. Figure 28-B

shows the fully assembled permanent magnet-type magnetizer on a 4-inch external tendon. 

Source: FHWA. 

A. Opened condition. 

Source: FHWA. 

B. Assembled condition. 

Figure 28. Photos. Permanent magnet-type magnetizer.

A dedicated controller was also developed for the permanent magnet-type MMFM system. 

Powered by a 9-V battery, the controller transmits the driving voltage to the Hall effect sensors, 

receives a signal from each of the sensors, and integrates the signal from the search coil. The 

collected signals are saved on a memory card for subsequent data analysis with a laptop 

computer equipped with MMFM software. 

Comparison of Two MMFM Systems

The solenoid type and the permanent magnet type have their pros and cons from physical and 

operational standpoints. Table 2 provides a comparison chart of the two systems.

Table 2. Comparison of solenoid-type and permanent magnet-type MMFM systems.

Category Solenoid-Type (second-generation) Permanent Magnet Type

Major capability Locate corrosion damage areas Locate corrosion damage areas

Estimation of 

section loss Quantify actual section loss accurately Estimate section loss qualitatively

Magnetic strength Ability to change magnetic strength by 

changing the magnetizing current Limited magnetic strength

Magnetizer 

polarity Can change current polarity Cannot change magnet polarity 

Permanent magnets Search coil

 ect sensors

Roller Roller

Hall effect sensors

Search coil Permanent magnets
28 

Category Solenoid-Type (second-generation) Permanent Magnet Type

Additional 

capabilities

Monitor stress in the object and 

development of new corrosion damage — 

Magnetizer 

weight 77 lb (from first-generation 220 lb) 56 lb

Measurement 

Method Point measurement; scan measurement Scan measurement

Magnetizer 

exterior Aluminum bobbin External shell casing

Introduction of 

magnetism Wrapped electrical wire 4 semicircular-shaped permanent magnet sets 

Sensor holder Cylindrical plastic casing Half-split plastic casting

Search coil 1 search coil 1 search coil

Hall effect sensor 1 Hall effect sensor 16 Hall effect sensors

Number of rollers 2 each 4 each

Encoder type Separate wire encoder One built-in digital encoder

Components Flux meter Battery-powered one-unit 

controller

Components Gauss meter Signal cable

Components DC supplier; polarity switch; data 

reader; power cables — 

Data type 

Magnetic field; magnetic flux 

hysteresis (point); magnetic flux chart 

(scan) 

Induced voltage signal of search

coil; change of magnetic flux; 

signal of Hall effect sensors

Power 

requirement

Generator for three-phase AC 

240-V/30-A None 

Auxiliary 

equipment Electrical wire wrapping machine None 

Required number 

of operators and 

time

4–5; 60~90 min 2–3; 10~20 min

—No data.

The number and orientation of Hall effect sensors are the major differences between the solenoid 

type and the permanent magnet type. While the solenoid type has a single Hall effect sensor at 

the center of the sensor holder, the permanent magnet type has evenly distributed eight Hall 

effect sensors in the radial direction and eight Hall effect sensors in the longitudinal direction, as 

shown in figure 23. 

The solenoid type produces a nearly flat magnetic flux line in a defect-free metallic object, but 

the permanent magnet type exhibits a curved magnetic flux line for the same condition. As a 

result, a reduction of magnetic flux is not proportional to a reduction in the object’s 

cross-sectional area. Also, different magnetic field patterns cause different patterns of the 

magnetic flux line.
29 

The solenoid-type MMFM system produces two types of data based on the measurement 

method. A point measurement gives an absolute magnetic flux value associated with a hysteresis 

loop between the magnetic flux and the magnetic field at each measurement location. Therefore, 

this method can estimate accurate and quantifiable section loss compared with other NDE 

methods. A scan measurement yields a magnetic flux chart that shows relative flux values along 

the specimen length with respect to absolute magnetic flux. This method provides an overall 

superior corrosion damage detection capability for considering both speed and accuracy. 

However, during the field trials, the solenoid type required considerable preparation time 

(60~90 min per tendon), and at least four people were needed for each measurement. These 

requirements were a significant drawback in the field.

The permanent magnet-type MMFM system produces three types of data: an induced voltage 

signal from the search coil, relative changes in magnetic flux by integrating the induced voltage, 

and voltage signals from the Hall effect sensors. Although the permanent magnet type needs

minimal preparation and measurement time (10–20 min per tendon) and two to three people, it 

cannot change the magnet’s north-south polarity and has limited magnet strength. Therefore, the 

permanent magnet type can only run for scan measurements and detect relatively larger section 

losses than the solenoid type. 

Based on the strengths and weaknesses of each MMFM system, an appropriate system should be 

chosen for the objective of a particular field-testing program. For example, the permanent 

magnet type is preferred for tasks requiring rapid scans and an overall condition assessment. In 

contrast, the solenoid type is suitable for accurate damage assessment in critical areas.

Field Evaluations in the Varina-Enon Bridge

The Varina-Enon Bridge was chosen for evaluating the field performance of the MMFM systems 

because there could still be many tendons that were vulnerable to further deterioration due to 

poor grout conditions. Also, full support and cooperation could be obtained from VDOT and the 

FHWA division office. 

The main objective of the field evaluations was to determine whether the MMFM systems could 

be used in the field without affecting the accuracy and performance observed in the laboratory. 

Therefore, two rounds of field evaluations were made on some external tendons of the bridge. 

First Visit With the Second-Generation Solenoid-Type MMFM System

The second-generation solenoid-type MMFM system was tried on some of the Varina-Enon 

Bridge tendons in 2010. A team of TRM engineers, FHWA researchers, and VDOT personnel 

evaluated the MMFM system over 4 d. 

The second-generation solenoid type was powered by an industrial-grade generator providing

three-phase AC at 240-V/30 A.

Figure 29 shows the magnetizer being installed on a test tendon using a motorized wire-wrapping 

system (figure 29-A) and an operator pulling the magnetizer with a rope during a scan 

measurement (figure 29-B).
30 

Source: FHWA. 

A. Motorized wire-wrapping system. 

Source: FHWA. 

B. Scan measurement. 

Figure 29. Photos. First MMFM testing in the Varina-Enon Bridge. 

A small pulley was installed at the test tendon’s highest point. The upward scan was made by 

pulling the other end of a rope attached to the operator-facilitated magnetizer at a constant speed, 

and the downward scan was done by gravity while releasing the rope. 

Before each scan measurement, a group of short wires was attached to the tendon bottom near 

the scan’s starting and ending points. The extra mass added by these wires introduced the flux 

spikes in the magnetic flux chart, which served as the measurement reference points. 

In most cases, two consecutive scans were made before completing the scan measurement per 

tendon. The first scan magnetized the tendon, and the second scan collected MMFM data. 

Operators learned that maintaining a constant speed and smooth glide over an HDPE duct 

manually was difficult to execute in the field. Neoprene couplers and hose clamps on the duct 

frequently impeded the smooth magnetizer movement. These obstacles contributed to variable 

magnetizer moving speed and jerky motion that resulted in unstable MMFM data. For these 

reasons, the in situ MMFM testing was more challenging for the operators than the laboratory 

testing. If suspicious areas were identified, the magnetizer was parked at each of those areas and 

performed a point measurement. 

After the MMFM testing was completed, some of the suspected corrosion areas and intact 

control areas were excavated to verify actual physical conditions against the MMFM data. If 

good quality grout covered an excavated area without exhibiting rust stains or cracks, the strands 

in the grout were assumed to be in good condition, and no further excavation was made. If an

excavated area exhibited an uncertain condition, the grout was removed for inspecting strand 

condition. As the final step, the excavated sections were restored with the original duct pieces, 

followed by wrapping them with heat-shrinkable tape or sheets. 

Second Visit With the Permanent Magnet-Type MMFM System

Soon after developing the first permanent magnet-type MMFM system, a second visit to the 

Varina-Enon Bridge was made in 2015 to try the new system. In less than 3 d, 22 tendons were 
31 

scanned due to its much quicker installation and removal process than the solenoid-type 

magnetizer. It was estimated that the solenoid-type magnetizer could have taken nearly 2 w to 

cover the same number of tendons. Based on the MMFM data, five sections were opened to 

verify their conditions. The sixth one was already opened during the previous corrosion 

investigations conducted by VDOT.

Figure 30 shows a closeup view of the magnetizer installed on an external tendon (figure 30-A) 

and a scan measurement in progress using a pulley and rope (figure 30-B).

Source: FHWA. 

A. Closeup view. 

Source: FHWA. 

B. Scan measurement. 

Figure 30. Photos. Second MMFM testing in the Varina-Enon Bridge. 

TEST RESULTS AND DISCUSSION

This section presents test results from the laboratory and field evaluations, followed by an 

in-depth discussion. 

Laboratory Evaluations 

Figure 31 shows photographs of artificial damage introduced into the tendon A specimen, and 

figure 32 shows a matching graph of magnetic flux versus scan distance data. 
32 

 

 

 

 

 

 

 

 

 

 

 

Source: FHWA. 

A. R1 artificial damage in the 

tendon A sample. 

Source: FHWA. 

B. R2 artificial damage in the 

tendon A sample. 

Source: FHWA.

C. R3 artificial damage in the 

tendon A sample. 

Figure 31. Photos. Artificial damage conditions of tendon A specimen.

Source: FHWA. 

1 m = 3.28 ft. 

Figure 32. Graph. Scan measurement data from tendon A specimen.

As listed in table 1, artificial damages at R1 and R3 were introduced with the impressed current 

method, and the other damage at R2 was introduced by three saw cuts. Figure 32 contains two 

sets of magnetic flux data by scan measurement (curve) and point measurement (dots). The dots 

labeled R1, R2, and R3 correspond to the point measurement locations for artificial damage 

shown in figure 31. The remaining three dots indicate the point measurement locations at intact 

locations, that is, no artificial damage. 

