Flexural strength of FRP reinforced UHPC beams considering size effects

Available studies have shown that the contribution of steel fibers to the flexural capacity of ultra-high-performance concrete (UHPC) beams is significant and inherently size-dependent. This study proposes a design-oriented method to determine the flexural strength of reinforced UHPC beams internally with fiber-reinforced polymer (FRP) bars. To this end, a validated analytical model was first developed for estimating the moment-curvature of FRP-reinforced UHPC sections. Subsequently, a parametric investigation was carried out to examine the effects of steel fiber inclusion in UHPC and section depth on flexural strength. The results showed that crack-bridging steel fibers significantly enhanced the flexural capacity of tension failure-controlled sections, whereas their contribution was insignificant in compression failure-controlled sections. Neglecting the steel fiber contribution may lead to incorrect predictions of the failure mode. A pronounced size effect was evident in the flexural strength as well as in the balanced reinforcement ratio. At an FRP reinforcement ratio of 1.0%, increasing the beam depth from 250 mm to 2000 mm resulted in an approximate 27.5% reduction in normalized flexural strength. Finally, a design-oriented flexural strength method that accounts for both steel fiber contribution and size effect was proposed. The accuracy of the proposed method was validated through comparison with available experimental data from flexural tests on 42 UHPC beams.