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United States Department of Transportation United States Department of Transportation

Public Roads - Summer 2021

Date:
Summer 2021
Issue No:
Vol. 85 No. 2
Publication Number:
FHWA-HRT-21-004
Table of Contents

How 10 States Use, Evaluate, and Implement Nondestructive Evaluation Technology

by Hoda Azari

State DOTs are adopting NDE techniques to manage pavement and bridge deck maintenance and repairs.

Historically, State departments of transportation have relied primarily upon visual inspections to evaluate the condition of bridge decks. For suspect areas identified during the visual inspection, engineers use hammer sounding or "chain dragging" to further evaluate potential subsurface damage, such as delamination. Delamination occurs when deicing materials and water penetrate the concrete bridge deck down to the steel reinforcing bar. The steel corrodes until the force from the buildup of corrosion product cracks the concrete along the rebar mat, resulting in spalls and similar damage.

Roadway workers inspect a bridge deck. © Oregon DOT.
A bridge inspector uses the chain dragging technique to help identify delamination. While some states continue to prefer traditional methods like chain dragging, others are beginning to employ advanced, nondestructive evaluation (NDE) technologies instead of, or in addition to, these methods.

Advances in nondestructive evaluation (NDE) technology have enabled States to save inspection time and better assess the actual condition of bridge decks. The Federal Highway Administration NDE Program, in collaboration with the U.S. Department of Transportation's Technology Transfer program, recently conducted a comprehensive review of current practices and policies of those States that are using NDE technologies to complement visual inspections. FHWA researched published papers and conference presentations and also conducted interviews with key State asset owners.

The FHWA review primarily addressed bridge deck inspection and presented highlights of how bridge owners use NDE technologies to assist in managing bridges more effectively. The most common NDE methods used by States, primarily on bridge decks, pavement, and tunnels, are ground-penetrating radar (GPR), infrared thermography (IR), and impact echo. As part of the research process, each State also answered the question: "How can FHWA help with NDE knowledge and implementation?"

NDE Method

Description of Physical Principle

Ground-Penetrating Radar (GPR)

Ground-penetrating radar uses electromagnetic wave pulses in the microwave band of the radio spectrum to image the subsurface features in bridge decks or pavement. Radar reflections from inside the material are then imaged for evaluation. GPR can be deployed by hand or on push carts ("ground coupled") for a slow, more detailed inspection, or attached to a vehicle at traffic speeds ("air coupled"), but with some lost resolution in exchange for speed.

Infrared Thermography (IR)

Infrared thermography uses a camera that images the temperature of an object in lieu of inspecting the visual appearance of the object. Flaws in the bridge deck, pavement, or tunnel liner heat and cool at a different rate, providing a surface image of the location of the flaw.

Impact Echo

Impact echo is an acoustic or stress wave-based method in which stress waves are transmitted into the material and then reflected from features and flaws in the material.

The States discussed in this article also participated in at least one of the second Strategic Highway Research Program (SHRP2) projects by the American Association of State Highway and Transportation Officials (AASHTO): Nondestructive Testing for Concrete Bridge Decks and Nondestructive Testing for Tunnel Linings. Details of each State's participation, results, and reports can be found on the AASHTO SHRP2 program website at http://shrp2.transportation.org/Pages/R06_NondestructiveTesting.aspx.

While it is difficult to call any one State a lead NDE adopter because different States have different needs, examples of successful applications and case studies are provided in the following summaries and tables.

State

Chain Dragging

Ground-Penetrating Radar (GPR)

Infrared Thermography (IR)

Impact Echo

California

  • Evaluating suspect areas on bridge decks
  • Investigating automated sounding
  • Inspecting pavement
  • Estimating recycled volumes for asphalt milling
  • Determining asphalt thicknesses for concrete overlay
  • Detecting delamination in suspect areas on bridge decks and in tunnel liners
  • Locating subsurface utilities
  • Measuring cover depth prior to milling operations to avoid equipment damage
  • Identifying clear areas for coring
  • Using in conjunction with GPR
  • Detecting moisture in tunnel liner
  • Detecting delamination in suspect areas on bridge decks and tunnel liners

Colorado

  • Evaluating suspect areas on bridge decks
  • Hammer sounding tunnel liners
  • Evaluating suspect areas on bridge decks
  • Inspecting tunnel liners
  • Using in trial evaluations of suspect areas in tunnels

Summary of NDE Technology Applications in California and Colorado.

