Linear and nonlinear creep of UHPC under compression: Experiments, modeling, and verification

Both the linear and nonlinear compressive creep behaviors of ultra-high performance concrete (UHPC) are experimentally investigated and modeled using cylindrical specimens under sustained compression and reinforced UHPC beams under sustained bending. The 90-day creep tests on 16 groups of cylindrical specimens explore the influences of the curing procedure, initial loading age of the UHPC, and sustained stress levels. The tested creep coefficients are compared with predictions provided by existing creep models. The NF P18-710 model performs best for creep coefficient predictions of UHPC specimens under a sustained stress level of 0.3fc. The average relative error of the results predicted by NF P18-710 model is 11.65% . However, all existing creep models for normal concrete overestimate the creep results. The mean values of relative error of results predicted by ACI209, MC2010 and B4 models are 140%, 147.2% and 145.6%, respectively. On the basis of the test results, a nonlinear creep model is proposed in which the sustained stress threshold of nonlinear creep for UHPC is 0.3fc. The mean value of the ratio between the calculated results by using the proposed model and the measured results from other individual sources is 1.04 with a coefficient of variance of 23.26%. All existing nonlinear creep models for normal concrete underestimate the nonlinear creep of UHPC. The 180-day creep tests on the UHPC reinforced beams are used to investigate the influences of sustained stress levels and the reinforcement ratio. The creep strain and time-dependent displacement of seven reinforced UHPC beams are calculated using the proposed creep model. Comparisons between the calculated and measured results verify its accuracy.