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TECHBRIEF |
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Publication Number: FHWA-HRT-14-071 Date: September 2014 |
Publication Number: FHWA-HRT-14-071 Date: September 2014 |
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FHWA Publication No.: HRT-14-071 |
This document is a technical summary of the unpublished Federal Highway Administration report Nondestructive Inspection Protocol for Reinforced Concrete Barriers, available through the National Technical Information Service at www.ntis.gov.
The National Transportation Safety Board (NTSB) investigation of the accident that occurred on August 10, 2008, on the William Preston Lane, Jr., Memorial Bridge, which crosses the Chesapeake Bay near Annapolis, MD, identified significant corrosion of the anchor bolts attaching the bridge railing to the bridge deck. A subsequent NTSB recommendation stemming from that investigation, H-10-18, was to "expand the research and development of nondestructive evaluation technologies to develop bridge inspection methods that augment visual inspections; offer reliable measurement techniques; and are practical, both in terms of time and cost, for field inspection work; and promote the use of these technologies by bridge owners." (1) This document summarizes a response to that recommendation.
Reinforced concrete barriers are generally anchored to the deck of a bridge or retaining wall using reinforcing steel protruding from the main structure or by anchored bars or bolts added during retrofits. Corrosion of steel bars or bolts can weaken this attachment and reduce the capacity of the barrier. The most direct damage resulting from corrosion is the reduction of steel diameter and cross-sectional area. Steel corrosion in concrete is caused primarily by chloride- or carbonation-induced corrosion. Barriers are generally located at or very near the gutter-line of a roadway and may have significant long-term exposure to corrosive deicing materials and other corrosion-inducing environments.
Anchorages and adjacent voids are typically embedded in concrete structural elements and consequently cannot be fully inspected visually. Research has been done on nondestructive evaluation (NDE) methods to evaluate reinforced concrete and embedded steel reinforcement.(2,3) Four NDE technologies were examined and tested in this project, and the results of those test are reviewed and summarized in this document.
The four NDE technologies considered in this project were ground-penetrating radar (GPR), low-frequency ultrasonic tomography, infrared (IR) thermography, and digital radiography.
GPR is a high-resolution electromagnetic technique designed primarily to investigate the shallow subsurface of the earth, building materials, roads, and bridges. GPR is routinely used to locate and map reinforcing steel in concrete structures. Detecting damage to embedded reinforcing steel, however, is a difficult challenge.
The general principle of GPR is illustrated in figure 1. The electromagnetic wave is radiated from a transmitting antenna and travels through the material being tested at a velocity determined primarily by the material's dielectric permittivity. When the wave encounters an object having different dielectric properties than the surrounding medium, it produces a scattered pulse that is detected by a receiving antenna. The resulting output voltage signal is amplified, processed using different filters, and displayed on a computer monitor.
For a single scan of the antenna, a standard GPR display is referred to as a B-scan or radargram image (figure 2).(4) The horizontal axis indicates the antenna position, and the vertical axis displays either signal arrival time or inferred penetration depth. The gray-scale image depicts echo strength, with white depicting a strong positive voltage value and black depicting a strong negative voltage.
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1 m = 3.3 ft |
When the antenna is scanned across a concrete fixture containing reinforcing steel, the reinforcing steel is sensed at many different antenna positions (figure 3). The arrival time of the reinforcing steel echo depends on the distance between the antenna and the reinforcing steel. (The arrival time is shortest when the antenna is directly above the reinforcing steel.) Because of the dependence of echo arrival time on antenna position, regions of high reflected amplitude in B-scans have a hyperbola-like shape, as illustrated in figure 2.
Low-Frequency Ultrasonic Tomography
The low-frequency ultrasonic tomographer creates a three-dimensional (3-D) representation (tomogram) of internal defects that may be present in a concrete structure. The instrument is based on the ultrasonic pitch-catch method and uses an antenna composed of an array of dry point contact transducers. The transducers emit shear waves into the concrete. One transducer sends out a stress-wave pulse, and a second transducer receives the reflected pulse (figure 4). The time from the start of the pulse until the arrival of the echo is measured. If the wave speed C is known, then the depth d of the reflecting interface can be calculated.
Once data are acquired in the explore-and-scan mode, a signal processing technique called synthetic aperture focusing technique (SAFT) is used to reconstruct a 3-D tomographic image of the interior of the concrete member.
Using SAFT, the pulse-echo measurements made at a multitude of transmitter-receiver locations are combined to form a map of the ultrasonic reflectivity of the region of interest. The method takes advantage of both spatial and temporal correlations to enhance the resolution and the signal-to-noise ratio of the reconstructed 3-D image. The reconstructed 3-D image is stored in the computer, and the user can view a 3-D picture of the locations of all detected interfaces, or the user can view the projection of the interfaces on three orthogonal planes with specialized software supplied by the manufacturer.