California

While the California Department of Transportation (Caltrans) is researching and starting to implement newer NDE technologies, the agency's bridge deck inspections remain primarily visual. Like most State DOTs, Caltrans supplements and enhances its visual inspections with spotting scopes and similar visual aids. As a followup to the visual bridge deck inspections, hammer sounding and chain dragging continue to be the primary methods to further evaluate suspect areas.

Caltrans has some high-speed inspection programs in place for pavement inspection and crack detection. Cameras collect images of pavement and bridge decks and apply artificial intelligence technology to identify and determine details like crack locations, widths, and density. The program has proved highly effective, especially in urban areas where it is difficult to close lanes for conventional visual inspection.

An aerial image with GPR data overlaid onto a street. Original image © Google Earth. Overlay © Caltrans.
 
An aerial image with GPR data overlaid onto a street. Original image ©Â© Google Earth. Overlay© © Caltran
Caltrans uses GPR data overlaid on maps to help identify service line locations and depths prior to roadway milling.

As a result of its participation in the SHRP2 program, Caltrans is deploying some NDE techniques such as IR cameras and GPR.

Caltrans reported that air-coupled GPR enables an increased data acquisition speed of up to 50 miles (80 kilometers) per hour; however, this rate is not appropriate for every investigation. The agency also investigated automated hammer sounding. This technique can be done faster than walking speed and is yielding similar results to chain dragging. The main advantage of this technique is that it uses a rolling closure at about 20 miles (30 kilometers) per hour in lieu of complete lane closures.

In terms of barriers to additional implementation of advanced NDE techniques, Caltrans indicated that the primary limiting factor is resources. With the number of bridge decks in California, current applications of advanced NDE technologies are better focused on specific project-level investigations responding to known or anticipated problems. Caltrans is not yet routinely using GPR, IR, or any other common NDE technologies for bridge deck evaluations.

The agency indicated that GPR is a good tool, but it requires significant training and experience to use it effectively. It is not clear how to make GPR simpler or more accessible, but it is a valuable tool, and Caltrans says it should be pushed forward to become a more routine, turnkey-type inspection. Similarly, IR is also helpful and complementary to GPR, but it needs some continued internal development to deploy. Other NDE techniques, such as impact echo, are waiting in the wings but not quite ready for broad implementation. These techniques should continue to be researched and moved ahead as a complement to GPR and IR.

"Caltrans would like to see improvements and enhancements in existing NDE techniques," says Erol Kaslan, the State bridge maintenance engineer in the Division of Maintenance at Caltrans. "More research is required to make NDE methods more economical to use and less subjective, so that the advanced NDE methods can be routinely used. Cheaper versions of technology with little or simplified postprocessing of data would be the goal in that alternative."

Caltrans performs return-on-investment calculations to decide when and where to use NDE and to determine if it is worth implementing additional NDE technologies.

How Can FHWA Help?

When asked how FHWA can help Caltrans with NDE knowledge transfer and implementation in California, the agency noted that there seems to be a core group of States implementing these technologies. On a Federal level, it would be good to bring States together and share results so that they can hear about lessons learned, develop core experiences, apply certain best practices, transfer knowledge, and implement group training. Improved dissemination of information among the States would be beneficial so that they can keep in touch with each other to share experiences.

Caltrans also feels more work needs to be done in validating and evaluating the effectiveness of the various NDE methods. An unbiased assessment of the advantages and limitations of the methods would be more useful than relying on assessments from consultants and equipment manufacturers.

Colorado

The Colorado Department of Transportation (CDOT) applied NDE techniques as part of its SHRP2 tunnel inspection program. CDOT employed both IR and LiDAR (light detection and ranging) methods in the initial scans of some tunnels.

"CDOT ran the IR and LiDAR system at traffic speed (up to 35 miles per hour, or 56 kilometers per hour) so that the tunnel could initially be inspected much faster than through manual inspections," says Tyler Weldon, a maintenance engineer with CDOT. "Where IR identified potential problems, CDOT manually investigated the areas with the conventional tunnel lining inspection techniques of visual inspection and tapping."

The agency found that the time required to collect, correlate, and analyze the data was not necessarily cost-effective relative to the information obtained from the inspections, and noted some data analysis repeatability issues. Correlating the images to the correct locations proved to be particularly cumbersome and time-consuming.

