The only reinforced-concrete bridge deck inspection technique required by the Federal Highway Administration (FHWA) is visual inspection.
The process of visual inspection includes an inspector looking at the surface of a deck for cracks, potholes, efflorescence and other signs of deterioration. The inspector then rates the deck based on the extent of deterioration viewed and recorded, and provides pictures of any damage noted. This is then used to make a decision as to the repair or replacement needs of the structural element.
While visual inspection can provide useful information, it does not provide a complete understanding of the deck’s condition, because subsurface information cannot be acquired. Other methods are used for this purpose (e.g., hammer tapping and chain dragging); however, these methods are only useful for detecting subsurface deterioration that is in an advanced stage, and can only be used in ideal conditions, such as on a bridge deck without an asphalt overlay, or those without significant traffic noise. Most of the bridge decks in Rhode Island, specifically, have asphalt overlays.
Nondestructive evaluation tools
In an effort to obtain a more comprehensive bridge deck evaluation, the Rhode Island Department of Transportation (RIDOT) is working with Roger Williams University to assess the possibility of incorporating nondestructive evaluation techniques such as ground-penetrating radar (GPR) and infrared thermography (IR) into bridge deck inspections. There are two GPR systems that RIDOT is looking to employ: a ground-coupled system and an air-coupled system. The results presented in this article were completed with data collected using the ground-coupled system (Figure 1), but bridges are currently being selected to scan with the air-coupled system. The ground-coupled system requires lane closures but is exceptionally useful for detailed assessments. The air-coupled system can be operated without any lane closures, and can collect enough data to provide an accurate deck assessment. The purpose of using the ground-coupled system for this project was to obtain as much detail as possible for this initial investigation.
Independent of the system used (ground-coupled or air-coupled), GPR works by sending electromagnetic pulses from a high-frequency antenna (1.6 GHz) through the subsurface of a bridge deck. When those signals encounter a material of differing dielectric property, like the concrete/reinforcing steel (rebar) interface, the signal is reflected and captured by the receiver in the antenna. For bridge decks, this signal reflection from the rebar is one often used for analysis, because it is corrosion of the reinforcing steel that is the main cause of bridge deck deterioration. Information at the rebar level can provide significant insight into the condition of the deck. Below are collected radar signals (B-scans) from two different bridge decks. Figure 2 is an assembly of signals collected from a healthy deck, and Figure 3 from a deteriorated deck. The rebar reflections (represented by hyperbolas) collected from the healthy deck are bright, clear, and easy to see. Those from the deteriorated deck are fuzzy, and in many cases are not as easy to pick out of the image. This decrease in image quality (represented by a decrease in signal amplitude at the rebar level) is associated with deterioration.
Infrared thermography works by collecting surface temperatures of the deck using an infrared camera. Figure 4 shows a typical IR camera mount setup for collecting data from a bridge deck. The camera sits about 12 ft above the surface of the bridge deck, pointing directly down, so that an entire lane’s width of data can be collected, at posted speeds, in one pass while minimizing image distortion. The IR camera measures thermal radiation emitted through the deck’s surface, and displays it as a temperature.
As rebar corrosion progresses, delaminations, or air/water pockets, develop above the rebar. The air pocket acts as an insulator, so when the deck is heating up as the day begins, more energy is emitted from locations that contain delaminations below the surface, and appear as hotter temperatures in comparison to those deck areas free from delaminations.
Data collection and testing
During the summer of 2015, concrete bridge decks were scanned with ground-coupled GPR and IR. Ground truth data was obtained from the decks as well (to verify the GPR and IR findings) using a number of different testing methods, including half-cell potential, impact echo, hammer sounding, chain drag, concrete powder (for chloride testing) and coring. The GPR and IR results of two of the bridge decks are presented in this article.
The main selection criteria for this project were that the decks did not have a concrete or asphalt overlay (to minimize variables for this initial investigation); they were on a road with a very low average daily traffic count; and there was a variety of deterioration levels among them. For the IR testing, decks with shade due to surrounding trees or overhead structural elements were not ideal candidates. Also, if one day was predicted to have more of a temperature differential than another, that day was selected as delaminations with the IR camera would be easier to identify. Both GPR and IR data collection requires the deck to be free of moisture for at least 24 hours, and the IR requires sunny skies.
Data collection from each deck began after a 1-ft x 1-ft grid was painted onto each deck so that defects identified by each method could be properly located on the deck, once post-processing was completed. Typically the GPR and IR systems were used to collect data first, as any method using water will distort both sets of data. Depending upon the bridge location, size and age, not all ground truth methods were used on every deck. Therefore, the data collection could take anywhere between 4-8 hours.
Post-processing, visualization and results
Once the data was collected, a number of post-processing steps were required to extract pertinent information. The GPR data was post-processed mostly using commercially available software to extract the rebar reflection amplitudes. Additionally, a custom in-house software was used to indicate which amplitudes were indicative of healthy regions and which were indicative of deteriorated regions.
The IR surface temperatures were extracted from the commercial software used to collect the data. Minimal to no post-processing was performed on these data sets. Once the GPR amplitudes and IR surface temperatures were extracted, coordinates were assigned to each data point, based on the deck dimensions, data-collection starting location, and the grid painted on the deck prior to data collection. The purpose of assigning coordinates is so that plan view maps of the data can be created for each method (Figure 5 and 6).
The GPR color contour plot of bridge deck 1 (Figure 5b) shows the variation in rebar reflection amplitudes (dB). While this bridge deck is newer, the concrete cover varied significantly throughout the deck. As a result, those areas of thicker concrete cover have lower amplitudes, and appear to be deteriorated (colored red), but are not. The IR color contour plot for the same deck (Figure 5c), shows the variation in the surface temperatures (°F). There was very minimal temperature variation on the surface of this deck, indicating that this deck is free from delaminations. Areas free from delaminations are represented by the color green.
The GPR rebar reflection amplitudes for bridge deck 2 (Figure 6b) were deemed all healthy based on in-house developed software, resulting in an all-green color contour plot. This is expected as this bridge deck is brand new. The IR color contour plot does have areas of red, only because there were shadows cast on the deck from nearby trees. However, if those shadows did not exist, this entire plot would be green as well. This deck surface also had minimal temperature variations.
In addition to qualitatively viewing locations of potential damage with the contour plots, the deterioration levels are represented quantitatively as well. This helps transportation agencies in prioritizing which decks do not need any attention, which will need repairs in the near future, and which will be requiring total replacement. The spatial deterioration quantities for the GPR and IR data essentially count how many coordinates are considered deteriorated by that method and divides that value by the total number of coordinates in that plot. The spatial correlation between GPR and IR is determined by comparing coordinates between the GPR and IR plots, and looking for coordinates that either both agree that the location is deteriorated or that both agree that the location is not deteriorated. The spatial correlation between GPR and IR provides an added level of confidence. For example, if a high correlation exists between both methods, then both methods are pointing to similar areas of deterioration and/or similar areas that are free from deterioration. The transportation agency can then use the contour plots to locate those areas of deterioration, if necessary.
Currently, RIDOT and Roger Williams University are employing these methods on decks that have an asphalt overlay, and are including the air-coupled GPR system. The intent is to compare the ground-coupled and air-coupled GPR data, to ensure deteriorated areas are clearly visible in both data sets so that the air-coupled system can be the default system for future deck inspections. The end goal is to obtain detailed, compressive and accurate subsurface bridge-deck evaluations for decision making and funding allocation without disrupting traffic flow.