Coefficient of thermal expansion of cementitious mortars: new testing methodology

Abstract Given the inherent porosity of cementitious materials, fluctuations in temperature disrupt the hygrothermal equilibrium, thereby influencing the accuracy of the coefficient of thermal expansion (CTE) measurements. This results in variations in the observed expansion magnitudes, which are contingent on the material's moisture content. Many existing methodologies for measuring CTE do not adequately control for moisture or consider procedural nuances, leading to inaccuracies and often necessitating expensive equipment. This study evaluates two methodologies for determining the CTE of cementitious mortars, focusing on the influence of moisture and the use of accessible equipment. Among the two methodologies tested, one was recommended for its higher precision in determining the CTE of a prismatic mortar sample by measuring length variations due to temperature changes. In this method, the sample is heated to 80ºC at a fixed relative humidity until hygrothermal equilibrium is reached, after which measurements are taken during cooling to 22 ± 2°C. Adjustments were made to eliminate the influence of studs, and the CTE is calculated using linear regression to correlate strain with temperature variation. The methodology demonstrated a repeatability standard deviation of 0.08×10−6/ºC, comparable to the precision of AASHTO T 336 (0.12×10−6/ºC) and superior to ASTM C 531 (1.62×10−6/ºC). Evaluating CTE at moisture levels of 50%, 75%, 98%, and saturation revealed a parabolic behavior, with a maximum CTE near 75% relative humidity, aligning with existing literature. This methodology offers a promising approach for assessing the CTE of mortars, particularly those exposed to environmental conditions, emphasizing the importance of compatibility between repair mortar and substrate across different moisture levels.