Fire-induced spalling mitigation in high-strength concrete: development of a reproducible torch method and evaluation of keratin waste fibers

This study addresses two challenges in fire-induced spalling of high-strength concrete (HSC): the limited reproducibility of small-scale fire testing methods and the need for sustainable alternatives to polypropylene fibers (PPFs). An Improved Torch Method (ITM) was developed as a low-cost comparative assessment method by introducing a temperature-stabilization stage prior to specimen exposure, and validated against the Traditional Torch Method (TTM) using control and PPF-reinforced concrete mixtures. The validated ITM was then applied to evaluate keratin-based dog hair fibers (DHFs) as a sustainable alternative to PPFs. The ITM reduced the coefficient of variation (COV) of residual strength results from 19.5% (TTM) to 6.5%, demonstrating enhanced reproducibility. DHFs effectively mitigated spalling damage, achieving reductions of roughly 70% in spalling depth and more than 90% in spalled volume compared to unreinforced concrete, performing comparably to PPFs. Thermal and chemical analyses suggest that DHFs' efficacy is attributed to protein dehydration and dimensional changes between 232 and 385 °C, creating permeability pathways analogous to the melting mechanism of PPFs. These findings establish the ITM as a reliable screening tool and highlight the potential of keratin waste fibers for sustainable spalling mitigation.