Study On Damage Of Concrete Under The Coupling Action Of Salt Attacks,Freeze-thaw Cycles And Load

The western region of Jilin province is a typical soda saline soil areas in China.The concrete projects in this region are subjected to salt attacks,freeze-thaw cycles and other negative factors,which make the service life of concrete structures in this region far lower than design service life.The previous durability study mainly consider chloride ion as the main corrosive ion,which has the greatest damage on the concrete matrix.Through the field investigation on the northeast saline soil area,it is found that under the action of freeze-thaw cycles and load,the durability damage of carbonate and sulfate ions is very serious.This research work was financially supported by the State Key Program of National Natural Science Foundation of China?Grant No.41430642?,the Special Fund for Major Scientific Instruments of the National Natural Science Foundation of China?Grant No.41627801?,and China Postdoctoral Science Foundation?Grant No.2015M581403?.The following theoretical and experimental research work is carried out on the change of durability damage of concrete under the action of salt attack-freeze-thaw-load.?1?Through field sampling in the saline soil region of Northeast China,different salt attack tests,freeze-thaw cycle tests,and coupling tests under salt attack,freeze-thaw cycles,and load were designed in respect of the practical environment of concrete structure.?2?Based on SO₄^2-and CO₃^2-attack test,it was found that when the concentration of solution was 5%,the concrete mass and relative dynamic elastic modulus first increased and then decreased.Compared the results of two kinds of salt,it was found that the damage of mass and the relative dynamic elastic modulus of concrete under the CO₃^2-solution were more serious than those of SO₄^2-solution.The relative elastic modulus loss of the two was relatively similar,but the mass loss of the former was far greater than that of the latter.?3?For the first time,a new concrete freeze-thaw damage model was established based on Logistic function and damage law under the condition of freeze-thaw cycles,and three stages of concrete freeze-thaw damage were described in detail.?4?Based on the coupling test under salt attacks and freeze-thaw cycles,the effect of salt type on the frost damage of concrete was investigated.The damage of the concrete under the SO₄^2-solution was mainly caused by the swelling material in the concrete,which make the relative elastic modulus of concrete drop rapidly.In addition to generating expansive substance,and then making the relative dynamic modulus decreased rapidly,CO₃^2-attacked the surface of concrete at the same time.The relative elastic modulus loss of SO₄^2-and CO₃^2-was relatively similar,but the mass loss of the latter was far greater than that of the former.?5?The curing concrete was taken as the basic state,and the relative dynamic elastic modulus of the concrete was taken as the damage variable according to the damage mechanics and strain equivalence principle,and the theoretical damage model of the concrete under the coupling action under salt attack and freeze-thaw cycles was established.The damage of concrete under salt attack and freeze-thaw cycles was considered as the meso-scopic damage;the development of concrete cracks under load as the macroscopic damage,and the coupling damage model of concrete under the action of salt attack,freeze-thaw cycles and load was established combined with macro-mechanical coupling conditions.?6?According to the above theoretical and experimental studies,it was found that the coupling damage of concrete structures under multiple factors was not a linear superposition relationship in respective environment,but was determined by the coupling effect between various factors.The coupling damage of salt attacks and freeze-thaw cycles depend on the coupling effect of salt attacks damage and freeze-thaw cycles damage;The coupling damage of concrete under the effect of salt attack-freeze thawing-load is determined by the coupling effect of macroscopic damage caused by the salt attack and freeze-thaw cycles as well as the macroscopic damage caused by the load.The coupling damage shows different evolution rules according to factors such as salt corrosion time,freeze-thaw cycles and strain.The coupling damage model of concrete under the action of salt attack-freeze thawing–load is established in this paper,which provide a theoretical foundation for the durability damage evaluation and service life prediction of saline soil in cold region,and offer research basis for the durability design of concrete structures in complex environment.