超高性能混凝土圆环约束收缩试验研究

以约束水平、环境条件（密闭或干燥）和钢纤维等为参数，开展了超高性能混凝土（UHPC）圆环约束收缩试验。研究了钢环应变随龄期的发展规律；分析了各参数对圆环约束下的残余应力与各关键龄期的力学性能的影响；采用了拉应力水平和应力松弛率来评价UHPC的开裂性能。为配合圆环收缩试验，开展了自由收缩与基本力学性能试验。试验表明，未掺钢纤维的UHPC早期开裂风险大，在14 d前均发生开裂，裂缝平均宽度大于0.25 mm，含钢纤维试件均未开裂。不同约束程度对拉应力水平与应力松弛率的影响均显著，降低约束程度能有效降低开裂风险。与自由收缩测试结果不同，圆环约束UHPC在密闭条件下后期的开裂风险会高于环向干燥条件。建议以密闭条件下14 d的抗裂性能作为控制指标评价圆环约束下UHPC的开裂性能。

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.