Effect of Polyoxymethylene (POM) Fibers on the Flexural Performance of UHPC T-Beams

To investigate the influence of polyoxymethylene (POM) fibers as a substitute for steel fibers (SF) on the flexural performance of ultra-high-performance concrete (UHPC) T-beams, material property tests and four-point bending tests on structural members were conducted. A theoretical formula for calculating the ultimate flexural capacity was derived based on cross-section internal force equilibrium analysis. Material test results indicate that the compressive strength of POM-UHPC is approximately 20% lower than that of SF-UHPC. POM-UHPC presents strain-softening characteristics under uniaxial tension with an initial cracking strength of 3.48 MPa, and its stress decays rapidly after cracking. In contrast, SF-UHPC exhibits distinct strain-hardening behavior, with an ultimate tensile strength of 7.81 MPa. Member test results demonstrate that the flexural capacity of both series of T-beams increases with the longitudinal reinforcement ratio. Under the same reinforcement condition, the bearing capacity of SF-UHPC beams is 8.3–11.1% higher than that of POM-UHPC beams. Crack pattern observations show that POM-UHPC beams develop fine, uniform, network-distributed cracks with excellent crack dispersion and deformation capacity, whereas SF-UHPC beams generate fewer cracks with larger main-crack widths. Based on the plane-section assumption and force equilibrium, calculation formulas for ultimate flexural capacity of both POM-UHPC and SF-UHPC T-beams are derived, in which a fiber type influence coefficient is introduced. Good agreement is achieved between calculated and experimental results. Although POM fibers reduce the compressive strength of UHPC and the bearing capacity of beam members, they significantly improve crack-control performance and structural toughness. POM-UHPC possesses obvious advantages in corrosion resistance and lightweight performance, and shows prospective advantages for potential corrosive-environment applications, while relevant durability tests remain to be further investigated. The strength loss can be compensated via reasonable structural design. The proposed modification coefficients are calibrated for the investigated parameter range, and further validation is needed for extended engineering conditions.