Research On Destruction Of Concrete Under Condition Of Wetting-drying And Freezing-thawing Cycles Based On Nuclear Magnetic Resonance Technology

Cement concrete is the most widely used construction engineering material in construction engineering.The durability and service life of concrete is seriously affected by the complicated environment,and salt corrosion and freezing-thawing cycles are two key factors leading to performance degradation.Therefore,it is necessary to study the destruction of concrete under the condition of wetting-drying and freezing-thawing cycles.This paper analyzes the mechanical properties and microscopic pore structure of concrete under the condition of wetting-drying and freezing-thawing cycles,when the concrete is with different air content and water-to-binder ratio when circulating in tap water and sulfate solution.Macroscopic indicators include mass loss rate,relative dynamic elastic modulus,uniaxial compressive strength,etc.;while microscopic pore structure indicators include T₂ relaxation value,T₂ spectrum integral area and porosity.Through the gray correlation analysis method,the law between the above indicators is searched for correlation analysis.Research have shown that under the same water-to-binder ratio,the addition of air entraining agent can increase the number of tiny pores.When the strength class is C30,the ratio of pore whose size is less than 100nm in the concrete with 5‰air entraining agent is 80%,which is more than non-air entraining agent concrete which is 73%.Under the condition of no air entraining agent,the porosity of C50 concrete is lower than that of C30 concrete,and the ratio of harmless pores of C50 is about20%,which is greater than 10%of C30 concrete.The presence of sulfate accelerates the destruction of concrete.With no air entraining agent,the increase of the T₂spectral area of the C30 concrete specimen in water is slightly smaller than that in the sodium sulfate solution under the 5th cycle of wetting-drying and freezing-thawing.Through gray correlation analysis,the pores in concrete are divided into four categories according to the pore size:harmless pores,less harmful pores,harmful pores,and more harmful pores.And the gray correlation analysis is used to find out the relation of T2 relaxation value whit porosity,compressive strength,and relative dynamic elasticity.The research found that the harmless pores and the more harmful pores have a good correlation with the relative dynamic elastic modulus,and the correlation between the harmful pores and the compressive strength is good,and the average correlation coefficient is higher than 0.8.The dry-wet-freeze-thaw resistance of concrete can be preliminarily predicted by the change of porosity.When the proportion of harmless pores with pore diameter less than 20nm is high and the proportion of harmful pores of 100-200nm is low,the durability of concrete is good.On the contrary,it is poor.