While both the scan and point measurement data at R1 and R3 showed reasonably good 

agreements, a large discrepancy was observed at the middle damage location (R2). The actual 

number of wires cut at this location was 16.5, but the MMFM estimated only a 2.7-wire cut, 

which is equivalent to a 2.0-percent section loss. 

After discovering a noticeable discrepancy between the scan measurement data and point 

measurement data at R2, TRM carried out a small-scale laboratory study to investigate the effect 

of cut length on the MMFM data’s accuracy. The TRM engineers fabricated a laboratory 

specimen by using the same number (19) of 0.6-inch seven-wire strands as the Varina-Enon 

tendon A specimen. Then, one strand was completely cut (equivalent to 5.3-percent section loss) 

to have different cut lengths ranging from 0.4 inches to 15.7 inches at eight locations. Repeated

R1

R2

R3
33 

point and scan measurements were made with the specimen. The following section summarizes

the study findings.

Figure 33 shows two examples of the scan measurement data and point measurement data 

superimposed at three locations for a 0.4-inch cut (figure 33-A) and a 7.9-inch cut (figure 33-B). 

The middle points correspond to the cut locations and the left and right points for intact areas. 

Source: FHWA. 

A. Magnetic flux chart for defect length of 

0.4 inches. 

Source: FHWA. 

B. Magnetic flux chart for defect length of 

7.9 inches. 

Figure 33. Graphs. Magnetic flux versus defect length data. 

As expected, scan measurement data and point measurement data collected at the intact areas 

showed a high degree of agreement. The 0.4-inch cut produced a small and narrow magnetic flux 

reduction (figure 33-A), but the degree of agreement (ratio of two data) between the two sets of 

data was low. On the other hand, the 7.9-inch cut produced a noticeably deep and wide magnetic 

flux reduction (figure 33-B) and a higher degree of agreement between the two sets of data than 

the 0.4-inch case was observed. 

Figure 34 summarizes the relationships between the cut length and the reduction of magnetic 

flux in terms of the magnetic flux rate obtained by the scan and point measurement methods. 
34 

Source: FHWA. 

1 mm = 0.039 inches. 

Figure 34. Graph. Relationships between cut length and reduction of magnetic flux. 

A horizontal line indicates an actual section loss of 5.3-percent, equivalent to one strand removed 

out of 19 strands and can serve as the reference to judge the estimates’ accuracy against the 

actual section loss. The point measurement method yielded more accurate estimates than the scan 

measurement method in all cases. The latter underestimated the section losses by 0.6-percent. 

The most accurate estimation was made by the point measurement method when the cut length 

was 6 inches or longer. Therefore, it was confirmed that the discrepancy observed at R2 in 

figure 32 was caused when magnetic flux jumped over the narrow saw cuts. 

Figure 35 presents the change of magnetic flux and the search coil signal of the permanent 

magnet-type magnetizer versus scan distance data from the tendon D specimen. The other two 

datasets, that is, the radial and HGx signals, are omitted here. 
35 

Source: FHWA. 

1 m = 3.28 ft; V·s= volt-second.

Figure 35. Graph. Permanent magnet-type data from the tendon D specimen. 

The permanent magnet type missed the 0.4-percent section loss (figure 26-A) but detected the 

1.4-percent section loss (figure 26-B). Although the permanent magnet type’s change of 

magnetic flux data is a good indicator for locating the suspected corrosion areas, it is not as 

accurate as the solenoid type. 

Discussion on Laboratory Data 

The first round of laboratory evaluation employed the first-generation solenoid-type system on 

the Varina-Enon Bridge’s three tendon specimens. The magnetic field strength of 55 kA/m was 

used for the magnetizer. Table 3 summarizes the point measurement data, which were divided 

into intact and damaged areas. The mean magnetic flux value of each tendon’s intact areas was 

calculated. Also, the overall mean magnetic flux value of all three tendons’ intact areas was 

calculated to approximate the true magnetic flux of the undamaged tendons. 

-3.0 

-2.0 

-1.0 

0.0 

1.0 

2.0 

3.0 

4.0 

5.0 

6.0 

7.0 

8.0 

120

140

160

180

200

220

240

260

280

300

320

340

0 0.5 1 1.5 2 2.5

Signal ofsearch coil (m

V)

Change of magnetic flux (V・s)

Distance (m)

Magnetic Flux Signal of Search Coil

20 V or 1.0 mV

1.4 percent

section loss
36 

Table 3. Summary of the first point measurement dataset collected with the 

first-generation solenoid-type magnetizer.

Specimen 

ID

Intact 

Area 

ID

Magnetic 

Flux at 

an Intact 

Area 

(kMx)

Damaged Area 

ID (Degree of 

Damage) 

Magnetic 

Flux at a 

Damaged 

Area 

(kMx)

Magnetic Flux at a 

Damaged Area as a 

Percentage of Mean 

Magnetic Flux at Intact 

Areas of the Tendon 

(Estimated Section Loss, 

Percent) 

Tendon A S1 519.07 R1 (minor 

corrosion) 

519.02 99.89 (0.11) 

Tendon A M1 519.30 R2 (3 saw cuts) 509.45 98.04 (1.96) 

Tendon A E1 520.47 R3 (minor 

corrosion) 

519.05 99.89 (0.11) 

Tendon A Mean 519.61 — — — 

Tendon B S1 519.19 R1 (1 saw cut) 515.65 99.24 (0.76) 

Tendon B M1 519.61 R2 (2 saw cuts) 514.13 98.95 (1.05) 

Tendon B M2 519.97 R3 (1 saw cut) 518.98 99.88 (0.12) 

Tendon B E1 519.61 — — — 

Tendon B Mean 519.60 — — — 

Tendon C S1 520.58 R1 (1 saw cut) 520.21 99.92 (0.08) 

Tendon C M1 520.54 R2 (1 saw cut) 514.74 98.86 (1.14) 

Tendon C M2 520.91 R3 (2 saw cuts) 495.99 95.26 (4.74) 

Tendon C E1 520.55 R3′ (5 wires 

removed from 

R3)

497.76 95.60 (4.40) 

Tendon C Mean 520.65 — — — 

Overall

mean 

— 519.98 — — — 

— No data.

ID = identifier; kMx = kilomaxwell. 

As expected, the latter group (damaged areas) exhibited less magnetic flux than the former

(intact areas). In each damaged area, a section loss was estimated by subtracting the magnetic 

flux percentage at the location with respect to the tendon’s intact areas’ mean magnetic flux from

100-percent. As listed in table 3, the estimated section losses ranged from 0.11 to 4.7-percent.

The second round of laboratory evaluation employed the second-generation solenoid-type 

magnetizer on the Varina-Enon Bridge’s tendons A and C. No measurements were made on the 

tendon B specimen. Magnetic field strength was decreased from 55 kA/m with the 

first-generation magnetizer to 38 kA/m. As a result, the overall mean magnetic flux in the intact 

areas decreased from 519.98 to 510.67 kMx. 

Table 4 summarizes the second round of point measurement data. Because the artificial damage 

was introduced in two steps, the table divides the magnetic flux data into three groups: the intact 

area group, the initial damage group, and the final damage group. 
37 

Table 4. Summary of the second point measurement dataset collected with the 

second-generation solenoid-type magnetizer.

Specimen ID (Tendon)

Intact Area ID

Magnetic 

Flux at an Intact 

Area (kMx)

Initial Damaged Area ID

(Damage Level) 

Magnetic Flux at an Initial 

Damaged Area (kMx) 

Initial Damaged Area 

Magnetic Flux % of Intact 

Areas’ Mean Magnetic 

Flux (estimated section loss, 

percent) 

Final damaged area Area 

ID (Damage Level) 

Magnetic Flux at a Final 

Damaged Area (kMx) 

Magnetic Flux at a Final 

Damaged Area as % of 

Mean Magnetic Flux at

Intact Areas (Estimated 

Section Loss, %) 

A S1 510.10 R1 (minor 

corrosion)

505.30 99.37 (0.63) R1′ (1 wire 

removed)

500.70 98.47 (1.53)

A M1 508.70 R3 (minor 

corrosion)

501.20 98.57 (1.43) — — —

A M2 506.20 — — — R2′′ (14 

wires 

removed)

452.07 88.91 (11.09)

A E1 508.90 — — — R3′ (1 wire 

removed)

502.40 98.80 (1.20)

A M 508.48 — — — — — —

B S1 511.50 — — — — — —

B M1 511.80 — — — — — —

B M2 512.20 — — — — — —

B E1 511.80 — — — — — —

B M 511.83 — — — — — —

C S1 512.00 — — — — — —

C M1 511.10 — — — R2′ (1 wire 

removed)

508.70 99.41 (0.59)

C M2 511.80 — — — — — —

C E1 511.90 R1 (1 saw 

cut)

507.80 99.24 (0.76) R3′′ (7 

wires 

removed)

485.10 94.80 (5.20)

C M 511.70 — — — — — —

— No data.

ID = identifier; M = middle point; E = ending point; S = starting point. 

Figure 36 shows linear regression analysis results of the first and second point measurement

datasets listed in table 3 and table 4 to determine the relationship between estimated section 

losses and the corresponding magnetic flux data. 
38 

Source: FHWA. 

Figure 36. Graph. Linear correlation between magnetic flux data and estimated section 

losses of the laboratory data. 

Both datasets exhibited almost perfect one-to-one relationships with a coefficient of 

determination (R2

) greater than 0.99: as the section loss increased, the magnetic flux decreased 

linearly. There was a difference of approximately 10 kMx between the parallel regression fit 

lines because two datasets were collected with two different magnetic field strengths, 519.98 and 

510.67 kMx. 

Figure 37 shows a direct relationship between estimated section losses in table 4 and the actual 

section losses, which were determined based on the ratio of the number of physically removed 

wires to the total number of wires in the tendon. 

