Behaviour of Self-Compacting Concrete Members Reinforced with Small Dimension Square Steel Tubes

<p dir="ltr">Columns play a pivotal role in transferring all types of loads imposed on a structure to the foundations of that structure. In an effort to improve the efficacy of concrete columns, this study proposes an innovative way of reinforcing concrete columns by replacing traditional steel bars with small dimension square steel tubes filled with self-compacting concrete (SCC). The main aim of this research study is to investigate the behaviour of square steel tube-reinforced self-compacting concrete (SSTR-SCC) specimens under different loading conditions.</p><p dir="ltr">The research study was divided into four different sets of experimental programs. The first set of the experimental program focuses on understanding the effect of the unsupported length-to-width (L/B) ratio on the structural stability of square steel tubes with small cross-sectional dimensions subjected to axial compression and tension. Sixty-three small square steel tube specimens with three different cross-sectional dimensions (25 mm × 25 mm, 30 mm × 30 mm, and 35 mm × 35 mm) were tested under compression and tension. Specimens with varying L/B ratios ranging from 2 to 12 were tested under axial compression.</p><p dir="ltr">The second set of the experimental program focuses on investigating the effect of the unsupported length-to-width (L/B) ratio on the axial compressive behaviour of square self-compacting concrete-filled steel tube (SSCFT) specimens. The SSCFT specimens with three different cross-sectional dimensions (25 mm × 25 mm, 30 mm × 30 mm, and 35 mm × 35 mm) and L/B ratios of 2–12 were tested under axial compression. The performance of SSCFT specimens was compared with that of square unfilled steel tube (SUT) specimens of identical cross-sectional dimensions.</p><p dir="ltr">The third set of the experimental program introduces an innovative approach to replace traditional steel bars with small cross-section steel tubes in self-compacting concrete columns. The structural behaviour of square steel tube-reinforced self-compacting concrete (SSTR-SCC) columns was investigated experimentally. Sixteen specimens were cast and tested under concentric, eccentric, and flexural loads. Four specimens reinforced with 12 mm diameter deformed steel bars were considered as reference specimens, and the remaining 12 specimens were reinforced with small square steel tubes of three different cross-sectional dimensions (25 mm x 25 mm, 30 mm x 30 mm, and 35 mm x 35 mm) and a constant thickness of 2 mm. All the specimens were laterally confined with 10 mm square-shaped steel ties at 50 mm spacing.</p><p dir="ltr">In the last set of the experimental program, twelve small square steel tube-reinforced self-compacting concrete (SSTR-SCC) columns were tested under symmetric and asymmetric biaxial eccentric loads and their structural behaviour was investigated. Three specimens (reference specimens) were longitudinally reinforced with 12-mm diameter deformed steel bars, and the remaining nine specimens were reinforced with self-compacting concrete filled small square steel tubes of three different cross-sectional dimensions (25 mm x 25 mm, 30 mm x 30 mm, and 35 mm x 35 mm) and a constant thickness of 2 mm. All the specimens were laterally confined using 10-mm diameter steel bar ties at 50-mm centre-to-centre spacing.</p><p dir="ltr">The experimental findings revealed that the L/B ratio significantly influenced the performance of small square steel tubes. Experimental results of SSCFT specimens revealed that steel tubes effectively confined the infill concrete and demonstrated a 20–40% increase in compressive strength compared with the respective SUT specimens.</p><p dir="ltr">The SSTR-SCC specimens, under concentric, eccentric and flexural loads, demonstrated the superior load-carrying capacity and ductility compared to the reference specimens. Under biaxial eccentric loads, the SSTR-SCC specimens also demonstrated better performance than the reference specimens in terms of maximum axial load capacity and ductility. The SSTR-SCC specimens were found to be a scientifically significant and structurally efficient alternative to conventional reinforced concrete columns.</p>