Predicting relative humidity of early-age concrete under sealed and unsealed conditions

The decrease of internal relative humidity (IRH) in early-age concrete induces autogenous shrinkage. Autogenous shrinkage usually increases the risk of cracking if the concrete is restrained from shrinking freely at an early age. Predicting autogenous shrinkage of early-age concrete requires knowledge of IRH. Therefore, investigation of IRH is fundamental to evaluate cracking resistance. Although investigations of IRH in hardened concrete have been conducted, a prediction model for IRH in early-age concrete that considers the water/cement ratio (w/c) is still lacking. Therefore, experimental studies and analysis of IRH in early-age concrete with different values of w/c were conducted. Results showed that: (i) the IRH in concrete at 28 d after casting increased with increasing w/c under sealed and unsealed conditions; (ii) the average rate of IRH decrease in early-age concrete decreased with increasing w/c; (iii) the critical time of IRH (the length of the water-vapour saturated stage with 100% IRH) in early-age concrete increased with increasing w/c. Models for early-age IRH are proposed in consideration of w/c, critical time and age of concrete under sealed and unsealed conditions.