For bridge deck inspections, CDOT starts with a visual inspection and uses chain dragging to further investigate any suspect areas. Because NDE inspections provide the bridge inspectors with more detailed information about the condition of a bridge, CDOT looks at the NDE results when rating the bridge. For bridge decks with overlays, the agency brings in maintenance personnel for more examination and repair if the visual inspection identifies any suspected membrane problems. For inspections prior to a bridge deck rehabilitation, CDOT employs chain dragging and GPR to help with rehab quantity estimates.

Advanced nondestructive evaluation equipment mounted on the roof of a truck in front of a tunnel. © Colorado DOT.
CDOT used this truck-mounted high-speed IR and LiDAR system in tunnel inspections.

In addition, CDOT applies various monitoring technologies, including accelerometers, tilt meters, and water flow meters, when the agency has particular concerns about a bridge or wall—often because of scour impacts.

While CDOT does not have any new NDE technology investigations in its plans, it keeps up with the latest NDE developments through vendors, other States, and the Transportation Research Board to help identify potential applications. The agency has investigated drone use, but because of the involvement required (such as trained pilots) and high costs, CDOT does not anticipate a broad implementation of drones for its inspections.

How Can FHWA Help?

When asked about what else FHWA can do to help CDOT with NDE knowledge and implementation in Colorado, the agency responded that it always seeks to keep up to date on new NDE technology. Peer exchanges are particularly valuable for this effort. CDOT noted that the Western Bridge Preservation Partnership may be a group to further investigate for partnership or information exchange opportunities because NDE fits into part of its objectives.

CDOT cautioned that while new NDE technology offers some useful applications, it is not always a cost-effective solution, and forcing new technology on the States should be avoided. While FHWA guidance helps, States should determine if and when new NDE technology would be useful and cost-effective for a particular project or program.

State

Chain Dragging

Ground-Penetrating Radar (GPR)

Infrared Thermography (IR)

Impact Echo

Indiana

  • Detecting debonding in overlays
  • Detecting where chlorides have migrated into bridge deck using high-speed GPR
  • Predicting when and where problems can occur
  • Evaluated to identify suspect areas, but not currently implemented
  • Detecting debonding in overlays

Iowa

  • Using manual and automated sounding on bridge decks to evaluate suspect areas
  • Not applicable (high salt buildup in concrete, due to Iowa's climate, precludes the use of GPR)
  • Evaluated, but found polished rock aggregate causes reflection problems
  • Uses cart-based system on bridge deck overlays

Louisiana

  • Evaluating automated sounding to better plan hydro demolition
  • Inspecting decks and deck joints using high-speed GPR
  • Inspecting bridge decks and deck joints
  • Assessing fire damage to bridge decks
  • Measuring pile depths for scour evaluations

Summary of NDE Technology Applications in Indiana, Iowa, and Louisiana.

Indiana

The Indiana Department of Transportation (INDOT) participated in the SHRP2 study and scanned over 200 decks using air-coupled GPR. The agency used the results to assist engineers with programming decisions. INDOT viewed the SHRP2 meetings as helpful to better learn what other States have been doing with their evaluations and implementations of NDE technology.

"Bridge NDE inspection is becoming a larger part of INDOT's program to manage bridges. The NDE data are used to predict when and where problems can occur and to define deteriorated areas, including delamination," says Anne Rearick, the director of bridge management at INDOT.

While INDOT does have the ability to conduct NDE itself, it has found that consultants bring much more experience and sometimes apply proprietary software that does a good job. The agency enters the results into a database, and bridge inspectors may then use that information as part of their condition rating process. While INDOT does not perform return-on-investment calculations for NDE technologies, it does use the NDE data to develop bridge degradation curves and to make decisions about whether to repair or replace a bridge deck.

For future plans, INDOT came away from the SHRP2 study with questions about when and how to use GPR. As a result, the State is conducting further studies with IR, aerial IR, pole-mounted IR, and impact echo technologies with the intention of developing both project- and network-level NDE programs.

How Can FHWA Help?

When asked how FHWA can help further NDE knowledge and implementation in Indiana, INDOT noted that more emphasis should be given to support practitioners on how to best apply the various NDE technologies, including the collection, interpretation, and management of the data produced. Different recommendations are needed to address variables such as State geographies, weather, road treatments, and aggregates. INDOT is always looking to keep abreast of what is new in NDE technology and how it may help Indiana.

