Many of existing structures were built using reinforced concrete (RC) as the main constituent material. Despite a considerable history of success in the adoption of this material, RC based structures do not last forever due to many factors; however, their service life can be extended through structural rehabilitation such as strengthening. RC structures can be strengthened using carbon fibre reinforced polymer (CFRP) composites to increase both their structural and long-term performance. The combined effects of hygrothermal conditioning and sustained loads on the structural and long-term performance of RC structures strengthened with CFRP composites are not fully understood. In this regard, the evolution of the mid-span deflection of RC slabs strengthened with different CFRP systems is investigated when these slabs are simultaneously subjected to a sustained load (approximately 40% of their ultimate capacity) (i) in a climatic chamber controlled at 20 °C and 55% of relative humidity, and (ii) in tap water immersion at 20 °C, for up to 6 years. In this paper, the experimental campaign is described, the ongoing results are presented and discussed, prediction of instantaneous mid-span deflection, and estimation of long-term mid-span deflections up to 100 years are provided.