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Title: Bridge Deck Condition Assessment with Electromagnetic, Acoustic and Automated Methods for Proceedings of the 6th International Workshop on Structural Health Monitoring 2007 Authors: Dryver Huston1, Nenad Gucunski2, Ali Maher2, Jianhong Cui1, Dylan Burns1, Frank Jalinoos3 1
School of Engineering, University of Vermont, Burlington, VT, USA Civil & Environmental Engineering, Rutgers University, Piscataway, NJ, USA 3 Federal Highway Administration, Turner-Fairbanks Highway Research Center, McLean, VA, USA 2
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ABSTRACT Reinforced concrete bridge decks are high-performance structural elements with designs that require survival of decades of harsh loading. Early detection and accurate assessments of distress can lead to more effective and economical maintenance and management. A difficulty with bridge deck assessments is that much of the important damage is subsurface and hidden from sight. An ideal assessment system would detect the presence, location and extent of subsurface damage, with a minimum of detection errors, without impeding the flow of traffic and require only modestlyskilled personnel to operate. This paper will present the results of a study that used three primary types of sensors to assess bridge deck condition. The sensor types include: 1. Electromagnetic in the form of ground penetrating radar (GPR), 2. Acousto-elastic wave in the form of impact echo and chain drag, and 3. Electrochemical in the form of half-cell measurements. Some of the sensor systems were automated – others were manually operated. INTRODUCTION Concrete bridge decks are high-performance structures with many challenging and unresolved maintenance issues. One of the more important technical questions is effective nondestructive measurement of subsurface conditions, including sub-surface cracks, air-filled delaminations, rebar corrosion and the physical state of the concrete. _____________ Dryver Huston, Dylan Burns, Jianhong Cui; School of Engineering, University of Vermont, Burlington, VT 05405-0156 U.S.A.. Nenad Gucunski, Ali Maher; Civil & Environmental Engineering Department, Rutgers University, Piscataway, NJ 08854 U.S.A. Frank Jalinoos; Federal Highway Administration, 6300 Georgetown Pike, McLean, VA 22101 U.S.A.
A variety of technologies can assess bridge deck condition with varying degrees of reliability and ease of use. Three classes of methods include electromagnetic – ground penetrating radar (GPR); elastic wave – such as impulse echo and chain drag; and electrochemical – half-cell corrosion potential measurements. The overall goal of this study is to evaluate the performance of these testing techniques for concrete bridge deck condition rating, ease of use and the migration into automated systems with sufficient speed to not require major traffic disruptions. The first step in a bridge deck evaluation is a visual survey and/or receipt of complaints of distress from motorists. A visual survey identifies spalling, patching, surface cracks, and potholes, but does not reveal subsurface deterioration. Delamination surveys are a typical next step. Several standard methods for locating and quantifying delaminations, including the hammer tap, chain drag, and rotary percussion, chain drag, half-cell, impact echo, GPR, ultra-sound, and infrared. GPR systems use electromagnetic radiation at microwave frequencies (typically in the range from 900 MHz to 3 GHz) to characterize subsurface features. Microwaves readily penetrate and interact with dielectric materials, such as concrete; and reflect off metallic materials, such as rebars. Microwave propagation characteristics vary depending on the properties of the dielectric material. Equipment that transmits and receives microwave pulses, i.e. radar systems, can detect and locate changes in propagation velocities. Elastic waves provide another means of testing concrete bridge decks. Chain drag testing is one of the most common elastic wave test techniques. The method is to drag a chain across the concrete bridge deck while listening to the audible response. Sweeping the chain over a healthy solid section of concrete with no delaminations or voids causes the technician to hear an audible ringing sound. When the chains are dragged over a section of concrete with delaminations, the technician will notice a dull hollow sound. Using this method a technician can sweep the entire bridge deck and mark out the areas containing delaminations. This method is easy and inexpensive to perform. Due to the slow nature of the chain drag it is often difficult to get the entire bridge shut down, so it is common practice to just shutdown one lane at a time. The noise and vibration from the traffic in the other lane can greatly affect the technician’s interpretation during the chain drag. The testing is very subjective and renders it difficult to compare the results from different technicians [1]. The impact echo (IE) method uses a mechanical impact to excite and to launch high frequency elastic waves that interact with subsurface features serves as a means of subsurface evaluation. There are several variant IE test devices. These devices can measure both surface wave speeds to indicate elastic module and bottom surface reflection reverberations to determine slab thickness and presence of delaminations, Figure 1.
Severe Delaminations Cause 3 to 6 kHz DrumLike Vibrations, Detectable by PSPA and Chain Drag
Milder Closed Delaminations Cause > 18 kHz Spectral Peaks, Detectable by PSPA but not by Chain Drag
No Delamination Causes 6 to 18 kHz Spectral Peak, Detectable by PSPA and Chain Drag
Figure 1 Detection of delaminations in concrete with Portable Pavement Seismic Analyzer [2] [3]
A half-cell meter measures the presence of electrochemical activity. The half-cell method can map areas of active corrosion. As such, it often forms part of an overall structural assessment, particularly in reinforced concrete [4]. Using a half-cell to assess reinforced concrete involves attaching a copper electrode to steel reinforcing bars and pressing a porous wet Cu/CuSO4 electrode to the concrete surface. This arrangement sets up a half-cell where the difference in oxidation-reduction potentials of iron and copper ions provides a measure of corrosion activity. The amplitude and sign of the voltage generated by the half-cell indicates the presence of corrosion and the reliability of the measurement. TEST SYSTEMS Two different ground penetrating radar (GPR) systems provided an automated noncontacting electromagnetic evaluation of the bridge deck. One was the FHWA PERES/HERMES II system. This system uses micro-impulse radar and operates in the frequency range between 500 MHz and 5 GHz, Figure 2 [5]. A single antenna pair mounted on a linear drive mechanically rasters across a 1.5 meter span with a transverse lateral data capture resolution of 1 cm. 3-D migration techniques correct the raw data for hyperbolic nonlinearities and reconstruct underlying structures [6]. The second GPR system was the 3d-Radar, Figure 3. This is a step-frequency ground penetrating radar with 47 antennas that allows high-resolution measurements in a frequency range 100-2000 MHz. The system is capable of generating high-resolution 3-D images of buried objects and natural interface layers down to 2-3 meters depth.
