PSI - Issue 64

954 A. Cagnoni et al. / Procedia Structural Integrity 64 (2024) 951–958 4 Alessandro Cagnoni, Pierluigi Colombi, Marco A. Pisani, Tommaso D’Antino / Structural Integrity Procedia 00 (2019) 000–000

Table 1. Efficiency of anchorage systems available in the literature. Anchor system Material Authors

Anchor efficiency η [%]

Wedge-barrel

Metallic

Al-Mayah et al. (2006), Schmidt et al. (2012), Al-Mayah et al. (2013), Heydarinouri et al. (2021)

100

PPS

Terrasi et al. (2011) Shi et al. (2022) Züst et al. (2022) Ye and Guo (2011)

73 85 68 82

BFRP

Polymer

Clamped

Steel

Burningham et al. (2014)

100 100

Bonded

Cement

Zhang and Benmokrane (2004)

Concrete dry mix Expansive cement

Jia et al. (2022)

45

Saeed et al. (2020)

98 ( Φ 10), 100 ( Φ 13) 96, 94 (epoxy added quartz)

Epoxy

Jia et al. (2022)

Composite bonded

Epoxy Epoxy

Cai et al. (2015) Mei et al. (2020)

98

95, 100 (epoxy added sand)

3.1. Creep Creep is defined as the tendency of a solid material to deform under sustained loads. For the FRPs, creep is induced by the straightening of uneven fibers, the viscoelastic deformation of the resin matrix, and the creep of fibers themselves (Yang et al. (2018)). This phenomenon typically progresses through three distinct phases: primary, secondary, and tertiary, as illustrated in Fig 1a. This phenomenon depends on the applied load level and the period of loading time. In the case of FRP tendons, if the stress level remains low, fiber damage is confined to the secondary phase, which can potentially increase service life indefinitely (ACI Committee 440 (2004)). Several studies have been conducted to quantify the creep phenomenon for CFRP products. Najafabadi et al. (2018) conducted an experimental campaign to study the short-term creep phenomenon on bars made of epoxy and vinylester matrices. The authors found that the creep phenomenon may be neglected for both types of matrices with a load level lower than 60% of the tensile strength. Zou (2003) conducted an experimental campaign and proposed an analytical model to predict the long-term behavior of CFRP rods. The experimental results confirmed that the creep coefficient is practically zero when the tendons are loaded at stress levels lower than 60% of their tensile strength. Based on this, the author predicted that CFRP tendons can sustain a stress level of up to 79% of their tensile strength without failure after 100 years. Yang et al. (2018) proposed an additional analytical model after conducting tests on CFRP rods with various stress levels. The model showed that after one million hours (114 years), the increments of strain caused by creep were the 1.83% and 1.91% for stress levels equal to 69% and 85% of the ultimate tensile strength of the rods, respectively. Grace et al. (2023) studied the effect of creep on CFCC strands on two different diameters subjected to three different stress levels. The authors proposed an analytical model and predicted a residual minimum tensile strength 88% of the unconditioned tensile strength after one million hours of sustained load. The studies presented indicate that when the CFRP tendons are subjected to stress levels equal to or less than 75-80% of their tensile strength, they have a lifespan exceeding 100 years. 3.2. Relaxation Related to the concept of creep, relaxation is another phenomenon caused by the viscoelasticity of the material, which represents the tendency of a solid material to gradually reduce stress over time under constant strain. The typical relaxation stress-time curve is characterized by an initial drop and then an asymptotic behavior, as illustrated in Fig 1b. A limited number of studies were conducted to establish the load loss in CFRP tendons varying the prestress level. Saadatmanesh and Tannous (1999) conducted an experimental campaign using CFRP rods. The authors confirmed