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Non-destructive corrosion monitoring of prestressed concrete HPC by Time Domain refelectometry
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Wei Liu et al. 1
Nondestructive Corrosion Monitoring of Prestressed HPC Bridge Beams Using
Time Domain Reflectometry
Wei Liu, Robert Hunsperger,
Dept. of Electrical & Computer Engineering, Univ. of Delaware, Newark, DE 19716
Michael Chajes, Degang Li,
Dept. of Civil & Environmental Engineering, Univ. of Delaware, Newark, DE 19716
Eric Kunz
VETEK Systems Corp. 6 Oak Road, Elkton, MD 21921
Abstract. A novel nondestructive evaluation technique for corrosion detection of embedded or encased steel
reinforcement or bridge cables using time domain reflectometry (TDR) is developed and demonstrated in
this paper. TDR has traditionally been used to detect faults in electrical transmission lines. It involves
sending an electrical pulse along the line and analyzing the echoes. By applying a sensor wire alongside of
steel reinforcement (such as a prestressing strand), a transmission line is created. Both analytical models
and small-scale laboratory tests have shown that TDR can be effectively utilized to detect, locate and
identify the extent of damage in steel reinforcement in this manner. After reviewing the TDR theory and
presenting laboratory results, the paper will discuss an actual field implementation of this technology. A
TDR system has been has been installed in Bridge 8F, a prestressed high-performance concrete (HPC)
adjacent box beam bridge recently constructed in Fredrica, Delaware. The primary purpose of the TDR
installation was to demonstrate that the system can be successfully applied to actual structures. After the
TDR instrumentation was installed, initial readings were taken to verify that the system had survived the
fabrication process. Differential comparisons of subsequently measured signals to the baseline readings can
reveal damage to the strands. Initial experimental results from this project are reported in this paper.
Wei Liu et al. 2
INTRODUCTION
In 1999, the Delaware Department of Transportation (DelDOT) received funds through the Federal
Highway Administration’s Innovative Bridge Research and Construction Program (IBRC) to design and
construct a high-performance concrete bridge in Fredrica, Delaware (later referred to as Bridge 8F). HPC is
concrete that is optimized for a specific application and often possesses qualities such as high strength, low
permeability, good workability, and excellent long-term durability. A two-span, prestressed concrete,
adjacent box beam bridge utilizing HPC in both the beams and deck was designed. The beams for the
bridge were fabricated by Concrete Building Systems (CBS) in Delmar, Delaware. Prior to casting, long-
term monitoring instrumentation was installed in several beams. Bridge 8F, which replaced a deteriorated
four-span structure, was completed in October 2000.
The performance of the HPC beams is being evaluated through long-term monitoring. The monitoring
began at the time of fabrication, and is continuing now that the bridge is in-service. One of the many
aspects of the long-term monitoring program is corrosion detection using time domain reflectometry
(TDR). In this case, if the HPC performs as expected, TDR measurements will confirm that no corrosion
damage is occurring.
ANALYTICAL MODEL AND LABORATORY RESULTS
Time Domain Reflectometry
Time domain reflectometry is a proven method of detection of physical problems in electrical and
optical transmission lines. A transmission line is a wave guiding system along which electromagnetic
waves can travel. It typically has at least two parallel conductors. Examples are telephone lines and
television cables. The key difference between transmission lines and conventional circuits is the size. A
transmission line is long compared to the signal wavelength. As a result, signals cannot travel
instantaneously from one end to the other, because there will be a propagation delay and phase change.
TDR is a well-established technique in the field of electrical engineering that has been used since
1940’s to detect faults in transmission lines. It involves sending an electrical pulse along the transmission
line and using an oscilloscope to observe the echoes returning back from the line. Any discontinuity will
Wei Liu et al. 3
cause a reflection. From the transit time, magnitude, and polarity of the reflection, it is possible to
determine the spatial location and nature of the discontinuity. (1)
A time domain reflectometer is usually configured as shown in Figure 1. The pulse generator generates
a fast rising step wave or pulse. This wave is launched into the transmission line. A high impedance
oscilloscope is connected to monitor the wave.
