Research On Leaching Behavior Of Hydraulic Concrete Under Applied Electric Field

In recent years,the damage on hydraulic concrete caused by calcium leaching has been a matter of great concern.Most researchers have studied the single leakage dissolution behavior or single alternative freezeing-thawing behavior of concrete while damage rules of hydraulic concrete under coupling effect between leaching and freeze-thaw cycling has not been reported.The natural diffusion is so slow that it is quite significant to design the accelerated leaching method and investigate the dissolution performance during the service process for optimal mix proportion of hydraulic concrete.The research about deterioration mechanisms under leaching coupled with freeze-thaw cycling combined to the special cold regions can reflect the real engineering practice and has important theoretical and practical significance in service life prediction of leached concrete.In order to address the research gaps,the correlation between electrochemical accelerated leaching and natural leaching was explored based on the electrochemical accelerated leaching method(EALM).Furthermore,the electrochemical accelerated leaching schemes were optimized according to test results.The quantitative relationship between mechanical properties,leaching parameters and electrical resistivity of leached concrete at different leaching periods was then discussed in detail and meanwhile the critical dissolution indicator was confirmed.Besides,the effect of different factors on the electrochemical accelerated leaching performance of cementitious materials was investigated,such as fly ash replacement,coarse aggregates as well as exposure areas.Finally,the quantitative relationship between calcium leaching rate and electrical resistivity of concrete under single leaching factor as well as coupling factor between leaching and freeze-thaw cycling was built and the electrical resistivity was therefore proposed to characterize the leaching damage extent of concrete as a nondestructive examination method for leached concrete.The main contents of this thesis are summarized as follows:(1)Based on the principle of electrochemical acceleration,the self-designed electrochemical accelerated leaching device(EALD)was used to carry out electrochemical accelerated leaching test under different applied voltages for typical slab concrete.The percent of dissolved calcium oxide mass up to 25% from concrete matrix was taken as the critical leaching index and the diffusion time needed to reach the critical state was then forecast through applied voltage extrapolation method and natural diffusion method.The results of diffusion time of leached concrete needed to reach the critical state were compared which was helpful to build the correlation between these two leaching methods.Furthermore,the electrochemical accelerated leaching schemes were optimized and the reasonable water exchange cycle and applied voltage were also confirmed.(2)The electrochemical accelerated leaching rule of common concrete without fly ash was studied.Firstly,the kinetic equation of dissolution referred to the p H value of solution in cathode room,calcium leaching rate as well as electrical resistivity of leached concrete was discussed.Then,the mass loss,compressive strength and dynamic elastic modulus of leached concrete at different leaching periods were also monitored and the relationship between these index and elect rical resistivity validates the electrical resistivity as an effective and sensitive way to characterize the damage degree of leached concrete.What's more,the critical leaching indicator was confirmed for C30 and C40 concrete.(3)The effect of fly ash replacement,coarse aggregates and exposure areas on the leaching behavior of cementitious materials was studied based on EALM.The calcium leaching rate was adopted as the main index to evaluate the dissolution degree of concrete specimens and residual mortar specimens with different sizes.Meanwhile,mercury intrusion porosimetry(MIP)and phenolphthalein spraying method were also chosen to explore the differences between electrochemical accelerated leaching performance of large diameter mortar specimen s and small diameter mortar ones.Moreover,the optimal additive amount of fly ash for corrosion resistant concrete for water-to-binder 0.5 and 0.6 was determined through the experimental verification and theoretical calculation.(4)The leaching test coupled with freeze-thawing test was conducted to discuss the durability damage features of concrete under different coupling test schemes.The effect of leaching damage on the frost resistance of concrete and the effect of freeze-thawing damage on the leaching resistance of concrete were studied respectively according to such characteristic parameters as calcium leaching rate,p H value of cathode solution and electrical resistivity of concrete.In addition,the results under single leaching and couping effect between leaching and freeze-thawing were compared,including electrical resistivity,ultrasonic fight-time,mass variation as well as leaching depth which can be applied to judge the damage extent of concrete durability caused by different environmental factors.Finally,two main conclusions that the calcium dissolution was more harmful for the concrete durability and the improvement effect of fly ash was more notable on the anti-erosion capability of hydraulic concrete were revealed.