Source: FHWA. 

Figure 37. Graph. Linear correlation between actual section losses and estimated section 

losses based on the second point measurement dataset. 

The linear regression line falls close to the line of equality such that the second-generation 

solenoid-type overestimated section losses up to 0.27-percent. Conservatively speaking, this 
39 

level of overestimation should be better than an underestimation, considering typical corrosion 

damage observed in real PT bridges. 

With these linear relationships established for the solenoid-type MMFM system, comparing the

magnetic flux values measured at the suspected areas to a known intact area’s mean magnetic 

flux of a specific tendon can estimate the section losses in the suspected areas. 

Even though the solenoid-type MMFM system could detect section loss as low as 0.2-percent 

based on the regression equation for the flux data associated with 55 kA/m in figure 36, a 

0.4-percent section loss would be considered the lowest detection limit for the point 

measurement method. This limit is equivalent to losing about 50-percent of a wire among 

133 wires in 19 strands. As discussed with the data presented in figure 34, the scan measurement 

method produced up to a 0.6-percent error compared with the point measurement method. 

Therefore, a 1.0-percent section loss (0.4-percent plus a margin of error of 0.6-percent) would be 

considered the lowest detection limit for the scan measurement method. 

Field Evaluations

The first round of field MMFM testing was conducted with five external tendons. One of them, 

labeled P12S12TW-3, was chosen because MFL testing in 2007 revealed that it suffered from 

severe corrosion damage. Because the tendon’s corroded strands were already exposed without a 

duct, the magnetizer wheels could not roll smoothly over the exposed bare strands, and operators 

had to vigorously push and pull the magnetizer. Figure 38 shows the second-generation 

solenoid-type magnetizer on the exposed section of the tendon. 

Source: FHWA. 

Figure 38. Photo. MMFM testing of the P12S12TW-3 tendon.
40 

Figure 39 shows the scan measurement data of the tendon, including the exposed bare strand 

section. 

Source: FHWA. 

1 m = 3.28 ft. 

Figure 39. Graph. Scan measurement data from the P12S12TW-3 tendon.

The MMFM data exhibited reduced magnetic flux over the exposed bare strand section, 

indicated by the solid red ellipse. The recognizable corrosion damages were also visually 

confirmed in the section. Figure 40 shows two closeup photographs of the observed corrosion 

damages.

Source: FHWA. 

A. Corroded strands. 

Source: FHWA. 

B. Another area of corroded strands. 

Figure 40. Photos. Closeup views of corroded strands. 

Table 5 summarizes the point measurement data collected from the Varina-Enon Bridge tendons. 

As shown in this table, the scanned tendons did not contain any recognizable corrosion-damaged 

areas except for the most severely corroded tendon (P12S12TW-3), which produced four-point 

measurement data.
41 

Table 5. Summary of point measurement data from the Varina-Enon Bridge tendons. Tendon ID Intact Area ID Magnetic Flux at 38 kA/m at Intact Areas (kMx) Magnetic Flux at 55 kA/m at Intact Areas (kMx) Damage Area ID (Physical Condition) Magnetic Flux at 38 kA/m at Damaged Area (kMx) Magnetic Flux at 55 kA/m at Damaged Area (kMx) Magnetic Flux at a Damaged Area as a % of Overall Mean Magnetic Flux at Intact Areas: 38 kA/m (Estimated Section Loss, %) Magnetic Flux at a Damaged Area as % of 

Overall Mean Magnetic Flux at Intact 

Areas: 55 kA/m (Estimated Section Loss, %) 

P14S13TE-1 U 471 499 — — — — — 

P14S13TE-1 M 471 499 — — — — — 

P14S13TE-1 D 472 500 — — — — — 

P14S13TE-1 M 471 499 — — — — — 

P14S13TE-2 U 470 496 — — — — — 

P14S13TE-2 M1 471 497 — — — — — 

P14S13TE-2 M2 470 496 — — — — — 

P14S13TE-2 M3 470 496 — — — — — 

P14S13TE-2 D 472 498 — — — — — 

P14S13TE-2 M 471 497 — — — — — 

P14S13TE-3 U 473 497 — — — — — 

P14S13TE-3 M 472 498 — — — — — 

P14S13TE-3 D 472 495 — — — — — 

P14S13TE-3 M 472 497 — — — — — 

P12S11TE-2 U 472 500 — — — — — 

P12S11TE-2 M 473 501 — — — — — 

P12S11TE-2 D 471 500 — — — — — 

P12S11TE-2 M 472 500 — — — — — 

P12S12TW-3 M1 472 498 U (minor 

corrosion)

469 496 99.50 

(0.50)

99.61 

(0.39)

P12S12TW-3 M2 470 497 M3 (minor

corrosion)

469 496 99.50 

(0.50)

99.61 

(0.39)

P12S12TW-3 D 470 497 M4 (wire 

break)

462 488 98.02 

(1.98)

98.00 

(2.00)

P12S12TW-3 — — — M5 (minor 

corrosion)

468 495 99.29 

(0.71)

99.41 

(0.59)

P12S12TW-3 M 471 497 Mean 467 494 — — 

Overall mean — 471.35 497.95 — 467.00 493.75 — — 

— No data.

D = downward point; ID = identifier; M = middle point; U = upward point. 
42 

The estimated section losses observed on the P12S12TW-3 tendon varied between 0.39 and 

2.0-percent, roughly equivalent to the loss of 0.5 to 2.5 wires. Because the tendon was active 

under significant stress, actual section losses could not be measured accurately after acid 

cleaning of the corroded wires, followed by pit depth measurements using a digital pit gauge. 

However, the MMFM data seemed to quantify the section losses reasonably well compared with 

the visual appearance of the corroded areas.

Figure 41 shows linear regression analysis results of the P12S12TW-3 tendon to determine a 

relationship between estimated section loss and the corresponding magnetic flux. For comparison 

purposes, the laboratory datasets shown in figure 36 were overlapped in the same graph. 

Source: FHWA. 

Figure 41. Graph. Linear regression fits between magnetic flux and estimated section losses

of the P12S12TW-3 tendon in relation to the laboratory datasets. 

The field MMFM data of the P12S12TW-3 exhibited perfect fits with an R2 of 1. The regression

analysis results suggested that a specific one-to-one relationship between magnetic flux data and 

the section losses could be established to estimate any section loss in the field PT tendons having 

the same tendon configuration (i.e., strand size and the number of strands), provided that their 

mean magnetic flux value at the verified intact areas was determined in advance. 

Another potential benefit of the MMFM methods is to estimate tensile stress being exerted in the 

tendons and stay cables qualitatively. This possibility is based on the fact that magnetic flux 

decreases as stress in the metallic object increases because stress induces physical deformation of 

a metal, that is, a strain that changes the metal’s magnetization property. For example, the 

P12S12TW-3 tendon in service showed the field mean magnetic flux values of 498.0 kMx at 

55 kA/m and 471.4 kMx at 38 kA/m at the intact areas. These values were 95.5-percent (versus 

520.0 kMx at 55 kA/m) and 92.3-percent (versus 510.7 kMx at 38 kA/m) of the mean flux values 

measured in the “no-stress” laboratory tendon specimens retrieved from the same bridge. 

Figure 42 shows unpublished data from an internal research study that TRM conducted. 
43 

© 2016 Tokyo Rope Manufacturing Co. 

N/mm2 = Newton per square millimeter; 1 N/mm2 = 145.04 psi. 

Figure 42. Graph. Influence of stress on the reduction of magnetic flux.

Even though direct comparisons between the flux data presented in figure 42 and the 

Varina-Enon Bridge’s MMFM data cannot be made due to different test conditions, it is still 

interesting to see a unique relationship between increasing tensile stress and decreasing magnetic 

flux, as a function of the magnetic field. Therefore, the solenoid-type MMFM system may be 

able to monitor stress variations at a tendon’s target locations by measuring the change in 

magnetic flux over time. 

The permanent magnet-type MMFM system did not yield clear indications that any of the 

22 scanned tendon sections contained apparent corrosion damage. To be sure, five sections 

exhibiting unusual or uncertain MMFM responses, such as reduced magnetic flux and disturbed 

signals from Hall effect sensors, were chosen for excavation. When the selected areas were 

opened, large gout voids and partially exposed strands existed along the length of the tendon. 

Figure 43 shows a typical tendon condition upon excavation. 
44 

Source: FHWA. 

Figure 43. Photo. Uncovered condition of a tendon exhibiting grout deficiencies.

While the tendons’ bottom portion was filled with normally hardened grout, the grout in the top 

appeared weaker than the bottom grout, and white grout powder covered the exposed surfaces of 

grout void and strands. These conditions were likely caused by the grout segregation when 

excessive water could be mixed with cement during the grout operation. However, no 

recognizable corrosion damage was found on the exposed strand surface. 

Figure 44 shows an example of uncertain MMFM data in terms of magnetic flux and search coil 

data obtained from a tendon section (SP12T10) that was later opened to examine its interior 

condition. 
45 

Source: FHWA. 

V·s = volt-second.

A. Magnetic flux and search coil data. 

Source: FHWA.

1 m = 3.28 ft.

B. Axial Hall effect sensor (HGx) data.

Source: FHWA.

IC = integrated circuit.

C. Radial Hall effect sensor (HGy) data.

Figure 44. Graphs. Permanent magnet-type MMFM data of a tendon.

Although there was a low probability of finding corrosion-damaged strands buried in the grout, 

the grout could not be removed for inspection because the tendons carried the live load. Thus, the

damage detection capability of the permanent magnet-type MMFM system could not be 

determined. 

More improvements were made to the permanent magnet-type MMFM system’s hardware and 

the data analysis methodology based on the field experience. Since then, a couple of field 

projects—unrelated to the present study—established a 3.0-percent section loss as a conservative 

damage detection limit for the latest version of the permanent magnet-type MMFM system.

