Residual strength of hybrid‐fiber HPC after elevated temperatures: Process‐based models consistent with SEM / EDX evidence

This study quantifies the residual mechanical performance of hybrid polypropylene–steel fiber high‐performance concrete (HFHPC) after exposure to 20–1000°C under controlled heating and cooling. Relative compressive, splitting tensile, flexural strengths, and static modulus of elasticity were measured and interpreted alongside SEM/EDX observations. Two response functions were calibrated: a quadratic surrogate and a mechanistic process model with a knee temperature T 0 , a decay span T 2 , and a tail exponent n . Hybrid mixes with moderate PP–ST dosage retained over 90% of their baseline strength and stiffness up to ~400°C and consistently outperformed fiber‐free HPC at intermediate temperatures. Compression exhibited a knee at T 0 ≈570°C with a moderate tail, whereas splitting tensile strength initiated earlier degradation at T 0 ≈220°C. A steeper tail ( n = 1.7) and broad span ( T 2 ≈650°C) reproduced the observed decline. Process models improved the adjusted coefficient of determination ( R 2 ) and reduced the residual sum of squares relative to quadratic fits while mapping parameters to plausible mechanisms (PP melting/venting, steel–matrix debonding, C–S–H decalcification). SEM/EDX microstructures corroborated these mechanisms. The resulting laws provide compact, physically interpretable predictors for post‐fire strength retention in HFHPC within the tested range and can support rapid assessment. Transfer to other mixture designs should follow targeted re‐calibration.