Electrical Resistivity and Formation Factor of Air-Entrained Concrete

This paper studies the influence of air content on the electrical resistivity and formation factor of concrete as these measures are often used in specifications for acceptance and payment. Experimental measurements are conducted on 30 air-entrained concretes with three water-cement ratios (w/c = 0.40, 0.45, and 0.50) over a large range of air contents (2.5 to 9.0%). The porosity of the concrete is measured, which is comparable to the theoretical estimation from the Powers-Brownyard model. Electrical resistivity measurements are performed on saturated concrete samples, and samples submerged in simulated pore solutions. The samples submerged in a bucket of simulated pore solution achieve a degree of saturation that relates to the filling of matrix pores, (that is, the Nick Point). The degree of saturation at the Nick Point ([S.sub.NK]) decreases as the air content increases. A formation factor ([F.sub.SAT]) is calculated for the saturated concrete, as well as the apparent formation factor [F.sub.NK] for the samples submerged which reach Nick Point saturation ([S.sub.NK]) in the simulated pore solution. As the air content increases, [F.sub.SAT] decreases due to the increased porosity (air voids) that are filled with conductive fluid, while [F.sub.NK] is independent on the air content (as the air voids are filled with nonconductive air). As the w/c increases, both [F.sub.SAT] and [F.sub.NK] decrease due to the increased porosity and connectivity. For the concrete with the same w/c, the addition of a high-range water-reducing admixture (HRWRA) results in higher values of [F.sub.SAT] and [F.sub.NK] due to the refined micro-structure in mixtures containing HRWRA. A saturation function is used to provide a powerful tool in quality control to back-calculate [F.sub.SAT] that relates to different transport properties. Keywords: air content; apparent formation factor; electrical resistivity; formation factor; Nick Point.