Study On Corrosion Mechanism Of Pile Foundation Concrete In Saline Soil Environment Of Taitema Lake In Xinjiang

In the saline soil environment,the bridge pile foundation concrete is prone to the corrosion of the rebar caused by the Cl^-erosion in the saline soil or the salt lake.Plasma erosion of SO₄^2-,HCO₃^-and Mg²⁺leads to the macro-fine-micro deterioration of concrete materials.Multi-ion erosion of the concrete is a complex physical-chemical-mechanical process coupled with ion diffusion,chemical reaction and mechanical damage.In this paper,the influence of the macro performance and microstructure of concrete under the internal corrosion of SO₄^2-was studied through laboratory experiment.The variation rules of appearance and mechanical properties of concrete corroded by Na Cl-Na₂SO₄、Na Cl-Mg SO₄、Na Cl-Ca SO₄ and Na Cl-Na HCO₃ solutions with different concentrations were analyzed.The law of ion migration and product distribution in concrete during erosion were obtained.Based on the finite difference method,the calculation model of corrosion products,volume expansion and crack density on the ion diffusion coefficient under Cl^--SO₄^2-coupling erosion was established.The specific work and main conclusions are as follows:1.Through the Na₂SO₄ internal corrosion test,the compressive strength,relative dynamic elastic modulus,microstructure and corrosion products of concrete after different concentrations of internal corrosion of SO₄^2-were studied.The influence of different concentration of SO₄^2-on cement hydration process was analyzed.The result shows:（1）With the increase of curing time,the compressive strength and relative dynamic elastic modulus of concrete gradually increase,and the relationship between the relative dynamic elastic modulus and curing time conforms to the logarithmic function.The compressive strength of SO₄^2-internally corroded concrete are lower than that of the control group.The relationship between the compressive strength and SO₄^2-concentration after 28 days standard curing presents a quadratic decreasing function.（2）With the increase of SO₄^2-concentration,the corrosion products AFt and Gyp fill into pores and microcracks.Interpenetrating cracks appear in concrete after internal erosion at relatively high level SO₄^2-concentrations.（3）In the early stage of erosion,SO₄^2-solution promotes cement hydration.In the late stage of deceleration and stable stage,SO₄^2-solution restrains cement hydration.2.The evolution process of pore structure and microstructure of concrete after the erosion of composite solution of Na Cl、Na Cl-Na₂SO₄、Na Cl-Mg SO₄、Na Cl-Ca SO₄,Na Cl-Na HCO₃with different concentrations was studied by the natural immersion external erosion test.The damage characteristics and erosion mechanism of concrete under external erosion with different concentration of composite solution were obtained.Test results indicate that:（1）The degree of influence of the composite solution on the compressive strength change rate,mass change rate,relative dynamic elastic modulus and appearance of concrete from high to low is as follows:Na Cl-Mg SO₄,Na Cl-Na HCO₃,Na₂SO₄-Na Cl and Ca SO₄-Na Cl.（2）The binding ability of SO_42-,Mg²⁺and HCO₃^-to hydration products in composite solution is higher than that of Cl^-.（3）Corrosion products such as AFt,Gyp,Mg（OH）₂,and Ca CO₃ reduce the alkalinity and cementitious properties of concrete by consuming hydration products Ca（OH）₂,C-S-H,etc.Increased damage to the concrete matrix.（4）In the early stage of erosion,with the concentration of the composite solution increases,corrosion products are deposited in pores and cracks,and the pore structure of concrete is refined and the ion diffusion rate is reduced.In the later stage of erosion,the corrosion products cause damage to the matrix,resulting in cross cracks,which induce the deterioration of the macroscopic properties of concrete.The micro-morphology and pore distribution of concrete eroded by the solution containing only Na Cl were less changed.（5）Based on grey correlation entropy analysis method,the influence of pore size distribution on the relative dynamic elastic modulus of concrete corroded by multi-salt is discussed.It is found that the influence degree of pore size on the relative dynamic elastic modulus from high to low is as follows:small pore（<0.05μm）>medium pore（0.05μm-10μm）>large pore（10μm-63μm）.3.In the full immersion environment of Cl^--SO₄^2-composite solution with different concentrations,the diffusion process and content of Cl^-after Cl^--SO₄^2-erodes concrete are calculated by theory.（1）Based on Fick’s second law of unsteady diffusion,a coupled diffusion model for Cl^-and SO₄^2-was constructed considering the secondary chemical reaction between SO₄^2-and hydration products.（2）The effects of Cl^--SO₄^2-concentration,corrosion products,crack density and erosion time on the diffusion of Cl^-were analyzed,and the theoretical Cl^-values were compared with the experimental ones.（3）Through the comparison of Cl^-content in the surface layer（0-5mm）of concrete after the composite solution erosion,it can be found that the influence on Cl-diffusion from high to low under the same mole ratio and erosion time is as follows:Na Cl-Mg SO₄、Na Cl-Na₂SO₄、Na Cl-Na HCO₃ and Na Cl-Ca SO₄.