Experimental investigation on the behaviour of hybrid HPFRC flat slabs

Reinforced concrete flat slabs is a widespread structural solution in office, commercial and residential buildings around the world. Punching shear often governs the design of these structures, leading to increased self-weight and material consumption when thickness increase is sought as a solution or increased labor costs in the case of punching shear reinforcement utilization. New possibilities are arising with the development of advanced concrete materials such as High-Performance Fiber Reinforced Concrete (HPFRC) to improve the behavior of flat slabs subjected to punching shear. HPFRC offers the advantages of both High Strength Concrete (HSC) and Fiber Reinforced Concrete (FRC) as well as an increased performance in terms of self-compactness and durability. Motivated by the scarcity of previous experimental research on slabs with realistic thickness and the promising results obtained in the past from specimens with FRC and HSC, this paper describes an experimental program with the aim of studying the behavior of HPFRC flat slabs under monotonic punching loading. To study the possibility of minimizing HPFRC consumption, the specimens were designed and produced as hybrid specimens, with HPFRC used only in the vicinity of the support column, being the rest of the slab casted with a Normal Strength Concrete (NSC). The experimental variables were the flexural reinforcement ratio (0.6% and 1.0%) and the extent of the HPFRC zone (1.5 and 3.0 times the effective depth of the slab, measured from the face of the column). A total of five specimens were tested, including a NSC reference specimen. The results show that HPFRC applied over a limited region near the column can increase the cracking load, maximum load carrying capacity as well as the displacement at failure. These benefits are apparent even with a small extent of the HPFRC zone (only 1.5 times the effective depth from the face of the column).