Punching shear and load-deflection performance of RC flat slab made with hybrid concrete

Due to their various benefits, flat slabs are among the most commonly used flooring solutions worldwide.However, their use has been limited because of the risk of punching shear failure between the slab and the columns.Researchers have explored different solutions to address these issues, including using steel fiber-reinforced concrete (SFRC).Although this type of concrete offers advantages, its high cost can increase the overall expense of the structure compared to conventional concrete.An effective solution proposed was using hybrid concrete that combines SFRC in specific areas of the slab -namely, the center and around the columns -while utilizing conventional concrete for the remaining area.This study aims to investigate using hybrid concrete in flat slabs to enhance their punching shear resistance.The areas of fiber-reinforced concrete examined were circular shape.Limited research has explored the application of such hybrid concrete for shear strength, and even fewer studies have focused on the circular shapes of SFRC zones.Two parameters were considered in this study: (1) the radius of the hybrid concrete zone, which was set at two and three times the diameter of the column, and (2) the added percentage of steel fibers, specifically 0.5%, 1%, and 1.5%.Seven slabs were cast; one slab was made entirely of normal strength concrete (NSC) and served as a control, while six slabs were composed of hybrid concrete, a combination of NSC SFRC.The punching shear capacity, load-deformation behavior, crack patterns, failure mode, stiffness and ductility properties were investigated.The results indicated that hybrid concrete significantly enhanced punching shear resistance and other characteristics of flat slabs at both hybridization zones, specifically at two and three times the column radius.Additionally, the rate of improvement was directly proportional to the steel fiber content used.For example, with a fiber content of 1.5%, the punching shear capacity improved by 41.98% and 66.67% for the zones at two and three times the column radius, respectively.Furthermore, the uncracked stiffness increased by 30.9% and 44.6%, while the load capacity was enhanced by 42% and 66.7% for the same zones.