Early-age tensile creep and shrinkage-induced cracking in internally restrained concrete members

Experiments were carried out under ambient conditions and in a temperature-humidity control room to assess the influences of early-age shrinkage and tensile creep on cracking in reinforced-concrete (RC) members subjected to internal restraint. Two concrete mixes were considered, with compressive strengths of 36 MPa and 47 MPa. The evolution of the tensile creep coefficients was measured using unreinforced dog-bone-shaped specimens subjected to sustained axial tension. The shrinkage-induced stress tests were performed on RC prisms internally restrained by one concentrically placed reinforcement. Free shrinkage and restrained shrinkage were measured on companion plain concrete prisms and on unloaded RC prisms, respectively, to determine the degree of restraint. The results show that the 36 MPa concrete had a higher tensile creep coefficient than the 47 MPa concrete, but that there were no significant differences in early-age free shrinkage. A lower humidity results in more free shrinkage strain, but leads to more tensile creep and, consequently, increased relaxation of the tensile stresses. The magnitude of the restrained shrinkage depends on the reinforcement ratio. The development of the tensile strength of concrete is a governing factor influencing the time to cracking. The tensile ageing coefficient was calibrated for the two concrete mixes.