Flexural capacity of reinforced concrete slabs retrofitted with ultra-high-performance concrete and fiber-reinforced polymer

In this study, a new method for strengthening and retrofitting the flexural capacity of reinforced concrete (RC) slabs using ultra-high-performance concrete (UHPC) and fiber-reinforced polymer (FRP) is proposed. The effectiveness of the strengthening method was investigated through experimental studies. Eight RC slabs were tested consisting of the control slab (without strengthening), a slab strengthened with FRP at the bottom, and six slabs retrofitted with 30-mm-, 40-mm-, and 50-mm-thick UHPC layers on the top and also strengthened with FRP at the bottom. The flexural capacities of the slabs were determined using four-point bending tests. Detailed flexural behaviors including cracking patterns, strain distribution, and ductility of the test slabs were examined and compared. The findings demonstrated that the incorporation of UHPC layers in the compressive zone of the RC slabs played a crucial role in substantially increasing their strengths (from 31.48 to 64.09%) and changing the modes of failure and cracking patterns within the composite slabs. Moreover, the combined use of UHPC in the compressive zone and FRP in the tension zone resulted in a remarkable increase (from 10.35 to 40.08%) in the flexural capacity, and about 1.9-2.3 times reduction in the maximum deflection compared to slabs using only UHPC in the compressive zone.