The views of the three orthogonal planes in figure 5 have formal names. A C-scan shows the reflecting interfaces projected on a plane parallel to the test surface; that is, a C-scan is a "plan view." A B-scan shows the reflecting interfaces projected on a plane perpendicular to the test surface and perpendicular to the scan direction; that is, a B-scan provides an "end view." A D-scan shows the reflecting interfaces projected on a plane perpendicular to the test surface but parallel to the scan direction; that is, a D-scan provides an "elevation view." The user can also look at specific "slices" through the object in each of the three directions by defining the Z-coordinate for a C-scan image, the X-coordinate for a B-scan image, and the Y-coordinate for a D-scan image.
IR Thermography
IR thermography is a non-contact method for detecting the difference in temperature between a problem area and its surroundings. Depending on the underlying cause, the problem area may be warmer or colder than its surroundings. This temperature difference is clearly visible in the camera's image display. A trained operator can immediately hone in on the source of the problem and its cause without having to cut into walls or disassemble equipment.
Digital Radiography
Because of isotopic sources such as Caesium (Cs) 137, Cobalt (Co) 60, and Iridium (Ir) 192, conventional radiography applied in the field is typically considered hazardous.(5) The physical size of these sources makes them very convenient for remote location inspection. Because the sources are not collimated, however, they require a large area to be cordoned off to prevent radiation exposure to personnel in the area. In addition, these source types in the past have been used with films (as the detector) that can take minutes of exposure to get the correct radiographic image.
Advances in both the source and the detector technology in recent years have led to radiographic images that can be obtained in much less time and with lower exposure. These advances are primarily due to the development of a pulsed X-ray source. Pulsed X-ray sources produce radiation only when the source is activated and the X-rays are highly directional. Furthermore, the sources are man-portable and can be operated using batteries equivalent to those used in hand-held electric tools.
Laboratory testing was conducted on two concrete barrier configurations used in the Washington, DC, metropolitan area: the F-shape bolt down barrier and the New Jersey free standing portable barrier. The testing was of barrier connectionsbolt connections in the case of the F-shape bolt down barrier, and embedded reinforcing steel connections in the case of the New Jersey barrier. Additional tests are proposed on reinforcing steel rebars within barriers to quantify damage. A third type of barrierthe Fshape free standing portable barrierwould be added for these tests. A mockup bridge deck was constructed for the testing, and connecting rebars and bolts were altered by machining away steel to produce varying levels of crosssection losses. Future testing should include the placement of hollow balls, loose gravel, and foam in barriers to simulate voids and delamination and to quantify their effects. Future testing should also be conducted on specimens where true sectional loss due to corrosion of steel reinforcement has occurred. Specimens for this testing can be prepared by inducing electrical current to speed up the corrosion process.
In the completed testing of barrier connections, the four selected NDE technologies were applied to the barrier connections. GPR provided images of the response of rebars in concrete for the F shape bolt down barrier at all crosssection loss levels. The thicker concrete cover of the New Jersey barrier, however, prevented penetration from the output of the highfrequency antenna. Consequently, the GPR images showed no indication of a response. Although the low frequency ultrasonic tomographer detected reinforcing steel in concrete, it was difficult to differentiate between different crosssection losses in the rebars without prior knowledge. The responses from 0 and 10 percent cross-section losses were clear for both rebar sizes used, but the larger cross-section losses of 25 and 50 percent were not evident. IR thermography was not successful in locating rebars in either type of concrete barrier, mainly because of thick concrete cover. Lastly, digital radiography with a low power pulsed Xray system also did not penetrate deep enough into the thick concrete to produce images of the rebars. Figure 6 and figure 7 are examples of the GPR images and the ultrasonic tomographer images at different cross-section loss levels in the Fshape bolt down barrier, respectively.
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1 m = 3.3 ft |
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1 mm = 0.04 inches |
To understand the current capacity protocols for determining deterioration thresholds that should trigger advanced inspection and possibly rehabilitation of concrete barriers, a short survey was developed and distributed among FHWA Division Bridge Engineers. The survey results collected are listed in table 1. The table indicates that there is currently little or no guidance available on acceptable and unacceptable deterioration levels in concrete barriers, or at what point NDE methods should be used for assessment of barrier conditions.
Table 1. Deterioration threshold and guidance survey.
