SHEAR BEHAVIOUR OF REINFORCED AND PARTIALLY PRESTRESSED STEEL FIBRE CONCRETE BEAMS

Abstract

Shear failure of concrete beams is usually sudden and brittle in nature, and should be avoided in practice by proper design. Conventionally, stirrups are provided in beams to fulfil the requirements for resistance against shear failure. Recent investigations however, indicated that the addition of small discrete steel fibres could lead to a significant improvement in the shear behaviour of beams. The main objective of this study is therefore to investigate the effectiveness of discrete steel fibres as shear reinforcement in reinforced and partially prestressed concrete beams. The investigation involved both analytical and experimental works. In the analytical works, two methods were considered for the evaluation of the shear response of steel fibre concrete beams. For this purpose, principal stress - strain relations were developed and verified with the available test data for steel fibre concrete under biaxial state of stress. The first proposed method to predict the shear response is based on the modified compression field theory (MCFT). In this method, basic equilibrium and compatibility equations and material laws for the cracked concrete accounting for the effect of steel fibres and the softening of concrete in compression were considered. The analysis, which assumed principal stress and strain directions to coincide, involved an iterative process to evaluate the deformation and stress in the beam under a given load. The second method, which is more rigorous, involves a non-linear finite element formulation based on a rotating smeared crack model. It treats the steel fibre concrete as an orthotropic non-linear elastic material. The finite element formulation was checked for convergence characteristics and numerical stability. To verify the validity of the proposed analytical methods, tests were carried out on fully reinforced and partially prestressed steel fibre concrete beams under shear and heading. The major test parameters were the steel fibre content, partial prestressing ratio and shear span to effective depth ratio of the beams. The steel fibres used were hook-ended, 30 mm long and 0.5 mm in diameter. Beam deflections, concrete and steel strains and cracking were monitored during the tests. The experimental study indicates that the stiffness of the beams after web cracking significantly improved with the addition of steel fibres, leading to enhanced shear strength and this trend is found to be independent of the partial prestressing ratio. The shear carrying capacity of the beams was also found to increase with reduction in the shear span to effective depth ratio. The test results, together with those available in the literature were compared with the predictions of the proposed methods. In general, the strain measurements and the ultimate strength of the test beams compared well with the analytical predictions. Based on the study, a simple equation is derived for the calculation of the contribution of steel fibres to the shear capacity of the beams. With this, beams with steel fibres alone or in combination with stirrups as shear reinforcement can be designed using the provisions of the existing codes. Three beams were designed in this manner and tested to failure. The beams exhibited shear strength in excess of the design values.