Structural behaviour of elements combining ultra-high performance fibre reinforced concretes (UHPFRC) and reinforced concrete

Ultra-High Performance Fibre-Reinforced Concretes (UHPFRC) have high mechanical strengths (fU,c > 150 MPa, fU,t > 6 MPa) and exhibit quasi-strain hardening in tension. Their very low permeability prevents the ingress of detrimental substances. In composite structural elements formed of normal strength reinforced concrete and Advanced Cementitious Materials (ACM), UHPFRC offer a high potential in view of the load carrying and protection function of the ACM layer. The objectives of the study described in this thesis are to determine the performance and structural behaviour of composite "UHPFRC-concrete" elements in bending. Towards this end, the current knowledge of UHPFRC properties is to be extended and modelling tools are to be developed in order to predict the structural behaviour of such composite elements and to make recommendations for their design. The experimental program is performed in order to characterize the UHPFRC and determine the structural behaviour of composite "UHPFRC-concrete" elements through 15 full-scale beam bending tests. The material tests focus on UHPFRC early age behaviour and on the determination of its outstanding tensile properties with an original uniaxial tensile test. They show that the UHPFRC properties become virtually constant after 90 days. The time-dependent behaviour of the composite beams is investigated during 11 weeks starting from the casting of the UHPFRC layer. After these long-term tests, the beams are subjected to bending until failure with the UHPFRC layer in tension. The parameters are the thickness of the UHPFRC layer, the presence of rebar in the UHPFRC and the static system. The time-dependent behaviour of composite "UHPFRC-concrete" members is investigated focusing on early age by means of test results and an existing numerical model. Using the results of the material tests as input, the numerical model is able to predict the behaviour of composite "UHPFRC-concrete" elements with the exception of self-desiccation of the UHPFRC. However, the deformations of composite "UHPFRC-concrete" members are correctly modelled by introducing autogenous shrinkage directly as volumetric deformation in the model. The structural response of the composite members under bending with the UHPFRC layer in tension is investigated with an original analytical model, which is an extension of the classical bending model for reinforced concrete. The influences of cross-section geometry, the UHPFRC tensile properties, the compressive strength of the substrate and the type of reinforcement are studied. This study demonstrates that the use of UHPFRC enhances the performance of composite "UHPFRC- concrete" elements in terms of resistance and stiffness. Furthermore, durability is extended due to the low permeability and tensile strain hardening properties of UHPFRC. The incorporation of rebar in the UHPFRC layer leads to a further increase in resistance and stiffness of the composite element and to a higher apparent magnitude