Experimental analysis of the shear strength of composite concrete beams without web reinforcement

Composite concrete members without web reinforcement are often used in precast construction. The contribution of the cast-in-place concrete topping slab to vertical shear strength has been traditionally disregarded. However, significant cost savings can result from designing and assessing these structures if this contribution is considered. This paper presents the experimental study of a series of 21 monolithic and composite (precast beam and cast-in-place slab) specimens without web reinforcement, and with rectangular and T-shaped cross-sections, failing in shear. The vertical shear strength was analysed by the following test variables: cross-section shape, the existence of an interface between different aged concretes, strengths of the two concretes and the differential shrinkage effect. From these experimental tests, it was concluded that the slab contributed to shear strength, the use of high-strength concrete slightly increased specimens' shear strength and the differential shrinkage did not reduce shear strength. Specimens' failure modes were analysed based on their shear transfer mechanisms, noticing that the arching action in the slab was considerable after critical shear crack formation. The vertical shear strength experimental results were well predicted by the codes' formulations (Eurocode 2, Model Code 2010 and ACI 318-19) when composite beam depth was taken for the calculations instead of beam depth. Codes significantly underestimated the horizontal shear strengths of the composite specimens.