Experimental Study On Performance Of Concrete Cylinder Confined By CFRP Under Sulfate Continuous Semi Immersion

Northwest China is located in the plateau inland,the climate is bad,hot in summer,cold and dry in winter,and the air is dry,and a large number of salt lakes are distributed,the surrounding saline soil is rich in sulfate,these environmental conditions are great damage to the durability of bridge piers and concrete columns half buried in saline soil.In recent years,carbon fiber reinforced polymer（CFRP）has become an effective means to strengthen concrete columns or piers due to its superior performance.In the saline soil environment,most of the concrete members are in service and have been damaged to varying degrees.The key of confined concrete is the interaction between confined materials and concrete.Considering the deformation compatibility of CFRP and concrete matrix,the initial damage due to sulfate attack should also be considered when using CFRP to repair and strengthen concrete members in practical engineering.In this paper,the macro mechanical properties and micro deterioration of CFRP confined concrete columns under sulfate semi immersion are analyzed in detail by simulating concrete columns or piers half buried in saline soil in Western China.The experimental contents and results are as follows（1）In order to determine whether CFRP confined concrete columns are strongly confined in natural environment,the mechanical properties of 0,1,2,3-layer CFRP confined concrete columns in natural environment are compared and analyzed.The test results show that the ultimate bearing capacity and strain of concrete confined by CFRP increase with the increase of the number of CFRP layers,so one or more CFRP layers are considered as strong constraints.Considering the economy and durability of concrete members,two layers of CFRP are used to strengthen concrete columns.（2）In order to study the macro-mechanical properties of CFRP-reinforced concrete columns in semi-buried saline soil,a concrete column with a diameter of 150mm and a height of 300 mm was used for three working conditions（ordinary concrete column,CFRP constrained normal concrete,and CFRP constrained degraded concrete）.In the indoor simulation test,a 10%sodium sulfate solution was used to simulate the western saline soil environment,the strength and strain changes under three working conditions were analyzed,and the corresponding stress-strain curves were drawn.The test results show that with the continuous increase of erosion age,the strength and ultimate strain under the three working conditions all increase first and then decrease.（3）In order to understand the micro-erosion of the CFRP-reinforced concrete column in the semi-buried saline soil,the Ba Cl₂ titration test was used to test the sulfate ion content at different depths from the surface and the inside of the concrete column under three working conditions to characterize the sulfate ion content.Distribution law in concrete.The test results show that the sulfate ion content gradually decreases as the erosion depth continues to increase.X-ray diffraction technology was used to analyze the concrete corrosion products under three working conditions,and it was found that the main corrosion products were gypsum and ettringite.The SEM electron microscope was used to conduct in-depth analysis and research on the surface concrete micro morphology under three working conditions.And the maximum erosion depth model under three working conditions was predicted.（4）In order to explore the pore size distribution in the concrete after the CFRP reinforced concrete column in the semi-buried saline soil is eroded by sulfate.The low-field nuclear magnetic technology is used to analyze the change law of pore structure and the evolution law of pore size distribution under three working conditions.And established the correlation between the compressive strength and porosity of ordinary concrete column and CFRP constrained ordinary concrete under continuous half immersion of sulfate.（5）In order to construct a mechanical model of CFRP reinforced degraded concrete columns semi-buried in saline soil in the west,based on the experimental research in Chapters 3 and 4,strength retention and maximum erosion depth are introduced as quantitative indicators of concrete damage.Further establish the correlation between the maximum erosion depth and the compressive strength under the continuous half-immersion of sulfate.Through the regression analysis of the compressive strength and strain law of the degraded concrete constrained by CFRP under the action of two stages,it is established based on the strength retention rate and the maximum Ultimate strength and ultimate strain model of erosion depth.