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
University of Delaware, 1999
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
University of Delaware, 1999
Transmission Line
Distributed parameter equivalent circuitZ Z+ ∆ Z
R ∆ Z L ∆ Z
G ∆ Z C ∆Z
Z+∆ZZ
An incremental length of transmission line
University of Delaware, 1999
Characteristic Impedance
CjG
LjRZ
ωω
++=0
Discontinuity: Impedance Mismatch
0ZZ ≠
University of Delaware, 1999
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
University of Delaware, 1999
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.
University of Delaware, 1999
Impedance vs Radius a
Impe
danc
e (O
hm)
Radius of the Steel Cable (cm)
University of Delaware, 1999
Time Domain Reflectometry
Pulse Generator
Oscilloscope
Transmission LineZl
Load
University of Delaware, 1999
Excitation Signals
Step wave Pulse
Rise Time: the time required for the voltage to rise from 10% to 90% of the final value
University of Delaware, 1999
How TDR works?
University of Delaware, 1999
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
University of Delaware, 1999
Experimental Results
University of Delaware, 1999
Severity of Defect
Seven-strand prestressing cable with broken strands
Six broken strands Two broken strands
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Multiple Defects
University of Delaware, 1999
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
University of Delaware, 1999
Noise in the Measurement
• Random noise
• Repeatable noise ------ nearby conductorsvariations of d
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Baseline Comparison
New MeasurementBaseline Measurement
University of Delaware, 1999
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