Punching shear strength of recycled aggregate-steel fibrous concrete slabs with and without strengthening

This paper presents an experimental study on the punching shear behavior of reinforced concrete flat slabs fabricated using recycled coarse aggregate (RCA) in addition to the presence of steel fibers strengthened by carbon fiber reinforced polymer (CFRP). The ratios of replacement natural coarse aggregate by RCA were (0, 35, 55, and 75%) and the volume fraction of the steel fiber used in this study was 1.2%. Two strengthening methods were used, namely, externally bonded reinforcement (EBR) and externally bonded reinforcement on groove (EBROG). 16 square slabs were cast with overall dimensions of 800 mm by 800 mm by 80 mm thickness. The slabs were designed to fail in punching shear only. All the slabs were simply supported on four edges and tested for punching via a vertical load applied through the square central column. The test results showed that the mechanical properties, ultimate loads, and first crack loads decreased with increasing proportion of RCA replacement, while the incorporation of steel fibers improved the compressive strength and ultimate loads by the ranges of 12%–17% and 16%–25%, respectively. Strengthened slabs lead to considerable improvements in the punching behavior of slabs, with an increase in punching load ranging from 22–30% compared to unstrengthened slabs when using the EBR technique, while the increase in the punching loads ranged between 40 and 55% when using EBROG strengthening technique. However, using half amount of CFRP with the EBROG method led to an increase in punching shear of 20–28%. This research indicates the particular improvement offered by the EBROG with regard to the punching shear capacity of fibrous recycled coarse aggregate concrete (RCAC) slabs compared to the EBR method. In addition, the mode of failure of reinforced concrete (RC) slabs changed from one of debonding failure to concrete cover separation when using the EBROG method.