Nondestructive Evaluation of Concrete Bridge Deck with Overlays
|Application of a bridge deck overlay system.|
Overlay systems have been used by State departments of transportation (DOTs) since the 1960s to extend the service life of deteriorated concrete bridge decks by protecting the underlying concrete substrate. An overlay is a thin layer of material—such as asphalt concrete, portland cement-based concrete, latex modified concrete, epoxy polymer concrete, or polyester polymer concrete—that is placed over existing concrete. A bridge deck overlay system can improve the ride quality for drivers, add protection for embedded reinforcement, and/or modify the transverse profile and vertical alignment of the existing roadway to improve deck drainage. More than 10,000 bridges in the United States have been successfully rehabilitated using overlays. However, the overlays on bridge decks can deteriorate and debond from the underlaying concrete decks.
Debonding, a separation between overlays and the deck, is a common defect in concrete bridge decks. Even if the overlay looks intact, the underlying concrete deck may have hidden deterioration (e.g., rebar corrosion and delamination). Because the underlying concrete is not accessible for direct inspection, other methods must be used to identify these deteriorated areas throughout the overlay service life.
|From fall 2018 until December 2020, the National Park Service rehabilitated the Arlington Memorial Bridge for the first time since it opened in 1932. The rehabilitation replaced the steel bascule span and concrete deck, and restored the deteriorated portions of the struts, piers, and concrete arch approach spans.|
Nondestructive Testing Advantages
Destructive and nondestructive testing are widely used by engineers to evaluate the structural integrity and characteristic differences in material properties, defects, and discontinuities of bridge structures. The sampling required for destructive testing damages a structure. In contrast, nondestructive evaluation (NDE) technologies enable assessment of structures without causing damage. Nondestructive testing also enables more comprehensive inspection since the tests can be repeated, and several technologies can be used together to better identify and characterize underlying defects.
“NDE technologies provide data not otherwise available to bridge owners to support well-founded decisions concerning investments in preservation, maintenance and rehabilitation,” says Hari Kalla, associate administrator, Federal Highway Administration (FHWA) Office of Infrastructure.
|The UT-EyeCon was mounted on a robotic arm for scanning specimens. Scans were performed at dense 2-inch by 2-inch (5.08-centimeter by 5.08-centimeter) grid points to develop high-resolution condition maps of specimens.|
Through laboratory specimens under controlled conditions and in the field under actual conditions, FHWA’s NDE Laboratory at the Turner-Fairbank Highway Research Center identified promising technologies for assessing concrete bridge decks with different types of overlays. The following nine technologies were considered for investigation:
- Impact echo (IE)
- Ultrasonic surface waves (USW)
- Ultrasonic shear-wave tomography (Ultrasonic Testing (UT)-MIRA and UT-EyeCon)
- Infrared thermography (IRT)
- Ground-penetrating radar (GPR)
- Electrical resistivity (ER)
- Half-cell potential (HCP)
- Impulse response (IR)
The researchers designed the experiments based on the results from the finite element (FE) simulations of IE, USW, and ultrasonic shear-wave methods. These FE analyses ensure that the specimen’s dimension provides reliable stress-wave propagations and temperature distribution without significant disturbances from boundary effect reflections.
|The frequency spectrum signals from FE simulations show the reflection waves from the specimen’s boundary can be negligible.|
The researchers designed and manufactured eight identical specimens with various artificial defects. The artificial defects included delamination at upper and lower rebar levels, honeycombing, voids, vertical cracks, and precorroded rebars within an elevated chloride content environment. After fabrication, the researchers first tested bare concrete specimens with nine NDE technologies to assess their performances in detecting defects before placing the overlays.
The NDE Laboratory experts evaluated the specimens before applying overlays.
|The condition maps were developed using the dominant frequencies measured by the IE method. IE detected shallow and deep delaminations, honeycombing, and voids in all eight specimens.|
Based on the NDE of eight bare concrete specimens, the researchers summarized the effectiveness of nine NDE technologies on uncovered decks.
|The UT-based C-Scans with EyeCon. The C-Scan is a two-dimensional section reconstructed from multiple A-Scans in the X-Y plane. The UT method detected shallow and deep delaminations and voids in all eight specimens, but only some honeycombing in some specimens.|
Specimen Testing with Overlays
The NDE Laboratory researchers studied the effectiveness of nondestructive methods in evaluating reinforced concrete bridge decks rehabilitated by seven types of widely used overlays: asphalt with a liquid membrane (S5AL), asphalt with a fabric membrane (S4AS), asphalt without a membrane (S7A), silica fume-modified concrete (S6S), latex-modified concrete (S3L), epoxy polymer concrete (S1E), and polyester polymer concrete (S8P). Epoxy-, latex-, silica fume-, and polyester polymer-based overlays were constructed by experts from the Virginia Transportation Research Council. A piece of plastic sheet covered half of each specimen to create the debonding defect between concrete and the overlays. The other half was shot-blasted to prepare the surface for the proper bonding of the overlays. Each overlay was placed within 24 hours after shot blasting to minimize the carbonation of the blasted concrete and ensure proper bonding. Epoxy polymer overlay was placed on S1 in two layers. Polyester polymer overlay was placed on S8 with a thickness of 0.75 inches (1.90 centimeters) and a 28-day compressive strength of 6,240 pounds per square inch (43.02 megapascals) from concrete cylinder tests. Latex- and silica fume-modified concrete overlays with a thickness of 1.5 inches (3.81 centimeters) were placed on S3 and S6, respectively. The concrete surface was saturated with water before the overlays were placed. These two overlays contained typical mixtures used on concrete bridge decks in Virginia. The 28-day compressive strengths of the latex- and the silica fume-based overlays from concrete cylinder tests were 5,490 pounds per square inch (37.85 megapascals) and 9,430 pounds per square inch (65.01 megapascals), respectively. The liquid membrane was used on sample S5, and the sheet membrane was incorporated in sample S4. Parchment paper that can withstand temperatures up to 420 degrees Fahrenheit (215.55 degrees Celsius) was used to create debonding under half of each asphalt overlay.
