Fiber reinforced polymer (FRP) bars have lower modulus of elasticity than steel bars. For this reason when FRP bars are used as flexural nonprestressed reinforcement in concrete sections, the stress in the FRP is limited to a relatively small fraction of its tensile strength. This limit, necessary to control width of cracks at service, governs design of the required cross-sectional area of the FRP. Parametric studies on rectangular and T-sections are presented to show that the design based on allowable strain in the FRP results in sections that exhibit large deformation before failure. The concept of deformability, given in the Canadian Highway Bridge Design Code, as a requirement in the design of sections is discussed and modifications suggested. Using the new definition, it is shown that when, in addition to the crack control requirement, an upper limit is imposed on the cross-sectional area of the FRP, no calculations will be necessary to check the deformability.Résumé : Les barres en polymères renforcés de fibres (FRP : « fiber reinforced polymer ») ont un module d'élasticité plus bas que celui de barres d'acier. Pour cette raison, lorsque des barres en FRP sont employées en tant que renforcement non précontraint en flexion dans des sections en béton, il est nécessaire de limiter la contrainte dans le FRP à une fraction relativement petite de sa résistance en tension. Le respect de cette limite, nécessaire au contrôle de la largeur des fissures durant le service, détermine quelle est l'aire de la section transversale du renforcement en FRP qui est requise. Des études paramétriques sur des sections rectangulaires et en T sont présentées afin de montrer que la conception basée sur le respect de la tension limite permise dans les FRP résulte en des sections qui présentent de larges déformations avant la rupture. Le concept de l'état de déformation, donné dans le Code canadien sur le calcul des ponts routiers en tant qu'exigence dans la conception de sections, est discuté et des modifications sont suggérées. En utilisant la nouvelle définition, il est montré que, si une limite supérieure est imposée sur l'aire de la section transversale de FRP en plus de l'exigence du contrôle des fissures, alors il n'est pas nécessaire de procéder à des calculs afin de vérifier l'état de déformation.
Reinforced and prestressed concrete columns with one or two layers of carbon fibre reinforced polymer (CFRP) wrap were tested to failure in axial compression. When the results were compared with the maximum load predictions of two proposed design methods, the predictions consistently underestimated actual loads. The design methods are thus conservative. A simple analysis for circular columns reveals that the confining effect of the wrap is not engaged until the concrete actually starts failing and dilating. A finite element model of a chamfered square column confirms this analysis, as do strain readings from the tests. It is shown that strength gains are not linearly related to wrap thickness. The failure mechanism suggests that design should not be based on the ultimate strength or strain of the wrap and that strength gains can be expected to reduce with increasing brittleness of the concrete and with increasing eccentricity.
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