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Ultra High-Performance Fibre-Reinforced Concrete (UHPFRC) combines high strength, deformation capacity and low permeability. This makes UHPFRC particularly suitable as a strengthening and protective layer for application to existing reinforced concrete structures (RC-UHPFRC composite members). The structural behaviour of UHPFRC layers in composite members is investigated with a comprehensive experimental program. In a systematic approach the behaviour of plain UHPFRC and reinforced UHPFRC (R-UHPFRC), the bond behaviour of reinforcement in UHPFRC and the flexural behaviour of composite beams are analysed. The study is limited to thin UHPFRC layers (≤ 50 mm) and small diameter bars. Special attention is given to the representativeness of the experimental specimens. In order to avoid preferential fibre orientations, all UHPFRC and composite specimens are cut from larger elements. The high scatter of the tensile behaviour of plain UHPFRC, quantified by the testing of a high number of specimens, confirms the reasonableness and necessity of additional bar reinforcement. The additional reinforcement increases not only the resistance but also improves the deformation capacity and strain hardening behaviour of UHPFRC. The use of reinforcing bars with different steel grades and surface characteristics, namely ribbed and smooth bars, improves the current knowledge base on the bond behaviour. It is shown that the pre-peak behaviour is independent of the bond strength and that the crack spacing is dominated by the fibre reinforcement. The reinforcing bars with low bond strength allow for large post-peak deformations and avoid the localised stress concentration in the UHPFRC macro-crack. The use of high yield strength steel is beneficial in view of the increased deformation capacity of R-UHPFRC. The apparent strain hardening of UHPFRC is significantly increased by the reinforcement. The strengthening of reinforced concrete beams with a R-UHPFRC layer is characterised by the significant increase of the resistance and stiffness (depending on the reinforcement) combined with an important reduction of the rotation capacity. With a good balance between resistance increase and loss of rotation capacity the smooth reinforcing bars of the UHPFRC layer represent an interesting alternative to conventional reinforcement. The use of high yield strength steel proves to be advantageous due to the higher deformation capacity. Through the use of an analytical model for plain UHPFRC the origin of the scatter of the tensile behaviour and the conditions for the occurrence of strain hardening are identified. The modelling of R-UHPFRC gives an insight into the interaction of fibres and reinforcing bars and the apparent UHPFRC tensile behaviour. Further, it is shown that the characteristic structural behaviour of composite sections can be described with the extended sectional analysis. Finally, the main experimental and analytical results are summarised in recommendations for the application of R-UHPFRC to existing structures. This thesis contributes to the understanding of the behaviour of R-UHPFRC and composite members and emphasises the necessity of additional reinforcing bars in UHPFRC in order to achieve a reliable structural behaviour.