Quality Assurance of Bridge Deck Concrete Overlays using Surface Resistivity Testing
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Abstract
Through years of heavy usage, critical transportation components such as bridge decks are repeatedly subjected to stresses from freeze-thaw cycles, chemical attack and the physical loads. These continual stresses and deterioration mechanisms acting on bridge decks contribute to the fact that 11.1% of NCDOT bridges are currently considered structurally deficient (NCDOT 2019). Once a bridge deck becomes moderately to severely deteriorated, an overlay rehabilitation is often employed. Since traffic closures must be minimized, it is essential that the rehabilitation efforts be completed rapidly, potentially resulting in errors. Issues could occur despite quality control (QC) efforts on the part of the contractor, and acceptance inspection and testing on the part of the agency. Errors made in the placement of concrete overlays are often hidden under the finished surface, only later to appear after wear, scaling, and/or loss of cover. It is a challenge to determine the overlay’s quality once it is placed and Quality Assurance (QA) inspection and testing measures must be used when evidence of a defective installation is present or is suspected to be present. In this study, surface resistivity (SR) testing is studied for its viability as an inspection approach for determining the quality of concrete overlays. Bridge deck mockups were developed with regard to potential influencing variables and tested in a methodical sequence. Each mockup is composed of a base concrete, which acts as the existing deck after overlay preparation, the top reinforcing steel mat (epoxy-coated and non-coated), placed defects, and a different overlay mixture with varying thickness assigned to each mockup. It was expected, through observation of trends evident in heatmaps created from the data, that characteristics of the mockups such as placed voids, overlay thickness variation, details of edge effects, and determine the influence of steel reinforcement could be identified by changes in SR. Through preliminary testing of cylinders, it was found that although SR readings are affected by non-coated steel reinforcement, SR readings were not significantly affected by epoxy-coated steel reinforcement likely due to an insulating effect of the epoxy coating. The preliminary specimens were also used to quantify the influence of concrete cover and provide cover distance correction factors. Although specific to the particular mixtures used in this study, a similar approach could be used with other mixtures. The SR meter was effective in determining overlay thickness in the bridge deck mockups and adjustment factors were developed for each overlay. Additionally, it was found that edge effects have more influence on SR than those that have been previously published in the literature, and the meter’s orientation with respect to the edge is also a factor influencing the readings. Voids and reinforcement were also detectable through SR readings, although not as readily as expected. Statistical techniques using an array of measurements may be necessary to link SR readings to the presence of voids and reinforcing bars. As a result of this research, a procedure was developed to be used for field use, borrowing from the SR Testing of Bridge Deck Mockup section within this thesis. Potential modifications and recommendations are offered for adapting this procedure to other potential situations. The procedure developed could potentially be utilized to enhance common concrete overlay testing and inspection protocols, as well as serve as a forensic check on the quality of overlay construction.