Effect of various polymer fibers on spalling mitigation of ultra-high performance concrete at high temperature

Polymer fibers have been commonly used to prevent spalling of ultra-high performance concrete (UHPC) at high temperature. Although different polymer fibers have been adopted in UHPC, the critical properties of polymer fibers required for spalling resistance remained unanswered. This paper investigated the behavior of commonly used polymer fibers, including linear low-density polyethylene (LLDPE), ultra-high molecular weight polyethylene (UHMWPE), polypropylene (PP), polyamide 66 (PA) and polyester (PET) fibers, in spalling mitigation of UHPC through spalling tests, permeability measurements and microscopic characterizations. It was found that LLDPE, PP and PA could prevent spalling of UHPC, while UHMWPE and PET fibers were not effective in spalling prevention. Melting point of fiber could not determine its effectiveness on spalling prevention and empty channels left by molten fibers did not govern spalling resistance of UHPC. In fact, high coefficient of thermal expansion of polymer fibers is required for enhancing permeability of UHPC samples. Thermal expansion of polymer fibers created microcracks even before melting of fibers. This affected pore size distribution and increased permeability significantly, leading to high spalling resistance of UHPC.