Ground Penetrating Radar (GPR): Using Radio Waves to Detect Salt on Asphalt Overlaid Bridge Decks

As part of bridge maintenance, evaluating deterioration in asphalt overlaid concrete bridge decks is crucial in determining the timing and nature of any maintenance. Chloride contamination caused by de-icing salt is one of the primary contributors to bridge deck deterioration. Early detection can trigger proactive maintenance rather than more expensive, reactive rehabilitation. Ground Penetrating Radar (GPR) technology is an efficient and reliable testing method that collects data to evaluate deterioration in asphalt overlaid concrete bridge decks. When comparing the results of the GPR evaluation to that of ASTM D6087 and traditional test methods (Chloride samples, AC resistance, and CSE), the results proved more accurate when compared to ASTM D6087, and correlated well with traditional methods.

GPR can identify and quantify the chloride contamination by looking for reductions in the amplitude of the returning signal due to attenuation. Attenuation is affected by the conductivity (chloride content) of the concrete. The challenge is that the amplitude of the returning signal is also affected by geometric spreading of the signal. A method to account for geometric spreading (by normalizing the amplitude to the 90th percentile of the data) has been developed and is extensively used on bare concrete decks, but less work has been done on data correction for asphalt overlaid bridge decks. On asphalt overlaid bridge decks, the GPR signal must pass through both the asphalt and concrete, both of which have different electromagnetic material properties. Consequently, this produces less reliable results due to the addition of amplitude variability caused by inconsistent signal reflection at the asphalt/concrete interface. To address this, the amplitude was corrected for geometric spreading using the 90th percentile method and applying Fresnel's Law to correct for the variability in the asphalt/concrete interface reflection.

A total of five structures in Ontario with asphalt overlays, waterproofing membranes, and epoxy rebar were tested in 2020 using GPR to determine chloride contamination. The results from the proposed GPR method were compared to the traditional test methods, and to the analysis method specified in ASTM D6087. The results of this GPR approach suggested that accounting for geometric spreading, in conjunction with interface reflection correction, not only correlates with the traditional test methods, but produces more accurate deterioration mapping than ASTM D6087. This increased accuracy allows for better decision making on the timing and nature of any recommended bridge repairs, leading to improved cost savings over the lifespan of the bridge.