Bridge Cable Modeling
There are obvious similarities between transmission lines and steel strands used for prestressing
concrete and for cables in cable supported bridges. A steel strand is a good conductor embedded in a
dielectric (concrete). It can be modeled as an asymmetric, twin-conductor transmission line by adding a
sensor wire alongside of the cable. The cross sectional view of the strand/wire system is shown in Figure 2.
Physical defects of the strand, such as abrupt pitting corrosion, general surface corrosion, and voids in the
surrounding concrete, will change the electromagnetic properties of the line. These defects, which can be
modeled as different kinds of discontinuities, can be detected by TDR. (2)
For a thorough analysis of the wave propagation in this transmission line, one needs to solve Maxwell's
equations with boundary conditions imposed by the physical nature of the system under investigation. It is
also possible to represent a line by the distributed parameter equivalent circuit, as shown in Figure 3, and
discuss wave propagation in terms of voltage and current. The distributed parameter equivalent circuit
possesses a uniformly distributed series resistance R, series inductance L, shunt capacitance C, and shunt
conductance G. (R, L, C, and G are defined per unit length.) By studying this equivalent circuit, several
characteristics of the transmission line can be determined.
The characteristic impedance of the line, Z0, is given by
The propagation constant, γ, which defines the phase shift β and attenuation α per unit length, is.
CjGLjR
Zωω
++
=0
( )( )CjGLjRj ωωβαγ ++=+=
Wei Liu et al. 4
ω is the radian frequency, which is given by ω=2πf, where, f is frequency in Hz. The velocity at which
the voltage travels down the line can be defined in terms of β:
The distributed parameters of the transmission line can be calculated from the geometry and material
parameters of the cable. After making several necessary simplifications, the characteristic impedance is
given to a high degree of accuracy by the simplified expression (3)
The characteristic impedance of the line is a function of a, b, and d. Note that b is much smaller than a
and d (see Figure 2), and it remains the same value along the line. However, the radius of the steel cable, a,
may be changed if corrosion occurs. When b<<d,
This expression has a negative value. This means that the characteristic impedance will increase for a
small decrease of a. Since radius a always decreases at a corrosion site, corrosion will cause higher
characteristic impedance. This change of impedance can be detected by time domain reflectometry.
In order to utilize TDR to detect corrosion, the damage sites of a strand need to be modeled as electrical
discontinuities in a transmission line. Several physical defects are of great interest when considering the
durability of steel strands. Among them are abrupt pitting corrosion, general surface corrosion, and voids in
the grout.
General surface corrosion tends to reduce the radius of the strand on the order of a few percent over a
part of length of the line. It is modeled as a section of transmission line with different characteristic
impedance. The extent and length of the corrosion can be determined from the magnitude and duration of
the reflection, respectively. Abrupt pitting corrosion is a severe localized damage. Its length is small
compared to wavelength of the excitation signal. Therefore, it is modeled as an inductor in series with the
−−= −
abbadZ
2cosh
21 222
10 ε
µπ
22
220 1
21
adad
adadZ
−+−≈
εµ
π
βω=pv
Wei Liu et al. 5
line. A positive reflection from the site of pitting corrosion is expected. The location of the corrosion site is
obtained from the transit time. The reflection amplitude indicates the magnitude of the damage. Voids in
the concrete tend to reduce the dielectric constant and therefore increase the characteristic impedance. Also,
voids will change the velocity of propagation in the transmission line. Therefore, voids can be detected by
TDR, as well as corrosion.
Small-scale Laboratory Tests
Small-scale laboratory tests have been conducted to verify the effectiveness of TDR in locating and
characterizing simulated corrosion sites. Several 1-meter and 3-meter specimens, made from standard rebar
and seven-wire strand, with built-in defects were used to study the ability of TDR to detect damage sites.
TDR tests were performed on both grouted and ungrouted specimens. Preliminary testing indicated that
damage detection for the ungrouted specimens was very similar to that of grouted specimens since the
relative reflection magnitude did not depend on the dielectric constant of a uniformly filled medium.
