Durability Of Prestressed BFRP Bars Embeded In SSGC In Seawater Environment

With the rapid development of the emerging marine industry,more and more coastal and marine infrastructure such as ports,bridges and offshore wind farms are ushering in a new wave of construction.However,most of the marine projects are far away from the land.During the construction process,there is a shortage of river sand and fresh water,and the construction is greatly affected by the tidal action.Moreover,the buildings will also be corroded by the chloride ions in the seawater during the service process.In addition,traditional cement produces a large amount of CO₂ during the production process,which produces a greenhouse effect.To address these challenges,a new type of concrete structure was developed:the basalt fiber reinforced polymer(BFRP)reinforced seawater sea sand geopolymer concrete(SSGC)structure(BFRP-SSGC structure).The advantages of this structure include:(1)Seawater and sea sand are used as raw materials,and local materials are used to alleviate the depletion of freshwater and river sand resources;(2)BFRP bars have high performance,low cost,wide range of raw materials,and corrosion resistance.The use of BFRP bars to replace ordinary steel bars can fundamentally solve the problem of corrosion.(3)Geopolymer has early strength and high strength,excellent anti-seawater erosion performance,and its production raw materials are widely used,and its carbon emission is low.It is a good substitute for Portland cement.In order to ensure the safety of BFRP-SSGC structures in coastal areas,it is urgent to prove the durability of BFRP bars in SSGC structures in seawater environment,which is of great scientific significance and engineering value for promoting the application of this new type of structure.Therefore,this study took the prestressed BFRP bars embedded in seawater sea sand geopolymer concrete as the research object,designed the artificial seawater accelerated aging test method,and explored the prestressed BFRP bars embedded in SSGC in the seawater environment at differenttemperatures(room temperature.,40?,60?),different alkali content(4%,6%),different prestress(no prestress,20%prestress)and different corrosion cycles(30 days,60 days,120 days,240 days)The time-varying laws of tensile properties,interlaminar shear properties and transverse shear properties of SSGC,combined with advanced microscopic detection technology,reveal the degradation mechanism of its mechanical properties,and use the Arrhenius model to predict the residual life of the prestressed BFRP bars embedded in SSGC.The main research contents and conclusions of this paper are as follows:(1)The experimental study on the time-varying law of the prestress level of BFRP bars embedded in SSGC was carried out,and the influence law of temperature and SSGC alkali content on the prestress loss of BFRP bars in seawater environment was proved.The prestress loss of SSGC embedded prestressed BFRP bars was monitored by FBG fiber grating sensor.The results showed that the prestress loss of BFRP bars was concentrated within 24 hours after unwinding(loss rate was about 10.2%).The prestress loss of 7 days is gradually stable(loss rate is about 19.4%),and the immersion ambient temperature and alkali content have little effect on the prestress loss of BFRP bars.After 120 days of seawater immersion,the prestress loss of BFRP bars does not exceed 35%.(2)By implementing the artificial accelerated aging test of multi-time-scale prestressed BFRP bars(SSGC-wrapped),and comprehensively adopting multi-scale testing technology of micro-mesostructure and macro-mechanical properties,the coupling effect of seawater environment and prestress is discussed.The damage evolution law of resin matrix degradation,glass fiber etching,and water molecule plasticization in the lower BFRP bars reveals the deterioration mechanism of the tensile properties of BFRP bars in SSGC components under seawater immersion service environment.The research results show that the tensile properties of BFRP bars degrade faster with higher temperature,and the effect of alkali content on the tensile properties is limited.Under the coupling effect of prestress and environment,prestress plays a leading role in reducing the tensile strength of BFRP bars.SEM observed that some fibers fractured in BFRP bars with 20%prestress,which greatly reduced the tensile strength of BFRP bars.The effect of different concrete on the tensile properties of BFRP bars is SWSSC>NC>SSGC.The tensile modulus of elasticity under each working condition has no obvious regularity,because the longitudinal modulus of elasticity of FRP bars mainly depends on the modulus of fibers.(3)The experimental research on the shear performance of prestressed BFRP bars in seawater environment was carried out,and the influence of different ambient temperatures,different alkali contents and different corrosion cycles on the shear performance of prestressed BFRP bars was found out.The research results show that when there is no prestress,the temperature has little effect on the interlaminar and transverse shear properties.When there is 20%prestress,the temperature will increase the effect of the prestress on the BFRP reinforcement,thereby increasing the gaps.No matter the interlaminar shear strength or the transverse shear strength,the difference between the two alkali dosages is very limited.The interlaminar shear strength is more affected by the prestress than the transverse shear strength,because the interlaminar shear performance depends on the interface between the base resin and the fiber,and the horizontal stress is stronger than the transverse shear strength.Shear stress is more favorable to induce interface degradation.The shear performance degradation laws of BFRP bars wrapped by SSGC,SWSSC and NC are similar.SWSSC wrapping has the greatest effect on the shear performance of BFRP bars,while SSGC wrapping has the least effect.The EDS energy spectrum results show that the Si and Fe elements at the interface between the fiber and the resin substrate change significantly after 240 days,which indicates that the BFRP bars have undergone deterioration reactions such as fiber-resin interface damage and basalt fiber erosion.(4)The long-term life prediction of prestressed BFRP bars wrapped with SSGC is carried out by using a common model based on the Arrhenius equation.The results show that for BFRP bars wrapped with SSGC with 4%alkali content in seawater environment at room temperature,when the strength retention rate is degraded to 70%,the tensile strength needs113 days(no prestress)and 30 days(20%prestress),and the interlaminar shear takes 227days(no prestress)and 66 days(20%prestress),and transverse shear takes 151 days(no prestress)and 56 days(20%prestress).For BFRP bars wrapped with 6%alkali content SSGC in seawater environment at room temperature,when the strength retention rate is degraded to 70%,the tensile strength needs 100 days(no prestress)and 34 days(20%prestress),and the interlaminar shear needs 168 days(no prestress)and 118 days(20%prestress),and 50 days(no prestress)and 40 days(20%prestress)for transverse shear.The interlaminar and transverse shear strengths takes more time than tensile strength at the same retention rate.This shows that the tensile strength is more affected by the prestress and the environment.