A new test setup for measuring early age coefficient of thermal expansion of concrete

摘要

Coefficient of thermal expansion (CTE) of concrete changes at early ages mainly due to microstructural modifications that occur during cement hydration. This paper proposes an innovative way to assess the CTE of concrete since very early ages, with the original inclusion of an internal heating/cooling system, that allows to speed up the thermal balance cycles, and increase the number of CTE measurements during the early ages. The method comprises a specimen of 154 mm diameter and 300 mm height, submitted to successive heating and cooling steps with thermal variations of 5 degrees C, right after casting. A spiral heating/cooling tube, embedded in concrete, is used to circulate the controlled temperature water bath inside the specimen. This paper demonstrates the feasibility of the strategy for accelerating thermal equilibrium of the specimen, both experimentally and through numerical simulation. The short duration of the thermal cycle in this proposal strongly increases the ability of the method in capturing the early kinetics of CTE evolution, allowing to measure the CTE as soon as 4.1 h after mixing. With durations of each full temperature cycle of 1.5 h, it was possible to obtain as many as 12 CTE measurements within the first 24 h of testing, which is an important improvement in regard to similarly sized specimens in the literature. Furthermore, when the information obtained from the proposed test method is combined with the continuous feed about E-modulus of concrete that can be obtained through application of the EMM-ARM technique, very definite conclusions can be attained about the expectable stress levels that can be attained in concrete elements at early ages, due to the early variation of CTE. Three concrete mixtures are used to verify applicability of the proposed method for measuring early age CTE of concrete. The calculated values for these concrete mixtures are in agreement with the data found in the literature.