Short- and long-term deflections of RC slabs strengthened with CFRP systems under sustained loading and different environmental conditioning

This study investigates the combined effects of sustained loading and environmental conditioning on the short-and long-term mid-span deflection behavior of reinforced concrete (RC) slabs strengthened using different CFRP-based techniques: (i) Externally Bonded Reinforcement (EBR), (ii) Near-Surface Mounted (NSM), (iii) EBR with prestressing via mechanical anchorage (EBR-MA), and (iv) EBR with prestressing via gradient anchorage (EBR-GA). Over a period of up to six years, each of the 24 studied slabs was subjected to a sustained load equivalent to approximately 40 % of its ultimate capacity and exposed to one of the following six environmental conditions: (i, ii) two controlled laboratory settings– 20 ◦C and 55 % relative humidity in a climatic chamber, and full immersion in tap water at 20 ◦C in tanks; (iii to vi) four outdoor exposure sites across Portugal, each characterized by distinct humidity levels, temperature variations, freeze-thaw cycles, and UV radiation exposure. Instantaneous and long-term mid-span deflections were recorded and compared with the adopted predictive models. The results demonstrate that both the axial stiffness of the CFRP and the selected strengthening technique significantly influence deflection magnitude. For the instantaneous response, the Gilbert approach combined with the fib MC (2020) formula provided improved agreement with the measurements. Regarding long-term behavior, the Age- Adjusted Effective Modulus Method with a modified tension-stiffening model enabled derivation of slab-specific bond-dependent coefficients. Furthermore, the observed deflections closely align with the predicted values, and the proposed time-dependent factor curves (spanning up to 50 years), developed using a generalized Kelvin model together with ACI 318R-19, offer a valuable tool towards establishing simplified predictive models.