A Micro Structural Study of Passive Confined Self-Compacting Concrete under Compression at Elevated Temperatures

Abstract

This research investigates the impact of elevated temperatures on the load-carrying capacity of Self-Compacting Concrete-Filled Steel Tubes (SCCFST) with varying concrete grades, and slenderness ratios. The study begins with the selection of two key materials: self-compacting concrete (SCC) and steel tubes. Three concrete grades, namely M60, M70, and M80, along with different slenderness ratios 8 and 10, are considered as variables. The specimens undergo 28 days of curing before being subjected to controlled heating in a 1000°C-capacity oven at intervals of 100°C. Subsequently, uniaxial loads are applied by using a universal testing machine until failure occurs. The study aims to determine, first the load-carrying capacity of SCCFST specimens, as well as to analyze the corresponding stress and strain characteristics and second Microstructural analysis, to identify chemical compound which effect the load-carrying capacity at different temperatures. The results of this investigation reveal a distinct trend in the load-carrying capacity of SCCFST specimens as temperatures range from 30°C to 800°C. Within the temperature range of 30°C to 400°C, a moderate decline in load-carrying capacity, ranging from 5% to 11%, is observed. However, beyond the critical threshold of 400°C, a notable and rapid reduction in load-carrying capacity is documented, with an average decrease of 22% noted from 400°C to 500°C. Microstructural analysis of failed specimens from the buckling zone provides insights into the mechanisms responsible for these temperature-induced changes. The primary contributors to load-carrying capacity, including C-S-H (Calcium Silicate Hydrate), Ca(OH)₂ (Calcium Hydroxide), and CaCO₃ (Calcium Carbonate), are identified and studied in detail. At 400°C, both Ca(OH)₂ and CaCO₃ begin to decompose, with CaCO₃ exhibiting a slower decomposition rate compared to Ca(OH)₂, reaching a decomposition rate of 55%. Conversely, the decomposition of C-S-H is observed at 500°C, with a decomposition rate of 60%.