Accurately estimating the water-to-cementitious materials ratio (w/cm) of hardened concrete is one of the most difficult tasks frequently requested of concrete petrographers. Different methods have been developed and implemented with varying degrees of success. To increase the reliability of estimated w/cm, the indentation hardness of laboratory prepared specimens of neat paste, mortar, and concrete was evaluated using a Buehler semi-automatic testing machine. Preliminary results indicate good correlations between the average indentation hardness measured on the Rockwell HRL scale and the w/cm of the specimens. The data demonstrate a potential for using these correlations to estimate the w/cm of hardened concrete. Testing protocols, sample preparation requirements, and the effects of aggregate and of the composition of concrete mixtures (portland cement and fly ash content) are illustrated. Directions for future work are also discussed.
A linear regression equation is developed between the paste volumes via the point count method and total cementitious materials contents of 31 concrete samples with known mix proportions. The equation has a coefficient of determination (R2) of 0.96 and a standard deviation of 13.6 kg/m3. The equation can be used to calculate the total cementitious materials content based on the paste volume measured via point count and the water-to-cementitious-materials ratio estimated petrographically, and the result is more accurate than the traditional method currently used by many concrete petrographers. The equation is equally effective for portland cement concrete and concrete containing fly ash. In addition, the effect of fly ash, and the traditional method for estimating the cementitious materials content are further discussed.
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