Behaviour of circular high strength concrete columns reinforced with micro, macro and hybrid steel fibres under different loading conditions

This study investigates experimentally and analytically the behaviour of High Strength Concrete (HSC) columns reinforced with micro steel fibres, macro steel fibres and hybrid steel fibres under different loading conditions (concentric axial load, 25 mm and 50 mm eccentric axial loads and four-point bending). The influence of the type, volume content and aspect ratio (length to diameter ratio) of steel fibres on the strength and ductility of HSC columns were investigated by testing 16 circular specimens of 200 mm diameter and 800 mm height. All the specimens were reinforced with the same amount of longitudinal and helical steel reinforcement. The specimens were divided into four groups (Group RC, Group MI, Group MA and Group HY) of four specimens. Group RC specimens contained no steel fibres, Group MI specimens contained 3% by volume of micro steel fibres, Group MA specimens contained 2% by volume of macro steel fibres, and Group HY specimens contained 2.5% by volume of hybrid steel fibres (1.5% of micro steel fibres and 1% of macro steel fibres). A total of 12 specimens were tested as columns with different eccentricities (concentric axial load, 25 mm and 50 mm eccentric axial loads) and four specimens were tested as beams under four-point bending. In addition to the experimental work, analytical studies were conducted to develop axial load-bending moment interaction diagrams of Groups RC, MI, MA and HY using the equivalent rectangular stress block method and layer-by-layer integration method. Also, bending moment-curvature diagrams of the specimens tested under 25 mm and 50 mm eccentric axial load were developed using layer-by-layer integration method. The experimental results showed that the inclusion of micro steel fibres and hybrid steel fibres into HSC (Group MI and Group HY) enhanced the strength and ductility of the specimens under eccentric axial loads. The results also showed that the addition of macro steel fibres into HSC (Group MA) enhanced the ductility but reduced the strength of the specimens compared to Group RC specimens. The analytical axial load-bending moment interaction diagrams underestimated the corresponding experimental axial load-bending moment interaction diagrams. Also, the analytical axial load-bending moment interaction diagrams constructed from the layer-by-layer method were close to the experimental axial load-bending moment interaction diagrams. In addition, the analytical bending moment-curvature diagrams were in good agreement with the corresponding experimental bending moment-curvature diagrams.