EXPERIMENTAL RESEARCH ON SHRINKAGE OF ULTRA-HIGH PERFORMANCE CONCRETE UNDER RESTRAINED RINGS

Shrinkage of ultra-high performance concrete (UHPC) was investigated by restrained ring tests taking the restraint degree, sealed or dry circumferential condition and steel fiber as the main parameters. The development of steel ring strain with age was studied. The influences of various parameters on the residual stress under the restrained ring and the mechanical property at key age were analyzed. The tensile stress level and the relaxation rate were used to evaluate the cracking performance of UHPC. Free shrinkage and mechanical properties were also investigated to match the restrained ring test. The results showed a high early cracking risk of UHPC without steel fiber. All specimens cracked with an average width of more than 0.25 mm before 14 d, while the specimens with steel fiber did not crack. Various restraint degrees all have significant effects on both the tensile stress level and stress relaxation rate, indicating that decreasing the restraint degree can reduce the cracking risk effectively. Different from the free shrinkage test results, UHPC under ring restraint showed higher cracking risk at the late stage in a sealed condition than in a dry circumferential condition. The crack resistance at 14 d under a sealed condition is recommended as a control index to evaluate the cracking performance of UHPC under restrained ring.