Because the researchers have access to the damage site when measuring specimens that are not embedded
in concrete, such specimens are more convenient to use to study the electromagnetic properties of the
simulated corrosion. As a result, bare specimens were mainly used in small-scale TDR tests.
The specimens were connected to the time domain reflectometer through standard 50Ω coaxial cables.
The far end of the specimen was connected to a terminating resistive load. A pulse was then sent down the
sample and the reflections shown on the oscilloscope. The terminating load was changed from an open to a
short to determine where the end of the sample was. The propagation velocity was then calculated.
Figure 4 shows the TDR reflection from a 3-meter steel rebar sample. This sample has 50% pitting
corrosion in the middle (1.55m from the front end). Pitting corrosion was simulated by locally grooving the
rebar specimens. The TDR measurement was made on a bare specimen (surrounded by air and not
embedded in concrete). The first step in the waveform corresponds to the generation of the step wave (point
A). The wave is launched into a coaxial cable, which is used to connect the sample to the measuring
system. The characteristic impedance of this coaxial cable is 50Ω . However, the sample has higher
impedance. As a result, there is a positive reflection at the beginning of the sample (point B). The wave
travels down the line at vp, the velocity of propagation. At every point that the excitation signal crosses, the
transmission line equations must be obeyed. However, there is a simulated corrosion site at point C. The
Wei Liu et al. 6
physical damage changes its electromagnetic properties. Therefore, the transmission line equations are not
satisfied and a reflection is generated at this point. The reflected wave is separated from the incident wave
in time. This time, T= TC - TB, is the transit time from point B to the mismatch and back again. At the end
of the sample, the wave goes up because the line is terminated by an open circuit (point D). The time
interval between points B and D is 23.0ns, which gives a propagation velocity of 2.61x108 m/s, i.e. about
87% of the speed of light. The location of the damage site is determined as 1.58m from point B since TC -
TB =12.1ns, which is the correct location.
TDR can not only locate the corrosion, but also reveal the severity of corrosion. Figure 5 shows TDR
returns from two seven-wire steel strand samples. The strands are 0.95m long and 1.27cm (1/2 inch) in
diameter. Corrosion was simulated by severing several wires of the strand specimen. The damage was
produced over a 7.5cm length, 44cm from the end of the sample. The TDR readings were taken on bare
strands. The first marker indicates the initial reflection from the front of sample, and the third marker
indicates the reflection from the end of sample 9.94ns later. The propagation velocity is 1.91x108 m/s. The
initial reflection is positive compared to the baseline before the peak, which shows that the characteristic
impedance of the sample is larger than 50Ω . The impedance is measured as 56Ω . It is close to 52Ω , which
is predicated by our model. Because the sample was terminated by a short circuit, the reflection from the
end of the sample is negative. The second marker indicates the reflection from the simulated corrosion site.
Note that an accurate location is identified. Experimental results indicate that the magnitude of the
reflection depends on the severity of the damage. The sample on the right has severe damage in which six
strands are severed, while the other sample has two severed strands.
The sensitivity and accuracy of TDR measurement depends on several parameters. A TDR parametric
study has been conducted. The following parameters have been identified through small-scale laboratory
tests:
• diameter of the sensor wire,
• distance between sensor wire and steel element,
• relative position of the sensor wire and damage site,
• system rise time of the measuring system, which describes how fast the signal is,
• water content of the surrounding concrete.
Wei Liu et al. 7
These factors need to be considered before TDR installation and measurement. (4)
Differential TDR Measurements
In field applications involving complex structures like an actual bridge, noise will be present in the
TDR measurements. One type of noise is random noise. Whether in the lab, or in the field, small amounts
of random noise will be present. However, it has been shown that one can average the results of several
measurements to effectively mask the random noise.
Another type of noise in the signal can be created by
• electric field disturbance caused by steel components near the cable being tested,
• variations in d, the distance between the steel cable and the sensing wire, since the characteristic
impedance depends on d.