(24,25) 

-1.5 

-0.5 

0.5 

1.5 

2.5 

3.5 

-30

-10

10

30

50

70

90

110

130

150

170

3 5 7 9 11 13 15 17

Signal ofsearch coil (m

V)

Change of magnetic flux (V・s)

Distance (m)

SP12T10

Magnetic Flux Signal of Search Coil

20 V or 0.5 mV
46 

Discussion of MMFM Systems’ Field Performance

The field MMFM evaluations did not yield information as quantitative as the laboratory data for 

several reasons. First, the tendons tested in the Varina-Enon Bridge—except for one tendon—did 

not contain naturally occurring corrosion damages that exceeded the detection limits of the 

MMFM systems: 0.4-percent section loss by solenoid types’ point measurement method, 

1.0-percent section loss by solenoid type’s scan measurement method, and 3.0-percent section 

loss for permanent magnet type. Secondly, the PT tendons were in service with tremendous 

stress, and only exposed strands were visually inspected. As a result, the strands buried in the 

grout could not be examined by invasive methods. Lastly, the magnetizer’s inconsistent moving 

speed and extruded obstacles on the tendon surface caused signal distortions and disagreements 

between actual corrosion damage locations and those that appeared in the MMFM data. 

Nonetheless, field evaluations of the MMFM systems demonstrated that they could be used in 

the field as an effective in situ NDE tool if corrosion damage in assessable sections of external 

PT tendons exceeded the detection thresholds established for the MMFM systems. 

Some areas for improvement were identified to build efficient and practical MMFM systems 

readily available for field applications. For example, an autonomous scanning capability would 

be desirable to climb and descend the inclined tendons (and stay cables) at a constant speed. A 

commercially available motorized self-propelling system equipped with a synchronized digital 

distance encoder would be a good place to begin. A more challenging design feature for an ideal 

MMFM system may be the flexibility and adaptability designed for the ducts that have different 

diameters and extruded objects such as joint couplers, hose clamps, and air vents. 
47 

CHAPTER 4. RFM FOR INTERNAL TENDONS

This chapter describes the second FHWA study to develop and validate a laboratory prototype 

for internal PT tendons using the RFM. This in-house NDE study was launched after completing

the MMFM systems developed for physically accessible metallic objects such as external PT 

tendons and stay cables. 

PREVIOUS RESEARCH EFFORTS

The literature search of NDE technologies applicable to internal PT tendons identified research 

work performed by FIU and the University of Toledo. Since their studies explored the same 

concept of no-contact testing of the physically separated tendons as the RFM, their key findings 

are presented to understand what has been achieved before the RFM system is discussed. 

FIU Studies

FIU explored two NDE systems by using MFL and inductance methods as described in the 

following sections. 

MFL System

The FIU researchers developed two MFL systems powered by permanent magnets for internal 

tendons.(15,16) The later version—also a more sophisticated MFL system—was tested on a 

laboratory mockup and a decommissioned precast box girder segment retrieved from Ramp D, 

which was located near Fort Lauderdale-Hollywood International Airport. The laboratory 

mockup was built with a 4-inch-diameter plastic duct using PVC pipe and number 3 transverse 

rebars. Figure 45 shows the laboratory testing setup. 

© 2017 FDOT.

A. Overview.

© 2017 FDOT.

B. Plywood platform. 

© 2017 FDOT.

C. Cross section. 

Figure 45. Photos. Laboratory setup for MFL testing.(16)
48 

A 20-ft-long plywood platform was constructed, and rails were installed to guide the MFL 

carriage along with the platform (figure 45-A and figure 45-B). The clear cover between the 

platform and the duct was 2 inches (figure 45-C). The duct housed 10 0.6-inch strands containing 

three section losses of 14-, 50-, and 100-percent. The strands were magnetized by moving the 

permanent magnet over the strands in one direction several times. Figure 46 shows some 

laboratory test results: ND2 (active MFL test with undamaged strands), D2 (active MFL test with 

damaged strands), and D2−ND2 (difference of the two datasets).

© 2017 FDOT.

Figure 46. Graph. Laboratory test results obtained with the mockup.(16)

The Hall effect sensors detected disturbances of the magnetic field at the damage locations, as 

shown in figure 46. The D2−ND2 obtained with the active magnetic field method took the shape 

of a peak and valley at the defects. The observed inflection could be ascribed to many factors, 

including the defect size, magnetic strength, the distance between the Hall effect sensors and 

strands, and so on.(16) The researchers reported that their MFL system detected 50- and 

100-percent section losses, but missed the 14-percent section loss. A wide transverse rebar 

spacing (~18 inches estimated by signal peaks in the graph) might help detect the 50-percent 

section loss by avoiding magnetic interference between the transverse rebars and the strands. 
49 

Figure 47 shows the schematic of the decommissioned box girder segment that was used to test 

the MFL system in a manner similar to the laboratory experiments. 

© 2017 FDOT.

Figure 47. Illustration. Details of the decommissioned box girder segment.(16)

Galvanized 3-inch-diameter metal ducts were buried in the cantilever section of the top slab 

beneath approximately 3.5 to 4 inches of concrete cover and number 3 transverse rebars. The 

existing strands in the ducts were removed and replaced with seven new 0.6-inch strands for the 

MFL testing. Three cases of no damage (designated as SND), 100-percent section loss (SD), and 

no strands (SN) were tested with the active magnetic field method. The test strands prepared for 

the SD case were completely cut for 4 inches in the midlength to simulate complete metal loss.

Figure 48 shows three MFL datasets and the transverse rebar locations.

© 2017 FDOT.

Figure 48. Graph. MFL test results of a bridge segment.

(16)

The active MFL method identified the location of the 100-percent cut in this particular setup, 

where there was no transverse rebar at the defect location. The researchers speculated that the 

presence of a secondary ferromagnetic source (transverse rebars) could have affected the 

amplitude of signals at a smaller section loss due to incomplete magnetic saturation by the low 

magnetizing capability of the permanent magnet. In other words, disturbances and masking 

effects caused by the transverse rebars could yield complicated MFL signals that make it difficult

to detect corrosion damage in real situations. 
50 

However, the researchers also indicated that data analysis techniques could reduce the effect of 

mild reinforcement observed in the laboratory test data, and the postprocessing of the segment 

test data could identify the damage. A numerical study validated the laboratory test data. Also,

the detailed finite-element simulation could be used to improve the MFL system by determining

sensor positions and choosing an appropriate magnetic field to achieve saturation. The 

researchers concluded that using an electromagnet was a more practical approach to improve 

defect detection capabilities. There was also a need to develop MFL testing protocols specifically 

designed for PSC bridges.(16)

Inductance Measurement Method

Another laboratory study was conducted with another mockup to determine the effectiveness of 

an electromagnetic-based inductance method. The laboratory setup consisted of a 20-ft-long 

tendon mockup made with multiple strands encased in a 4-inch PVC pipe. The strands had three 

section losses along the length: a 100-percent cut at 2.5-ft, a 50-percent cut at 10-ft, and a 

14-percent cut at 16-ft. Since the penetration depth of the magnetic field around the inductor coil 

increased with the diameter of the coil, multiple inductor coil configurations were tested to find 

the best combination of penetration depth and resolution. A sheet of plywood was placed 

3 inches above the mockup tendon to support the testing equipment. Inductance measurements 

were made at uniform intervals along the strand length, as shown in figure 49.

© 2017 FDOT. 

A. Overview. 

© 2017 FDOT. 

B. Induction measurement system. 

Figure 49. Photos. Laboratory setup for induction measurement method.(16)

When inductance measurements were made in the vicinity of the defect (low inductance), a 

larger diameter coil picked up additional inductance from the undamaged section of the strands 

(high inductance). Therefore, the resolution of the inductance readings at a section loss decreased 

as the coil diameter increased. Multiple tests led to a good balance of penetration depth and 

resolution. A high-precision inductance meter produced the necessary test signal to generate the 

fluctuating magnetic field through the coil. Inductance measurement data collected at equally 

spaced points along the mockup strands matched well with the corresponding section losses, as 

shown in figure 50. 
51 

© 2017 FDOT.

Figure 50. Graph. Inductance measurement data from the laboratory mockup.(16)

Figure 51 shows an inductance measurement in progress on the decommissioned box girder 

segment.

© 2017 FDOT.

Figure 51. Photo. Modified second prototype elliptical coil used for inductance testing on 

the top slab of a precast segment.

The in situ measurements were difficult to make due to the variable clear cover and interference 

from steel components above the strands. Therefore, the coil dimensions were changed to have 

an elliptical shape such that the narrow dimension was parallel to the longitudinal axis of the 

strands, as shown in figure 51. To factor in these nonideal testing conditions, baseline inductance 

measurements were made at 107 equally spaced points along the 10-ft segment while adding one 

to six strands in the duct. The inductance measurement results are shown in figure 52.
52 

© 2017 FDOT.

Figure 52. Graph. Inductance readings along the length of the segment for an increasing 

number of strands.(16)

The spikes in inductance in figure 52 were due to transverse rebars and transverse prestressing 

strands. The decreasing inductance from left to right was caused by the increasing distance 

between the inductor probe and the strands as the metal duct slopes down (increasing clear 

concrete cover) relative to the top of the concrete. The data showed there was an increased 

inductance for the larger (more strand) samples and an increased inductance at transverse rebar 

locations. This trend suggested that if a calibration dataset were made for future numerical 

comparisons, the location of a section loss could be detected even under transverse rebars. 

The major factors affecting the inductance readings included the quality of the inductance meter, 

coil geometry, number of turns of the coil, test signal frequency, concrete cover over the tendon, 

and the metal duct’s wall thickness. More importantly, the inductance method was very sensitive 

to magnetic interference (shielding) from secondary transverse rebars. Future potential 

improvements to the inductance method include the following:

• Modifying inductor coil geometry to sharpen (localize) the inductance reading.

