Detection and Characterization of Corrosion of Bridge ...

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

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Typical TDR Reflections

Shunt-RC

Shunt-RL

Series-RC

Series-RL