Vertical Impedance Testing for Assessing Protection from Chloride-Based Deicing Salts Provided by an Asphalt Overlay System on a Concrete Bridge Deck

In cold regions, chloride-based deicing salts are commonly applied to roads and bridges during winter to melt ice. The objective of this work was to demonstrate the utility of a new technology, vertical impedance testing, for assessing the condition of concrete bridge decks treated with membranes and asphalt overlays to reduce chloride-induced corrosion of the reinforcing steel. The vertical nature of the impedance testing allows interrogation of all layers from the deck surface down to the reinforcing steel, specifically permitting evaluation of the protection against chlorides provided by any deck surface treatments, the full depth of the concrete cover, and any rebar coatings. Spatial variations in vertical impedance measurements on a bridge deck in Wendover, Utah, were compared to the occurrence of cracking in the asphalt overlay, half-cell potential values, and chloride concentrations measured at specific locations before deck rehabilitation. The data were also compared to the occurrence of actual deck repairs completed after the asphalt overlay and membrane were removed. The impedance data were well aligned with the distress and half-cell potential data and were used as a basis for strategically selecting locations for coring and chloride concentration sampling, which enabled further interpretation of the impedance data. Of the 29 patches corresponding to delaminations identified by the contractor, nine were positioned at locations originally characterized by cracking of the asphalt overlay surface, 18 were positioned at locations characterized by low impedance magnitude, and eight were positioned at locations characterized by uncertain half-cell potential classifications. The vertical impedance testing method therefore shows great promise as a tool for evaluating the protection against chlorides provided by an asphalt overlay system. Furthermore, contour plots of impedance magnitude may possibly be used to estimate the scope of work, including the location and extent of deck repairs, that may be required during deck rehabilitation.