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Beams reinforced with surface-mounted fiber-reinforced polymer (FRP) can fail prematurely either by FRP delamination or by cover delamination. Most existing models for these two failure modes predict failure based on stress concentrations that are dependent of the properties of the bond layer. The best existing model for cover delamination is found to perform poorly for beams retrofitted using soft adhesives in the bond layer. This paper describes an experimental investigation of the effect of bond layer properties and concrete strength on cover delamination. The results demonstrate that bond layer properties do not significantly affect cover delamination. A new model that is independent of bond layer properties is developed and provides excellent correlation with all data available in the literature.
Keywords: cover; delamination; fiber-reinforced concrete; polymer.
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INTRODUCTION
Premature failure mode
Reinforced concrete (RC) beams strengthened with surface-mounted fiber-reinforced polymer (FRP) reinforcements can fail prematurely by loss of bond between the FRP and the concrete substrate.1 Bond failure occurs either by FRP delamination due to separation of the FRP from the concrete substrate (refer to Fig. 1), or by cover delamination due to separation of the concrete beneath the bottom layer of steel reinforcement (refer to Fig. 2).
Definition of cover delamination
Tumialan et al.2 identified two different failure modes for cover delamination. They defined Failure Mode I (CDI) as failure initiated from the FRP cutoff and Failure Mode II (CDII) as failure starting from an intermediate crack and developing toward the beam midspan.2 Failure Mode II, however, is a secondary failure mode that can only occur after FRP delamination initiates at a flexural crack and progresses to the FRP curtailment. Therefore, cover delamination for the purpose of this paper is defined as concrete cover separation that initiates at the FRP curtailment. Cover delamination may also be called cover debonding or cover separation. The term cover delamination is chosen herein because it appears to be the most commonly accepted.
Failure models
Numerous theoretical and experimental studies2-24 have been developed to predict cover delamination failure, but none of them provides a fully satisfactory failure model for all available data. In a review paper, Smith22 states that existing models fall into three categories: (a) shear capacitybased models; (b) concrete tooth models; and (c)...