Iowa

For bridge deck inspections, the Iowa Department of Transportation (Iowa DOT) employs sounding as their most widely used and trusted technique, with repeatable and readily understood results. While almost all inspections are done manually, Iowa DOT has also investigated some automated hammer sounding systems to speed up the inspections. The agency concluded that while it does not necessarily save money performing automated inspections, inspections are completed much faster. Iowa DOT carried out validation tests and found that automated system results reasonably match the manual sounding results. In addition, the agency found the impact echo method—typically a cart-based system—to be the most reliable to inspect through concrete overlay bridge decks.

A man pushes equipment in a small cart on a bridge. © 2020 Iowa Department of Transportation.
A bridge inspector uses a cart-based impact echo system on a bridge deck in Iowa.

"The agency found that validation testing is the key to determine if an NDE technique does what consultants and equipment manufacturers claim," says Mike Todsen, a special projects engineer for Iowa DOT.

In one study, Iowa DOT compared the results from an automated system scan to their manual sounding results, with almost no correlation. "Without physical validation of the actual condition when using methods such as coring, the NDE results cannot be relied on," says Todsen.

Iowa DOT believes NDE can help in planning and decisionmaking, but has not done formal return-on-investment calculations to quantify the added value of NDE.

How Can FHWA Help?

Iowa DOT reported that it would like to see more work done by FHWA to evaluate the effectiveness and validation of the various NDE methods. An unbiased assessment of the advantages and limitations of the methods would be more useful than relying on consultants and equipment manufacturers. The agency would also welcome having advice available to pick the best NDE system and solution for a particular inspection problem.

Louisiana

The Louisiana Department of Transportation and Development (LADOTD) has experienced a culture shift toward the use of NDE technology. The agency believes chain dragging to be an aging technology, and typically only the more experienced inspectors still use it. The new generation of inspectors does not view chain dragging as the best technique.

The agency has employed high-speed GPR and IR to minimize or eliminate the need for lane closures, and as a way to prioritize which bridge decks need work.

"LADOTD has the third-highest bridge deck area in the country, and with such long bridges—some more than 17 miles [27 kilometers] long—it is not practical to chain drag all the decks," says Haylye Brown, an assistant bridge maintenance engineer for LADOTD Section 51. "LADOTD is proactive in the use of NDE and relies heavily on NDE techniques to help identify potential problems areas."

LADOTD noted that its NDE applications are working well. Program managers use the NDE data to plan repairs to bridge decks. Currently, LADOTD does not use NDE results as part of the bridge condition rating; however, it would like to pursue this capability and is investigating the possibility.

Regarding the use of other advanced NDE technology in the State, LADOTD is evaluating unmanned aerial vehicle (drone) technology and plans to provide a drone to every district for bridge inspection. The agency also noted that NDE technology is improving. High-speed GPR deck inspection used to be limited to around 34 miles (55 kilometers) per hour, and now, inspections are performed at up to 60 miles (96 kilometers) per hour.

How Can FHWA Help?

LADOTD indicated that FHWA could increase NDE knowledge and implementation in Louisiana by working to define what NDE personnel requirements should be specified and how to correlate NDE data to bridge element conditions. The agency would also appreciate ways to keep abreast of advances in NDE technology, including knowledge sharing about what is new and where best to use it.

A truck on the side of a bridge pulls nondestructive evaluation equipment behind it. © LADOTD.
LADOTD used this truck-mounted, high-speed impact echo system as part of its SHRP2 project.


 

State

Chain Dragging

Ground-Penetrating Radar (GPR)

Infrared Thermography (IR)

Impact Echo

Nebraska

  • Evaluating suspect areas on bridge decks
  • Developing automated system with local universities
  • Evaluating bridge decks to help estimate repair work
  • Evaluating bridge decks
  • Used infrequently

New Mexico

  • Detecting debonding in overlay
  • Checking for correct rebar placement when unexpected cracking occurs
  • Detecting delamination in bridge decks (used infrequently)
  • Used infrequently
  • Used infrequently

Oregon

  • Evaluating bridge decks with high-speed automated sounding
  • Inspecting bridge decks at higher speeds
  • Checking for correct rebar location and depth
  • Supporting identification of coring locations for concrete chloride testing
  • Locating subsurface utilities
  • Evaluating bridge decks
  • Performing experimental projects using high-speed impact echo system
  • Assessing bridge deck condition

Summary of NDE Technology Applications in Nebraska, New Mexico, and Oregon.

