PSI - Issue 11

Petr Stepanek et al. / Procedia Structural Integrity 11 (2018) 12–19 Petr Stepanek at al. / Structural Integrity Procedia 00 (2018) 000–000

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1. Introduction Fiber reinforced polymers (FRP) are in some cases a very interesting alternative to conventional concrete reinforcements. Among the negatives that reduce the use of GFRP reinforcement in real constructions, it is necessary to include creep and problems with high-temperature resistance Saafi (2002). For reliable design of concrete structures with FRP reinforcement we have to know the long term strength of reinforcement and, of course, other characteristics e.g. bond behavior and Young modulus. Significant influence on long term mechanical characteristics of FRP rebars has internal environment in concrete, pH value, temperature, humidity and type of loading. Some codes define the long term strength of FRP reinforcement with simply and (sometimes excessively) reliable equations – see Table 1. Those restrictions are often very strong and limit the using of FRP reinforcement in concrete structures from economical point of view. The code or recommendation restrictions are used because of lack reliable long term data from experiments. Table 1. Long term strength of FRP reinforcement Reduction of long term strength Code Environment Material of reinforcement Note Glass (GFRP) Carbon (CFRP)

0,2×C E f fl,d ; C E = 0,8 0,2×C E f fl,d ; C E = 0,7 0,25×f fl,d 0,3×f fl,k /γ f

0,55×C E f fl,d ; C E = 1,0 0,55×C E f fl,d ; C E = 0,9

dry

ACI 440.1R-15 (2015)

wet

0,65×f fl,d 0,8×f fl,k /γ f

CSA S806-12 (2012) fib MC 2010 (2013) CNR-DT 203/06 (2007) fib Bulletin No.40 (2007)

γ f = 1,25

0,3×η a f fl,k ; η a = 0,8 0,3×η a f fl,k ; η a = 0,7

0,9×η a f fl,k ; η a = 1,0 0,9×η a f fl,k ; η a = 0,9

dry wet

f fl,k /(η env,t γ f ); η env,t ≈ 1,1÷3,0; γ f = 1,25

= 0,8 × characteristic value long term strength determined by testing

≤ 0,7×f fl,k

JSCE

f fl,d granted short term strength (0.1% quantile), f fl,k characteristic short term strength (5% quantile), η a , η env,t , C E reduction coefficients (environmental influences), γ f material coefficient

2. Influence of Alkalinity and Ambient Temperature on Long-Term Properties of GFRP Reinforcement The limiting factor for long term strength of FRP reinforcement is also the fact that the properties of the composite can be negatively affected by the alkaline environment of the concrete in the long term, especially for the frequently used E and ECR glass fibres (see Girgle at al. (2016), Karbhari (2007)). Therefore, glass fibres must be protected by a well-chosen matrix (vinyl ester, epoxide). The ability of polymer matrices to protect against alkali is key in case of GFRP reinforcement. If the aggressive medium penetrates the matrix to the fibres, it causes them to become brittle and reduces their threshold tensile strength; e.g. Benmokrane at al. (2015). To determine the degree of GFRP reinforcement degradation by alkalis there are several types of accelerated tests that can be used. The principle of accelerated tests (e.g. ACI 440.3R (2012), CSA S806/12 (2012)) is used. A theoretical basis for the prediction of long-term properties from the accelerated tests is the Arrhenius equation. 2.1. The experimental program The performed experiments (more information about texts and results at Girgle at al. (2017)) were focused on testing the behaviour of the GFRP reinforcement with a diameter of 10 mm (value without sand surface layer) from