Ultra high performance fibre reinforced concrete for highway bridge applications

It has been two decades since Ultra High Performance Fibre Reinforced Concrete (UHPFRC) has come to the market and, so far, it has been used in only a limited number of highway bridge structures. This is due to many unanswered questions related to its structural behaviour for highway bridge structures and its high initial cost. The lack of knowledge regarding the structural behaviour is down to unavailability of appropriate test methods for this special type of concrete which behaves differently compared to normal concrete. Furthermore, the current precast production contributes to its high initial cost significantly. Therefore, this study has investigated various aspects that are restricting the potential use of UHPFRC for highway bridge applications. The investigation included extensive experimental studies and numerical modelling. In the experimental programme, various parameters ranging from material and mechanical properties, potential use of the concrete for cast in-situ applications to ductility behaviour were investigated. Furthermore, the numerical analyses were carried out to identify appropriate finite element models to predict the flexural and shear behaviour of the concrete. In the experimental work, simple and reliable test methods for material characterisation were developed. The validity of two non-destructive testing methods in studying the elastic properties of the concrete was confirmed. From this, the UHPFRC constitutive material model was obtained and used in numerical modelling. The reliability of the test methods were established by performing numerous experimental tests and similar results obtained at all times. Furthermore, the suitability of the concrete for cast in-situ applications at various temperatures by monitoring the strength development from an early age (12 hours) up to 360 days were investigated. The results showed significant strength gain of the concrete in both compression and tension within 7 days when cured at temperatures similar to site conditions (20 and 30 oC). A phenomenon related to the 90 oC curing temperature in precast production was reported and showed to have caused loss of flexural strength and toughness of the concrete. In addition, for the first time, an effective test method for studying the punching shear strength of the concrete with minimal influence of flexural stress was developed and used successfully. The results showed a reduced effective punching shear perimeter of UHPFRC slabs by half compared to normal concrete. The numerical analyses were carried out using the Abaqus finite element software. In this study, the Concrete Damaged Plasticity (CDP) and Concrete Smeared Cracking (CSC) material models with minor modifications were used to simulate the flexural and shear behaviour of UHPFRC beam and slab specimens, respectively. The results obtained here were validated against experimental studies with good agreement. The CDP model was found to replicate the linear and nonlinear structural response of the concrete with better accuracies than the CSC model. This study presents significant findings on the suitability of UHPFRC for structural applications with a lower initial cost compared to its current precast production. Furthermore, results obtained on its excellent structural behaviour in flexure and shear provides structural designers great confidence in using this concrete for highway bridge applications. The findings reported in this study contribute to the literature of UHPFRC significantly.