While noise created due to these reasons can be relatively large, it is repeatable. Once a concrete girder is
instrumented, the location of the steel components causing noise, and the distance d between the steel
strand and sensing wire will remain constant. Therefore, the noise will be repeatable. Differential TDR
measurement can be used to effectively distinguish corrosion sites from repeatable noise. If several TDR
measurements are made for the same strand over a long time period, the later TDR results should be
identical to the former ones except for the corrosion sites. A differential comparison of stored signals with
newly measured ones can reveal corrosion that occurred between the two measurements. The differential
TDR method has been tested experimentally. Figure 6 shows TDR results obtained from a 1-meter seven-
wire strand bare sample. This sample has two severed strands over a 4.0cm length, 48cm from the front end
of the sample. From waveform 1, it is hard to tell whether or not the sample is damaged and where the
damage is. However, if this waveform is differentially compared with waveform 2, which is the TDR return
obtained from the same sample when it did not have any electrical discontinuities, the damage site can be
easily identified.
BRIDGE 8F EXPERIMENTAL PROGRAM
The effectiveness of the TDR corrosion detection method has been proven through laboratory tests. In
order to ready this technology for field implementation, full-scale experiments and field demonstrations are
Wei Liu et al. 8
necessary. Bridge 8F is the first field demonstration of the TDR corrosion monitoring technology. The
primary purpose of the demonstration was to show that the TDR system was robust enough to survive the
fabrication and erection process, and to show that usable signals could be achieved. While no corrosion is
anticipated in the HPC bridge in the near future, the repeatability of the signals over time will help to
validate the technology and confirm the performance of the HPC.
The complete experimental program involving Bridge 8F consists of two parts. First, a material testing
program has been initiated to determine the properties of the HPC used for the bridge beams. This program
involves a series of material tests conducted over a one-year period. Next, the performance of the HPC
beams is being evaluated through long-term monitoring. One of the many aspects of long-term monitoring
is corrosion detection. Corrosion monitoring with TDR began at the time of fabrication, and is continuing
now that the bridge is in service. Important aspects of the experimental program are provided in the
following sections.
History and Details of Bridge 8F
The existing bridge consists of four 8.67 m (28 ft. 5 in) spans supported by concrete encased I-beams
resting on solid concrete piers and abutments. The bridge was built in 1920 and rehabilitated in 1992. It
was classified as structurally deficient due to considerable spalling and salt infiltration of the piers,
abutments, and fascia beams. Soil conditions (clay layers beneath the marsh), have caused the structure and
approach roadway to settle over the years. These problems led DelDOT to pursue the replacement.
The HPC replacement bridge consists of two 19.0 m (62 ft. 4 in) spans consisting of adjacent
prestressed concrete box beams with a concrete deck to make the structure continuous for live load (see
Figure 7). HPC is being used throughout the bridge with low permeability limits to reduce the amount of
salt penetrating the concrete over the life of the structure and high strength to optimize structural members
and stretch span lengths.
Each of the 22 adjacent prestressed concrete box beams is 19.0 m (62 ft. 4 in) long and 0.686 m (27 in)
deep. The beams were fabricated by Concrete Building Systems (CBS) in Delmar, Delaware, and were all
produced in the same bed. The test program focuses on three beams, which are labeled B5, B7(4), and
B7(8) and can be seen in Figure 7. Each beam was prestressed using 12.7 mm (0.5 in) diameter, Grade 270
(1863 MPa), seven-wire low relaxation strands. Beams B5, B7(4), and B7(8) have 16, 18, and 18 straight
Wei Liu et al. 9
strands respectively. The strands were jacked to an initial tensile force of 138 kN (31 kips). After
approximately 18-hours after curing, the strands for each beam were released. Beam B5 was cast on August
30, 1999, while beams B7(4) and B7(8) were cast on October 4, 1999 and October 14, 1999, respectively.
Prior to casting, long-term monitoring instrumentation was installed in each beam.
TDR Instrumentation
A typical beam cross-section is shown in Figure 8. TDR monitoring wires have been installed alongside
a total of five strands in beams B5, B7 (4), and B7 (8). The wire is fully insulated, silver-coated copper
wire, which is commercially available. Both silver and copper are more corrosion resistive than steel.
