Ultra-high performance concrete (UHPC) is a novel class of concrete that has superior mechanical properties and durability characteristics as compared to that of conventional concrete. When structural members made of UHPC are used in building construction, the provision of appropriate fire resistance is a key safety consideration. Since UHPC is a new construction material, there is limited information, as well as limited research on the fire performance of UHPC members. Preliminary research at the material and structural level have shown that UHPC members exhibit comparatively poor fire performance as compared to conventional concrete due to fire-induced spalling resulting from its dense microstructure as well as faster degradation of mechanical properties with temperature. At present, there is a lack of experimental data and numerical models for evaluating the fire resistance of UHPC structural members.To overcome some of the current knowledge gaps, the behavior of UHPC under fire conditions is studied at both the material and structural levels. As part of material characterization, thermal and mechanical property tests were carried out in the 20-800℗ʻC temperature range on two types of UHPC mixes (with and without polypropylene (PP) fibers). Data from measured property tests were utilized to propose empirical relations for high-temperature material properties of UHPC. As part of structural level characterization, four UHPC beams were tested under simultaneous application of loading and fire exposure. The test variables included the presence of polypropylene fibers, load level, and type of fire exposure. As part of the numerical study, a macroscopic finite element (MFE) model, originally developed to evaluate the fire resistance of reinforced concrete (RC) beams made of conventional concrete, was extended to predict the thermo-mechanical response of UHPC beams under fire conditions. The novelty of the developed numerical model lies in the consideration of stresses resulting from pore pressure, structural loading, and thermal gradients for evaluation of spalling, instead of evaluating spalling based on only stresses due to pore pressure as in the previous studies. Further, the fire resistance analysis model was also modified to carry out a member-level structural analysis rather than an analysis of a single critical section. In addition, an expression for variation in permeability of concrete resulting from cracking patterns across the cross-section is proposed. The program also accounts for permeability variation due to the addition of polypropylene fibers. The model was validated by comparing thermal and structural response, the extent of spalling, and fire resistance predictions against measured test data on UHPC beams.The validated model was further applied to conduct a set of parametric studies to quantify the effect of critical parameters on the fire response of UHPC beams. Results from the studies indicate that load level, fire scenario, cover thickness, specimen shape, sectional dimensions, and dosage of steel and polypropylene fibers have a significant influence on the fire response of UHPC beams. Further, among beams of different concrete types, the fire resistance of UHPC beams was significantly lower due to higher spalling levels resulting from their lower permeability, than normal strength concrete (NSC) and high strength concrete (HSC) beams, where permeability is relatively higher. Finally, results from the studies are used to develop a set of broad guidelines for the fire design of UHPC beams. By adopting the design guidelines, spalling in UHPC beams can be minimized and fire resistance can be improved.
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