External bonding of Carbon Fiber Reinforced Plastic (CFRP) sheets has come to be regarded as a very effective method for the strengthening of reinforced concrete structures. The behavior of CFRP-strengthened RC structures is mainly governed by the interfacial behavior, which represents the stress transfer and relative slip between concrete and the CFRP sheet. In this paper, the effects of bonded length, width, and concrete strength on the interfacial behavior are experimentally verified and a bond-slip model is proposed based on the experimental results. From the pull test for CFRP-bonded concrete, it is found that the bond strength increases as the bond width increases and the effect of concrete strength is minor. The proposed bond-slip model has nonlinear ascending regions and exponential descending regions, facilitated by modifying the conventional bilinear bond-slip model. Finite element analysis results of interface element implemented with bond-slip model have shown good agreement with the experimental results performed in this study. It is found that the failure load and strain distribution predicted by finite element analysis with the proposed bond-slip are in good agreement with results of experiments.
The softening response properties of plain concrete, which directly affect the structural capacity of the concrete containment building of nuclear power plants, were studied via direct tension tests using large-size concrete specimens with two notches. A variation of two and three levels of the concrete strength and the maximum size of coarse aggregate, respectively, was considered. Two independently controlled actuators were used to ensure a homogeneous increase of crack mouth opening displacement (CMOD) on the top and bottom notches of the specimen and to avoid secondary flexural stresses. From the tests, the complete load鈥揅MOD responses with stable post-peak descending curves were obtained for each specimen. The fracture energy data obtained using the complete load鈥揅MOD responses and one evaluated using a classical prediction equation were compared. The results, owing to size effect, indicated that the fracture energy obtained in the current investigation is larger on average than that obtained using a classical prediction equation. The direct tensile strength of the large-size specimens was, however, about half the splitting tensile strength of the cylindrical specimens.
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