• Modifying the coil turns and wrapping method to increase the penetration depth of the 

test field.

• Using a more precise inductance meter with a larger voltage test signal.

• Collecting and processing datasets is also an important issue to be investigated because 

averaging several datasets may provide a better-quality dataset to cancel the data outliers 

caused by electromagnetic noise.(16)
53 

The University of Toledo Study—Induced Magnetic Field Detection (IMFD) Method

The University of Toledo published several documents related to the IMFD method in 2010 and 

2011.(26,27,28) The researchers investigated how the magnetic properties of the prestressing strands 

were affected by the onset of corrosion when the strands were magnetized using an externally 

placed electromagnet. An IMFD method field trial was also conducted using a prototype the 

University of Toledo developed as part of another concurrent project.

The researchers developed an IMFD system using a yoke-shaped electromagnet and 

accompanying Hall effect sensors. The IMFD method used the principle that when a metallic 

object is magnetized externally by using an electromagnet, the change in magnetic properties is 

reflected in the amount of magnetic flux induced in the object (figure 53 illustrates imaginary 

magnetic flux lines passing through a steel specimen induced by a yoke-shaped electromagnet). 

© 2010 University of Toledo.

Figure 53. Illustration. Magnetic flux lines passing through the steel specimen induced by a 

yoke-shaped electromagnet.(26)

Hall effect sensors were mounted on the pole faces of the electromagnet to measure the axial 

component of the induced magnetic field accurately at a particular point on the steel. 

For the laboratory study, the researchers used the AISI 1018 steel rods that had 11 

cross-sectional areas (diameter 1/16–3/4 inches; length 18–24 inches), new seven-wire strands 

that had four cross-sectional areas (diameters 1/16~3/4 inches) and corroded seven-wire strands

that were obtained from a demolished bridge (original diameter was 0.5 inches). Tests were 

conducted to study the magnetic reluctance of concrete by using different airgaps and concrete 

thicknesses for all the specimens. Hollow wooden blocks and concrete blocks were used to 

create physical separations of different sizes between the pole face and the steel specimens. 

Figure 54 shows the IMFD data from AISI 1018 steel rods and new seven-wire strand steel with 

different cross-sectional areas as a function of airgap from 0.75 to 2.0 inches.
54 

© 2010 University of Toledo.

A. AISI 1018 steel rods. B. New seven-wire steel strands.

Figure 54. Graphs. Induced magnetic field versus cross-sectional area of steel specimens as 

a function of airgap.(26)

The induced magnetic field was proportional to the cross-sectional areas of both types of 

specimens. Also, the new seven-wire steel strands exhibited a higher degree of magnetization 

than the AISI 1018 steel rods. The researchers attributed this to the higher carbon content in the 

seven-wire steel strands and suggested that the metallurgy affected the amount of magnetic field 

induced in the steel specimens. Similar data obtained from five different thicknesses of concrete 

blocks are plotted in figure 55. 
55 

© 2010 University of Toledo.

A. AISI 1018 steel rods. B. New seven-wire steel strands.

Figure 55. Graphs. Induced magnetic field versus cross-sectional area of steel specimens as 

a function of concrete thickness.(26)

A comparison between the induced magnetic field data presented in figure 54 and figure 55

showed that concrete had a slight damping effect on the magnetization for both types of steel 

specimens. Also, the seven-wire steel strands had a higher amount of induced magnetic flux in 

concrete. Finally, the magnetic field induced in the steel specimens could be related to the 

corresponding cross-sectional area, regardless of airgaps and concrete block sizes.

Figure 56 shows a corroded seven-wire strand specimen’s estimated cross-sectional areas 

determined by the IMFD method and actual cross-sectional areas measured along its length. 

© 2011 University of Toledo.

Figure 56. Graph. Estimated cross-sectional areas of the corroded seven-wire strand versus 

actually measured cross-sectional areas.(27)
56 

Two curves representing variations of remaining cross-sectional areas estimated by the IMFD 

method using two concrete blocks (1.0 and 1.8 inches thick) overlapped closely with each other 

in many measurement locations. Furthermore, the degree of agreement between the estimated 

and actually measured cross-sectional areas was reasonably high. 

After the successful development of the IMFD prototype in the laboratory, a field trial of the 

prototype was conducted to validate the prototype’s performance on the soon-to-be-demolished 

Washington Waterloo Road Bridge in 2010. After the bridge was demolished, the strands were 

extracted and visually assessed for their effective cross-sectional areas. A commercially available 

MFL system was also employed for comparison. Figure 57 shows both types of equipment being 

used under the PSC box beams.

© 2011 University of Toledo. 

A. IMFD prototype. 

© 2011 University of Toledo. 

B. MFL equipment. 

Figure 57. Photos. Field trial of two NDE systems.(28)

When the field IMFD data and actual section losses were compared, it was apparent that the 

IMFD method needed to improve accuracy (in the laboratory, it could be accurate within 

±7-percent of the cross-sectional area). Even though the researchers had advanced the IMFD 

method to a level where theoretical ability had been proven, further refinement would be 

necessary to improve its accuracy of estimating section loss. On the other hand, the MFL system 

could detect hidden corrosion damage and strand breaks with sufficient accuracy. 

The researchers identified the following areas for improvement to obtain accurate readings and 

upgrade physical features: 

• It is necessary to locate the strands as closely as possible, and the distance between the 

strands and the Hall effect sensors mounted on the electromagnet pole faces should be 

known accurately. 

• It would be desirable to have stronger magnetization, even though it would require a 

heavier electromagnet. 

• It is recommended to use a thermocouple to monitor the temperature and determine the 

magnet’s ultimate strength for specific test scans. This step is recommended because 

when the sensor system was turned on for a considerable amount of time, the 

electromagnet’s self-heating caused an observed drift in the measured values and variable 

strength of the electromagnet.

• It is recommended to suspend a good support system under the bridge to enable the 

IMFD system to move along the length of the box beam in a controlled manner, 

magnetize the strands, and take measurements. Its heavy weight was a problem in the 

field when performing a scan under the bridge.

Permanent Magnet Permanent 

Magnet

Hall Sensors
57 

PRINCIPLE OF RFM

When a solenoid-type magnetizer for RFM is placed in the air over a metal object and is 

energized, a closed magnetic loop is formed around the metal object. Figure 58 illustrates this 

situation with a hypothetical case of an internal tendon containing the damage. 

Source: FHWA. 

Figure 58. Illustration. Principle of RFM. 

The airgap between a pair of yokes and the tendon can be replaced with a clear concrete cover in 

real PT structures. The magnetic flux passes through yoke number 1, enters the internal tendon 

via an airgap or concrete as the main flux (φM), and then returns to yoke number 2 through the 

other airgap or concrete. A portion of the magnetic flux leaks into the air (φA), travels in the air, 

and enters back to yoke number 2. Also, some of the φM can leak at the tendon damage into the 

air, and its magnitude depends on the size of the damage. The sum of φM and φA constitutes the 

return flux (φR). If the gap is very small and φA is negligible, φM and φR become equal. 

To compare φM and φR as a function of section loss, a small experiment was carried out with four 

seven-wire strands containing 0-, 25-, 50-, and 75-percent section losses via a fixed airgap of 

0.8 inches. Figure 59 shows the results graphically.
58 

Source: FHWA. 

Figure 59. Graph. Relationship between magnetic flux and cross-sectional area. 

If section loss increased, reduction of main flux (ΔφM) increased proportionally. A similar

proportionality was observed for the reduction of return flux (ΔφR), but a lesser degree, due to 

increasing amount of air leaked flux (ΔφA). As a result, the difference between ΔφM and ΔφR

grew proportionally as the section loss increased. Therefore, the size of the corrosion damage 

affects the accuracy of the RFM at a fixed airgap unless there is a measure to minimize the φA. 

DEVELOPMENT OF RFM PROTOTYPES

TRM was responsible for designing, fabricating, and testing four prototypes during the study. 

The engineers built two identical units of each prototype: one for the TRM laboratory in Japan 

and the other for the FHWA NDE laboratory. As the study progressed, the units kept in the 

FHWA NDE laboratory were constantly updated whenever any changes and improvements were 

made to the units left in Japan. TRM research staff visited the FHWA NDE laboratory six times 

to update the prototypes and perform necessary tests during the 21-mo contract period. 

Soon after the study began, the first RFM magnetizer prototype was built to determine the 

optimum configuration of solenoid coils and yokes. The magnetizer had two core configurations: 

a single coil between two 2-inch-thick rectangular-shaped yokes and two identical solenoid coils 

in parallel between two 2-inch-thick inverted-Y-shaped yokes. Figure 60 shows the core 

configurations schematically. 

0

10

20

30

40

50

60

70

80

0 10 20 30 40 50 60 70 80

Reduction of flux (percent)

Loss of metallic area (percent)

Main flux Retun flux
59 

Source: FHWA. 

Figure 60. Illustration. Schematic of the first two prototypes. 

The prototypes were designed for yoke number 1 to send magnetic flux into a metallic object 

(tendon) through the air and yoke number 2 to receive magnetic flux through the air. The overall 

dimensions of the coil were 4.0 inches in diameter and 10.2 inches in length. A search coil was 

wrapped around the tip of yoke number 1, and an HGx was mounted on yoke number 2. A search 

coil (search coilM, where “M” is “main flux”) and an HGx (HGxM) were also installed on the 

surface of a mockup internal tendon made with a 4-inch plastic pipe containing a varying number 

of steel strands. Different airgaps between the tips of the yokes and the mockup were tried using 

2- by 4-inch wooden pieces. Even though this arrangement was not realistic for an NDE system 

intended for internal PT tendons, changes in the sensor location, steel mass, and the airgap 

helped the researchers understand how magnetic flux flew in the noncontact metal object under 

the given experimental setups. 

