Effect of hybrid fibers on shear behaviour of geopolymer concrete beams reinforced by basalt fiber reinforced polymer (BFRP) bars without stirrups

The shear response of thirteen slender geopolymer concrete (GPC) beams reinforced with basalt fiber reinforced polymer (BFRP) bars as longitudinal reinforcements without stirrups was investigated. Different fiber combinations were used to enhance the shear capacity of the beams, including (1) single type of macro-steel fibers (SF) or (2) macro-synthetic polypropylene fibers (PF), (3) hybridization of SF and micro-polyvinyl alcohol fibers (PVF) or (4) PF and micro-carbon fibers (CF). The experimental results indicated that the presence of SF yielded the greatest improvement in the cracking behavior, post-cracked stiffness, and shear capacity of the beams. The addition of 0.5% SF to the GPC and OPC beams increased the normalized shear strength most by 56% and 14%, respectively. The higher contribution of SF to the shear strength of GPC can be attributed to the superior adhesive bonding strength of fiber and GPC matrix. Despite being less effective than SF, adding PF also improves significantly the normalized shear strength up to 33% with 0.5% fiber content. The hybridization of SF and PVF showed a good synergy in enhacing the shear capacity. Meanwhile, the combination of PF and CF did not improve the shear strength but enhanced considerably the ductility of the beams. In addition, three analytical models were proposed to estimate the shear capacity of the SF reinforced concrete beams. The comparison of prediction and experimental results demonstrated the model derived from the modified compression field theory (MCFT) achieved the best correlation.