Figure 2. PERES/HERMES II system on Carter Creek Bridge, Dumfries, VA, USA
Figure 3. 3d-Radar system mounted on test vehicle Carter Creek Bridge, Dumfries, VA
Acoustic elastodynamic measurements in the form of Impact Echo (IE) measurements, implemented in a Portable Seismic Property Analyzer (PSPA), chain drag and hammer tapping provided complementary data. The PSPA is an ultrasonic/high frequency seismic device that incorporates ultrasonic surface wave (USW) measurement for elastic modulus evaluation and IE technique for delamination detection and characterization, Figure 4. The IE analysis uses spectral pattern recognition algorithms to determine the presence and extent of underlying delaminations.
Figure 4. Portable Seismic Property Analyzer
TEST PROCEDURES Initially a series of lab tests evaluated the capability of the PERES/HERMES II GPR system to detect air gaps in concrete test slabs. A concrete slab was created in two molds with a non-uniform gap, these two sections of the concrete slab were then set on top of each other creating a very small air gap, Figure 5. This sample was then placed under the PERES system for scanning. Two bridges in Virginia (USA) were the subject of the field test portion of this study, the Carter Creek bridge and the Van Buren bridge. The Carter Creek Bridge is a 5 span bridge located in Irvington, VA. The bridge sees moderate to heavy traffic. The Van Buren Bridge is a three span bridge located in Dumfries, VA. This bridge is on a dead end road and sees very little traffic. Upon arrival at a testing location several testing procedures were followed. First the PERES II system was unloaded from the truck, and setup at the beginning of the deck diction of the bridge that was to be tested. Next a grid of 305 mm (1 ft) was laid out on the bridge deck. The grid served as a reference frame for aligning and registering all the different testing methods. This was used to mark the delaminations found from chain drag and hammer tap, as well as locations for the PERES, and the half Cell readings. The half-cell data were taken at each grid point and recorded. The 3-d Radar and PSPA tests recorded and registered the data relative to the grid. TESTING RESULTS The laboratory tests confirmed the reliability of the PERES/HERMES II system. Figure 5 shows typical results from a concrete slab with simulated delaminations. The field tests were largely successful in that all of the instruments operated properly and gathered nominally good data. Figure 6 shows slices across the deck of images taken with the PERES/HERMES II system on the Carter Creek Bridge. Figure 7 shows the result of half-cell potential measurement taken at approximately the same location on the Carter Creek Bridge. Both figures show an corresponding feature of interest in the center of the span. Figure 8 shows a 3d-Radar assessment of the Van
Buren Bridge. Figure 9 shows impact echo data taken on the Van Buren Bridge. Note that the impact echo data indicate that this bridge is severely delaminated.
(a)
(b)
(c)
Figure 5. (a) Test slab with air gap; (b) PERES data processed using LLNL software; (c) 3-D PERES data shown in Slicer Dicer© software.
Figure 6. Top image is the top surface of span 5 Carter Creek Bridge; bottom is the top rebar mesh.
Figure 7. Half-cell potential measurement of Carter Creek Bridge
Figure 8. 3-D radar GPR condition assessment on Van Buren Bridge
Figure 9. Impact echo data and chain drag-identified delaminations on Van Buren Bridge
CONCLUSION The testing procedures went quite smoothly. The chain drag method is relatively inexpensive, and involves very little equipment, but it is slow and involves shutting down the bridge deck, this method is also subjective and depends on the technician’s interpretation. Visual inspection can only addresses surface feature, nothing subsurface. Impact echo can be a very accurate method if the grid spacing is small enough, but again the presently-available instrument is slow and requires manual placement. The half-cell method is similar. Both methods involve shutting down the lane of the bridge being tested. The PERES/HERMES II system is similarly slow. The 3d-Radar system can collect data with a fair level of resolution, while traveling at speeds of approximately 20 km/hr. The GPR systems have difficulty locating in the concrete bridge decks. An initial examination of the data indicated that there was some agreement between the data sets. The impact echo and chain drag tests agreed on the high level of delamination present at the Van Buren Bridge. The GPR and half-cell agreed on the presence of a hot-spot on the Carter Creek Bridge. It is apparent that no single current technology, on its own, can be used to quickly and acutely, without shutting down the lane of traffic, asses a concrete bridge deck or detect small delaminations. One approach to resolving this difficulty is to fuse the data from several sensor types to estimate the potential for damage. Data fusion is generally defined as the use of techniques that combine data from multiple sensors and gather that information in order to achieve inferences, which will be more efficient than if they were achieved by means of a single sensor [7]. A possible multi-sensor data fusing system would involve the combination of a GPR system, an automated impacted echo system, and possibly ultrasound and IR systems as well. All of these systems would be mounted on the same platform and the data will be overlaid to achieve the best possible indication of concrete bridge deck condition and delamination. ACKNOWLEDGEMENT This research was funded in part by the US Federal Highway Administration with support from the TFHRC Nondestructive Evaluation Validation Center.
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