Study On Bond Property Between FRP Bars And Coral Concrete Under Marine Environment

Coral islands account for most of the islands in the South China Sea,and because the geographical location,their economic and strategic significance are very important.As our country pays more and more attention to the ocean rights and interests,the constructions on the islands of South China Sea will increase gradually.However,most of the islands are far from the mainland,so it would be a waste of time and money if all kinds of building materials,including the fresh water,were to be transported from the mainland.Coral aggregate concrete uses dead coral reefs as the coarse and fine aggregate,and can be mixed with seawater.Materials other than cement can be obtained locally.At the same time,in order to solve the serious corrosion problem of steel bar caused by the chloride salt in coral reefs and sea water,the FRP(fiber reinforced plastics)bar can replace the traditional steel bar and form the FRP bar-coral concrete structure,which provides a new way for the constructions on the islands.The bond behavior between FRP bars and coral concrete is the foundation of their cooperative work,but the research on this problem has not been open published yet.The descriptions of the adhesion mechanism,the bond-slip process,the failure morphology,and the establishment of bond-slip constitutive relation model are the basis of theoretical researches and practical engineering applications of FRP bar-coral concrete structure.In view of the poor impermeability of coral concrete and the harsh environment of the South China Sea,it is also important to study the deterioration of bond strength of FRP bar-coral concrete in marine environment.This paper studies the following questions:1.The bonding properties were studied between FRP bars and coral concrete.Two types of FRP bars(CFRP,GFRP)are involved.The key parameters such as bond strength,relative slippage,bond stress and so on were obtained by pulling-out testes of a batch of eccentric cube specimens of FRP bar-coral concrete.The stress process and failure mode of FRP bar-coral concrete are studied experimentally.The effects of embedding length,diameter of bars and thickness of protective layer on the bond strength are clarified.The formation and transfer mechanism of bond force between FRP bars and coral concrete are summarized.The different performances of the specimens between these two kinds of FRP bars are also discussed.There are three kinds of failure modes:pull-out failure,FPR bar fracture and concrete splitting.The experimental results show that the bonding mechanism and experimental phenomena of the FRP bar-coral concrete are similar to the ordinary concrete.The adhesive force is mainly composed of chemical adhesive force,friction force and mechanical bite force.The bond-slip curve can be divided into four stages:micro-slip section,slip section,descending section and residual section.However,the bond strength of FRP bar-coral concrete is lower than that of ordinary concrete and the slippage is large because of its lighter weight,porous,fragile,lower strength and elastic modulus.The average bond stress of FRP bar-Coral concrete increases with the increase of cover layer,and decreases significantly with the increase of the embedment length.When the cover layer is small,the fracture of specimen occurs,and with the increase of embedment length,the failure form is gradually changed from the pull-out to the bar fracture.In addition,the bond strength of CFPR bar specimen is better than that of GFRP bar specimen under the same condition.In the stress distribution test curried out next,the distribution of bond stress in the anchorage zone of two groups of CFPR specimens with different diameter and different embedment length were measured by using advanced Bragg fiber grating technique.The stress of FRP bars,the relative slip and other parameters were also analyzed.It was found that when the embedment length was short,the stress value of FRP bars at each measuring point was close to each order and increased linearly with the raise of the pull-out load,and the corresponding bond stress distribution was relatively uniform along the anchorage zone.On the other hand,under the influence of the uneven distribution of aggregate and the processing technology of FRP bars,the distribution of bond stress along the anchorage zone was very inhomogeneous when the embedment length was longer,and this non-uniform distribution was also reflected in the nonlinear growth of relative slip.2.The bond-slip constitutive model of FRP bar-coral concrete was studied.the relative experimental studies are plenitudinous but the theoretical researches are rare.In view of the difference of physical and mechanical properties between FRP bars and steel bars,and between coral concrete and ordinary concrete,the existing bond-slip theory of reinforced concrete cannot accurately describe the bond-slip behavior between FRP bars and coral concrete.On the basis of experimental data and existing research results,this study established a bond-slip constitutive model for FRP bars and coral concrete.Then we solved the bond-slip differential equation solution based on the model.Finally,the theoretical formula of the minimum embedment length of FRP bars in coral concrete is derived by using the model.The results show that this constitutive model is in good agreement with the experimental data,and the calculated value of embedment length is close to the standard value.The research results provide a useful theoretical basis for the study of bond between FRP bars and coral concrete.3.On the basis of the sufficient decomposition of the existing finite element modeling methods of reinforced concrete members and the characteristics of various material elements,combined with the constitutive model of bond-slip proposed in this paper,the model of freely supported coral concrete beam with CFRP bars and GFRP bars considering bond slip is established by using the nonlinear finite element structural software of ANSYS.The stress failure process is simulated and the deformation and force of the connecting spring element are analyzed.The results show that the bond between FRP bars and coral concrete can be effectively realized by using the bond-slip constitutive relation model proposed in this paper.The calculation results are good and reliable.4.The deterioration of bond properties of FRP bar-coral concrete in marine environment was studied.The accelerated aging experiment of FRP bars in artificial seawater at different temperatures was carried out,and analyzed the corrosion degradation mechanism of FRP bars with temperature and immersion time.The results showed that the corrosion of FRP bar are degraded to different degrees due to the reaction of resin composition with seawater and corrosion damage of fiber.Compared with the soaking time,the effect of temperature was more obvious than immersion time,and the deterioration of the GFRP bar was much greater than that of the CFRP bar.Then the bond behavior of FRP bar and coral concrete in marine environment was tested to reveal the degradation mechanism of bond behavior between FRP bar and coral concrete under the coupling of temperature humidity and salinity.The experimental results show that the ultimate bond strength of FRP reinforced coral concrete increases first and then decreases with the increase of soaking time.Increasing the thickness of the protective layer can effectively reduce the erosion of FRP bars by seawater and help to maintain the bond property between FRP bars and coral concrete.Finally,based on the experimental study,a formula for the reduction coefficient of bond strength between FRP bars and coral concrete under high temperature marine environment is proposed.The coefficient includes two key variables:immersion time and immersion temperature.The simulation results are in good agreement with the experimental results.In a word,the research results of this paper not only have a positive effect on the improvement and development of the theory of FRP bar-coral concrete structure,but also provide a reference for the application of FRP bar-coral concrete structure in the marine environment of southern China.It has certain theoretical and engineering value.