Therefore, the corrosion of steel strands will occur well before the corrosion of the monitoring wire.
Corrosion monitoring wires were wound loosely around the strands. Since the TDR signal wavelengths are
much shorter than the twisting length, this did not affect the transmission line geometry. The monitoring
wires were held in place with cable ties, as shown in Figure 9.
A section of coaxial cable was used for each strand to provide electrical access to the strand/wire
system. The connecting coaxial cable is about 8.5 m in length, which is longer than necessary and allows
for installation variances. To minimize noise, the coaxial cable to strand/wire connection was made inside
the concrete beam and was covered by insulating tape. A standard BNC connector was installed on the
other end of the coaxial cable. An HP54750A digitizing oscilloscope was used to take TDR measurements,
which is capable of generating a fast rising step wave with rise time less then 100 picoseconds.
The energy loss can be a major concern when TDR is applied to long samples. Fully insulated
monitoring wire is used to reduce energy loss. At high frequencies, even the standard coaxial cable used to
connect the measuring system and the steel cable will consume some energy. Therefore, the coaxial cable
should be as short as possible to reduce energy loss and improve the sensitivity of TDR measure ments.
Upon the completion of the bridge construction, the connecting coaxial cable was cut to the shortest length
allowable.
Experimental Results
Corrosion monitoring began at the time of fabrication. During the fabrication of each beam TDR
measurements were taken before and immediately after the concrete pour. Since then, TDR readings were
Wei Liu et al. 10
taken on each of the 5 strands on roughly a monthly basis. All TDR results were converted and stored
digitally for future comparisons.
Figure 10 shows the TDR return from stand #2 of beam B7(4) before the pour. The sample is
terminated with a short circuit (electrical contact between strand and sensor wire). A voltage drop marks
the end of the sample. The electrical length is 142.7 nanoseconds. The signal is noisy due to the presence of
other electrically connected conductors. Note that the time scale used in this figure is different from the one
in Figures 11 and 12.
Figure 11 shows the TDR return from the same sample immediately after the pour. Theoretical analysis
shows that both characteristic impedance and propagation velocity depends on the dielectric constant of
concrete. When the water content of concrete is high, energy loss is significant because of the conductance
between steel strand and sensor wire. As another consequence, the characteristic impedance is small. From
Figure 11, it is hard to tell where the sample ends.
As a side benefit, it is possible to measure the moisture content of the concrete by TDR measurements.
As expected, signals indicated that water content changed most significantly in the first day after concrete
was poured.
Figure 12(a) and 12(b) are TDR readings taken 9 days and 177 days after the pour, respectively. The
end of the sample can be easily identified indicating that energy losses for the embedded transmission line
are not a problem. The electrical length of the sample is measured to be 266 nanoseconds, which is
significantly longer then the one without concrete.
Figure 12(c) is the differential comparison of Figure 12(a) and 12(b). It shows the total changes
occurred in the 168-day time period. TDR returns from this strand are repeatable with minor changes likely
due to the small changes in the concrete’s water content. Furthermore, the beams have been moved around
in the fabricator's yard during the monitoring process. This change in the environment also can cause
minor changes in the waveform. The repeatability of the TDR returns demonstrates the effectiveness of
differential comparison.
As discussed in the previous section, electrical connections were remade upon beam installation. As
expected, there were changes on TDR reflections due to the change of environment and connecting coaxial
cables. Initial measurements were taken after the bridge construction was finished. These results are used as
Wei Liu et al. 11
new baselines. If the steel strand is corroded in the future, a differential comparison of newly measured
signals to the baseline should reveal the corrosion damage site.
Since Bridge 8F is an HPC bridge, corrosion is not likely to occur within the next 20 to 50 years.
However, the purpose of this Bridge 8F experimental program is to transit TDR technology from laboratory
to the field. Even with standard concrete, a new bridge is not likely to corrode within the first 10 years,
which is much longer than the lifetime of a typical research project. However, it has been demonstrated that
TDR systems can be implemented and made operational. Furthermore, the stability of the signal over a
long time period will provide useful validation regarding the integrity of the system, and of the HPC.
