Case Studies in Construction Materials (Dec 2024)
Punching shear of sustainable recycled aggregate FRC slabs strengthened with NSM FRP bars
Abstract
Given the rise in sustainable use of recycled aggregate concrete (RAC) in new construction, weaker properties may compromise the integrity of structural components, including the slab-column connections. Low-strength concrete can be encountered, not only by using weak RAC in new construction, but also in the case of normal concrete in aged and deteriorated historic or conventional structures. To uphold the sustainable use of such materials in critical structural components, the need for restoring the strength via external strengthening using near-surface-mounted (NSM) fiber-reinforced polymer (FRP) bars is presented in this study. The paper experimentally investigates the behavior of NSM FRP strengthening against punching shear failure in slab-column connections produced from RAC and steel fibers with low structural strength. The three variables studied are steel fiber addition, recycled aggregate replacement, and FRP bar material type – Glass FRP (GFRP) versus Carbon FRP (CFRP). Five tests were conducted, utilizing three and two specimens strengthened with GFRP and CFRP bars, respectively. The three GFRP bar strengthened specimens differ in the type of concrete used, namely, ordinary concrete without steel fibers, ordinary steel fiber reinforced concrete (FRC), and RAC without steel fibers. The two CFRP bar strengthened specimens were of ordinary concrete and RAC, both with steel fibers, resulting in recycled aggregate FRC. RCA’s incorporation positively affected the results of the ultimate punching shear, flexural stiffness, and the maximum displacement due to the likely role of the recycled aggregate angularity in resisting the punching shear better. Due to higher flexural stiffness of the NSM CFRP, it entails more likelihood of debonding failure that limits its full utilization in increasing the punching shear capacity compared to that of NSM GFRP. However, both the NSM strengthening materials appear promising in low-strength (f’c ≤ 17 MPa) concrete slabs to reach acceptable capacities. Assessment based on different design code provisions and an analytical model revealed better accuracy of the Critical Shear Crack Theory (CSCT) and the fib Model Code in predicting punching shear capacity for NSM strengthened ordinary concrete column-slab connections, but with some slight deviations when applied to specimens made of RAC.
