DOT SPECIFICATION DEVELOPMENT BASED ON EVALUATION OF GROUND-PENETRATING RADAR SYSTEM PERFORMANCE IN MEASURING CONCRETE COVER (REINFORCEMENT DEPTH) ON NEW BRIDGE DECK CONSTRUCTION

Concrete cover, an important parameter affecting the protection of reinforcing steel in a structure, allows the alkaline nature of cement to act as a barrier to the corrosion process. New Hampshire Department of Transportation (NHDOT) designed a method to investigate the effectiveness of a high-resolution, ground-coupled Ground Penetrating Radar (GPR) system for accurately measuring concrete cover depth, which helps assess the quality of new bridge deck construction. Based on the results of its one-year evaluation, NHDOT modified its quality control/quality assurance (QC/QA) specifications to require bridge deck cover measurement using this new technology on all new construction. Contractor pay factor associated with deck cover measurements is determined by GPR measurement results. One GPR profile is collected in each lane for all sections of the bridge deck. A compact 1.5 GHz ground-coupled sensor (antenna), controlled by a data acquisition unit, collects radar scans at 0.33 in. (8.38 mm) intervals along each profile. Raw data are transferred to a laptop and quickly reviewed to select a single calibration hole location. A core is drilled to the top of the bar and its depth is measured relative to the concrete surface. The bar location and depth is entered into the data processing software program. Now calibrated, the software automatically determines individual bar depth, mean cover depth, and standard deviation of all bar depths within the scan path. These values are output to an ASCII file for reporting. Initial comparisons of GPR and pachometer (hand-held rebar depth indicator) measurements over a series of newly constructed decks determined accuracy and repeatability of each technology. Verification holes drilled into the deck (ground-truth measurements) were used for reference. Both technologies were also spot-checked to compare for measurement repeatability. Based on initial evaluation of a handful of bridge decks, NHDOT decided to continue investigating the new GPR tool. Afterward, NHDOT modified its QC/QA cover specification, replacing pachometer measurement of deck concrete cover with GPR. Temporary, then permanent, phase-in of the revised NHDOT specification for QC/QA cover allowed contractors time to adjust to this new technology. Given NHDOT's positive results with GPR, other DOTs have decided to evaluate its potential.