Characterization of New and Old Concrete Structures Using Surface Resistivity Measurements

This report describes the findings of concrete resistivity characterization using the surface resistivity (SR) test method. This characterization was conducted on Florida's new and old coastal bridge structures. The testing took place on partially immersed substructures on more than 60 bridges. The SR measurements were performed in areas above the mean tide line (marine growth). The SR profiles at several locations as a function of elevation were performed using a commercial Wenner probe. This Wenner probe is the same instrument used in Florida Department of Transportation's FM5-578 test method. A resistivity gradient from low to high proceeding from the submerged to atmospheric zone is usually observed. This gradient results from the elevation dependence of moisture saturation and pore solution composition. A conditioning procedure was designed and implemented as part of the field testing to increase the moisture content of the concrete being examined (tidal and splash zone). The characteristic SR after one or two days of monitoring is called herein the SRfield. This value was usually the minimum resistivity value observed at the second or higher examined elevation. During the field trips nominal 2-in. concrete cores were obtained in the vicinity of where the SR profiles were made. These cores were used either to perform wet SR measurements in the laboratory or to obtain chloride profiles. The chloride profiles were then used to estimate the apparent chloride diffusivity. The wet SR was achieved after a few weeks of exposure in a high humidity chamber, and is named herein the SRwet. A correlation between SRfield and SRwet was performed and it was found that SRfield was about 3 times that of SRwet; i.e., no full saturation was obtained with the conditioning method. A correlation was computed between chloride diffusivity (average per component) vs. SRwet. However, due to the field nature of the data there is some scatter, the R-squared varied between 0.62 and 0.69 depending on whether all bridges visited were included or if only a subset was used in the fitting. A second component of this project consisted of experiments on laboratory samples aimed at understanding how the resistivity varies over time depending upon the concrete mix and environmental exposure, as well as in the presence - and absence - of reinforcing steel. Numerical modeling was performed to obtain K (cell constants). The K values were used to normalize the SR measurements performed on laboratory samples. Additionally, numerical modeling was performed to investigate additional scenarios (different concrete cover thicknesses, multiple rebar presence, dimensions of the concrete block interrogated, presence of multiple concrete layers). Very useful insight was obtained from this research effort.