Nebraska

The Nebraska Department of Transportation (NDOT) reported that it conducted university-level research in NDE and completed some field implementations. NDOT inspectors routinely use visual inspection. If visual inspections indicate a concern, NDOT performs chain dragging or a more indepth inspection.

Equipment mounted to the back of a truck. © Nebraska DOT.
The Nebraska Department of Transportation uses a truck-mounted, air-coupled GPR system to evaluate bridge decks.

NDOT performs advanced NDE only at the project level and does not own its equipment for advanced NDE methods. The agency hires outside consultants for all advanced NDE. Under SHRP2, the consultants, along with a team from the University of Nebraska and Brigham Young University, carried out GPR and IR imaging on a variety of bridge decks. Because NDOT uses asphalt overlays extensively when rehabilitating bridge decks, it primarily wanted to determine how the NDE methods worked on overlaid decks.

"NDOT wants to apply NDE to help with estimating quantities for its rehabilitations," says Fouad Jaber, an assistant State bridge engineer for NDOT. "Overall, NDOT was satisfied with the technology's performance at estimating quantities on asphalt overlays."

While no specific return-on-investment calculations have been performed, NDOT believes these scoping inspections will save money by more accurately estimating the range of work.

"NDOT now has 2 years of pre-construction deck evaluation data that could be analyzed to see if 1) they correlate with lower unit cost for repairs, 2) they have been accurate when compared to paid quantities, and 3) there have been cases when repairs were not cost-effective because they were too extensive," says Kent Miller, a data management engineer at NDOT.

NDOT's university partners are also working on a traffic speed platform with several NDE methods.

How Can FHWA Help?

When asked how FHWA can help with NDE knowledge and implementation in Nebraska, NDOT noted that some NDE methods are easy to comprehend and interpret and some are more challenging. The agency does not have the training to interpret NDE data, so assistance in that area would be helpful. The agency must rely on the consultant and cannot, as the owner, do a good job on the quality control of the inspection process.

"The top NDE need for NDOT would be a way to determine when membranes are breached," says Miller.

The implementation of the SHRP2 project went well because NDOT received funding to apply new technologies, rather than having to fund the work internally. NDOT would like to see more of these kinds of projects to try new or proven technologies. It would be helpful if competitive funding were available that could be requested to evaluate and enable new NDE research, similar to the programs for evaluating ultra-high performance concrete. An FHWA program would enable the States to implement some of the new ideas they would like to see to fruition.

NDOT indicated that the Midwest Bridge Preservation Partnership would be a good forum to help spread any NDE news. There are also annual bridge preservation partnership meetings at which NDE news could be shared.

New Mexico

The New Mexico Department of Transportation (NMDOT) primarily uses NDE to follow up on suspected areas identified in visual inspections. NMDOT reported that it employs NDE methods to help manage bridges on a project level, as well as to find problems that need repair, and then to establish projects to fix the problems. The agency also uses NDE results to help guide decisions on whether to preserve, repair, or replace bridge sections.

A man in safety equipment pushes a cart-mounted device on a bridge. © New Mexico DOT.
A student researcher performs a GPR evaluation of a bridge deck in New Mexico.

NMDOT identified a number of factors as barriers to implementing more advanced NDE techniques, including: 1) difficulty accessing all areas on elements other than bridge decks, 2) traffic control, 3) resistance from users because of the uncertainty of the data's usefulness, and 4) variability of results between inspectors. However, NMDOT indicated some successful NDE applications, in which NDE resulted in more precise results than chain dragging. For instance, the agency used ultrasonic testing to detect voids on bridge diaphragms and to inspect steel bridge pins to ensure that they were free of cracks.

"GPR is not a magic bullet, but it offers value when applied appropriately," says Kathy Crowell, a bridge design section manager for NMDOT's Bridge Bureau. "GPR can help determine whether a deck is a candidate for preservation versus replacement."

NMDOT found that GPR works well as a first pass, but a second, more indepth scan is necessary to produce the more detailed results critical to decisionmaking. For overlay inspections, NMDOT indicated that chain dragging is the most reliable technique it has used to detect debonded overlays. GPR seems to work in seeing through asphalt and epoxy overlay, but does not appear to work with some overlay materials such as latex-modified concrete.