CONCLUSIONS
A novel nondestructive evaluation technique for detecting damage in embedded or encased steel
reinforcement or bridge cables using time domain reflectometry has been developed and demonstrated.
Asymmetric transmission line models apply to steel elements with sensor wires. The effectiveness of TDR
in locating and characterizing simulated corrosion sites has been demonstrated through small-scale
laboratory tests, and TDR instrumentation has successfully been installed in beams of an HPC bridge
currently under construction. Differential TDR measurements are being used to monitor the onset of
corrosion of steel prestressing strands in the HPC bridge. The nature and repeatability of initial
measurements have demonstrated that the method is viable for actual field use. Additional research is
underway to investigate the application of this method for detecting damage in steel reinforcement of
cables of existing structures.
ACKNOWLEDGMENTS
This work was supported in part by the National Science Foundation under grant CMS-9700164 and the
Delaware Transportation Institute under grant No.929.
Wei Liu et al. 12
REFERENCES
1. “Time Domain Reflectometry Theory" Hewlett-Packard Application Note 1304-2, Hewlett-Packard
Company, Palo Alto, Ca lif. 1998.
2. Bhatia, S. K., R. G. Hunsperger, and M. J. Chajes. Modeling Electromagnetic Properties of Bridge
Cables for Non-destructive Evaluation. Proc. of Int. Conference on Corrosion and Rehabilitation of
Reinforced Concrete Structures, Orlando, Florida, 1998
3. Liu, W. Nondestructive Evaluation of Bridge Cables Using Time Domain Reflectometry. Master’s
thesis, University of Delaware, 1998.
4. Liu, W, R. G. Hunsperger, K. Folliard, M. J. Chajes, J. Barot, D. Jhaveri, and E. Kunz. Detection and
Characterization of Corrosion of Bridge Cables by Time Domain Reflectometry. Proc. of SPIE Vol. 3587.
Newport Beach, California, 1999.
Wei Liu et al. 13
FIGURE 1 Functional block diagram for a typical time domain reflectometer
Pulse Generator
Oscilloscope
Transmission LineZl
Load
FIGURE 2 Twin-conductor transmission line geometry of a bridge cable with sensor wire, where a is the
radius of the steel strand, b is the radius of the sensor wire, and d is the center-to-center distance between the
strand and wire.
a
b d
Z Z+ ∆ Z
R ∆ Z L ∆ Z
G ∆ Z C ∆Z
Z+ 2∆ Z
R ∆ Z L ∆ Z
G ∆ Z C ∆Z
FIGURE 3 Distributed parameter equivalent circuit of a transmission line.
FIGURE 4 TDR return of a 3-meter rebar sample. The sample has 50% pitting corrosion in the middle.
AB C
D
Wei Liu et al. 14
FIGURE 6 TDR results obtained from a 95cm seven-wire strand sample before (waveform 2) and after (waveform 1) a
simulated damage is made to the sample. The differential comparison in the bottom curve reveals the damage site.
FIGURE 5 TDR returns from 95cm seven-wire strand cable samples.
Six Severed Strands Two Severed Strands
Wei Liu et al. 15
FIGURE 7 Plan and cross-section of HPC bridge 8F.
(a) Instrumentation layout of beam B7(4).
(b) Beam B7(4) in the yard of the concrete plant.
FIGURE 8 Beam B7(4).
Wei Liu et al. 16
(a) Monitoring wire. (b) Connection to coaxial cable
FIGURE 9 TDR instrumentation.
FIGURE 10 TDR return of strand #2 of beam B7(4) on October 4, 1999. (before pour)
FIGURE 11 TDR return of strand #2 of beam B7(4) on October 4, 1999. (immediately after pour)
Wei Liu et al. 17
(a) TDR return on October 13, 1999.
(b) TDR return on March 29, 2000.
(c) Differential comparison.
FIGURE 12 TDR returns of strand #2 of beam B7(4).