Thermal conductivity evolution of early-age concrete under variable curing temperature: Effect mechanism and prediction model

The thermal conductivity and microevolution mechansim of early-age concrete (EAC) under variable curing temperature environments significantly influence the temperature cracks in massive concrete. An experiment was carried out to compare thermal conductivity evolution differences of EAC under variable curing temperature (VCT) and standard curing temperature (SCT) conditions. The concrete specimens of moisture and dry states were used to consider the effect of moisture content on thermal conductivity. Results showed that the moisture concrete thermal conductivity under VCT condition decreases with curing ages and is higher than SCT condition. For the dry samples, the thermal conductivity of VCT condition increases with curing ages and is lower than the SCT condition. The difference in thermal conductivity between VCT and SCT increases from zero to seven days and decreases after seven days. The scanning electron microscope (SEM) tests showed that the VCT leads to slower growth of pore structure and larger porosity of EAC due to the slower hydration rate under VCT condition. The thermal conductivity difference between VCT and SCT is mainly caused by the differences in water content, chemical components in solid phase, and porosity (solid-phase volume fraction). Meanwhile, the relation among thermal conductivity, porosity and most probable pore size shows that curing temperature has a great effect on hydration degree and controls the thermal conductivity evolution. Besides, a prediction model of EAC thermal conductivity under VCT condition was built based on hydration kinetics.