PUNCHING SHEAR STRENGTH OF STEEL FIBER-REINFORCED CONCRETE TWO-WAY SLABS

Concrete is a brittle material with a low strain capacity. The addition of randomly distributed discrete fibers in concrete significantly improves the flexural behavior of structural concrete elements because of its enhanced tensile properties and crack control. In this paper, an experimental program was carried out to study the punching shear strength of flat slabs reinforced with steel fibers. The addition of steel fibers generally contributes to the improvement of punching shear strength of flat slabs. The experimental work consisted of 8 rectangular specimens measuring 100 mm in depth, 1000 mm in length, and 1000 mm in width, six of which consisted of hooked-end steel fibers. The concrete mixtures were cast using ordinary Portland cement with a water-cement ratio of 0.41. During the punching shear tests, the load and the maximum deflection were recorded on a computerized data recording system; as a result, the load-displacement curve was drawn. The main variables considered include the volume fraction of steel fibers as well as the steel bar reinforcement ratio. The fibers were used at the volume fractions of 0.5\%, 1\%, and 1.5\%, and the reinforcement ratios of the slabs were 0.45\% and 1.02\%. After 28 days of curing, all the specimens were tested. The objective of this study is to investigate the modes of failure, the load-deflection relationships, and the effectiveness of adding steel fibers to flat slabs. Based on the results of this study, it was concluded that adding 1.5\% of steel fibers improved punching shear strength up to 34\% and 64\% depending on the ratio of reinforcement. In addition, it was shown that the depth of the flat slabs could decrease up to 38\% and 40\% in comparison to the slabs with plain concrete. Furthermore, increasing the ratio of reinforcement from 0.45\% to 1.02\% led to the enhancement of punching shear strength and improved toughness. The test results indicated that with fiber volume fractions of 0.5, 1, and 1.5, the shear punching strength increased by 49\%, 30\%, and 23\% due to the enhancement of flexural reinforcement from 0.45\% to 1.02\%, respectively.