State |
Type of Barrier |
Anchorage System |
Inspection Protocol |
Guidance on |
---|---|---|---|---|
Colorado |
F-Shape |
Rebar During |
Not Available |
Not Available |
Hawaii |
Jersey |
Rebar During |
National Bridge |
Not Available |
Illinois |
F-Shape |
Rebar During |
Visual, Cores |
Not Available |
Indiana |
F-Shape |
Rebar During |
Visual, Hammering |
Not Available |
Kansas |
F-Shape |
Rebar During |
CoRE Guide (AASHTO) |
Not Available |
Michigan |
Type 4 |
Rebar During |
Visual |
Not Available |
Minnesota |
J-Shape, |
Rebar During |
Bridge Inspection |
Not Available |
Montana |
F-Shape |
Rebar During |
Visual |
Not Available |
New Jersey |
F-Shape |
Not Available |
Not Available |
Not Available |
New York |
Not Available |
Not Available |
Visual |
Not Available |
North |
Jersey and |
Rebar During |
National Bridge |
Not Available |
Ohio |
Jersey, |
Rebar During |
National Bridge |
ODOT, Bridge Inspection Manual |
Pennsylvania |
F-Shape |
Rebar During |
Visual |
Not Available |
Tennessee |
Jersey, |
Rebar During |
Visual, Hammering |
Not Available |
Texas |
Single Slope, |
Rebar During |
National Bridge |
Not Available |
AASHTO = American Association of State Highway and Transportation Officials. |
The limited scope of the testing and the paucity of positive results preclude the drawing of definitive conclusions about the usefulness of the reviewed NDE technologies for barrier inspections. Further investigation is warranted. The thickness of the concrete in barriers appeared to affect all four of the technologies, although GPR and low frequency ultrasonic tomography somewhat less so than IR thermography or digital radiography. Among the many avenues for additional effort, the fusion of GPR and the low frequency ultrasonic tomography should be considered. Such a fusion might provide complementary information regarding the condition assessment of concrete barriers. Future research about such a fusion could include the quantification of measurements made in the tomographer and GPR tests already completed. Future work should include image processing techniques to quantify voids and cross-section loss.
1. National Transportation Safety Board, Safety Recommendations H-10-17 and H-10-18, Washington, DC, 2010.
2. Clemena, G., Lane, S., Freeman, T., and Lozev, M., Evaluation of Nondestructive Evaluation Methods for Application in Early Detection of Deterioration in Concrete Pavements, VTRC–R13, Virginia Transportation Research Council, 2000.
3. Al-Qadi, I.L., Lahouar, S., and Loulizi, A., Ground-Penetrating Radar Calibration at Virginia Smart Road and Signal Analysis to Improve Prediction of Flexible Pavement Layer Thicknesses, FHWA/VTRC 05-CR7, Virginia Department of Transportation, 2005.
4. Jensen, T., Wendt, S., Gray, J., Eisenmann, D., Margetan, F., Lo, C., Nakagawa, N., and Brasche, L., Feasibility Study for Detection and Quantification of Corrosion in Bridge Barrier Rails, InTrans Project 11-413, Center for Nondestructive Evaluation, Iowa State University, Ames, IA, 2013.
5. Cobb, A., Feasibility Study for the Inspection of Hidden Corrosion Damage in Multi-Layer Gusset Plates, SWRI Project No. 18.17643, Sensor Systems and Nondestructive Evaluation (NDE) Technology Department, Southwest Research Institute, San Antonio, TX, 2013.
Researchers This research was funded by Federal Highway Administration and was performed under contract no. DTFH61-08-D00014 at the Turner Fairbank Highway Research Center, McLean, VA. The principal investigator was Satish Chintakunta, the lead researcher was Pranaam Haldipur, and the program manager was Raghu Satyanarayana of Engineering Software Consultants, Inc. The FHWA program manager was Shane Boone. Distribution This TechBrief is being distributed according to a standard distribution. Direct distribution is being made to the Divisions and Resource Center. Availability This TechBrief may be obtained from the FHWA Product Distribution Center by email to report.center@dot.gov, fax to (814) 239-2156, phone to (814) 239-1160, or online at https://www.fhwa.dot.gov/research. Key Words Nondestructive evaluation, reinforced concrete barriers, safety, National Transportation Safety Board, groundpenetrating radar, ultrasonic pulseecho. Notice This document is disseminated under the sponsorship of the U.S. Department of Transportation in the interest of information exchange. The U.S. Government assumes no liability for the use of the information contained in this document. The U.S. Government does not endorse products or manufacturers. Trademarks or manufacturers’ names appear in this report only because they are considered essential to the objective of the document. Quality Assurance Statement The Federal Highway Administration (FHWA) provides highquality information to serve the Government, industry, and public in a manner that promotes public understanding. Standards and policies are used to ensure and maximize the quality, objectivity, utility, and integrity of its information. FHWA periodically reviews quality issues and adjusts its programs and processes to ensure continuous quality improvement. |