Crushing Performance and Crack Development in Polymer-Fiber-Reinforced Precast Concrete Manhole Rings

Precast concrete manhole rings are exposed to localized tensile stresses and cracking during handling, transport, installation, and service. This study evaluates the effect of macro-synthetic polymer fibers on the crushing behavior, crack development, and failure mechanism of precast rings. Three reference rings and three polymer-fiber-reinforced rings made of C45/55 concrete were tested horizontally under monotonic displacement-controlled loading in accordance with EN 1917. Deformations and cracking were monitored using LVDTs, strain gauges, and digital image correlation. Both series satisfied the serviceability crack-width requirement and the minimum crushing-load criterion while showing comparable initial stiffness. The mean ultimate crushing load increased from 30.70 kN for the reference rings to 41.03 kN for the rings reinforced with MP40 macro-synthetic polymer fibers, corresponding to a 33.6% increase. At comparable advanced post-cracking deformation, the fiber-reinforced rings carried 73.6% more load. Polymer fibers promoted distributed cracking, reduced damage localization, and enabled stable load transfer after matrix cracking, whereas the reference rings developed a dominant crack and showed rapid post-peak strength reduction. The deformation criteria adopted from the fib Model Code 2010 were also satisfied. For the tested MP40 macro-synthetic fibers at a dosage of 4.0 kg/m³, the results demonstrate improved post-cracking resistance and ductility, supporting further assessment of reduced conventional steel reinforcement in precast manhole rings.