Influence Of Cracking On Damage Behavior Of Concrete Subjected To The Salt Frost Cycles

Concrete is the most widely used civil engineering material for infrastructure construction.Although concrete has good technical and economical efficiency,but the cracks are often caused by loads,alkali aggregate reaction,shrinkage,etc.during the life cycle of concrete structures.The cracks providing a fast passage for the erosive medium entering the reinforced concrete in the external environment,shortening service life of reinforced concrete.It is worth noting that for concrete structures in cold marine environments in the north,concrete will undergo coupling failure between freezing and thawing and chloride salts,which will significantly increase the damage of structural concrete.In this paper,the development of the salt crack damage of concrete cracks caused by the initial cracks under the coupling of freeze-thaw and chloride salts,and the influence of chloride ion transport behavior in concrete after damage are studied.The initial cracks with widths of 20?m,40?m,60?m,80?m,100?m,130?m and 150?m were induced in the concrete specimens by the modified Brazilian Splitting Method,and the width information of cracks was characterized by image processing.The salt-freezing test was carried out on concrete with different initial crack widths,and the crack width expansion,concrete spalling mass loss and relative dynamic elastic modulus change were studied.At the same time,the effect of the above coupling on the transport behavior of chloride ion in concrete after damage was studied by using the rapid migration method of iodide ion.On the whole,with the increase of salt freezing cycles and the expansion of the initial crack width,the loss of concrete peeling loss and relative dynamic elastic modulus are gradually increased,and the damage is mainly concentrated in the crack and the surrounding area.In addition,the initial crack width and the water binder ratio have a significant effect on the salt-freezing performance of the concrete.However,when the water binder ratio is too high,the initial crack width has little effect on the salt-freezing performance of the concrete.The specific research results are as follows:(1)After the coupling of freezing thawing and chloride salt,the variation of crack width of concrete after salt freezing shows different regularity with the increase of initial crack width before salt freezing.As the number of salt freezing cycles increases to 28 times,the crack width of the concrete increases after salt freezing,but the crack width does not increase indefinitely,and the crack width remains stable or decreases when it reaches a certain value,and the maximum width that can be reached depends on initial crack width.In addition,increasing the water binder ratio will increase the crack width expansion of the concrete after salt freezing.According to the development trend of cracks in salt-freezing environment,the initial crack width(Wcr)of concrete can be divided into three categories: a.Wcr<40?m,the crack width increases slowly before the salt-freezing cycle reaches 16 times;After 16 cycles of freezing,the crack width increases rapidly.b.40?m<Wcr<100?m,the concrete crack width increases rapidly before the salt-freezing cycle reaches 20 times;after the salt-freezing cycle reaches 20 times,the concrete crack width as a whole shows a decreasing trend;c.Wcr>100?m,at the time,the initial crack width increase has little effect on crack propagation after salt freezing.(2)With the increase of the number of salt freezing cycles,there is a critical threshold for the influence of initial crack width and water binder ratio on the mass loss of concrete peeling after salt freezing.As far as the initial crack width is concerned,there are the following rules: a.Wcr < 80 ?m,the mass loss of concrete increases with the increase of the initial crack width.b.Wcr>80?m,the mass loss decreases with the increase of initial crack width.Secondly,in the range of initial crack width from 20?m to 150?m,when the number of salt freeze cycles reaches 8 times,the mass loss of concrete spalling increases significantly.As far as the water binder ratio is concerned,the following rules exist: when the water binder ratio is <0.38,the initial crack width has a significant effect on the mass loss of concrete spalling;when the water binder ratio is >0.38,the effect is smaller.(3)With the increase of the number of salt freezing cycles,the initial crack width and the ratio of water binder ratio to the relative dynamic elastic modulus of the concrete after salt freezing also have critical thresholds.This result confirms the mass loss of concrete.As far as the initial crack width is concerned,there are the following rules: a.Wcr < 80 ?m,the relative dynamic elastic modulus of the concrete gradually decreases as the initial crack width increases.b.Wcr>80?0,the relative dynamic elastic modulus of concrete increases with the increase of initial crack width.And,in the range of initial crack width from 60 ?m to 150 ?m,the relative dynamic elastic modulus of concrete is significantly reduced when the number of salt freeze cycles reaches 8 times.As far as the water binder ratio is concerned,the following rules exist: a.the water binder ratio <0.38,the initial crack width has a significant effect on the relative dynamic elastic modulus of the concrete;b the water binder ratio >0.38,the influence is negligible.(4)After experiencing the damage of salt freezing cycle,the iodide ion diffusion coefficient of concrete increases rapidly with the initial crack width.In terms of the initial crack width,the following pattern is observed: a.Wcr < 20 ?m,the concrete iodide ion diffusion coefficient increases slowly as the initial crack width increases.b.Wcr>20?m,the diffusion coefficient of concrete iodide ion increases rapidly with the increase of initial crack width.As far as the water binder ratio is concerned: When the water-to-binder ratio is less than 0.38,the initial crack width has a significant influence on the iodide ion diffusion coefficient;when the water-binder ratio is greater than 0.38,the initial crack almost no effect,and the permeability resistance of the test piece mainly depends on the number of salt freezing cycles.