Comparison between methods to determine the fibre orientation factor of an HPFRC bridge box girder

Abstract

Due to high volume fraction of steel fibres allied to improved rheology and packing density of cementitious matrices, materials like HPFRC and UHPFRC have been providing the possibility of partial or total replacement of passive reinforcement, expanding their applications to structures like footbridges, motorway bridges, and roof shells. When performing ultimate limit state design, the tensile strength curve obtained from tests of prismatic specimens moulded in laboratory must be adjusted by the so-called fibre orientation factor to consider the actual distribution and orientation of the fibres in the structural element. As this factor is influenced by the formwork geometry and the casting process, a representative mock-up of the actual structure must be constructed. In this context, this work aims to compare two methods for determining the orientation factor for a proposed bridge girder. The first method is based on 3-point flexural tests, and the second, on the number of fibres crossing a fracture plane. For their application, a box girder mock-up was built and prismatic specimens were extracted. The results showed both methods provided similar orientation factors in more than half the analysed specimens. However, in the remaining ones large differences were found which were then significantly reduced when the number of fibres crossing the tensile region of each specimen was considered in the calculation instead of the whole area. Moreover, the discrepancy shown amongst the post-cracking behaviour of the specimens indicates that steel fibre-reinforced concrete materials cannot be treated as isotropic without prior investigation.