Backscattered Electron Imaging to Determine Water-to-Cement Ratio of Hardened Concrete

Abstract

A methodology has been developed for the determination of the water-to-cement (w/c) ratio in hardened concrete using backscattered electron imaging (BEI) within a scanning electron microscope. Reproducible quantitative data are obtained by using standards from polished sections as well as by controlling brightness and contrast on the instrumentation. The method is based on concrete sections that have been vacuum impregnated with epoxy and polished to a flat surface. During impregnation, epoxy fills capillary porosity as well as cracks, voids, and other defects. The w/c ratio is directly related to the capillary porosity and degree of hydration and therefore can be measured. The epoxy-impregnated porosity appears black in BEI, whereas other phases such as calcium silicate hydrate, ferrite phases, and aggregate appear as brighter phases. Signal production, brightness, contrast, sample preparation, general methodology, and resolution are discussed.