Experimental study and theoretical prediction on shrinkage-induced restrained stresses in UHPC-RC composites under normal curing and steam curing

Ultra-high performance concrete (UHPC) has been recently developed as an innovative cementitious-based material, which exhibits excellent tensile and compressive strengths, as well as long-term durability. However, information regarding shrinkage-induced stress development in a composite system of UHPC-reinforced concrete (RC) is limited. The objective of the present research is to identify the restrained shrinkage of reinforced UHPC layer restrained by the RC substrate in UHPC-RC composites for 150 days. Free shrinkage of the UHPC layer was also measured for comparison. The UHPC layers were cured under normal laboratory (NC) temperature (5 degrees C-35 degrees C) and high-temperature steam (HC) at a temperature of 100 degrees C for 48 h. Shrinkage-induced restrained stresses composite members have been evaluated based on a preexisting model used for conventional concrete. As UHPC shows different properties such as strength, shrinkage, and viscous behavior compared to conventional concrete, the development of elastic modulus of UHPC was included in the model for verification of cracking potential. A parametric study was conducted to investigate the influence of creep coefficients of UHPC and NSC on the restrained stresses. Results showed that restrained shrinkage in UHPC overlay was decreased by 57% and 80% compared to free shrinkage under NC and HC, respectively. The five stages of UHPC shrinkage development under NC during the first three days were observed while UHPC shrinkage under HC continually and rapidly increased during the curing period. On the other hand, the same shrinkage value was obtained for both NC and HC of UHPC at the age of 150 days. The preexisting model could calculate the restrained stresses, which was compared with tensile strengths of the different ages. The parametric study indicated that increasing creep coefficients of UHPC and NSC decreased the magnitude of shrinkage-induced restrained stresses in UHPC-RC composite structures.