PSI - Issue 64

Manish Prasad et al. / Procedia Structural Integrity 64 (2024) 1524–1531 Manish Prasad / Structural Integrity Procedia 00 (2019) 000 – 000

1525

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Keywords: Fiber Reinforced Polymer, Concrete cover separation, Machine Learning

1. Introduction Structural strengthening with Carbon Fiber Reinforced Polymers (CFRP) of concrete structures can be performed to increase load capacity of structural members, for seismic retrofitting purposes, to support any increment in design load, to resist stress produced by design or construction errors, or to restore original load capacity. Externally bonding (EB) CFRP laminates is one of the most well-recognized strengthening methods (Rahimi & Hutchinson, 2001). However, occurrence of concrete cover separation (CCS), in which concrete cover along with the EB CFRP delaminates before reaching the ultimate capacity of the element, is one of the most prevalent failure modes in flexural strengthening of EB beams with CFRP. Various researchers have studied CCS phenomenon and proposed strength prediction models. According to shear strength-based models (Ahmed & Van Gernert, 1999; Jansze W., 1997; Raoof & Hassanen, 2000), the failure load at CCS is a function of the shear strength of the beam. On the other hand, interfacial stress models (Saadatrnanesh & Malek, 1998; Tumialan et al., 1999; Varastehpour & Hamelin, 1997) are based on the stress concentration at FRP end due to curtailment. Tooth models (Gao et al., 2005; D. Zhang et al., 2012), in turn, are based on the comb analogy (Kani, 1964). According to Kani [1], due to the bending moment, flexural cracks develop in the shear span that eventually led to shear cracks. Consequently, concrete teeth are formed between these cracks, see Fig. 1. A shear failure crack develops from the tip of one of the flexural cracks. According to the tooth model theory, these concrete teeth are assumed to behave as cantilever elements with base fixed at the internal reinforcement level and loaded at extreme tension concrete fiber of the beam through the tension force in the CFRP. These cantilevers fail simultaneously when the stress at the fixed base exceeds the tensile strength of concrete, resulting in CCS. Accordingly, the beam

Fig. 1. Kani's comb analogy

fails due to the loss of flexural strength. fib Bulletin 90 (fib, 2019) proposes the calculation of CCS with a formulation that is an extension of Jansze’s model (Jansze W., 1997), who suggests the application of a shear strap at the end of FRP strip to prevent CCS when the shear force at the end support exceeds the resistance V Rd,c,ef obtained from Eq.(1),

  

   

0.15

(100 ) 

0.75 1 19.6  +

(1)

V

V

=

s

, , Rd c fe

, Rd c

a

f

where V Rd,c is the design value for shear resistance for members not requiring shear reinforcement according to (EN 1992-1-1: Eurocode 2, 2004), ρ s is the longitudinal steel ratio, and a f is the distance between the support axis and FRP end in mm. Smith and Teng proposed Eq.(2) to estimate the shear capacity V c of the concrete in the beam without the contribution of internal shear reinforcement (Smith & Teng, 2002a), ( ) 1 ' 3 1.4 2000 c c s c d V b d f    =  −    (2)