Punching Shear Strength Improvement of Flat Slabs using Polypropylene Fibers

The punching shear failure at the region around the column is considered the most dangerous type of failure in the flat slab system, causing an instantaneous collapse of the buildings. It is a brittle failure mode that happens suddenly without any warning. Recently many techniques were introduced by researchers to avoid shear punching failure and enhance shear punching resistance of flat slab. Improving the mechanical properties of concrete is a powerful approach line using polypropylene fibers (PF) in concrete in which details are presented in this paper. Twelve slabs were made with dimensions of (1000*1000) mm, and 100mm in thickness. The flexural reinforcement was the same for all models; for the upper layer was (6ϕ6), while the lower layer was (10ϕ12) without shear reinforcement. The factors studied in this investigation involve the polypropylene fibers quantity, compressive strength value. The results showed a noticeable improvement in punching shear strength when adding (PF%=1%) by cement weight. However, the impact of polypropylene fibers was negligible when adding a ratio greater than 2%, and it was also observed that the failure changed from a brittle failure of non-polypropylene fibers models to a ductile failure in slabs containing polypropylene fibers. The failure cracks in a slab with polypropylene fibers are tiny and less in number than the NC slab. This is because fibers increase the internal bonding of the concrete material component. Also, increasing the ratio of polypropylene fibers reduced the perimeter and area of the punching shear region while the failure angle slightly increased.