Behaviour of UHPFRC-retrofitted RC beams with varying strengthening configurations under single and repeated blast loading

The objective of this study was to characterize the effects of various UHPFRC strengthening configurations on the blast performance of reinforced concrete (RC) beams. The methodology involved blast tests on eight as-built and UHPFRC-retrofitted RC beams using a pneumatically-driven shock-tube. The 150 mm x 200 mm x 2440 mm beams were first cast with 40 MPa concrete and were retrofitted by replacing the 20 mm cover with UHPFRC. The studied parameters included the influence of UHPFRC applied on the tension face, or in the form of U, Full or localized jackets, under single or repeated blast loads. The first set included one as-built beam and three specimens with tension-sided (T), U-jacket (UJ) or full jacket (FJ) retrofits, and were tested under repeated and gradually increasing blast pressures (Pr = 16, 38 and 50 kPa, for Blasts 1, 2 and 3). The second set included one as-built beam and two specimens with full-jacket retrofits applied over the full span (FJ) or in the middle hinge region (FJ-Hinge). The final beam had a hybrid of the T-sided and FJ-Hinge schemes. These beams were tested under single Blast 3 loads, and then tested under static bending to assess their residual capacity. In the first set, all retrofits resulted in reduced displacements (ranging from 30% to 60%) when compared to the as-built beam; however, the repeated blasts ultimately led to crack localization and rupture of the tension steel bars. In the second set, both the full-span and localized hinge schemes led to reductions in damage and displacements (ranging from 20% to 40%), without bar fracture, along with significant residual capacity. Finite element (FE) modelling using LS-DYNA was used to predict the blast behavior of the beams. After validation, the models were used to study the effects of the longitudinal steel ratio and blast load scenario on the behaviour and failure mode of the beams. Increasing the steel ratio was found to be effective in preventing bar rupture, while this failure mode was more likely under repeated blast loading.