University of Delaware, 1999

Detection and Characterization ofCorrosion of Bridge Cables byTime Domain Reflectometry

Wei Liu, Robert Hunsperger, Kevin Folliard, Michael Chajes,Jignesh Barot, Darshan Jhaveri, Eric Kunz

Dept. of Electrical & Computer Engineering, Univ. of Delaware

Dept. of Civil & Environmental Engineering, Univ. of Delaware

VETEK System Corp.

University of Delaware, 1999

Outline

• Introduction

• Transmission Line

• Modeling Bridge Cables

• Time Domain Reflectometry (TDR)

• Experimental Results

• Conclusions

University of Delaware, 1999

Introduction

• Corrosion of metallic reinforcement is amajor threat to aging infrastructure

• Current NDE methods

V isua l Mechanica l E lectrica l

N D E

University of Delaware, 1999

A new NDE technique usingTime Domain Reflectometry

(TDR)• TDR has traditionally been used to detect

discontinuities in a transmission lineTransmission line• Steel cable

Defects Discontinuities• TDR can detect, locate and identify the

extent of defects

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What is a transmission line?

• A wave guiding system which consists oftwo or more parallel conductors

• Examples: telephone lines, television cables

• What is the difference between transmissionlines and conventional circuits? --- SIZE

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Transmission Line

Distributed parameter equivalent circuitZ Z+ ∆ Z

R ∆ Z L ∆ Z

G ∆ Z C ∆Z

Z+∆ZZ

An incremental length of transmission line

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Characteristic Impedance

CjG

LjRZ

ωω

++=0

Discontinuity: Impedance Mismatch

0ZZ ≠

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Modeling Bridge Cables

Steelcable

Sensorwire

Grout

Tube

abd

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Characteristic Impedance

)(cosh2

22221

abbad

C−−−

= πε

CGandLR ωω <<<< ,

−−=≈ −

abbad

CLZ

2cosh

21 222

10 ε

µπ

Physical Defects Impedance Mismatch

)(cosh2 2

2221

abbadL −−−=

πµ

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Small Changes of Dimension

( )( ) ( ) 4222222

2220

2

121

aabdbd

abd

ada

dZ

++−−

+−−=εµ

π

22

220 1

21

,

adad

ada

dZ

dbwhen

−+−≈

<<

εµ

π

Impedance will increase for a small decrease of a.

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Impedance vs Radius a

Impe

danc

e (O

hm)

Radius of the Steel Cable (cm)

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Time Domain Reflectometry

Pulse Generator

Oscilloscope

Transmission LineZl

Load

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Excitation Signals

Step wave Pulse

Rise Time: the time required for the voltage to rise from 10% to 90% of the final value

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How TDR works?

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Analyzing Reflections

0

0

ZZ

ZZ

V

V

i

r

+−

==ρ

Voltage reflection coefficient

2

TvD p=

Open circuit

∞=ZiV

ir VV =

0 T

Short circuit

0=ZiV ir VV −=

0 T

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Experimental Results

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Severity of Defect

Seven-strand prestressing cable with broken strands

Six broken strands Two broken strands

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Multiple Defects

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Cable-to-wire Distance

abd

321 ddd <<

1dd =

2dd = 3dd =

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System Risetime Tr

Tr=500ps Tr=1.0ns

Tr=2.0ns Tr=3.0ns

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External Sensor Wire

• Suitable for existing bridges

• Larger distance less sensitive

• Able to detect serious corrosion

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Noise in the Measurement

• Random noise

• Repeatable noise ------ nearby conductorsvariations of d

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Baseline Comparison

New MeasurementBaseline Measurement

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Conclusions

• TDR can be effectively used as an NDEtechnique for defects detection;

• It can detect, locate and identify the extentof defects;

• It is suitable to both new and existingbridges;

• TDR can be applied to not only bridges butalso other steel reinforced structures.

University of Delaware, 1999

Modeling Different Types ofDefects

• Abrupt pitting corrosion

• General surface corrosion

• Void in grout

University of Delaware, 1999

Typical TDR Reflections

Shunt-RC

Shunt-RL

Series-RC

Series-RL