Accelerated Diffusion Testing of Concrete with External Electric Fields and X-Ray Imaging

Abstract

The apparent diffusion coefficient (Dic) is important for modeling the service life of concrete. Most diffusion-based tests require the sample to be ponded for at least 28 days, and significant labor is required to obtain the data. This study will present a novel test method to determine the Dic in less than 8 h of ponding and 5 min of measurement time. The test uses a dental X-ray scanner modified to image the ions penetrating the concrete. The study will present how the method has been adapted for concrete and then will present an accelerated diffusion test that uses a charge potential to move ions within the concrete. Twenty-seven concrete mixes were tested at two ages, including 20% and 40% replacement levels of Class C and Class F fly ashes. Half of the samples were tested with a nonaccelerated diffusion test, and the second half was tested with an accelerated diffusion test to validate the correlation statistically. By making this important parameter easier to measure, this work can potentially increase the use of diffusion coefficients in specification, performance-based designs, and research measurements. This work shows that the accelerated diffusion testing at 6 h with the right testing parameters can provide data comparable with nonaccelerated testing, which takes 28 d of testing for samples with a diffusion coefficient greater than 1×10−12 (m2/s). This means the test can be used on a wide variety of samples, as the only materials with diffusion coefficients this low were from mixtures that contained 40% fly ash replacement after 1,100 days of curing. The ability to obtain diffusion coefficients that are this accurate with only 6 h of testing time is useful and can make this test and the widespread measurement of the diffusion coefficient easier to implement. While these findings are important, testing is needed with a wider variety of materials and also diffusion coefficients. This work focused on mixtures with a narrow range of w/cm and SCM replacement. The parameters chosen in this work for cd and c0 may need to be adjusted for a wider range of materials.