Shear strength of HPFRC prestressed beams

Abstract

This study presents an experimental campaign aiming at studying the shear behaviour of prestressed HighPerformance Fibre Reinforced Concrete (HPFRC) beams. Six shear tests on 10 m long and 0.725 m deep HPFRC beams are performed varying the shear reinforcement ratio (0, 0.2 % and 0.4 %) and the fibre volumetric fraction (0 and 1 %). The compressive concrete strength of 100 MPa and the centered prestress value of 10 MPa were kept constant. The displacements field was measured using Digital Image Correlation (DIC) technique. The number of fibres crossing the critical shear cracks were counted and the local fibre content and orientation throughout the web were measured using a non-destructive method based on the magnetic properties of the steel fibres. It is found that the flow of the self-compacting material during casting induces preferential orientation of the fibres along the longitudinal direction. The average number of fibres per unit area of the critical shear cracks was found to be approximately half of that counted in the fracture surfaces of the notched prims used for characterizing the tensile behaviour of the material. The beams with fibres and without vertical shear reinforcement failed well after the onset of shear cracking with gradual opening of the shear cracks up to peak load. For the beams with shear reinforcement, the addition of steel fibres increases the number of cracks in the webs and the shear strength. The contribution of the fibres was shown to decrease with increasing reinforcement ratio. The results also indicate that shear failure occurs when the sum of the contributions to shear balanced by the web and flanges reaches its maximum, which for softening fibre reinforced materials may occur after the web capacity is reached. A newly developed analytical model to estimate the in-plane shear strength is used in the discussion of the experimental results taking into account the data concerning the fibre orientation. The model accurately predicts the contributions of fibres to shear strength across varying reinforcement ratios. The conservativeness of the estimates can be explained by the contribution of flanges to shear resistance, which was not accounted for in the model.