Corrosion of steel reinforcing bars (rebars) is the main cause of degradation of reinforced concrete structures. This issue may be addressed by using corrosion-resistant rebars made of Fiber-Reinforced Polymer (FRP). However, in France, the use of FRP rebars is currently limited to temporary applications due to a lack of comprehensive data on their durability in service conditions. Although many studies have investigated the durability of FRP rebars exposed to different alkaline environments (aiming to simulate the surrounding concrete), little is known about the combined effects of alkali ions and sustained loading. The use of FRP rebars is an alternative option to steel reinforcements investigated for some structural components in the Cigéo project. Reinforced concrete structures in the deep geological disposal are exposed to degradation by corrosion, but also subjected to aging under temperature (exothermic waste) and load (convergence of the geological environment). This paper presents preliminary findings from a durability study conducted on Glass FRP (GFRP) and Carbon FRP (CFRP) rebars subjected to alkali environments at different temperatures. Additionally, for some GFRP specimens, sustained tensile loading was applied at 20% and 40% of the tensile strength. The study included mechanical, physico-chemical, and microstructural characterizations of the various series of aged rebars. The experimental data obtained were subsequently analysed to assess the potential influence of sustained load on the degradation kinetics of GFRP rebars. This ongoing study is a collaborative effort between Gustave Eiffel University and the French National Agency for Nuclear Waste Management (Andra), aligned with the recent guidelines published by the French Civil Engineering Association (AFGC).