Study On The Durability Performances Of Fiber Reinforced Polymer (FRP) Bars Exposed To Simulated Seawater And Sea Sand Concrete Pore Solution

Fiber reinforced resin polymer(FRP)composites have a great potential to replace the traditional materials(e.g.steel and wood)for the reinforcement of civil structures due to its advantages,such as light weight,high strength,corrosion resistance,easy construction and so on.However,considering the long-term life design of structures in civil engineering(generally more than 50 years for building structures)and the complex and harsh service environment(e.g.moisture,acid,alkali,salt and so on),the durability of FRP in serviced environments is one of the important influence factors on the application of FRP in civil engineering.Recently,the higher sustainable seawater and sea sand concrete(SWSSC)was also proposed to replace the conventional concrete in some engineering fields.Meanwhile,to avoid the corrosion of steel,the reinforced steel bars in SWSSC need to be replaced by FRP bars.However,the degradation trend and mechanism of FRP under SWSSC environment(high-salt and high-alkali)are still uncertain,which delays the popularization and application of SWSSC.Therefore,this study focused the durability of fiber-epoxy interface,and further investigated the durability of FRP bars in simulated SWSSC pore solution environment.The main research contents and fundings of this study are as follows:(1)Research on bonding mechanism between fiber and epoxy resin: The effect of electrochemical oxidation and sizing treatment on the interfical bond strength between carbon fiber and epoxy resin was studied using microbond pull-out test,meanwhile,XPS(X-ray photoelectron spectroscopy),AFM(Atomic force microscope),SEM(Scanning electron microscope)and so on were adopted analyse the interfacial bonding mechanism.The test results show that the oxidation and sizing treatment can effectively increase the content of active oxygen-containing functional groups on the suface of carbon fiber,which significantly improves the interfacial adhesion and hygrothermal resistance between single carbon fiber and epoxy.In addition,oxidation and sizing treatment can also improve the tensile properties,surface morphology and wettability of single carbon fiber.(2)Effect of water immersion on interfacial adhesion between carbon fiber and epoxy resin: The effect of the combination of surface oxidation and sizing treatments on the fiber-resin interfacal bonding inside carbon FRP(CFRP)plate under extreme hygrothermal environment(immersed in distilled water at 60oC)was systematically studied.The research resutls show that the combination of oxidation and sizing can hugely improve the interlaminar shear strength retention(from 0.24~0.38 for untreated CFRP to 0.74~0.86 for treated CFRP).In addition,the relationship between the loss factor at fiber-resin interface between and interlaminar shear strength for CFRP plates in the hygrothermal condition was also established.(3)Effect of simulated SWSSC pore solution on fiber-epoxy interfacial bonding behavior: The accelerated corrosion method was used to test the interlaminar shear strength of basalt FRP(BFRP),glass FRP(GFRP)and CFRP bars immersed in simulated SWSSC pore solutions,moisture absorption,Fourier transform infrared spectroscopy(FTIR)and SEM were adopted to analyse the degradation mechanism of fiber-resin interface,and the Arrhenius theory was finally used to predict the longterm behavior of FRP bars under SWSSC environment.The reasearch results show that the durability of interface for GFRP bars is best,while that for BFRP bars is worst,and that of CFRP bars is in between.The decreased interfacial adhesive for FRP bars is mainly from the degradation of resin degradation and interfacial debonding.In addition,an improved interlaminar shear strength test method to avoid the local failure in the middle part of FRP bar was proposed.(4)Effect of ofsimulated SWSSC pore solution on tensile performance of FRP bars: The accelerated corrosion method was used to test the tensile properties of B-,G-,C-FRP bars under the SWSSC environment,SEM,Energy dispersive X-ray spectroscopy(EDS)and X-ray computed tomography(CT)scanning technology were adopted to analysis degradation mechanism of FRP bars,and the Arrhenius theory was used to predict the degradation rate of the long-term behavior of FRP bars.Research results shows that the durability of GFRP bars is better than that of BFRP bars,and the chemical reactions involving Cl-and Fe2+ that is exclusive to basalt fiber could be an important reason for the worse durability of BFRP bars than GFRP bars.Meanwhile,the results of two existing long-term life prediction models was proved to be more conservative compared with the real field results.(5)Coupling effects of sustained load and simulated SWSSC pore solution on tensile properties of FRP bars: The accelerated corrosion test with different temperatures was adopted to test the tensile properties of FRP bars under different stress levels(20%,30% and 40%)in the high performance seawater and sea sand concrete(HP-SWSSC)pore solution,and SEM was adopted to investigate the degradation mechanism of FRP bars.The research results show that 20% stress level is a threshold for BFRP and GFRP bars in simulated HP-SWSSC solution,which basically consistent with the request creep rupture stress limit(i.e.0.20 ffu)of GFRP bars in ACI440.1R-06.Meawhile,the long-term performance prediction model for FRP bars through considering the ambient and relative humidity facors of serviced concrete structure was proposed.Based on above researches,this thesis revealed the effect mechanism of oxidation and sizing treatments on the hygrothermal aging of fiber-resin interfacial bonding behaviour,obtained the evolution of mechanical properties of FRP bars exposed to simulated SWSSC pore solutions environment,and found that the coupling effect of sustained stress and aging environment accelerated the debonding of fiber-resin interface,resulting in the performance degradation of FRP bars.Finally,the long-term life prediction model of FRP bars under different atmospheric temperature and relative humidity conditions was established.Above research findings were the necessary theoretical and experimental basis for exploring the degradation regularity and mechanism of FRP bars in SWSSC environment and the development of high-performance/long-life FRP bars and their reinforced concrete structures.