Structural behaviour of steel stub column in-filled using self-compacting concrete with fly ash and silica fume

Abstract

Concrete filled steel tubular (CFST) column is a composite structural member that consists of a hollow steel tube and concrete core. Literature has indicated that the concrete core could carry up to 60% of the applied compressive load. The use of fly ash (FA) in concrete has attracted the attention of many researchers due to the global quest for sustainable and green materials in achieving an economical and low carbon footprint environment. By the same notion, silica fume (SF) is added, to enable the concrete to flow and fill narrow steel tube, which resulted in the use of self-compacting concrete. Notably, the strength of Portland cement concrete would be affected by the FA and SF as cement replacement. Researchers have a mixed understanding of the ductility of CFST columns infilled with mineral admixtures as core concrete. The aim of this research was to investigate the performance and behaviour of CFST stub columns infilled with self-compacting concrete containing FA and SF under axial load. Laboratory investigations were conducted to develop self-compacting high-performance (SCHP) concrete containing FA and SF with a target compressive strength of 60 MPa. Series of SCHP concrete were prepared at water-cement ratio of 0.3 with 0%, 25%, 35%, 50%, 60%, and 75% replacement of Portland cement (PC) by FA and SF while maintaining 10% replacement of SF. The properties of the SCHP concrete were evaluated in terms of fresh, hardened, and microstructural properties, while the properties of structural cold-formed steel were obtained through a coupon tensile test. The performance of the FA-SF self-compacting CFST stub columns were examined through the axial compression capacity, the load shortening response, and the failure mode. The compression capacities of the CFST columns were then compared with the theoretical values obtained using the international design codes. The CFST stub column's behaviour was also simulated through Finite Element (FE) modelling using ABAQUS software. The experimental results showed that concrete with 25%PC, 65%FA and 10%SF sustained a maximum compressive strength of 79.73 MPa at 28 days, and the cement content of this mixture was just 146.88 kg/m3. The microstructure of FA-SF self-compacting concrete was relatively less porous compared to the ordinary Portland cement concrete. Moreover, the incorporation of FA and SF in the concrete core has resulted in a higher stiffness index and concrete contribution ratio, indicating the enhancement of the CFST stub columns' stiffness. However, the control CFST stub columns demonstrated better ductility. Comparing the FE predictions with test results revealed that the FE models marginally underestimated the circular and square columns' ultimate strengths by an average of 1.3% and 1.43%, respectively. The proposed model forecasted the ultimate strength of the CFST stub columns with good prediction accuracy. The mean was 0.99, with a standard deviation of 0.01 for circular CFST columns, while a mean value of 1.05 was obtained for square CFST stub columns with a standard deviation of 0.12. Although CFST stub columns infilled with FA-SF self-compacting concrete demonstrated higher axial load capacity and concrete contribution ratio, the ductility was lesser. However, this condition may still permit the usage of CFST stub columns with FA-SF self-compacting concrete to the construction in low seismic zones.