Effect of Moisture Content on Strength and Strain Energy Release Rate of Cement Mortar

Abstract

Specimens with small cross sections, dried in vacuum desiccators, were used to determine the influence of moisture content on the strength of cement mortar. Bending and compressive strengths, strain energy release rate, elastic modulus, and shrinkage were measured. As expected, strength increased as the moisture content decreased; the effect was greatest at the lower moisture contents (provided drying conditions did not cause cracking). The influence of moisture on the surface energy of the gel solids and on the elastic modulus was investigated. Because surface energy is difficult to measure, the strain energy release rate was determined instead, and these parameters were assumed to be covariants with moisture. The strain energy release rate increased sharply when the monomolecular water layer on the gel solids began to evaporate. It is suggested that moisture influences strength through its effect on 2 mutually independent components, i.e. the strain energy release rate and the elastic modulus. Using equations of fracture mechanics, with typical values assumed for the depth of surface cracks and the width of body cracks, a fairly good correlation was found between theory and experimental results.