Change of the Coefficient of Thermal Expansion of Mortar Due to Damage by Freeze Thaw Cycles

Abstract

This paper presents the experimental methods and findings in obtaining the strain behavior of mortar during freezing and thawing cycles (FTC) at the meso-scale under fully saturated condition and the coefficient of thermal expansion (CTE) and elastic modulus of mortar after FTC tests. A heat-cool cycle test comparing an oven-dried and undried mortar was also performed prior to the FTC test, confirming that oven drying at 105°C does not affect the CTE of samples, thereby ensuring the reliability of the results. During FTC with constant moisture content, a limitation in the increase in tensile strain was observed and this tensile strain decreased until contraction was observed. The contraction is attributed to the removal of gel pore water arising from negative pressures. Due primarily to the absence of available water supply, the displaced pore water cannot be refilled, which results in contraction at the end of the FTC. More importantly, the results show that after FTC, the CTE of frost damaged mortar increases while its elastic modulus decreases, primarily owing to microcracking when frost damage sets in. Microcracks act as broken bridges that can detach the aggregate from the hardened cement paste and in effect reduces the thermal restraints that each part (fine aggregate and cement paste) exerts on the other. The hardened cement paste can then expand/contract more freely under temperature variation, and thus can significantly affect (increase) the CTE of the whole composite (mortar). Further, stress transfer in the material is prevented due to microcracking resulting in elastic modulus reduction.