Estimating the Capillary Porosity of Cement Paste by Fluorescence Microscopy and Image Analysis
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ULLA H. JAKOBSEN, VAGN JOHANSEN & NIELS THAULOW G.M. Idom Consult A/S, Bredevej 2, 2830-Virum, Denmark
ABSTRACT A semi-automatic fluorescence image analysis method to estimate the capillary porosity or the water/cement (w/c) ratio of concrete has been investigated. The method is compared to the usual manual method which also uses thin sections examined in fluorescent light. The two methods correlate very well in the w/c range of 0.35 to 0.5. For higher w/c ratios the semi-automatic method is more sensitive and gives more reliable results. Semi-automatic analyses of standards of different compositions showed that the green tone intensity varied depending on the amount of mineral admixtures in the sample. The fluorescent intensity of the cement paste decreases in the following sequence: OPC, OPC + fly ash and OPC + limestone fines. Analyses of various standards proved that the homogeneity of the paste can be reflected by the standard deviation of the measurement. Finally, the investigation of the semi-automatic method showed that the intensity of the fluorescence light of the paste decreases with the length of illumination and that the intensity of the light bulb changes from bulb to bulb. These observations proved that it is important to carry out the fluorescent w/c analyses, manually as well as semi-automatically, under standardized conditions.
INTRODUCTION The capillary porosity or the water/cement (w/c) ratio of the cement paste in concrete has for many years been estimated manually using fluorescent impregnated thin sections by assessing the intensity of the green colour emitted from the cement paste when illuminated by UV light" The intensity of the green fluorescence colour of the paste is manually compared to standards with different w/c ratios, normally in the range of 0.35 to 0.60. This method is generally a quick and reliable method. In recent years, however, new concrete types with for example very low or high w/c ratios and high amounts of additives and admixtures, have shown that the human eye sometimes fails to distinguish intensity differences, especially when the green tone of the cement paste is high. Attempts have been made to use image analyses to quantify the water to cement ratio. Mayfleld used fluorescent impregnated polished samples while Elsen et al. 3 and Wirgot and Van Cauwelaert4 used thin sections. In order to differentiate between the various extreme green tones of the cement paste a semiautomatic colour image analysis method has been developed. A number of concrete standards with various w/c ratios and different compositions have been analyzed semi-automatically. From the green tone values of the standards, various calibration curves covering the w/c range of 0.3 to 1.0 have been estimated. These calibration curves can be used to estimate the w/c ratio when analyzing concrete samples with unknown water to cement ratios. 227
Mat. Res. Soc. Symp. Proc. Vol. 370 0 1995 Materials Research Society
METHODS Sample Preparation Both the manual and the semi-automatic estimation of the w/c r
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