| Under various loads and erosion,the mechanical performance of the concrete structure will deteriorate,making it difficult to meet the use requirements.Compared with reconstruction,strengthening the structure can not only make it meet the reliability requirements,but also more econimical.Carbon fiber composite reinforced materials(CFRP)materials have the outstanding advantages of light weight,high strength,fatigue resistance,stable properties,etc.It can adapt to many harsh environmental conditions,such as high humidity,seawater tides,freeze-thaw cycles,etc.,especially suitable for building structure reinforcement.It has been gradually promoted and applied since the 1980 s.In areas where the temperature difference between day and night is extremely large and near lakes or offshore,CFRP-reinforced buildings or bridges will experience significant freeze-thaw cycles during their service life.Studies have shown that the CFRP material itself is not sensitive to the effect of freeze-thaw cycles.However,the deterioration of the CFRP-concrete interface after freeze-thaw cycles is very significant,resulting in the threat of the safety of CFRP reinforced concrete structures.On the other hand,China is located between the Pacific Rim seismic zone and the Eurasian seismic zone.The crustal activity is very active due to the compression between the plates.For example,Gansu,Qinghai,Ningxia and other provinces in the northwestern region generally have seismic fortification intensity above 7 degrees.It has been shown that the strain rate effect caused by dynamic load has an important influence on the bearing capacity of CFRP-concrete interface.Therefore,it is necessary to comprehensively consider the influence of freeze-thaw cycles and strain rate effect on the interfacial bonding performance,which has not been reported.In this paper,concrete strength,freezing and thawing times and strain rate were considered as control parameters.And 144 single-sided shear tests were completed.The influence of these three paramaters on interfacial fracture energy,interfacial shear bonding strength and the degradation law of interfacial bonding performance under freeze-thaw cycles was investgated and the degradation law of interface bond performance under freeze-thaw cycle was obtained.Tests have shown that the CFRP reinforcement method is not suitable for C30 concrete building structures under extremely severe freeze-thaw conditions.With increace of freeze-thaw cycles,the peak shear strain,interface fracture energy,and interface shear bond strength of the CFRP-concrete interface decrease to varying degrees.After 50 and 75 freeze-thaw cycles,the interface fracture energy of C40 concrete strength specimens under static load decreased by 31.11% and 63.56% respectively compared with the unfreeze-thaw specimens.Terefore,the deterioration became more serious after 50 times.However,the C50 concrete specimen still has some residual strength after 75 freeze-thaw cycles,and the interface fracture energy has only decreased by 24.80%.In addition,the above-mentioned interfacial bonding performance parameters increase significantly with increasing strain rate.The effective bonding length of the C40 concrete specimen after freezing and thawing for 75 times is increased to about 150 mm,and other specimens are between 50 mm and 80 mm,and the strain rate has little effect on it.The exfoliation process of CFRP-concrete specimens was simulated by the fine finite element method based on the elastoplastic damage model and the viscous contact finite element model based on the cohesive zone model.In general,the test results are similar to those of the two simulation methods,which proves the feasibility of the two methods.The precision of the finite element method and the test results are better than the other method.However,the overall modeling process is more complicated,and considering the computational efficiency,the cohesion model is significantly better than the fine finite element method. |
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