## 1 Introduction  
The use of fibre-reinforced polymers (FRP) to strengthen concrete structures has been steadily gaining popularity over the last decades. The externally bonded reinforcement (EBR) and near-surface mounted (NSM) techniques are currently the most well-known methodologies employed to enhance the load-carrying capacity of civil structures. Standards and guidelines such as the fib Bulletin 90 [1], the AC1 440.2R-17 [2], and the CNR-DT 200 R1/2013 [3] have already approved the design of FRP strengthening systems for concrete structures. Even though the short-term bond behaviour of EBR and NSM methodologies has been extensively studied over the past decades, less attention has been paid to their behaviour under service conditions. External circumstances, such as sustained loading or environmental temperature conditions, can cause significant damage, either to the concrete [4, 5, 6] itself or to the strengthening system during its service life, which can then lead to a reduction of the load-carrying capacity, resulting in premature bond failures [7, 8, 9]. It is extremely important, therefore, to investigate the behaviour of FRP strengthening systems under real service environmental conditions. The effect of sustained loading in NSM CFRP strengthening systems was experimentally evaluated in [PERSON] et al. [10] and [PERSON] et al. [11] for room conditions. In both studies, sustained loading caused a substantial increase in the relative slip between the CFRP and concrete, although the load applied was much lower than the ultimate load of the bonded joint. From these investigations, numerical models based on the degradation of the bond-slip law of the bonded joint were proposed to predict their sustained loading behaviour. On the other hand, high service temperatures have been proved to significantly affect the performance of FRP strengthening systems. In general, dramatic decreases in the bond strength and stiffness of the joint have been observed when temperatures are close to the glass transition temperature (\(T_{g}\)) [8; 12; 13; 14; 15; 16; 17; 18], along with an increase in the bonded length needed to carry the maximum load [13; 15], a redistribution in the FRP strain [13; 16; 18], and a change in the failure mode [17; 18]. Furthermore, because service temperatures do not remain constant with time in real applications but rather oscillate during the day-night cycle and during the different seasonal weather conditions, it is crucial that the effects such cyclic temperatures might have on the structural response of the bonded joint be investigated. [PERSON] et al. [7] performed durability tests of NSM CFRP-concrete specimens under cyclic temperature (20 to 80 \({}^{\circ}\)C) conditions for six months and obtained a significant variation in the stiffness and a similar value of the ultimate load in comparison with the instantaneous values. On the contrary, when [PERSON] et al. [9; 19] studied the effect cyclic temperature conditions have on concrete elements strengthened with NSM CFRP rods using temperature cycles ranging from \(-15\) to \(55\)\({}^{\circ}\)C they detected a decrease in the ultimate load of the bonded joint as the temperature cycles being applied were increased. Several other experimental studies focused their investigations on the effects of the combination of sustained loading performance and high service temperatures, evaluating their impact on the activated length, the residual strength, and the stiffness of the bonded joint. Some contributions [20; 21] have concluded that the combination of high service temperature with sustained loading caused an increase in the overall strength of the EBR bonded joint, while others [9; 22; 23] observed a reduction in the EBR joint strength. In the field of NSM strengthening, only a few investigations on the subject of the combination of sustained loading and high service temperatures are to be found in the literature. For instance, [PERSON] et al.