Decoupling Thermal and Autogenous Strain of Concretes with Different Water/Cement Ratios During the Hardening Process

At early age, the autogenous strain and the coefficient of thermal expansion (CTE) are two of the most important concrete properties that are responsible for volume changes of cement-based materials and therefore for possible cracking in concrete structures (especially massive structures). Autogenous strain and CTE are dependent on the water/cement (W/C) ratio. A new experimental approach was developed to define both properties as well as the initial and final setting time with a single test. Every two hours, thermal variations of 3 °C are applied on a concrete sample with the device so-called BTJADE (from the French acronym BéTon au Jeune Age, Déformation Endogène). The initial setting time is the moment when the value of the CTE no longer corresponds to a liquid. The final setting time corresponds to a zero value of the derivative of the autogenous strain before the swelling of the concrete. Results of the initial and final setting time are compared to the evolution of the transmission of the ultrasound P- and S-waves. Three different compositions are investigated to define the influence of the W/C ratio (0.4, 0.5, and 0.6), and a fourth composition, for which the binder is composed of only 25 % cement, is also used in order to validate all observations. A new model expressed in function of the equivalent time or the advancement degree of reaction (defined with isothermal calorimetry) is proposed for computational modelling of the autogenous strain and the CTE. Effect of the W/C ratio and the physical mechanisms are highlighted.