Mechanical properties of hybrid fibers and nano-silica reinforced concrete during exposure to elevated temperatures

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The increasing risk of fire poses a significant threat to the safety of structural concrete. While the mechanical properties of concrete after exposure to elevated temperatures are well-documented, its properties during exposure remain insufficiently understood, posing challenges for effective fire-resistant design of concrete structures. This study aims to address this knowledge gap by investigating the mechanical properties of hybrid fiber and nano-silica (NS)-reinforced concrete (HFNRC) during elevated temperature exposure. Using a novel multi-functional testing apparatus designed for simultaneous heating and mechanical testing, the effects of varying steel fiber volume ratio (SFs, 0-1.5% by volume) and NS dosages (0-1.5% by weight) were examined at temperatures of 200°C (473.15K), 400°C (673.15K), 600°C (873.15K), and 800°C (1073.15K). The experimental results indicate that compressive and flexural strengths initially decreased, then increased, and subsequently decreased again as temperatures rose from 200°C to 800°C, while splitting tensile strength consistently declined. At 800°C, the compressive, splitting tensile and flexural strengths decreased by 63%, 82% and 76%, respectively. Notably, the addition of 1.5% by volume SFs increased the splitting tensile strength by up to 119% at 400°C, providing the greatest improvement among all tested properties. Empirical equations were established to predict the compressive, splitting tensile, and flexural strengths of HFNRC during elevated temperatures. Microstructural analyses using scanning electron microscopy (SEM) and X-ray diffraction (XRD) demonstrated that NS enhances the compactness of HFNRC’s matrix and promotes the formation of calcium silicate hydrate (C-S-H) gel, contributing to improved performance under fire exposure. These findings are relevant to international research focused on enhancing the fire resistance of structural materials and improving the safety of concrete structures globally.