Flexural Damage Study of UHPC Layer in Steel–UHPC Composite Slab Under Cyclic Load

To investigate effects of cyclic loading on the flexural performance of steel–UHPC (ultra-high-performance concrete) composite slabs, this study designed specimens with two load amplitudes for fatigue tests and static comparative tests. Steel–UHPC composite slabs were susceptible to fatigue damage in the UHPC layer under cyclic negative bending moments. Static comparative tests revealed that the UHPC remained uncracked under a load of 1.15 times the linear elastic limit. Consequently, the peak loads for the two cyclic test series were set to 70% and 115% of the UHPC linear elastic limit, respectively. Fatigue tests indicated that tensile cracks developed in the UHPC layer under both cyclic load levels. Using the neutral axis offset as a fundamental variable, a theoretical model was established to analyze the degradation in flexural performance under cyclic loading. The analysis demonstrated that cyclic loading induces irreversible damage to the flexural behavior of steel–UHPC composite slabs. Compared with static test results, flexural performance degradation was observed in the first series after 2 million cycles, whereas in the second series, degradation occurred after only 10,000 cycles. After four million cycles, the flexural performance of the first series decreased by 7%, while that of the second series decreased by 12%.