Thermal degradation of polypropylene fiber-reinforced cementitious composites, tensile behavior and crack evolution assessed by digital image correlation

This study examines the tensile performance and crack evolution of polypropylene (PP) fiber-reinforced cementitious composites after exposure to elevated temperatures up to 400 °C. Specimens containing 0–3 % PP fibers (by volume) were subjected to controlled thermal regimes and evaluated using direct uniaxial tensile testing combined with digital image correlation (DIC). Results indicate that heating to 100 °C caused minimal strength reduction (<5 %), while exposure to 200 °C reduced ultimate tensile strength by approximately 18–25 %, depending on fiber content. At 300 °C and 400 °C, strength degradation became significant, reaching reductions of 40–65 %, accompanied by a marked decline in strain capacity and energy absorption. The tensile strain capacity decreased by up to 70 % at 400 °C relative to ambient conditions. Despite this overall degradation, elastic modulus increased by 8–15 % at intermediate temperatures, suggesting matrix densification prior to severe microstructural damage. Crack patterns transitioned from distributed multiple cracking at ambient temperature to localized single-crack failure beyond 200 °C, corresponding to fiber softening and melting observed in microstructural analysis. Composites with higher fiber volume fractions exhibited greater sensitivity to thermal exposure. Regression analysis confirmed a strong nonlinear correlation between temperature and tensile response ( R = 0.82–0.93).