Research On The Rebar Corrosion Mechanism In Concrete Under The Compound Action Of Carbonation And Chloride

The main factors affecting the durability of concrete are concrete carbonation and chloride salt corrison.Concrete carbonization is affected by many factors,such as the increase of carbon dioxide content in external environment,the accretion of population and the development of the industrialization process.Chloride salts derive from marine environment,chlorine salt additive and use of sea sands.Chloride ions corrodes the interior of concrete,thus the surface of rebars rust.In fact,most of concrete constructions are on service under a dual effect of carbon dioxide and chloride salt ions.Study of the dual effect on concrete is of great practical siginificance.In this dissertation,cement specimens with different water-cement ratio and different chloride content were set as research objects for studying the influence of chloride salts on the carbonization speed and the strength and exploring the effect laws.This dissertation also studies the effects of chloride salts on the microstructure of chloride salt cements,analyzes the influence of the carbonization of the pore structure,chlorine,sulfur and ettringite,finds the binding and the solidifying laws of chloride ions inside the cement which contains chloride salts and elaborate the corrosion mechanism of the interaction of the carbonization and the chloride salts.The X-ray computed tomograpHy(X-CT)and the electron probe microanalysis of elemental analysis(EPMA)technology,combined with other technologies such as the mercury analyzer,the field emission scanning electron microscopy,the carbonation tank and the porosity analyzer were employed to reveal the main laws and the achievements are summarized as follows:In order to explore the effects of different chloride salts content and different water-cement ratio on the strength of specimens as well as the interaction laws between these two components,the concrete specimens with different water-cement ration and different chloride content were on the compressive and the tensile strength test on the 3d,7d and 28d of maintance respectively.Secondly,to find the effects of chloride salts on the carbonization speed of concrete,the traditional pHenolpHthalein solution test combined with X-CT scan test were applied to carry out the intuitionistic tests.The X-CT and the electron probe microanalysis of elemental analysis(EPMA)tests were carried out after the accelerated carbonization of the concrete specimens,the results showed that the water-cement ratio is the significant factor which affects the intensity changes of the concrete specimens,and with the increase of the curing period,the law is more obvious.Both the rupture strengh and the compressive strength are improved as the increase of the content of chloridesalts,but the difference of the strength between these specimens is getting smaller as the increase of the curing period.The chloride salts have a certain effect on the strength of the concrete specimens in the ealry period as the water-cement ratio is getting larger.The microstructure inside the concrete contained chloride salts was characterized by the electrion microsopy scanner.XRD combined with the TG/DTA comprehensie analysis were carried out to examine the composition of the concrete,exploring the effects caused by chloride ions on the process of the hydration process of the concrete.Through the studyding of the influence of the flyash,chloride salts and the blend of the two substance on the microstructrue of the concrete,evaluate the influence of chloride salts on the microstructure and the mineral composition of the ordinary concrete and which of the fly-ash concrete.And the Mohr method was applied to detected the concentration of the water-soluble chloride ions of various types of chloride salts containing concrete with the curing period of 28 days.The content of alkali was resonablly evaluated during the alkali-aggregate reaction.The effect of carbonizaiton on the distribution of soudium ions was studied and the reaction characteristic of of the alkali-aggregate reaction was demonstrated.The influence of chloride salts on the carbonization speed of the cement paste and the concentration distribution of Cl,S and Na inside the cement paste were detected by the X-CT and the EMPA technologies,the structrue transformation of the chloride salts containing cement paste under the carbonizaiton was denmonstrated and the characteristics of the microstructure were evaluated,the transportation law of the ions in the cement slurry was detected.The content in this paper provides new theories for the design for the durability of the reinforced concrete structure,the norm establishment and the lefe prediction.This paper investigate the corrosion area rate and the corrosion levels of the reinforced cement structures mixed with the chloride salts under the carbonization,and compared with the single effect factor,denmonstrating the corrosion characteristic or reinforced cement structures under the dual effect of the carbonization and the chloride salts.The results show that the corrosion area rate and the corrosion level of the dual-effec materials are larger than that of the single factor effect mateirals.As the increase of n(NO2-)/n(Cl-),the corrosion area rate and the corrosion level of reinforced cement structures under the dual effect of the carbonization and the chloride salts decrease gradually,when the n(NO2-)/n(Cl-)is 1.2,the corrosion area rate of the rebar decreased from 62%to 1.8%.