Damage models for UHPFRC and R-UHPFRC tensile fatigue behaviour

• Damage evolution of UHPFRC is analysed based on elementary damage mechanics theory.
• Fatigue damage in UHPFRC grows at constant rate until fatigue fracture.
• Significant damage in the early fatigue life leads to fatigue fracture of UHPFRC.
• The remaining fatigue life of UHPFRC is determined by the damage evolution model.
• An empirical relationship is deduced for fatigue damaging of R-UHPFRC.

Ultra-High Performance Fibre Reinforced Composites (UHPFRC) is a cementitious material showing relatively high tensile strength and significant tensile strain-hardening behaviour (given a certain volume of fibres). Adding a layer of UHPFRC or UHPFRC combined with steel rebars (R-UHPFRC) to structural members is an efficient method for strengthening of reinforced concrete structures. This paper presents empirical fatigue damage models for UHPFRC and R-UHPFRC. The tensile fatigue behaviour of UHPFRC is analysed based on elementary damage mechanics theory. Damage grows at a constant rate until fatigue fracture, which is considered to be due to the capacity of UHPFRC to redistribute local deformation increases. Difference in damage evolution between fatigue fracture tests and run-out fatigue tests is highlighted, and it is understood that when significant damage is caused in UHPFRC in the early stage of the fatigue life, UHPFRC fractures due to tensile fatigue. An average curve of damage evolution of fatigue fracture tests is proposed as a bi-linear damage evolution model of UHPFRC. The damage evolution model is used to determine the remaining fatigue life of UHPFRC by correlating the damage–fatigue strain relationship for UHPFRC. Considering that stress transfer from UHPFRC to steel rebars is characteristic of the R-UHPFRC tensile fatigue behaviour and is caused by fatigue damaging of the UHPFRC part, evolution of the modulus of deformation, i.e. the ratio of stress to strain of the UHPFRC part of the R-UHPFRC specimens is investigated. Among all the R-UHPFRC specimens similar behaviour is observed in the fatigue damaging curves of the deformation modulus of the UHPFRC part. An empirical relationship between the modulus of deformation of the UHPFRC part in the R-UHPFRC element and the number of fatigue cycles is proposed to characterise the R-UHPFRC tensile fatigue behaviour.