Preliminary test results obtained with both configurations indicated that most of the magnetic 

flux bypassed the mockup tendon and transmitted from yoke number 1 to yoke number 2 through 

the air, irrespective of coil configuration. From these findings, it was concluded that the coil 

length was too short to minimize magnetic flow through the air. 

The second magnetizer prototype was fabricated with a pair of 2.95-inch-thick 

rectangular-shaped yokes and two 4-inch-diameter solenoid coils in series between the yokes. 

This configuration elongated the coil length to 20.6 inches to minimize the φA by increasing 

resistance in the air between the yokes. A stainless-steel rod was also added between the yokes to 

mount an HGx in the middle of the magnetizer. Also, a new HGy (HGy1) was added close to the 

search coil installed on yoke number 1. 

Based on the test results obtained with the second prototype, the third prototype was made with

the following additional changes: 

• The diameter of the solenoid coils was increased to 4.7 inches. 

• The second search coil was added to the tip of yoke number 2. 
60 

• The third Hall effect sensor (HGy) for measuring MFL was added in the middle of the 

magnetizer.

After repeating the experiments on the tendon mockup, the fourth and final magnetizer prototype 

was developed. The diameter and length of the solenoid coils in series remained the same as the 

third prototype, that is, 4.7 and 20.6 inches, respectively. Each yoke consisted of three stacked 

steel plates. The clearance between the bottom of the solenoid coils and the tip of the yoke was 

3.3 inches. The weight of the assembled prototype was 218 lb. Figure 61 shows the following

individual components and their functionalities schematically: 

• Yoke number 1 and yoke number 2 was for incoming and returning magnetic flux, 

respectively.

• Search coil number 1 on the tip of yoke 1 was to measure return magnetic flux, φ1. 

• Search coil number 2 on the tip of yoke 2 was to measure return magnetic flux, φ2. 

• HGy near search coil number 1 was to measure vertical magnetic intensity, HGy1. 

• HGy near search coil number 2 was to measure vertical magnetic intensity, HGy2. 

• HGy in the middle of the crossbar was to measure MFL, HGy. 

• An HGx in the middle of the crossbar was to measure horizontal magnetic intensity, HGx. 

• An encoder distance meter was to measure and record the traveled distance of the 

magnetizer.

Source: FHWA. 

Figure 61. Illustration. Schematic of the fourth prototype and its functionality. 
61 

EXPERIMENTAL PROCEDURE

During the development of the prototypes, three specimen types were employed: a laboratory

mockup to mimic a web section of a segmental box girder, a real precast box girder segment, and 

a grouted external tendon specimen. This section describes the experimental work. 

Test Specimens

First Laboratory Mockup

A simulated concrete web mockup was designed and built with lumbers and sheets of plywood 

to avoid interference with the electromagnetic field coming from the magnetizer. Its overall 

dimensions were 16-ft (length) by 4-ft (height) by 19.0-inches (web width). It was supported by 

a 16- by 5-ft wooden floor. Six 6-inch heavy-duty wheels were attached under the floor.

The design of the mockup included the following key features of a typical segmental box girder 

containing internal tendons: 

• Two duct materials: metal and plastic.

• A changeable separation (clear cover) between the internal tendons and the magnetizer. 

• A slightly draped tendon profile. 

• Vertical and horizontal mild rebars.

• A realistic number (18) of 0.6-inch seven-wire strands. 

• A real anchorage system, including a wedge plate, a bearing plate, a transition tube, and 

spiral confinement reinforcement. 

Figure 62 shows a bird’s-eye view of the mockup with vertical rebars sticking out of the web 

wall and a magnetizer on a height-adjustable cart, a pair of aluminum guiding rails on the 

wooden floor, and testing equipment (figure 62-A), an interior view of the mockup (figure 62-B), 

and the anchorage zone (figure 62-C). Figure 63 shows schematics of the laboratory mockup.

Source: FHWA. 

A. Exterior view. 

Source: FHWA.

B. Interior view. 

Source: FHWA.

C. Anchorage view. 

Figure 62. Photos. First laboratory mockup.
62 

Source: FHWA. 

Figure 63. Illustration. Schematics of first laboratory mockup (not to scale). 

As shown in figure 62 and figure 63, two internal tendons were installed inside the web. The 

upper tendon used a 4-inch-diameter corrugated polyethylene (PE) duct, and the lower one used 

a 3.5-inch-diameter galvanized metal duct. Each tendon was approximately 21-ft long and was

rested on four crossbeams. A bulky bearing plate and spiral confinement reinforcement resulted 

in an increasing distance between the magnetizer and a tendon section in the anchorage zone. 

Therefore, the distance from the ducts to the vertical wall’s exterior face was adjusted by sliding 

the tendons laterally over the crossbeams. Actual distances at several tendon locations are 

included in the plan views in figure 63. In addition, the cart heights were fixed at 30.0 inches for 

the upper tendon and 13.5 inches for the lower tendon to make the centers of the magnetizer’s 

yoke plates and the tendons aligned. 

A network of 3/4-inch-diameter PVC pipe was used to add and remove number 5 rebars as 

vertical and horizontal rebars in the mockup. The PVC pipe network allowed the quick 

transformation of the rebar configuration during the RFM measurements. 

Each mockup tendon contained 18 strands without grout. Seven levels of artificial section loss 

(27.6-, 18.4-, 16.0-, 12.3-, 9.2-, 6.1-, and 3.1-percent) were introduced in the free length zone to 

simulate a wide range of corrosion damage encountered in real PT tendons. The same damages 

 

 

 
63 

are also introduced in the anchorage zone. Each artificial damage was 6.0 inches long, and the 

center-to-center distance between two adjacent damage locations was 18 inches. 

Second Laboratory Mockup 

In 2019, additional laboratory testing was performed at TRM’s research center in Tsuchiura, 

Japan, after two more Hall effect sensors (HGy3, HGy4) were added to the latest RFM magnetizer 

prototype. Figure 64 shows the locations of the added Hall effect sensors. 

Source: FHWA. 

Figure 64. Illustration. Schematic of the modified RFM magnetizer.

The modified magnetizer was evaluated on a second laboratory mockup that simulated a 

16-ft-long section of a box girder bridge deck. It was constructed with two 4- by 8-inch sheets of 

plywood, 20 number 6 transverse rebars, a 3.25-inch galvanized metal duct, supporting plastic 

blocks and aluminum bars, and three bundles of 0.6-inch-diameter strands. A pair of aluminum 

rails and an electrical winch allowed the magnetizer to travel straight over the metal duct. An 

encoder distance meter registered the magnetizer’s precise travel distance into the RFM data. 

Figure 65 shows the schematic of the second laboratory mockup testing. 
64 

Source: FHWA. 

Figure 65. Illustration. Schematic of the second laboratory testing setup. 

The following test variables were included: 

1. Duct material: galvanized 3.25-inch-diameter metal duct.

2. Number of seven-wire strands in three bundles: 5, 10, 15. 

3. Number and spacing of ordinary rebars in the transverse direction (perpendicular to the 

duct): 20 at 4-inch spacing. 

4. Clear cover (airgap) to the transverse rebars: 2.0, 4.0, 6.0 inches. 

5. Clear cover from the top of the duct to the bottom of the yokes: 7.0 inches. 

6. Clear cover from the top of the strands to the bottom of the yokes: approximately 

8.5 inches. 

7. Artificial damage level: 6.7-, 10.0-, 20.0-percent section loss. 

Three identical artificial damages were introduced by cutting one strand completely in each 

bundle. Such a damage scheme introduced different damage levels in the strand bundles: 

6.7-percent section loss for the 15-strand bundle; 10.0-percent section loss for the 10-strand 

bundle; and 20.0-percent section loss for the five-strand bundle. In addition, extra measurements 

were made for 13.3- and 40.0-percent section losses in some cases. The cut length of each 

damage was 6 inches, and segmented strand pieces were held together with electrical tape. Some 

of the transverse rebars intersected with the artificial damage. Figure 66 shows photographs of 

the experimental setup (figure 66-A) and a closeup view of one artificial damage location 

(figure 66-B). 

 

 

 

 

 

 

 

 
65 

Source: FHWA. 

A. RFM testing setup. 

Source: FHWA. 

B. Artificial damage. 

Figure 66. Photos. Second laboratory mockup. 

Once the mockup was constructed, a series of RFM testing was conducted in sequence by 

changing the rebar cover and the number of the strands/section loss. Final RFM testing was 

performed without the strands (i.e., the metal duct only).

Grouted Field Tendon Specimen 

The tendon C specimen was employed again as a simulated grouted internal tendon. It was 

placed in the original position of the metal duct after the latter was pushed aside to the other 

vertical wall. The gap between the specimen’s center and the vertical wall’s exterior surface was 

8.0 inches (clear cover 6.0 inches). Arrows in figure 67 indicate the tendon specimen. 

Source: FHWA.

A. End view. 

Source: FHWA. 

B. Plan view from the top of the mockup. 

Figure 67. Photos. Tendon C specimen installed inside the mockup. 

The specimen’s previously introduced saw cut damages—12-wire cut (9.0-percent section loss), 

7-wire cut (5.3-percent section loss), 1-wire cut (0.8-percent section loss), and 1 shallow saw cut 

(unknown section loss)—were positioned at 32.0 to 35.0, 35.0 to 43.5, 59.0 to 64.0, and 

Magnetizer

Rail
66 

85.5 inches, respectively. The distance was measured from the end of the free length zone. The 

tendon specimen was scanned first without vertical rebars and then with them at 6-inch spacing. 

Precast Concrete Box Girder

After the first round of experiments was completed in the FHWA NDE laboratory, the 

prototypes were taken to a full-size precast box girder segment stored in the TFHRC’s backyard. 

