Study On Properties Evolution Of Basalt-Carbon Based Hybrid FRP Bars In Seawater Sea-Sand Concrete

The shortage of freshwater and river-sand resources is increasingly prominent with the development and construction of offshore and island infrastructure.Meanwhile,the premature failure of concrete structures caused by corrosion of steel bars in the marine environment has become an important issue that needs to be addressed urgently for marine engineering.Due to the excellent chlorine resistance of fiber reinforced polymers（FRP）bars,they can replace steel bars in conjunction with sea water sea-sand concrete（SWSC）.However,the brittle failure mode of traditional FRP bars and their susceptibility to corrosion in alkaline SWSC environments limit the large-scale application of the bars.Therefore,developing new FRP bars with plastic yield segments and more corrosion resistance,and obtaining their properties evolution in SWSC,is of great significance for the construction of long-life marine engineering.This paper focuses on basalt-carbon based hybrid-FRP（HFRP）bars,and conducts a series of studies on the optimal design,basic mechanical properties,and properties evolution of HFRP bars in SWSC.Firstly,based on the design principles of composite materials,the optimal design method of HFRP bars studied using micromechanics and finite element analysis（FEA）to obtain HFRP bars with excellent performance and plastic yield deformation of HFRP bars were obtained.Secondly,the properties evolution of HFRP bars in simulated SWSC pore solution,SWSC（indoor acceleration）and real sea exposure environments（stressless and stressed）was studied.The deterioration mechanism of HFRP bars in SWSC environment was explored through macroscopic and microscopic testing and analysis.The synergistic mechanism of carbon fiber hybrid on durability improvement of HFRP bars has been revealed.Finally,considering the actual engineering environment,a time-varying model for the performance of HFRP bars under different corrosion environments was established through accelerated corrosion test verification.The environmental reduction factor for HFRP bars in SWSC serving in the marine environment is proposed.The main research contents and conclusions are as follows:（1）Optimization design and basic performance of basalt-carbon based HFRP bars.Based on the design principles of composite materials,hybrid composite design and optimized layout of carbon fiber and basalt fiber were carried out using micromechanics and finite element analysis to prepare HFRP bars with excellent mechanical properties and plastic yield deformation,and tensile properties of the prepared HFRP bars were tested.The results show that the tensile properties of HFRP bars are in direct proportion to the carbon fiber hybrid content(V_CF,%).Compared to the circular dispersion arrangement of carbon fibers,when carbon fibers are concentrated in the core,HFRP bars have higher mechanical properties due to the full utilization of carbon fibers.When the V_CF is 10%,the failure mode of HFRP bars exhibits a yield segment with plastic deformation.The experimental results verify the feasibility of the concept of micromechanical and FEA optimization design of HFRP bars.（2）Performance evolution of basalt-carbon based HFRP bars in simulated SWSC pore solution.The evolution of tensile properties and interlayer interfacial properties of HFRP bars in simulated SWSC pore solution was studied.The degradation mechanism of HFRP bars was revealed through microscopic analysis at different scales.The results show that the degradation rate of tensile strength and interlaminar shear strength is inversely proportional to that of V_CF.The degradation of HFRP bars is improved when carbon fibers are distributed in a circular direction compared to when carbon fibers are distributed in a concentrated core.The durability of HFRP bars has been improved due to the effective moisture barrier at the BFRP/CFRP interface,the excellent corrosion resistance of carbon fibers,and the excellent interfacial resistance of CFRP materials.（3）Performance evolution of HFRP bars in SWSC under indoor accelerated corrosion conditions.The tensile properties and interfacial properties of HFRP bars in SWSC were investigated using accelerated corrosion testing methods.The microstructure of HFRP bars embedded in SWSC was systematically analyzed using microscopic testing methods of different scales.The results show that the degradation of mechanical properties of HFRP tendons in SWSC is alleviated with the increase of V_CF.Compared to the circular dispersion arrangement of carbon fibers,the hydrolysis amount of resin,the degree of fiber etching and the void ratio between fibers and resins in HFRP bars are reduced when the carbon fibers are arranged in a concentrated core.Compare with normal concrete,SWSC is more prone to induce HFRP bars degradation due to its high porosity.（4）Performance evolution of HFRP bars in SWSC under indoor acceleration conditions.The properties evolution of of HFRP bars subjected to stress in SWSC under real marine environment（tidal zone）environment was studied.The performance and microstructure evolution of resin matrix,fiber bundle,fiber/resin interface,and SWSC-HFRP bars interface transition zone（ITZ）before and after corrosion were quantitatively analyzed.Moreover,existing life prediction models are modified using accelerated corrosion performance degradation data.The results show that the hybrid of carbon fibers alleviates stress induced HFRP bars degradation,and the alleviation effect becomes more evident with the increase of V_CF.The degradation of HFRP bars is mainly due to the etching of the fibers causing ion dissolution,while the microstructure of SWSC-HFRP bars ITZ is not affected.The modified long-term performance evolution prediction results show that HFRP bars have a longer service life and a higher environmental reduction factor compared to BFRP bars.