Performance of Reinforced Concrete–UHPC Composite Slabs Under Four-Point Bending: Experimental Tests, Numerical Simulations and Analytical Methods

This study investigates the behaviour of normal-strength concrete (NSC)–ultra-high-performance concrete (UHPC) composite slabs under four-point bending through experimental tests, finite element simulations, and analytical modelling. A total of 12 slabs were tested under static loading to examine the effects of UHPC layer position, UHPC thickness, reinforcement ratio in the UHPC layer, and shear span ratio on load-deflection behaviour, crack development, strain evolution, and failure mode. The test results showed that, when UHPC was placed in the tension zone, the slabs generally exhibited higher post-cracking stiffness and load capacity. The specimens with the longitudinal reinforcement spacing in the UHPC layer reduced from 100 mm to 50 mm exhibited an approximately 16% increase in ultimate load, but they were more prone to develop localised cracks and shear failure with a high reinforcement ratio in the UHPC layer and a small shear span ratio. When UHPC was placed in the compression zone, the specimens showed greater deformation capacities. With the same UHPC thickness, reinforcement ratio, and shear span ratio, their failure deflections were approximately 1.5–4.8 times those of the corresponding specimens with UHPC in the tension zone, whereas their failure was mainly governed by interface debonding. Finite element models were also developed and validated against the test results, followed by parametric analyses. The numerical results indicate that interface bond strength plays a dominant role in failure mode and deformation capacity, particularly for slabs with UHPC cast in the compression zone. Increasing the UHPC thickness generally improves the stiffness and load capacity, whereas insufficient thickness increases the risk of shear failure or interface debonding. Finally, an analytical model was proposed for composite slabs with different UHPC layer positions, and the model was shown to provide a reasonable prediction of the load-deflection curve of flexure-dominated slabs.