Research On Damage And Failure Mechanism Of Concrete Under Chemical Expansion

Concrete is widely used in engineering construction because of its excellent performance.However,practice shows that concrete damage and deterioration caused by chemical factors such as carbonation,deicing salt damage and sulfate erosion have become problems that must be faced in engineering.The damage of concrete under various chemical actions will seriously affect the durability of the structure and directly determine the service life of the concrete structure.This topic selection of chemical action of admixture,alkali aggregate reaction,sulfate erosion magnesium oxide and three kinds of concrete chemical degradation reactions as the research object,combining macroscopic performance testing and microscopic analysis,the research development of micro cracks of concrete panoramic quantitative analysis technology for quantitative characterization of the internal micro cracks of concrete,analysis of concrete under the effect of several typical chemical expansion damage mechanism.Using standard curing with 80℃ high temperature accelerated curing method,studied the different dosage of magnesium oxide and magnesium oxide,the additive of fly ash concrete in the expansion of the standard curing condition and under the condition of high temperature curing deformation,the mechanical properties and evolution regularity of the internal structure of micro cracks by using elastic wave impact test accelerated curing under the conditions of magnesium oxide in the process of concrete expansive evolving law of the elastic wave velocity,studied the admixture of magnesium oxide and expansion ratio,compressive strength,splitting tensile strength and elastic wave velocity of the evolution law.Magnesium oxide prepared epoxy impregnation method using fluorescence microscopic analysis of concrete section,using the group independent research and development to get a panoramic view of a fluorescent microscope slices of panoramic images,and micro cracks on the fluorescence microscopic image extraction and quantitative analysis,the accelerated curing 14 d after magnesium oxide content of 15%,20% of the prism in the concretes with high content of magnesium oxide CT scans,extract the micro cracks in the concrete CT images and quantitative analysis,and the quantitative results of fluorescence microscopic image were compared.The research results show that the expansion of the magnesia concrete rate increased with the increase of magnesium oxide content increased,the micro structure characteristic parameters of crack increases with the increase of dosage of magnesium oxide,magnesium oxide of excessive incorporation can lead to uncontrolled expansion cement matrix deformation,micro cracks produced in the interface transition zone and even macroscopic crack visible to the naked eye.Even under the standard curing condition,the internal cracking phenomenon of concrete is still obvious.Under the condition of high-temperature curing,uncontrollable expansion will occur,and the concrete will even break down,and its mechanical properties will decline to a large extent.Using alkali active aggregate concrete specimens under 60℃ accelerated curing box to accelerate the curing reaction,remove the specimen at different age,concrete performance tests,the study found that the alkali aggregate reaction of concrete specimen elastic wave velocity,relative loss modulus,tensile splitting strength decreases with the increase of concrete expansion ratio,compressive strength is not significantly affected by alkali aggregate reaction.There is a good linear correlation between the crack tensile strength,expansion rate,natural vibration frequency and the damage degree based on elastic wave velocity.The quantitative analysis of micro-cracks in the concrete specimens subjected to alkali aggregate reaction shows that there are many micro-cracks in the concrete specimens subjected to alkali aggregate reaction,and the micro-cracks are mainly distributed linearly at the aggregate edges.With the increase of expansion rate,the total length,length density,average width and maximum width of microcracks inside AAR deteriorated concrete specimens all increased,and the microcracks were mainly distributed in the interface transition zone.The results show that the non-uniform expansion of concrete during the reaction of alkali aggregate leads to the initiation and propagation of micro-cracks,which is the fundamental reason for the decay of concrete’s macroscopic performance.The difference of the compressive strength and the characteristic parameters of microcrack structure of concrete before and after corrosion was studied.The quantitative analysis of microcracks in concrete slices before and after corrosion was carried out.The results show that the microcrack density in concrete increases significantly after the sulfate resistance level is reached.