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An experimental investigation was carried out to generate the complete stress-strain curves of steel fibre reinforced high strength concrete under axial compression. The experimental program consisted of testing 100 x 200 mm concrete cylinders. The experimental variables of the study were concrete strength levels (58.03 MPa and 76.80 MPa), volume fractions (0.5% to 2.0%) and aspect ratios (20 and 40) of flat crimped steel fibres. The effect of the mixed aspect ratio of fibres on the stress-strain behavior of steel fibre high strength concrete was also studied by blending short and long fibres. The effects of these variables on the stress-strain curves are presented and discussed. The results indicate that high strength concrete can be made to behave in a ductile manner by the addition of suitable fibres. It is concluded that short fibres are more effective in controlling early cracking, thereby enhancing the strength of the composite, whereas long fibres are more effective in providing post peak toughness. Concrete strength seemed to have an adverse effect on the deformability of fibre reinforced high strength concrete. Based on the test data obtained, a simple model is proposed to generate the complete stress-strain relationship for steel fibre reinforced high strength concrete. The proposed model has been found to give a good representation of the actual stress-strain response.

An experimental study was carried out to investigate the behavior of high strength concrete short columns confined by circular spirals and square ties under monotonically increasing concentric compression. The test variables included volumetric ratio, spacing and yield strength of transverse reinforcement, longitudinal reinforcement ratio, lateral steel configuration, shape of cross section and concrete compressive strength. The effects of these variables on the uniaxial behavior of high strength concrete columns are presented and discussed The results indicate that more confinement is required in columns of high strength concrete than in columns of low strength concrete to achieve the desired post-peak deformability. The behavior of high strength concrete columns is characterized by the sudden spalling of concrete cover, leading to a loss of axial capacity. A comparative study of existing confinement models of high strength concrete columns was also conducted to assess their capabilities of predicting the actual test behavior. To this end, the stress-strain curves of the specimens tested in the present study were compared with the ones predicted by the various models. It is shown that Legeron & Paultre (2003) model estimates the actual experimental curves more closely as compared to the other models employed in the study.