How Can FHWA Help?

NMDOT indicated that FHWA could contribute to NDE training and education opportunities—particularly because of turnover in NMDOT staff. FHWA could also assist with a better way to keep apprised of advances in NDE technology, including webinars, NDE-focused videos, and peer exchange meetings. NMDOT indicated that it will be important to keep expectations reasonable when comparing NDE lab performance and real-world NDE performance.

Oregon

The Oregon Department of Transportation (ODOT) reported that it extensively employs chain dragging on concrete decks, and regularly uses magnetic particle inspection and ultrasonic inspection on steel bridge elements.

After performing some experimental projects using advanced NDE methods on concrete, the agency concluded that chain dragging will perform the same inspection as the more advanced NDE methods—and do it faster and cheaper. ODOT also felt that manufacturers oversold the capabilities of some of the advanced NDE technology, and the techniques did not necessarily do a good job at detecting delamination.

ODOT noted that it has had some successes in the area of structural health monitoring. "The agency monitored 13 bridges because of known, existing problems," says Steven Lovejoy, a senior mechanical engineer with ODOT. "ODOT monitored some bridges for just a few months and others for approximately 20 years, generally using strain gages or displacement transducers."

ODOT also noted that NDE results do not directly tie into the condition rating of bridges because the NDE results were difficult to correlate to an actual bridge condition, primarily because of issues in interpreting NDE data.

ODOT did not present any recent examples of the value of NDE being quantified for any projects. The agency discussed future plans to assess the cost of hiring an NDE consultant to scan a bridge deck to help estimate the extent of required repairs. This cost would then be compared to the cost of material overruns.

How Can FHWA Help?

When asked to highlight gaps between NDE technology that is available and potential future implementations in Oregon, ODOT responded that the procedure to take large amounts of raw NDE data and process these data to a point where they could be used to manage an asset (such as a bridge) is too difficult and inconsistent between operators. Despite ASTM standards to help regulate the process and make it more consistent than chain dragging, the data post-processing seems to add inconsistency. ODOT feels that it would be valuable if universal references and standards could be created to make the tests more repeatable.

State

Chain Dragging

Ground-Penetrating Radar (GPR)

Infrared Thermography (IR)

Impact Echo

Pennsylvania

  • Evaluating bridge decks and tunnel liners using hammer sounding
  • Evaluating bridge decks to estimate repair work
  • Detecting deterioration under asphalt
  • Measuring asphalt thickness
  • Measuring rebar cover depth
  • Performing quality assurance inspections of the rebar cover depth on new bridge decks
  • Evaluating tunnel liners
  • Evaluating bridge decks
  • Detecting moisture in tunnel liners
  • Used infrequently

Virginia

  • Detecting delamination and overlay debonding in concrete decks
  • Evaluating bridge decks
  • Checking rebar placement on new decks
  • Evaluating suspect tunnel liner areas
  • Evaluating tunnel roadway
  • Evaluating bridge decks
  • Inspecting tunnel liners using combination of IR and high-resolution videos
  • Checking for water intrusion in tunnel liners and culverts
  • Evaluating suspect areas in tunnel liners

Summary of NDE Technology Applications in Pennsylvania and Virginia.

Pennsylvania

The Pennsylvania Department of Transportation (PennDOT) reported that it primarily uses visual inspection on bridge decks. However, it has used NDE data, such as GPR and IR, to manage bridges and tunnels on a limited basis on specific projects to help with decisionmaking.

Pennsylvania also participated in the SHRP2 program for bridge decks and considered it a success. The program familiarized the State with some of the advanced NDE equipment and technology. PennDOT has used the GPR unit it purchased as part of the SHRP2 project to scan bridge decks and help plan the type and amount of needed rehabilitation or repair work.

PennDOT concluded that GPR can be effective, but only on certain bridge decks and in certain circumstances, because getting bridge deck deterioration information takes a lot of analysis after the scanning.

"Uncertain accuracy in the NDE results and the time and cost involved to get the final NDE results are the main barriers to implementing NDE on bridge decks," says Tom Macioce, the chief bridge engineer in the Bridge Design and Technology Division of PennDOT's Bureau of Project Delivery. "PennDOT would like to see a standard GPR procedure or solution for data analysis and reporting, including detailed settings of equipment, better accuracy in the rebar identification aspect of the software and process, and more cooperation between academia and NDE equipment manufacturers."

