Experimental Study and Theoretical Analysis on Shrinkage Properties of UHPC under Different Constraint Conditions

In order to study the restrained shrinkage properties and mechanism of UHPC (Ultra-High Performance Concrete), 11 groups of normal temperature cured reinforced UHPC-RC(Ultra-High Performance Concrete-Reinforced Concrete) restrained shrinkage specimens were designed. The effects of interface treatment methods, UHPC layer thickness, reinforcement ratio, substrate thickness, and specimen size on the restrained shrinkage performance of UHPC were experimentally studied. The development characteristics of restrained shrinkage strain of UHPC layers and surface cracking of post-poured UHPC layers were investigated at different ages under normal temperature curing conditions. The research results show that roughening has a greater restraint effect on the restrained shrinkage of UHPC than grooving, while also increasing the risk of cracking. The thicker the post-cast UHPC layer, the smaller the restraint effect. The larger the reinforcement ratio of UHPC layer, the greater the restraint effect. The substrate layer can increase the restraint effect on the restrained shrinkage of UHPC, but the influence of substrate thickness is not significant. The restrained shrinkage of UHPC is also affected by the size of the specimen. Based on the restrained shrinkage mechanism governed by composite structural constraints and experimental results, the theoretical calculation formulas for shrinkage stress and interfacial shear stress were derived, which are consistent with the experimental observations. The research results can provide a design basis and engineering suggestions for designers and constructors, as well as references for the application of UHPC materials in strengthening existing RC structures.