Study On Durability Of Prestressed GFRP Bars Wrapped With SSC In Seawater Immersion Environment

With the development of ocean blue space,Fiber Reinforced Polymer（FRP）Reinforced Sea sand Concrete（SSC）has become an important development direction of ocean engineering structures.The research focuses on the durability of SSC structures enhanced by FRP bars in Marine service environment.FRP has excellent resistance to chloride ion erosion,but the strong alkaline environment in concrete will lead to its long-term performance degradation.At present,the research on the durability of FRP bars mainly focuses on direct immersion to simulate the service environment.However,in actual engineering,FRP bars are often placed inside the concrete structure,and the alkali environment inside the concrete is changing under service environment,and the FRP bars in the concrete structure generally maintain long-term stress holding state.Some research results show that the results of laboratory corrosion resistance research are too conservative compared with the real concrete environment,which restricts the development of FRP in civil engineering.Glass Fiber Reinforced Composite（Glass-FRP）bars have stable properties,cheap and great development potential.In this paper,the durability of prestressed GFRP bars encased in concrete in seawater environment was studied.The prestressed loss of GFRP bars and its residual properties at different time points were tested.Based on the microscopic test,the long-term degradation mechanism of GFRP bars was revealed.The Arrhenius equation was used to predict the long life of prestressed GFRP bars wrapped in SSC.The research results provide data and theoretical support for further expanding the application of GFRP bars,and have important scientific significance and practical value for the development of GFRP reinforced SSC structure design in Marine environment.In this paper,the pretensioning method is proposed to prestress the GFRP bars wrapped in concrete,so as to restore the real environment and stress state of the bars in the actual engineering.The laboratory accelerated simulation method of the Marine environment,the first comparative study of the outsourcing SSC prestressed GFRP reinforcement in different environmental temperature（room temperature,40℃,60℃）,the stress level（unstressed,20% stress level,40% stress level）in the corrosion time node（30 days,60 days and 120 days）on the mechanical properties（tensile strength,elastic modulus,transverse shear strength,Interlaminar shear strength）time-varying regularity,Secondly,the degradation mechanism of its mechanical properties was analyzed by means of SEM electron scanning microscope and EDS energy spectrum analysis.Finally,the long-term life of prestressed GFRP warpped in SSC was predicted based on Arrhenius equation.The main research contents and conclusions of this paper are as follows:（1）The prestress loss of prestressed GFRP bars was monitored by FBG fiber grating sensing technology.The monitoring results show that the prestress loss tends to be stable after two weeks,and the residual prestress level is 13.8%（20% stress level）and 32.5%（40% stress level）after 70 days.（2）Tests on tensile strength and elastic modulus of prestressed GFRP bars wrapped in SSC under accelerated aging conditions in the laboratory were carried out.The ultimate tensile strength and elastic modulus of unstressed GFRP bars were 49.5% and 77.7% after 120 days of immersion in room temperature.The ultimate tensile strength and elastic modulus are 18.2%and 35.4% in 60℃ temperature.Because of high temperature accelerates the damage of erosive media to fibers,Tensile properties decrease with increasing temperature.The ultimate tensile strength and elastic modulus of GFRP bars with 40% stress level soaked in seawater at room temperature for 120 days are 27.6% and 50.3%.The decrease of tensile property with the increase of stress level is attributed to the rapid deterioration of fiber under the coupling action of tensile stress and environment.（3）Tests on transverse shear and interlaminar shear properties of prestressed GFRP bars wrapped in seawater sand concrete under accelerated aging conditions in the laboratory were carried out.The transverse shear strength and interlaminar shear strength of unstressed GFRP bars are 69.9% and 68.8% after 120 days of immersion in room temperature seawater,The transverse shear strength and interlaminar shear strength are 55.6% and 30.9% in 60℃temperature seawater.The decrease of shear properties with increasing temperature is attributed to the accelerated hydrolysis of resin by high temperature erosion medium and the debonding of fiber-resin interface.At room temperature,the transverse shear strength of GFRP bars under unstressed,20% stress level and 40% stress level are 73.5%,81.3% and 64.3% after60 days.Under the same condition,the interlayer shear strength is 86.8%,89.0%,50%.Different from tensile properties.This is due to the shear strength depend on the strength of the resin and the fiber-resin bonding properties.The 20% stress level will close microcracks inside the GFRP and slow down the diffusion rate of the corrosive media.The 40% stress level will expand the GFRP bar internal micro-cracks and accelerate the diffusion of the corrosive media.This indicates that different levels determine the mechanism of shear strength deterioration.（4）The fracture of GFRP bars in tensile test was amplified by SEM and the interface between GFRP bars and concrete mortar and the element composition inside the mortar were analyzed by EDS.The results show that the corrosion degradation of GFRP bars is caused by the erosion of the glass fiber by water molecules and hydroxide ions and the hydrolysis of the resin.This leads to the phenomenon of pits,etching and even fracture on the fiber surface.And the bond strength between resin and fiber is weakened with the hydrolysis of resin.Elements in the fiber and resin precipitate continuously to the GFRP bar-mortar interface.（5）Based on the Arrhenius equation,the long-term life prediction of prestressed GFRP bars wrapped in SSC was carried out.The results show that the strength of mechanical properties of GFRP decreases completely after 2.6 years.This indicates that the preparation technology and material properties of GFRP bars need to be further improved.