PennDOT also performed some tunnel inspections as part of the SHRP2 program. The goal was to demonstrate and evaluate the ability of hand-held NDE methods and high-speed mobile scanning NDE methods, such as air-coupled GPR and vehicle-mounted IR, to detect deterioration and defects in concrete tunnel linings. The agency also tested traditional physical inspection techniques, including hammer sounding and coring, in some limited areas to evaluate the high-speed NDE results.

PennDOT concluded that no single technology by itself can provide all information needed to ascertain the condition of the tunnel lining. The agency believes that the best application for the NDE technology is to use one or two NDE methods in combination with traditional hammer soundings. PennDOT determined that while GPR correlated reasonably well with hammer soundings, GPR does not seem to be a practical solution over traditional physical methods for biennial inspections because of the cost and time required for data processing. However, cost and time may be reduced significantly during subsequent NDE inspections because test procedures would already be established.

A van with externally mounted equipment in front of a tunnel entrance. © PennDOT.
PennDOT used this van-mounted GPR system to inspect tunnel walls.

In the tunnel IR testing, PennDOT found it difficult to obtain reliable and useful results for tunnel liners because the surface of the liner is not directly exposed to the sun for more active heating and cooling cycles. IR may be better suited to detect moisture-related anomalies in which the evaporative cooling is detectable.

PennDOT indicated several obstacles to greater highway application of NDE technologies. The cost and uncertain accuracy of NDE results are concerns, and sounding is still required. The software used to process and analyze the large amounts of collected data is not user-friendly and an experienced operator is required to run the software. For example, PennDOT received an estimate of 2,500 man-hours to convert digital photogrammetry data to drawings. Typically, the agency receives final NDE results within about a month of an inspection. In one case, it took 7 months to get a final NDE report.

How Can FHWA Help?

When asked how FHWA might better contribute to PennDOT's applications, the agency indicated that it would like to receive more updates about the general state of infrastructure NDE and to learn what NDE technologies and vendors have been evaluated by FHWA. FHWA could also help by continuing to share through the SHRP2 program what other States are doing. PennDOT would be interested in obtaining FHWA sponsorship every few years to participate in the updates.

Virginia

The Virginia Department of Transportation's (VDOT's) bridge deck inspections are primarily visual and tactile in nature. When preliminary inspections indicate significant corrosion deterioration, the agency deploys more indepth methods such as chain dragging, sounding, and half-cell potentials. VDOT has used advanced NDE techniques like GPR, IR, impact echo, and ultrasonic tomography if the need arises.

VDOT's work on evaluating advanced NDE techniques concentrates on two areas. The first area of interest is the rapid screening of a bridge deck to quickly assess its condition without the need to close lanes. Lane closures not only interfere with normal traffic flow, but also are hazardous for highway workers on the bridge deck. The second area of interest is in using NDE methods to help determine the extent of damage to a bridge deck. This information becomes valuable in making repair decisions, estimating damage progression rates, and better estimating and planning repair material quantities.

"Virginia's bridge deck NDE research under the SHRP2 program evaluated GPR, IR, and high-resolution digital imaging NDE technologies on 25 bridges," says Soundar Balakumaran, the associate director at the Virginia Transportation Research Council, which is part of VDOT. "The inspections did not yield any significant new information when compared to conventional manual inspections. The tunnel lining interfered with IR, which did not appear to provide enough image contrast."

VDOT inspected a few of the suspect areas with a more thorough hand-held GPR scan and with impact echo. Those results correlated well with routine inspections.

VDOT also performed NDE research in several other areas such as time-lapse thermography and magnetic flux leakage inspections. Magnetic flux leakage is a method where a magnetic field is introduced in a steel cable or tendon and then magnetic field sensors are used to look for leakage fields that identify potential underlying damage. These are not routine inspections, and were used only for a few special investigations. VDOT employed a magnetic flux leakage NDE system to inspect some post-tensioned tendons with good results and verified the technology in the lab on a few mock-up tendons with prebuilt defects and found excellent correlation.

Equipment mounted to an internal wall of a bridge. © Virginia DOT.
VDOT used this magnetic flux leakage NDE technique to scan a post-tensioned tendon inside a segmental box girder of a Virginia bridge.

