Validation of Benefits of Automated Depth Correction Method

Current ASTM specifications provide an approach for directly correlating the top rebar reflection amplitude from ground-penetrating radar (GPR) to deck condition when corrosion is the primary mechanism for concrete deterioration. However, current specifications do not offer an approach to compensate for geometric spreading losses caused by the inevitable variability in as-built concrete rebar cover. Even when cover thickness meets specified tolerances, significant errors may occur in mapping the location and quantity of deteriorated concrete when these amplitude variations are unaccounted for in air-coupled or ground-coupled GPR investigations. The significance of this correction is demonstrated by comparing mapped deterioration quantities that have been corrected and not corrected for depth variation, versus those quantities from independent methods such as half-cell corrosion potential, chain drag, or impact echo. One manual and two automated processes for depth correction are presented. These three processes compare favorably with one another on several mapped decks. One of the automated methods, which sets a deterioration threshold calibrated with half-cell and chain drag results, has been shown to be as accurate as manual methods on numerous decks. This approach is recommended for further evaluation and incorporation within ASTM D6087-08: Standard Method for Evaluating Asphalt-Covered Bridge Decks Using Ground-Penetrating Radar.