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U.S. Department of Transportation U.S. Department of Transportation Icon United States Department of Transportation United States Department of Transportation
Date:
Autumn 2022
Issue No:
Vol. 86 No. 3
Publication Number:
FHWA-HRT-23-001
Table of Contents

Nondestructive Evaluation of Concrete Bridge Deck with Overlays

by Hoda Azari and Sadegh Shams
"Construction workers are shown applying asphalt on a bridge deck. The bridge is over a river. Construction vehicles are on the bridge. Image Source: © Roman023_photography / Shutterstock.com."
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.

"Asphalt pavement is shown with major surface cracks. Construction equipment and cranes are on the roadway, which leads to the Lincoln Memorial in the distance. Image Source: FHWA."
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.

"A robotic arm in the laboratory is attached to a moving frame that moves in the X and Y direction over a concrete specimen. An ultrasonic imaging system is attached to the head of the robotic arm. Image Source: FHWA."
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:

  • Sounding
  • 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)

Specimen Fabrication

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.

"Line graph with x-axis ranges between 0 hertz to 10 kilohertz, and y-axis ranges between 0 and 1. The y-axis is normalized (it does not have dimension). Two traces are shown: the reflecting and the absorbing lateral boundaries in solid black and dotted red lines, respectively, fluctuating within the same frequencies between normalized amplitude of 0 to 1. The peak frequency is around 7 kilohertz. The curve is similar to an impact function. Image Source: FHWA."
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.

"Contour map of the specimen’s surface. The x-axis is the longitudinal distance in inches, ranging from 0 to 120 inches. The y-axis is the lateral distance in inches, ranging from 0 to 35 inches. The diagram shows a color spectrum where red appears at the center of the defects, yellow color is shown around boundaries of defects, and green is shown in all other areas. A key above the contour map shows that shallow delamination is shown as a rectangular solid line, the void as a rectangular dash-dotted line, the honeycomb as a rectangular long-dash line, the deep delamination as a rectangular dotted line, and the vertical crack as a vertical dotted line. Image Source: FHWA."
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.

"Contour map of the specimen’s surface. The x-axis is the longitudinal distance in inches, ranging from 0 to 120 inches. The y-axis is the lateral distance in inches, ranging from 0 to 40 inches. The diagram shows a color spectrum where red appears at the center of the defects, yellow color appears around the boundaries of the defects, and blue appears in all other areas. A key above the contour map shows that shallow delamination is shown as a rectangular solid line, the void as a rectangular dash-dotted line, the honeycomb as a rectangular long-dash line, the deep delamination as a rectangular dotted line, and the vertical crack as a vertical dotted line. Image Source: FHWA."
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.
 

"A table with 11 rows and 8 columns is shown. The label, Methods, is above the rows. The rows are labeled Sounding, USW, IE, UT-MIRA, UT-EyeCon, IR, GPR, ER, HCP, and IRT. Above the columns is the label, Defects in concrete bridge specimen. The columns are labeled Shallow delamination, Honeycombing, Void, Deep delamination, Vertical crack, Active rebar corrosion, and Concrete corrosive environment. The cells under Shallow delamination read Yes, Yes, Yes, Yes, Yes, Yes, Yes, No, No, and Yes for Sounding, USW, IE, UT-MIRA, UT-EyeCon, IR, GPR, ER, HCP, and IRT, respectively. The cells under Honeycombing read No, Yes, Yes, Yes, Yes, No, Yes, No, No, and Yes*, respectively. The cells under Void read Yes*, Yes, Yes, Yes, Yes, No, Yes, No, No, and Yes*, respectively. The cells under Deep delamination read No, Yes, Yes, Yes, Yes, No, Yes, No, No, and No, respectively. The cells under Vertical crack read No, Yes, No, No, No, No, No, Yes, No, and Yes*, respectively. The cells under Active rebar corrosion read No, No, No, No, No, No, No, No, Yes, and No, respectively. The cells under Concrete corrosive environment read No, No, No, No, No, No, Yes, Yes, No, and No, respectively. Image Source: FHWA."
Applicability of NDE methods for concrete bridge specimens without overlays.
"A concrete specimen is shown. Half of the top surface is covered with a plastic sheet. The silica fume-modified concrete is being applied to the specimen's top surface. Part of the surface of the one end is shown overlayed. Image Source: FHWA."
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.

"Contour map of the specimen’s surface. The x-axis is the longitudinal distance in inches, ranging from 0 to 120 inches. The y-axis is the lateral distance in inches, ranging from 0 to 35 inches. The diagram shows a color spectrum at the top half of the diagram, where red appears at the center of the defects, yellow color around boundaries of defects, and green in all other areas. The bottom half of specimen’s surface shows an almost red color without a distinctive spectrum to identify defect areas. A color bar shows a color spectrum from red, indicating 0 kilohertz to blue, indicating 15 kilohertz. A key above the contour map shows that shallow delamination is shown as a rectangular solid line, the void as a rectangular dash-dotted line, the honeycomb as a rectangular long-dash line, the deep delamination as a rectangular dotted line, and the vertical crack as a vertical dotted line. Image Source: FHWA."
The condition maps of the S1E specimen using the dominant frequencies measured by the IE.
"Contour map of the specimen’s surface. The x-axis is the longitudinal distance in inches, ranging from 0 to 120 inches. The y-axis is the lateral distance in inches, ranging from 0 to 40 inches. In the top half of the diagram, red appears at the center of the defects, yellow color around boundaries of defects, and blue in all other areas. The bottom half of specimen’s surface shows an almost red color without a distinctive spectrum to identify defect areas. A key above the contour map shows that shallow delamination is shown as a rectangular solid line, the void as a rectangular dash-dotted line, the honeycomb as a rectangular long-dash line, the deep delamination as a rectangular dotted line, and the vertical crack as a vertical dotted line. Image Source: FHWA."
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.