VDOT reported that its approach is to conduct research first, perform some trial projects on the more promising technologies, and then look for the best potential applications of the technology. The agency usually performs the initial implementation of new technologies with consulting companies. VDOT wants to implement the new technologies to reduce lane closures and save money, but first needs to validate each technology's performance.

There were many cases in which NDE methods saved VDOT money. In one example, initial tests showed that there may have been an issue with the cover depth of the reinforcing bar on a new bridge deck. A more detailed scan with GPR revealed that the entire top rebar mat had sunk due to misplacement and left a thick, unreinforced layer of concrete at the surface. The contractor replaced the entire deck before the bridge was put in service, avoiding the problems and resulting repairs that the issue would have caused.

How Can FHWA Help?

When asked how FHWA could better support VDOT's NDE applications, the agency stated that open communication, such as the discussions and demonstrations in the SHRP2 meetings, helps States to keep up with which NDE technologies are being used and which new ones are emerging for potential future applications.

Summary

The FHWA review provides a snapshot of how 10 States use, evaluate, and implement both basic and advanced NDE methods as part of their bridge deck inspection and management programs. All States have applied NDE techniques at some level. Several of the sample States have successfully established advanced NDE technology. For example, Indiana and Louisiana have both improved their ability to efficiently manage bridge deck maintenance and repair through new NDE processes. While traditional chain dragging techniques remain the choice of some States, NDE technologies such as at-traffic speed GPR and IR have enabled some States to dramatically improve the value of their NDE information and inspection efficiency.

FHWA provided States with an opportunity to identify where they saw gaps between NDE knowledge and NDE implementation, and where FHWA can focus its support to close those gaps. FHWA is thoroughly evaluating and prioritizing these recommendations to ensure that the goals of the FHWA NDE program are addressing the needs of the States. The latest FHWA NDE Program Strategic Plan has incorporated these States' needs as the primary focus.

The graphic starts from the left side with three arrows in a column. From top to bottom, these arrows read as follows: "Strategic Goals" (SGs), "Strategic Objectives" (SOs), and "Strategies." Each of these arrows points to a different level in the flow chart. From left to right, the strategic goals are as follows: "SG1: Stakeholders" (represented by a schematic of three people), "SG2: Technology" (represented by a robot and a monitor), and "SG3: Implementation" (represented by a launching space shuttle). These SGs flow down to the SOs. From left to right, the SOs are as follows: "SO1: Support Effective Application of Existing Nondestructive Evaluation (NDE) Technologies to Address the Needs of Asset Owners," "SO2: Develop New and Improve Existing Technologies to Meet the Needs of Stakeholders," and "SO3: Implement Deployment-Ready Technologies." This row flows down to strategies. The strategies for SO1 are as follows: "S1.1: Develop and Maintain Open Communication Channels With Stakeholders" and "S1.2: Maintain Expertise in Established and Emerging Technologies." The strategies for SO2 are as follows: "S2.1: Conduct NDE of Highway Infrastructure," "S2.2: Perform Construction Quality Assurance of Highway Infrastructure," "S2.3: Develop a Holistic Approach to Integrated Technology Solutions," "S2.4: Characterize Capabilities and Applications of Existing NDE Technologies," "S2.5: Leverage Robotic Systems to Improve Reliability/Accuracy and Reduce Deployment Cost/Time," "S2.6: Turn Proven Innovations Into Market-Ready Technologies," and "S2.7: Consider Future advanced Tools and Platforms." The strategies for SO3 are as follows: "S3.1: Maintain, Refine, and Expand Current Web-Based Training Tools," "S3.2: Develop and Validate Implementation Strategies," "S3.3: Perform Technology Demonstrations," "S3.4: Capture and Disseminate Best Practices," "S3.5: Assist in Developing Training Curricula, Standards, and Certification Related to Assessment Technologies," and "S3.6: Develop an Outreach Program Through Webinars, Workshops, Training, Discussion Forums, and Technical Briefings." Source: FHWA.
The strategic plan for FHWA's NDE Program includes strategic goals and objectives for stakeholders, technologies, and implementation.

Hoda Azari is the manager of the NDE Research Program and FAST NDE Laboratory at FHWA's Turner-Fairbank Highway Research Center. She holds a Ph.D. in civil engineering from the University of Texas at El Paso.