PSI - Issue 64

956 A. Cagnoni et al. / Procedia Structural Integrity 64 (2024) 951–958 6 Alessandro Cagnoni, Pierluigi Colombi, Marco A. Pisani, Tommaso D’Antino / Structural Integrity Procedia 00 (2019) 000–000

challenging due to the numerous parameters affecting the fatigue behavior. However, this phenomenon should be considered during the design of a concrete element with prestressed FRP tendons, particularly during the determination of the prestressing level applied to the tendons, which should be based on the expected load range and maximum load allowed. Typically, the load applied to post-tensioned RC elements induce limited variation in the stress applied to the tendon. Within this context, high values of prestress may be applied to the composite tendon without affecting their fatigue resistance. 4. Exposure to aggressive environments Prestressed concrete with bonded tendons is one of the possible prestressing techniques. Concrete is usually characterized by a high pH value ranging from 12.5 to 14 and during its initial curing phase it generates heat that may increase the temperature over 60 °C. Within this context, assessing the combined effects of alkali and high temperatures on the behavior of CFRP tendons is crucial. The durability of CFRP composites significantly depends on the matrix type. The reduction of the tensile properties of the composite tendon should be attributed to the degradation of the fiber-resin interface rather than to the degradation of carbon fibers (Ali et al. (2015), Zhou et al. (2019)). Different studies on this topic are available in literature. Among them, the most valuable studies are compared in Table 3. Micelli and Nanni (2004) performed an extensive experimental campaign to study the durability of various FRP rods. The results highlighted that epoxy-CFRP rods did not suffer for the conditioning environments, while epoxy/vinylester-CFRP rods were damaged by the conditioning solutions. The shear strength of both rod types decreased when subjected to alkali environments, particularly at temperature of 60 °C. Chen et al. (2007) studied the behavior of CFRP bars conditioned in the same environmental conditions. After 70 days of exposure, the bars exhibited a residual tensile strength 4% lower than the corresponding unconditioned tensile strength. Ali et al. (2015) and Benmokrane et al. (2016) conducted a similar experimental campaign on CFCC strands with various exposure times. The residual tensile strength of the tendon decreased with increasing the temperature or exposure time. Moreover, the losses of the elastic modulus were negligible. Ali et al. (2019) evaluated the effects of alkaline exposure combined with sustained loads on the behavior of CFCC strands. The results confirmed that the residual tensile strength of the CFRP tendons decreased with an increase of the applied stress. These results were confirmed by Khalafalla et al. (2019) that evaluated the residual tensile, shear, and bond strength of CFRP strands subjected to alkali environments, elevated temperatures, and sustained loads.

Table 3. Residual tensile strength of carbon composite tendons subjected to different environments. Authors Tendon type Conditioning type Temperature [°C]

Exposure time [h]

Residual tensile strength [%]

Micelli and Nanni (2004) Chen et al. (2007) Ali et al. (2015), Benmokrane et al. (2016)

Epoxy rods

Alkaline Alkaline Alkaline Alkaline

60 60 60 60

1000 1000 1700 1000 3000 5000 7000

100

Epoxy/vinylester rods

91 96 98 97 96 93

Epoxy rods

CFCC strands

Micelli and Nanni (2004) Wang et al. (2007) Zhou et al. (2019) Grace et al. (2021) Scott and Lees (2012)

Epoxy rods

Freeze-thaw and high temperature Fire temperature Fire temperature Fire temperature

From -18 to 49

-

100

Epoxy rods Epoxy rods

From 0 to 600 From 0 to 500 From 0 to 350

.

Variable Variable Variable

- -

CFCC strands

Epoxy rods

Salt water

60

500

91

Other studies evaluated the behavior of CFRP composites subjected to various environmental conditions. Micelli and Nanni (2004) exposed FRP rods to freeze-thaw and high temperature cycles and observed that composite bars were