"A table with 11 rows and 8 columns is shown. The label, Methods, is above the rows. The rows are labeled Sounding, USW, IE, UT-MIRA, IR, GPR, ER, HCP, and IRT. Above the columns is the label, Defects in concrete bridge specimen with bonded overlay. The columns are labeled Overlay debonding, Shallow delamination, Honeycombing, Void, Deep delamination, Vertical crack, Active rebar corrosion, and Concrete corrosive environment. The cells under Overlay debonding read: Yes, Yes, Yes, No, Yes, No, No, No, and Yes for Sounding, USW, IE, UT-MIRA, IR, GPR, ER, HCP, and IRT, respectively. The cells under Shallow delamination read No, No, Yes, No, No, Yes*, No, No, and Yes*, respectively. The cells under Honeycombing read No, No, Yes, No, No, Yes*, No, No, and No, respectively. The cells under Void read No, No, Yes, No, No, Yes*, No, No, and No, respectively. The cells under Deep delamination read No, No, Yes, No, No, Yes*, No, No, and No, respectively. The cells under Vertical crack all read No. The cells under Active rebar corrosion all read No. The cells under Concrete corrosive environment read No, No, No, No, No, Yes*, No, No, and No, respectively. Image Source: FHWA."
NDE methods for bridge deck with asphalt without a membrane overlay (S7A).

Field Testing

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.

"A table with 11 rows and 8 columns is shown. The label, Methods, is above the rows. The rows are labeled Sounding, USW, IE, UT-MIRA, UT-EyeCon, IR, GPR, ER, HCP, and IRT. Above the columns is the label, Defects in concrete bridge specimen. The columns are labeled Overlay debonding, Shallow delamination, Honeycombing, Void, Deep delamination, Vertical crack, Active rebar corrosion, and Concrete corrosive environment. The cells under Overlay debonding read: Yes, Yes, Yes, Yes, Yes, Yes, No, Yes, No, and Yes for Sounding, USW, IE, UT-MIRA, UT-EyeCon, IR, GPR, ER, HCP, and IRT, respectively. The cells under Shallow delamination read No, Yes, Yes, Yes, Yes, No, Yes*, No, No, and Yes, respectively. The cells under Honeycombing read No, Yes, Yes, Yes, Yes, No, Yes*, No, No, and No, respectively. The cells under Void read No, Yes, Yes, Yes, Yes, No, Yes*, No, No, and No, respectively. The cells under Deep delamination read No, Yes, Yes, Yes, Yes, No, Yes*, No, No, and No, respectively. The cells under Vertical crack all read No. The cells under Active rebar corrosion read No, No, No, No, No, No, No, No, Yes, and No, respectively. The cells under Concrete corrosive environment read No, No, No, No, No, No, Yes*, No, No, and No, respectively. Image Source: FHWA."
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.

"Contour map of the specimen’s surface. The x-axis is the longitudinal distance in feet, ranging from 0 to 120 feet. The y-axis is the lateral distance in feet, ranging from 0 to 20 feet. The diagram shows a contour map with a red to blue spectrum. There is a continuous red area at the top of the diagram between 140 feet and 165 feet. A color bar shows a color spectrum from red, indicating 1,000 kilopounds per square inch, to blue, indicating 4,000 kilopounds per square inch. Image Source: FHWA."
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.

NDE Protocols

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).

"Impact Echo Testing Protocol for Bridge Decks w/Overlays. Lines to: Data Collected (DC); Onsite Equipment & Personnel Requirements (OEPR); Methodology (M). Line from DC to: •Evidence of concrete delam & other defects like overlay debonding; •Conduct IE testing on concrete decks w/asphalt overlays w/surface temps under 35°F. Results depend on temp at testing time & must be corrected based on ref. temp. Line from OEPR to: Equipment •PRE-PL-LO-004, Personal Health & Safety Plan; •Impact echo equip/device; any of these: -Cart-mounted & semiautomated to control data collection line length & spatial location, spacing between individual test measurements, triggering impact/source deployment, ease of use; -Hand deployed & triggered manually at test points; -Data acqu. controller & software to sample & record at 50 kHz min. rate. •Data controller/storage device: laptop or data logger; •Data conditioning & analysis software to filter, clip, segment, discard noise & waveforms unrelated to IE data; •Impact source to generate & receiver to record signals, w/acceptable signal-noise ratio in 2-25 kHz range; •Digital camera; •Personnel: PRE-PL-LO-005, Personnel Quals. Line from M to: •Use local rect. grid (FLD-OP-SC-001, Data Collection Grid & Coordinate System for Bridge Decks) to locate test points; •Test Prep: Clear debris; •Measurements: -Collect data along any test line, as long as location of every IE reading is recorded; -Place impact device & sensor in direct contact w/sampled surface; -During field ops, ensure data in time & freq. domains are consistent w/expected signals from structure. •Final forms for data: time histories & freq. response of measured raw data at all x & y test locations from gridded deck. Processed data consists of condition grading based on resonant freq. response of IE data (grades 1 to 4), also on x-y coordinate system. •Reporting: Transfer metadata, data, docs & images to FHWA, and/or upload into Long-Term Bridge Performance Bridge Portal. Image Source: FHWA."
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.