EFFECTS OF CASTING DIRECTION ON PUNCHING SHEAR STRENGTH OF ULTRA HIGH PERFORMANCE FIBER REINFORCED CONCRETE SLABS

Abstract

Punching shear, characterized by a brittle behavior, which is considered as the typical failure mode of slab structures. Recently, Ultra High Performance Steel Fiber Reinforced Concrete (UHPFRC) has been known as a new achievement in concrete technology with much superiority mechanical properties which can help people to deal with the problem of punching shear of the thin slab. For UHPFRC material, the properties before and post-cracking mainly depend on the orientation of the fibers, which much depends on the casting method. In this research, an experimental work was conducted on 8 square UHPFRC slabs to investigate the effects of casting direction on punching shear capacity and modify an equation to predict the punching shear strength. The main parameters among these specimens were fibers volume fraction and different casting positions. The results showed that increasing fibers volume can significantly increase the ultimate punching failure load as well as delay the appearance of first cracks. Fibers also change the failure mode of the UHPFRC slabs from brittle to more ductile behavior. The more fibers the less flexural cracks were created and the higher ultimate punching shear failure loads were obtained. The effects of casting direction were clearly illustrated based on the size of failure cone. The slabs with the random distribution of fibers have smaller failure cone while comparing to slabs with the deliberate distribution of fibers. The results from the test and literature review were used to formulate an equation for predicting the punching shear capacity of UHPFRC slabs which gave the good agreement with the experimental results in this study.