|Applicability of NDE methods for concrete bridge specimens without overlays.|
|Silica fume-modified concrete overlays with a thickness of 1.5 inches (3.81 centimeters) were placed on S6.|
As the defects were placed symmetrically with respect to the longitudinal axis of specimens, half of each overlay was bonded to the underlying concrete specimens, and the other half was debonded. This approach allowed researchers to examine if debonding can be recognized by each NDE technology, and if debonding can affect defect detections.
The investigation results identified effective and promising NDE technologies to detect and characterize deterioration in concrete bridge decks with overlays. The IE method, for example, detected debonding, shallow and deep delaminations, honeycombing, and voids for the bonded halves of S1E, S3L, S6S, and S8P. For the debonded halves of the same specimens, IE stress waves were reflected at the interface, resulting in the accurate detection of the debonded area.
|The condition maps of the S1E specimen using the dominant frequencies measured by the IE.|
|NDE C-Scan of S3L specimen with EyeCon. The C-Scan is a two-dimensional section reconstructed from multiple A-Scans in the X-Y plane.|
The researchers conducted IE tests at different temperatures in an environmental chamber to identify the temperature threshold at which the asphalt overlay remained in a high enough stiffness state to transfer stress waves. This action is important for the detection of defects. Otherwise, the stress waves substantially lose their energy, resulting in poor quality of the received signals. Results showed that temperatures at or below 32 degrees Fahrenheit (0 degrees Celsius) would be required for IE tests to image defects successfully using the dominant frequencies approach.
The IE method was able to detect shallow delamination and debonding in S4AS and S5AL. However, it could not detect deep delaminations, honeycombing, and voids in the bonded halves of S4AS and S5AL, because the membranes significantly reduced the propagation of waves into the underlying concrete specimens.
The IE method detected debonding, shallow and deep delaminations, honeycombing, and voids in S7A, because sufficient waves could propagate into the underlying concrete specimen without a membrane underneath the asphalt overlay.
The project outcomes identified and ranked available and promising NDE technologies for assessing concrete bridge decks with overlays.
|NDE methods for bridge deck with asphalt without a membrane overlay (S7A).|
Because of the differences between the construction of laboratory specimens and conventional bridge decks, large-scale field testing is required to assess the performance of the NDE methods in the field. The researchers investigated the field performance of IE and USW on the Arlington Memorial Bridge, located in Washington, DC. The bridge deck was rehabilitated with an asphalt overlay. The USW method detected debonding of the asphalt overlay in an area with lower moduli than the other regions. The USW method detected two large areas of defects.
|NDE methods for bridge deck with latex modified concrete overlay (S3L).|
The researchers also obtained condition maps based on the dominant frequencies measured by the IE method. An area with lower dominant frequencies compared with other regions indicated defects. The IE method detected two large areas compatible with defective areas revealed by the USW method. The intact areas have a thickness mode frequency of about 10 kilohertz. The debonding areas have dominant frequencies around 2 kilohertz.
|The condition map using the moduli measured with the USW method. The NDE assessment was performed on 250 feet (76.2 meters) of the bridge, starting from the north side on the left lane. The defected areas were found within 141–165 feet (42.9–50.2 meters) of the longitudinal direction.|
The FHWA Advanced Sensing Technology (FAST) NDE Laboratory used these testbeds to evaluate NDE technologies that can assess decks with overlays to supplement the information in the InfoTechnology web portal and the Long-Term Bridge Performance Program (LTBP) research on bridge decks with overlays. This research allows the FAST NDE Laboratory and the LTBP experts to provide bridge owners with field data collection protocols and information to identify NDE technologies to assess bridge decks with different types of overlays. The protocols provide distinctive, step-by-step instructions for data collection and comprehensive references for standards cited in the protocols (e.g., see the accompanying chart of the testing protocol for IE).
|A chart plot of the significant items from the IE method protocol. “These protocols are essential to maintain consistency in data collection and storage,” says Dr. Jean Nehme, Long-Term Infrastructure Performance team leader. “These protocols will be of interest to practitioners, researchers, and decisionmakers involved with the research, design, construction, inspection, maintenance, and management of bridges with overlayed decks.”|
Hoda Azari is the manager of the NDE Research Program and FHWA’s NDE Laboratory at the Turner-Fairbank Highway Research Center. She holds a Ph.D. in civil engineering from the University of Texas at El Paso.
Sadegh Shams is a contracted research engineer working in FHWA’s NDE Laboratory at the Turner-Fairbank Highway Research Center. He holds a Ph.D. in civil engineering from the University of Wisconsin-Milwaukee.
For more information, see https://highways.dot.gov/research/turner-fairbank-highway-research-center/facility-overview or contact Hoda Azari, 202–493–3064, Hoda.Azari@dot.gov.