Long Term Mechanical Properties Of Sea Water Seasand Concrete Confined By BFRP Under Axial Compression

With the implementation of the national marine power strategy,China needs to carry out a large number of marine engineering constructions,the amount of concrete increases,and the problems of ecological environment caused by excessive exploitation of concrete raw materials are becoming increasingly prominent.If fiber reinforced polymer（FRP）reinforced sea sand concrete（SSC）can be used to replace the traditional reinforced concrete in marine engineering structures,it is convenient to obtain local materials and save costs,and it can also reduce the environmental problems caused by excessive exploitation of sand and gravel resources.FRP has excellent chloride ion corrosion resistance,but the strong alkalinity in concrete is the key factor leading to the long-term performance degradation of it.In this project,high-performance green basalt-carbon fiber hybrid composites（C/BFRP,Carbon/Basalt FRP）are developed through carbon fiber hybrid,and the durability of C/BFRP constrained concrete is studied.At the same time,the hydrated calcium sulphoaluminate generated by sulfate in seawater will cause the volume expansion of concrete,resulting in the prestress of FRP and restraining the damage caused by SSC expansion.Using the methods of experimental research and theoretical analysis,considering the service load and seawater corrosion environment,this thesis explores the compressive strength,elastic modulus,stress-strain mechanical behaviors of C/BFRP constrained SSC,and establishes the bearing capacity calculation model of its limit state.The main contents and conclusions were as follows:（1）The alkali activity of basalt rock and basalt fiber was studied.The results showed that the average elongation of the cement slurry specimens on the fourteenth day of the basalt rock was about 0.4%,and the surface of the specimen forms a typical network of alkali silica reaction（ASR,Alkali Silica Reaction）cracks.The reaction products observed two forms:one was like the petal crystal,two was the honeycomb shaped amorphous gel.The average elongation of cement samples made of basalt fiber was about 0.093%on the fourteenth day.The elongation test results were conservative because of the tensile action of fibers.The surface of the fiber was covered with alkali silica gel,and its potential alkali activity was comprehensively judged.（2）The axial compression test of BFRP confined SSC was studied,and the degradation law of its mechanical properties under load and seawater corrosion was discussed.The results showed that BFRP can significantly improve the compressive strength of SSC,with an increase range of 35%～84%.With the increase of corrosion time,the compressive strength of the specimen showed a downward trend.At the same time,the axial deformation capacity of SSC could be increased by 3～5 times.Seawater was an important factor affecting the compressive strength of SSC.Seawater could enhance and corrode its compressive strength at the same time,and the enhancement effect was greater than the erosion effect in a short time.The load holding would not reduce the compressive strength of seawater sand concrete.When the load holding and seawater corrosion work together,the load holding could slow down the degradation of its compressive strength.Because there were many micro cracks and micro pores in SSC and the compactness was low,the load holding made its internal pore structure denser due to long-term axial compression,reduced the infiltration of corrosive ions,and improved the compressive strength.（3）The mechanical properties of C/BFRP confined SSC under seawater corrosion were studied.The results showed that C/BFRP can improve the mechanical performance of SSC better than single BFRP,about 21.7%～32.3%,because CFRP constraint layer played a good role in protecting BFRP and core concrete.With the increase of seawater corrosion time,its compressive strength showed a downward trend,but the decline rate of performance was slow,and it could still maintain the strength retention rate of more than 80%after one year.The circumferential strain stress curve of C/BFRP constrained SSC showed an obvious platform stage.Because the outer CFRP constraint layer with low elongation was destroyed first,and the load originally borne by carbon fiber was transferred to basalt fiber,the process of stress transfer forms the platform of stress-strain curve.（4）The calculation model of BFRP constrained SSC limit state was analyzed,and the applicability of Lam and Teng model and ozbakkaloglu model was discussed.The model is modified based on the experimental data.The results showed that the errors of the predicted values of the two models about the ultimate axial stress were within 15%,indicating that they had a good prediction effect on the ultimate axial stress of BFRP constrained SSC.At the same time,with the increase of seawater corrosion time,the two models still had a good prediction effect.As for the prediction of ultimate axial strain,the prediction results of Lam and Teng model were slightly conservative,while ozbakkaloglu model had a good effect on the prediction of ultimate strain of BFRP constrained SSC,and the prediction accuracy of C/BFRP constrained SSS was slightly deficient,with AAE of 13.9%.Considering the simplicity of the model,the calculation formula of limit strain was modified based on Lam and Teng model.The modified strain enhancement coefficient k₂ corresponded to BFRP constrained SSC and C/BFRP constrained SSC,which were 9.73 and 10.55 respectively.