Determination of water–cement ratio of hardened concrete by scanning electron microscopy

Abstract A methodology has been developed for the determination of the water–cement ratio (w/c) in hardened concrete using backscattered electron imaging (BEI) by a scanning electron microscope. The method is based on concrete sections that have been vacuum impregnated with epoxy and polished to a flat surface. During impregnation of a dried concrete specimen, epoxy fills capillary porosity, cracks, and voids. The epoxy-impregnated porosity appears dark in BEI, while other phases such as calcium silicate hydrate, unhydrated cement grain and aggregate appear as brighter phases. The backscattered intensity of the epoxy is the lowest compared to all other phases present within a concrete. By using image analysis program and setting an appropriate threshold of the gray scale the capillary porosity of the concrete can be quantified. Reproducible quantitative data is obtained for a concrete sample of unknown w/c by using a set of standardized instrument parameters such as brightness, contrast and working distance. The water–cement ratio, which is directly related to the capillary porosity, can therefore be measured. Signal production, brightness, contrast, sample preparation, and general methodology are discussed. The measured data using this method is compared with data generated by using optical fluorescence microscopy, according to Nordtest NT Build 361-1999.