DETERMINATION OF THE W/C RATIO OF HARDENED CEMENT PASTE AND CONCRETE SAMPLES ON THIN-SECTIONS USING AUTOMATED IMAGE-ANALYSIS TECHNIQUES

This article presents results of the w/c ratio determination of hardened cement paste and concrete samples on thin sections using automated image analysis techniques. The determination of the w/c ratio of hardened concrete is of prime importance for deterioration diagnosis as well as for concrete Quality Assurance in general. An Optical method to determine the w/c ratio of hardened concrete using thin sections has been developed in Denmark and is now in general use in Denmark and the other Scandinavian countries. Two series of samples prepared in laboratory conditions have been used as 'reference samples' to develop and to evaluate procedures for measuring the w/c ratio by image analysis techniques : a series of pure cement paste samples and one of concrete samples. Three different image analysis methods have been employed : an interactive, an automatic method with thresholding of grey values by the operator, and a fully automatic method without thresholding. The second method was retained for further measurements. The measurements on the pure cement paste samples confirm the direct relationship between the w/c ratio of the samples and the mean grey value measured on thin sections. The results of the measurements of the thin sections prepared out of the reference concrete series also indicate a clear relationship between the w/c ratio and the mean grey value but these measurements are less reproducible. The main cause of this variability with the concrete samples is the difficulty in selecting threshold level values.