Probabilistic calibration of environmental reduction and partial safety factors for the design of reinforced concrete beams strengthened by flax fibre reinforced polymers based on two-factor accelerated degradation tests

•The viability of flax-FRP with an environmental bio-sourced resin for the strengthening of RC elements was investigated.•A two-factor accelerated test increasing both temperature and relative humidity conditions has been conducted.•Reduction and safety factors related to performances indicators suchtensile strength, stiffness, shear and pull-off strengths, found in four international design codes, and representative of the different principles applied in most of the design codes, have been calibrated for the specific use of F-FRP.•The comparability of the reduction and safety factors between F-FRP and C- or G-FRP confirms the viability of using F-FRPs.•Recommendations about the factors' values are given for typical desired service lives and targeted levels of reliability. The main topic of this article is the probabilistic-based calibration of environmental reduction and safety coefficients for the design of reinforced concrete civil engineering structures repaired or strengthened by externally bonded flax fibre reinforced polymers (FRP). In view of the poor feedback on the durability of flax fibre composite materials used in civil engineering and the impossibility of conducting performance tests over several years, a two-factor accelerated test campaign based on the stimulation of composite degradation by increasing both temperature and humidity conditions was conducted. Flax-FRP (with a bio-based epoxy matrix) laminates and Flax-FRP-strengthened concrete slabs have been exposed over a period of two years to various couplings of temperature (from 20 °C to 60 °C) and humidity (from 50 % of relative humidity to water immersion), according to an asymmetrical Hoke design of experiments. Similar specimens were also exposed to outdoor natural ageing (climate of Lyon, FR) over a period of three years. Series of tests (more than 320 tests) aimed at monitoring the evolution of the degradation of mechanical characteristics (tensile capacity, tensile stiffness, shear strength, pull-off strength) directly associated with the possible modes of failure of structural elements repaired by composites, were carried out on control and aged specimens. A degradation model of these mechanical characteristics considering the competition of two mechanisms - inducing non-monotonic degradation-, and the influence of the temperature and the humidity - through the Eyring model -, was then developed and finally extended to consider the variability of the degradation under different