Impact of synthetic fibers on spalling and intrinsic pore structure of ultra-high performance concrete (UHPC) under elevated temperatures

Incorporating synthetic fibers into UHPC is an effective method to mitigate explosive spalling at elevated temperatures. The improvement in UHPC's resistance to high-temperature spalling by synthetic fibers is primarily attributed to the increased permeability under elevated temperatures, which also affects the internal pore structure of UHPC. To investigate the effects of various synthetic fibers on UHPC's high-temperature spalling and internal pore structure, this study conducted high-temperature tests on UHPC with various synthetic fibers (PET, PP, NY, PVA, and PAN fibers). Utilizing SEM technique, MIP technique, and fractal theory, the pore structures of UHPC with various synthetic fibers after exposure to high temperature were compared and analyzed. This study revealed that the application of synthetic fibers improved UHPC's resistance to high-temperature spalling to a certain extent, but the high-temperature anti-spalling performance of UHPC with various fibers varied significantly. Selecting synthetic fibers with a high thermal expansion coefficient and low melting point is crucial for mitigating high-temperature spalling in UHPC through the addition of fibers. The order of the porosity of UHPC from highest to lowest after elevated temperature aligned with the high-temperature spalling resistance of UHPC with synthetic fibers from best to worst, which was U-PP > U-NY > U-PVA > U-PAN > U-PET > U. This sequence also coincided to the order of synthetic fibers arranged by thermal expansion coefficient from highest to lowest. Specifically, the greater the thermal expansion coefficient of the synthetic fiber, the higher the porosity and permeability of the corresponding UHPC after experiencing high temperature, resulting in better hightemperature spalling resistance of UHPC. The proportions of macropore in the overall pore structure of UHPC exceeded 80 % mostly, indicating significant pore coarsening in concrete after high-temperature exposure. The internal pore structure of UHPC exhibited fractal characteristics in Region I (1 mu m-600 mu m) and Region II (50 nm1 mu m), but not in Region III (5 nm-50 nm). In this study, fractal dimension models based on the internal porosity and total pore volume of UHPC were proposed. The proposed models showed a good fit with the experimental data, with relevant coefficients all greater than 0.9. Through the models proposed in this study, the fractal dimension of the pore structure can be directly obtained from the porosity or total pore volume of UHPC, which facilitates the characterization and quantification of the complexity of the disordered pore structure of UHPC after elevated temperatures.