Initially, all internal tendons were empty. In preparation for the RFM testing, four 3.5-inch 

internal metal ducts were chosen. The clear concrete cover was 6.0 inches. Each duct was 

inserted with three to six 0.6-inch seven-wire strands. Each strand bundle received one of four 

damage levels: no damage (0-percent), minor damage (1-percent section loss), moderate damage 

(10-percent section loss), and severe damage (30-percent section loss). Figure 68 shows a front

view of the box girder specimen (figure 68-A) and damage information (figure 68-B).

Source: FHWA. 

A. Front view. 

Source: FHWA. 

B. Damage information. 

Figure 68. Photo and illustration. Precast box girder specimen. 

After installing the strands, the ducts were either ungrouted or partially grouted. Artificial grout 

voids were also introduced in two of four ducts by wrapping strands with different sizes of 

styrene foam. In addition, several damaged strands were installed in two interior blisters and then 

filled with grout.
67 

Experimental Setup and Testing

The RFM system requires a more complicated measurement system than the MMFM systems. 

Figure 69 illustrates an overview of the RFM system components. 

Source: FHWA. 

Figure 69. Illustration. Schematic of the RFM system components. 

Each solenoid coil had its own power supply and polarity switch. These power supplies were 

connected to a function generator. Their current monitor outputs were connected to a data logger, 

and three homemade sensors (HGy, HGy1, HGy2) were connected directly to the data logger. A 

commercial Hall effect probe (HGx) and two search coils (φ1, φ2) were also connected to the same 

data logger through a gauss meter and flux meters, respectively. Then, the data logger was 

connected to a laptop computer for real-time data collection and subsequent data analysis. The 

last component of the RFM system was a wire-type encoder distance meter that allowed accurate 

recording of the magnetizer movement. 

At the beginning of the first laboratory mockup testing, researchers investigated the effect of the 

six horizontal rebars running parallel to the PE and metal ducts. The RFM data obtained with and 

without the horizontal rebars were not much different. This test result suggested that adding the 

small mass of the horizontal rebars did not contribute significantly to the original magnetic field. 

On the other hand, the preliminary test results clearly showed the strong influence of vertical 

rebars. Based on these results, the first RFM testing was performed without vertical rebars, 

followed by the second testing after adding vertical rebars at 6-inch spacing. The horizontal 

rebars were always kept in their positions. For each test condition, two consecutive scans were 

made to obtain duplicate RFM data.

Hall effect sensor (probe)
68 

Figure 70 shows the RFM magnetizer parked at the starting position of the first laboratory 

mockup before scanning the PE duct (figure 70-A) and a scan measurement in progress near the 

anchorage zone (figure 70-B).

Source: FHWA. 

A. Starting position. 

Source: FHWA. 

B. Scan measurement in progress. 

Figure 70. Photos. RFM testing on the first laboratory mockup. 

Figure 71 shows a scan measurement over blister number 1 (figure 71-A) inside the box girder 

and another scan measurement over an internal duct embedded in the box girder’s deck section

(figure 71-B).

Source: FHWA. 

A. Scanning over blister number 1. 

Source: FHWA. 

B. Scanning over an internal tendon. 

Figure 71. Photos. RFM testing on the box girder specimen. 

To scan the ducts smoothly, detachable fittings, such as crossbars and casters, were added to the 

magnetizer, as shown in figure 71-B. These added fittings increased the magnetizer’s weight to

250 lb. The clearance between the bottom of the yokes and the concrete surface was 0.35 inches, 

including the rail thickness of 0.08 inches.
69 

TEST RESULTS AND DISCUSSION

With four RFM magnetizer prototypes being developed, more than 200 test data were collected 

over 18 mo. Most of the data were collected primarily for validating the performance of the 

progressively improved prototypes. Thus, this section presents key findings of the latest RFM

data obtained with the final prototype. The RFM data consist of six graphs showing variations of 

HGx, MFL, φ1, HGy1, HGy2, and φ2 as a function of scan distance. 

First Laboratory Mockup 

The analysis of the RFM data indicated that four graphs (HGx, MFL, φ1, and φ2) provided more

conspicuous changes than the others (HGy1 and HGy2) when the RFM detected the damage. The 

following section discusses some of the representative RFM data obtained from the first 

laboratory mockup. (Note: whenever damage was detected in the RFM data, a red ellipse was 

placed at the identified location on the graphs.) 

The RFM data from the PE duct and metal duct in the free length zone without vertical rebars are 

shown in figure 72 and figure 73, respectively. 
70 

Source: FHWA.

Figure 72. Graphs. RFM data from the PE duct in free length zone with no vertical rebars: 

27.6-percent at 14.5-ft was detected. 
71 

Source: FHWA.

Figure 73. Graphs. RFM data from the metal duct in the free length zone with no vertical 

rebars: 27.6-percent at 14.5-ft was detected. 

Among three damage levels (27.6-percent at 14.5-ft, 6.1-percent at 16.0-ft, and 3.1-percent at 

17.5-ft), the largest damage could be detected in both cases. Red ellipses indicate the damage 

appeared in HGx, MFL, φ1, and φ2 graphs. Interestingly, there was a lapse of 0.5-ft between φ1

and φ2 due to the separation of yoke number 1 and yoke number 2 to pick up the magnetic flux.

Similar RFM data from the PE duct and metal duct containing four damages (15.3-percent at 

14.5-ft, 12.3-percent at 17.5-ft, 6.1-percent at 19.0-ft, and 3.1-percent at 20.5-ft) are shown in 

figure 74 and figure 75, respectively. 
72 

Source: FHWA. 

Figure 74. Graphs. RFM data from PE duct in the free length zone with no vertical rebars: 

15.3-percent at 14.5-ft and 12.3-percent at 17.5-ft were detected. 
73 

Source: FHWA. 

Figure 75. Graphs. RFM data from the metal duct in the free length zone with no vertical 

rebars: 15.3-percent at 14.5-ft and 12.3-percent at 17.5-ft were detected. 

The two largest damage levels (15.3- and 12.3-percent) were detected in both cases, as indicated 

by red ellipses in the HGx, MFL, φ1, and φ2 graphs. Again, there was a lapse of 0.5-ft between φ1

and φ2. 

After adding vertical rebars at a 6-inch spacing, the follow-up experiments were conducted to see 

how the RFM data would change. The RFM data of the PE duct and metal duct in the free length 

zone with vertical rebars containing the same damage levels, such as the one shown in figure 72

and figure 73 (27.6-percent at 14.5-ft, 6.1-percent at 16.0-ft, and 3.1-percent at 17.5-ft), are 

shown in figure 76 and figure 77, respectively. 
74 

Source: FHWA. 

Figure 76. Graphs. RFM data from the PE duct in free length zone with vertical rebars at 

6-inch spacing: 27.6-percent damage was detected at 14.5-ft. 
75 

Source: FHWA. 

Figure 77. Graphs. RFM data from the metal duct in the free length zone with vertical 

rebars at 6-inch spacing: 27.6-percent damage was detected at 14.5-ft. 

Despite wavy data signals due to electromagnetic interference at each of intersecting vertical 

rebar locations, the largest damage, 27.6-percent, could still be detected at 14.5-ft in both cases. 

The distance between φ1 and φ2 was somewhat reduced to less than 0.5-ft in the presence of 

vertical rebars. 

The RFM data from the PE duct and metal duct in the free length zone with vertical rebars 

containing the same damage levels as the ducts without the rebars shown in figure 74 and 

figure 75 (15.3-percent at 14.5-ft, 12.3-percent at 17.5-ft, 6.1-percent at 19.0-ft, and 3.1-percent 

at 20.5-ft) are shown in figure 78 and figure 79, respectively. When the vertical rebars are 

present, 15.3-percent could still be detected, but missed the lower damage levels (i.e., 12.3-, 6.1-, 

and 3.1-percent). Although the 12.3-percent section loss was detected in HGx and MFL graphs, 

their outputs were small. A distance of approximately 0.5-ft appeared again between φ1 and φ2. 
76 

Source: FHWA. 

Figure 78. Graphs. RFM data from the PE duct in the free length zone with vertical rebars 

at 6-inch spacing: 15.3-percent at 14.5-ft was detected. 
77 

Source: FHWA. 

Figure 79. Graphs. RFM data from the metal duct in the free length zone with vertical 

rebars at 6-inch spacing: 15.3-percent at 14.5-ft was detected. 

It was obvious that the presence of vertical rebars affected the RFM data in two ways. The first 

change was wavy data signals due to electromagnetic interference at each of intersecting vertical 

rebar locations. The second change was the decreased detection capability, contrary to the 

insignificant effect of the horizontal rebars. 

Figure 80 shows the RFM data from the metal duct in the anchorage zone as having three 

damage levels with vertical rebars at 6-inch spacing. Although HGx, φ1, and MFL graphs 

indicated a possible detection of 27.6-percent section loss at 5.0-ft, it was not conclusive. 
78 

Source: FHWA. 

Figure 80. Graphs. RFM data of the metal duct in the anchorage zone with vertical rebars

at 6-inch spacing. 

The RFM data collected in the anchorage zone could not detect artificial damage, regardless of 

damage level or adding vertical rebars. This observation was somewhat predicted even before 

this study began. Consequently, test results pertaining to the anchorage zone are not discussed

further in this report. 

Second Laboratory Mockup 

Figure 81 shows the center magnetic field (HGx) data of the second mockup without any strands 

in the duct. 
79 

Source: FHWA. 

Figure 81. Graph. HGx graph for the metal duct without any strands under two gaps.

When the metal duct did not contain any strands and the airgap (clear cover) between the 

transverse rebars and the bottom of the yokes was 2 inches, the presence of the transverse rebars 

could be seen clearly. As the airgap increased to 4 inches while the other conditions remained the 

same, the transverse rebars were nearly unrecognizable. Figure 82 shows other HGx data of the 

mockup when the five-strand bundle containing three 20-percent section losses was present in 

the duct. The columns in figure 82 represent the length and location of the damages. 

