Shear strength evaluation of synthetic microfiber and macrofiber reinforced concrete beams

Abstract This study investigates the use of synthetic fibers—glass, polyester, polypropylene, and polyethylene—in reinforced concrete beams to assess their contribution when the structural element is subjected to shear stress. The methodology began with the characterization of the concrete constituents (cement, fine and coarse aggregates, and fibers), following the EPUSP-IPT Concrete Dosing method. Subsequently, the beams were cast as follows: one (1) reference beam without added fibers and twelve (12) beams with added macrofibers—three (3) beams for each fiber type (fiberglass, polyester, polypropylene, and polyethylene). Shear performance was then evaluated through experimental beam failure testing. Glass and polyester microfibers contributed to post-peak or post-cracking stability, reducing the likelihood of sudden failure. Their contribution to shear strength gain was 15.1% for VFV01 (beam with 0.024% glass microfiber) and 21.6% for VFPO02 (beam with 0.032% polyester microfiber), both relative to the reference beam (VR). Regarding the polypropylene and polyethylene macrofibers, it was observed that increasing the fiber content in the concrete mix led to greater ductility. Among the tested fiber contents, 0.36% and 0.48% polyethylene and 0.16% polypropylene represented critical percentages, indicating values close to the optimal fiber content. This suggests that the composite's load-bearing capacity approached the matrix failure threshold. Therefore, this study demonstrates that fiber reinforcement can be effectively used to control and enhance the shear performance of structural concrete. This composite presents itself as a promising partial substitute for transverse reinforcement—namely, stirrups in beams—by reducing the likelihood of sudden failure. Glass, polyester, polypropylene, and polyethylene fibers, at certain percentages, contributed to increased shear strength and toughness of the concrete.