| Near-surface mounted FRP bars method(NSM FRP)can significantly improve the bearing capacity of reinforced structures and components,effectively improve the stiffness of the structure,avoid the effects of unexpected loads such as abrasion and impact,improve the utilization of FRP materials and the efficiency of reinforcement,so it has been widely used in building structures,bridge structures,underground engineering and other practical projects.Strenghtening structures and components will also suffer from humidity,temperature changes,and corrosion of various corrosive media during the service period as well as reinforced concrete structures,exposing problems such as reduced bonding strength and deterioration of FRP materials.The foundation of FRP and concrete working together for a long time is their reliable bonding performance.Although there are many research results on the bonding performance of NSM FRP bar-concrete interface,they are mostly based on experimental analysis and models research in the un-corroded state,few studies under corrosive environment.The thesis through the bonding performance studies of NSM FRP bar strengthened concrete interface under salt and alkali corrosion in order to providing a certain reference for the practical engineering application of NSM FRP reinforced concrete technology in harsh environments.In the thesis,pull-out test research was performed on 40 specimens of NSM FRP strengthened concrete in salt and alkali environments,and the loading process,failure mode,and failure mechanism of the specimens,the macroscopic characteristics of the surface of the specimen in the corrosive environment and the main factors such as the bond length,type of FRP bars,type of corrosion solution,and corrosion time influence of interfacial bonding performance of pull-out specimen are analyzed.The tests show that the failure modes of the specimens are structural adhesive splitting failure,FRP bar breaking failure,FRP bar and structural adhesive peeling failure,structural adhesive splitting while FRP bar breaking failure,FRP bar and structural adhesive as a whole and concrete interface peeling damage five types.By analyzing of the failure mode find that the main factor affects the failure mode is bond length,Shorter bond lengths are prone to occur peeling failures at the interface,and longer bond lengths are more likely to cause FRP bar breaking failure.The influencing factors are analyzed,and found that under salt and alkali corrosion environment,the maximum bonding stress of the specimen is related to the bonding length,FRP bar type,corrosion time,and corrosion solution type.Under salt solution corrosive environment,it is recommended to choose a moderate bonding length of 6d、8d for NSM GFRP bar and a shorter bonding length of 5d for NSM BFRP bar to strengthen the component.Under alkali solution corrosive environment,NSM GFRP bar and NSM BFRP bar strengthen the component respectively choose 6d and 8d as their optimal bond length in order to achieve the purpose of economic and effective increase of bonding stress.In the salt environment,60 days as corrosion time boundary.With the increase of the corrosion time before 60 days,the bond stress of the specimen continued to decrease.After 60 days,the bond stress increased with the increase of the corrosion time.In the alkaline environment and the same bonding length,the bond stress of NSM GFRP bars concrete specimens has no clear change regular pattern with the increase of corrosion time,while the bond stress of NSM BFRP bars concrete specimens decreased with the increase of corrosion time.Compared with the NSM GFRP bar strengthen the component,the NSM BFRP bar strengthen the component is more resistant to salt corrosion and the salt corrosion resistance is about 1.5 times that of the NSM GFRP bar strengthen the component,but the alkali corrosion resistance is relatively low.Based on the model of bond-slip constitutive relationship,a mathematical model of bond-slip relationship of NSM FRP concrete under salt and alkali corrosion is established by analyzing the bond-slip curve of the specimen.By comparing with the test results,the model is verified to be correct.Using reasonable material constitutive relations,selecting concrete and FRP bar element types as solid elements,and inputting the eigenvalue coefficients obtained from the NSM-FRP concrete bond-slip constitutive relation expression into the Fric subroutine contact surface relation,a numerical model of bond-slip at the interface strengthened with NSM FRP bars in salt and alkali corrosion is established.The simulation results are compared with the experimental results,and the two curves fitted well.On this basis,the influence of FRP bar diameter on the bond slip performance is analyzed.The research shows that as the diameter increases,the ultimate load increases and the bond stress decreases under salt and alkali corrosion environment.The thesis analyzes the chemical reaction mechanism of corrosive solution to FRP bars,concrete,and binder from a micro perspective,and explores the effect of carbonization of concrete on the interfacial bonding performance of NSM-FRP bar and concrete.The analysis shows that the chemical reaction in the corrosive environment leads to the degradation of each component of the FRP bars,which causes the silicon-oxygen bonds in the FRP bars to break,forming an intumescent Si-OH,and destroys the combination of resin and fiber.Under the catalysis of the corrosion solution,the unsaturated polyester resin in the structural adhesive will undergo a hydrolysis reaction.–COOR and-CH2-O-will be broken.However,the Van der Waals force is generated when the molecules in the corrosion solution collide with the cement molecules in the concrete and the structural adhesive molecules,making the adhesion between concrete and structural adhesive interface increases.This shows that the influence of the corrosive environment on the bonding failure mechanism of concrete interface strengthened with NSM FRP bars cannot be ignored. |
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