Source: FHWA. 

Figure 82. Graph. HGx graph for the five-strand bundle containing three 20-percent section 

losses under different rebar covers.

-150

-140

-130

-120

-110

-100

-90

-80

1.5 2.5 3.5 4.5 5.5 6.5 7.5 8.5 9.5 10.5

Magnetic field (HGx, G)

Distance from center of magnetizer (ft)

Gap = 2 in.

Gap = 4 in.

-1500

-1400

-1300

-1200

-1100

-1000

-900

-800

1.5 2.5 3.5 4.5 5.5 6.5 7.5 8.5 9.5 10.5

Magnetic field (HGx, G)

Distance from center of magnetizer (ft)

Gap = 2 in.

Gap = 4 in.

Gap = 6 in.

No rebars
80 

When the transverse rebars were absent, all three 20-percent damages were detected, even 

though the strands were 8.5 inches beneath the magnetizer. Conversely, when the rebars were 

present, electromagnetic interference hid some of the damages. This trend was more apparent 

when the rebars were closer to the magnetizer (smaller gap). As observed in figure 81, the 2-inch

gap data showed a clear pattern of the transverse rebar array picked up by the RFM, but the 

electromagnetic interference created by the transverse rebars obscured the artificial section 

losses. It was particularly apparent when the rebars were close to the magnetizer (e.g., when the 

gap was 2 inches) and the damage level was small. 

Researchers learned that the change of φ1 measured at the tip of the yoke number 1 and HGx

measured between the yokes were the critical parameters for detecting a section loss. For 

example, if a section loss exceeding 13-percent was present in the 15-strand bundle, the HGx

could identify it, regardless of the airgap. However, the φ1 could identify the same level of 

damage only if the airgap exceeded 4 inches. 

Figure 83 shows the polynomial regression analysis results of the relationship between the 

number of strands and airgap for HGx. The R2 was nearly 1.0 in every case, indicating a high 

degree of agreement between the variables.

Source: FHWA. 

Figure 83. Graph. Polynomial regression analysis results for HGx. 

For the 15-strand case, the most substantial reduction (18.3-percent) of the magnetic field 

occurred for the 2-inch gap compared with 9.0 percent (4-inch gap) and 4.3-percent (6-inch gap). 

If the number of strands decreased to 10, these numbers slightly increased to 18.5-percent (2-

inch gap), 9.3-percent (4-inch gap), and 4.6-percent (6-inch gap), respectively. These 

relationships suggested that the magnetic field does not change proportionally to different 

airgaps. 
81 

Grouted Field Tendon Specimen

Figure 84 shows the RFM data from the Varina-Enon Bridge tendon specimen with no vertical 

rebars. The actual distance from the mockup’s exterior wall to the specimen center was 

8.0 inches (or a clear cover of 6.0 inches).

Source: FHWA. 

Figure 84. Graphs. RFM data from the Varina-Enon Bridge tendon specimen with no 

rebars. 

The location of a detected section loss is indicated with red ellipses at around 3-ft. This damage 

corresponded to two adjoining section losses of 5.3- and 9.0-percent if the larger section loss was 

assumed to dominate the output. The 0.8-percent section loss and saw-cut damage could not be 

detected. 

The RFM data from the same specimen with vertical rebars at 6-inch spacing showed wavy 

signals compared with those in figure 84 and did not detect all section losses. 
82 

Precast Concrete Box Girder

The RFM data did not show any artificial damage hidden in two blisters and four internal ducts 

of the box girder. Figure 85 shows an example of the poor-quality RFM data, which missed the 

largest damage of 27.6-percent section loss in duct number 4. 

Source: FHWA. 

Figure 85. Graphs. RFM data from duct number 4 in the box girder. 

There were possible causes of the inability to locate even the largest artificial damage embedded 

in the precast box girder. The apparent cause was the electromagnetic interference from the 

ordinary transverse rebars. According to the rebar mapping done with GPR, the blisters also 

contained many rebars around the concrete surface, and the box girder deck slab contained

fifteen number 6 transverse rebars perpendicular to the ducts and eight number 5 longitudinal 

rebars running parallel to the ducts. Moreover, these identified rebars were not equally spaced. 

Therefore, some tightly spaced transverse rebars intersecting the ducts must have impeded the 

damage detection capability of the RFM. 
83 

Also, the number of strands (i.e., steel mass) installed in the box girder might have been too 

small. A small-scale experiment was performed to investigate the effects of the airgap and the 

number of 0.6-inch seven-wire strands on the RFM data. Figure 86 presents the test results: The 

RFM parameters (HGx, MFL, φ1, φ2) were greatly influenced by the airgap between the strands 

and the magnetizer. 

Source: FHWA. 

Figure 86. Graphs. Relationship between RFM parameters and number of strands as a 

function of the airgap. 

When the gap exceeded 1.4 inches, the magnetic field and flux decreased significantly, 

regardless of the number of strands. However, it was still possible to get the identifiable outputs 

except for one case of the four-strand (the smallest mass) at the largest gap of 6.9 inches. 

According to the data presented in figure 86, the RFM may require a minimum steel mass 

equivalent to eight 0.6-inch seven-wire strands and an airgap less than 7.0 inches to be a working 

NDE system for internal tendons. 

In summary, the RFM data from the PE duct and the metal duct were nearly identical under the 

same testing condition in the free length zone of the first laboratory mockup. Therefore, the same 

lowest detection limits were set for both duct materials: 9.0-percent section loss without vertical 

rebars and 15.3-percent section loss with vertical rebars spaced at 6 inches or wider. These limits 

were valid as long as the clear concrete cover (or airgap) was less than 6.5 inches for the PE duct 

and 7.5 inches for the metal duct. When similar tests were conducted with 3-inch spaced vertical 

rebars for the PE duct, only the largest damage level, 27.6-percent, could be identified in the free 

length zone. 

 

 

 

 

 
84 

Table 6 and Table 7 summarize the detection capabilities for the first laboratory mockup and 

Varina-Enon Bridge tendon sample, respectively. 

Table 6. Summary of detection capabilities for the first laboratory mockup. 

Duct 

Material Zone

Vertical 

Rebar 

Spacing

27.6% 

SL

18.4% 

SL

15.3% 

SL

12.3% 

SL

6.1% 

SL

3.1% 

SL

PE Free 

length No rebars Detected Detected Detected Detected UND UND

PE Free 

length 6 inches Detected Detected Detected UND UND UND

PE Free 

length 3 inches Detected Not tested

Not 

tested

Not 

tested UND UND

PE ANC No rebars UNC UNC UNC UND UND UND

PE ANC 6 inches UND UND UND UND UND UND

PE ANC 3 inches UND Not 

tested

Not 

tested

Not 

tested

Not 

tested

Not 

tested

Metal Free 

length No rebars Detected Detected Detected Detected UND UND

Metal Free 

length 6 inches Detected Detected Detected UND UND UND

Metal Free 

length 3 inches Detected Not tested

Not 

tested

Not 

tested

Not 

tested

Not 

tested

Metal ANC No rebars UND UND UND UND UND UND

Metal ANC 6 inches UNC UNC UNC UND UND UND

Metal ANC 3 inches UNC Not 

tested

Not 

tested

Not 

tested

Not 

tested

Not 

tested

SL = section loss; ANC = anchorage; UNC = uncertain; UND = undetected. 

Table 7. Summary of detection capability for the Varina-Enon Bridge tendon specimen.

Duct 

Material Zone

Vertical 

Rebar Spacing 9.0% SL 0.8% SL Note

PE Free length No rebars Detected UND 5.3% SL next to 9.0% SL

PE Free length 6 inches UND UND — 

— No data.

SL = section loss; UND = undetected.

Even though the RFM could not detect damage in the anchorage zone in any of the investigated 

conditions, the laboratory test results obtained with the final prototype magnetizer in the free 

length zone should be considered successful compared with the research outcomes from similar 

previous studies. All the magnetic-based NDE methods that were tried for internal PT tendons 

are still in an early stage regarding field applicability. Still, available laboratory data supported 

the hypothesis that the RFM system was superior to the MFL and inductance method (FIU) and 

IMFM (University of Toledo).

86 

CHAPTER 5. CONCLUSIONS

The solenoid-type MMFM system was the most accurate NDE system for external PT tendons 

despite the time-consuming preparation work in the field. Considering potential irregularities in 

the field, the conservative damage detection limits would be 0.4-percent section loss for the point 

measurement method and 1.0-percent section loss for the scan measurement method. 

On the other hand, the permanent type MMFM system was suitable for locating potential 

problem areas containing more than 3.0-percent section loss in external PT tendons. Ideally, both 

MMFM systems can be employed in sequence during the field investigations: The permanent 

magnet type identifies potentially problematic areas by quickly scanning all suspected tendons, 

followed by the point measurements in the suspicious areas with the solenoid type. 

An RFM prototype was developed for the internal tendons. It reached the immediate goal of 

detecting relatively small section losses hidden in internal tendons in laboratory environments 

and successfully proved its feasibility. Specifically, the prototype could detect a section loss 

larger than 15.3-percent in internal mockup tendons surrounded by vertical rebars at 6-inch or 

wider spacing when the clear concrete cover was less than 7.5 inches for metal ducts and 

6.5 inches for PE ducts. Without interference from the vertical rebars, as little as 9.0-percent 

section loss can be identified under identical testing conditions. When similar tests were 

conducted with 3-inch spaced vertical rebars for the PE duct, only the largest damage level, 

27.6-percent, could be identified in the free length zone. However, poor results were obtained 

from the anchorage zone of the laboratory mockup and the box girder specimen. 

It is recommended that the present RFM prototype be further improved to make it a 

field-deployable NDE system by reducing the weight and size of the magnetizer. Apart from 

hardware improvements, new data analysis methodologies should also be developed to identify 

electromagnetic interference from surrounding rebars. 

88 

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