Debonding of external CFRP plates from RC structures caused by cyclic loading effects

[摘要] ENGLISH ABSTRACT: This study set out to determine the debonding of externally applied Carbon Fibre ReinforcedPolymer (CFRP) plates from RC structures under cyclic loading. Triplet shear tests and finiteelement (FE) analyses were done on the epoxy to determine the bond stress between theCFRP plate and a reinforced concrete specimen. From these tests and analyses theaverage shear strength of the bond between the epoxy and concrete substrate wasdetermined and the shear strength of the epoxy specified by the supplier could be confirmed.A case study of a statically loaded beam was performed to verify the bond strength.Finally a reinforced concrete (RC) T-section was designed and pre-cracked to simulate adamaged beam in practice. These sections were then externally reinforced by bondingCFRP plates onto the face of the web. The sections were subjected to static and cyclicloading at different force amplitudes. Along with the experimental tests, FE models weredeveloped and analysed which had the same geometrical and material properties as theexperimental specimens. Due to time constraint a FE mesh objectivity study was not done,but the chosen element size is believed to be sufficiently small to replicate the experimentaltests objectively.The FE analyses and the experimental tests yielded results that were close to each other onboth the global scale and in terms of localised behaviour, thus it was decided that thecomputational approach could be used for the final design of a model of the debonding ofCFRP plates bonded onto RC beams under cyclic loading because the data can beanalysed more easily and a large variation of tests can be done.For the T-section 3 tests were conducted; a pull-off (static) test where the bonded CFRPplate was pulled from a specimen to get the ultimate failure envelope of the test specimens.The static test was followed by cyclic tests with force amplitude of 85% and 65% of theultimate pull-off strength. Different measurements were taken to get the global and localdisplacement behaviour of the section. The global displacement was measured by means ofa linear variable displacement transducer (LVDT, displacement meter) clamped onto theCFRP plate that pushed on the top of the concrete and the local displacement wasmeasured with the help of the Aramis system. The displacement was then compared to thesame displacements of nodes and elements in the FE models. The result was aconfirmation that the results from the FE models were sufficient to design a model for cyclicdebonding of CFRP plates from RC structures.From the FE models the relative displacement between the CFRP plate and concrete wasobtained in the vicinity of a crack. This relative displacement was then normalised by therespective stress range of the different tests, from which the normalised relativedisplacement was plotted against the number of cycles to get an equation limiting thenumber of cycles for a specific stress range.From the results, it appears that for cyclic load levels up to 65% of the peak static resistance,a threshold number of load cycles are required for delamination initiation. Subsequently, anear constant delamination rate is reached. The delamination rate is significantly lower forlower cyclic load levels. Finally, an unstable delamination stage is reached at a level ofabout 65 μm for all the analyses, after which CFRP pull-off is imminent.Service life design of CFRP reinforcement of RC beams should take into consideration thedelamination initiation threshold, the subsequent delamination rate and finally the initiation ofunstable delamination.

[发布机构] Stellenbosch University