Electrochemical Analysis Of Concrete Cement Hydration,Chloride Diffusion And Steel Corrosion

With the rapid development of civil engineering in China,concrete materials are widely used in large-scale engineering structures,such as high-rise buildings,sea crossing bridges,port terminals,reservoir dams,etc.Durability is the main factor affecting the service life of these concrete structures.However,these concrete structures are in a very harsh environment,especially marine concrete structures,which are subject to long-term erosion by sea breeze,sea fog and sea water,easily causing steel corrosion and other damage.The accumulation of these damages leads to the premature failure of the structure,which directly reduces the durability of the structure.Therefore,it is necessary to study the properties of concrete materials and the durability of reinforced concrete structures in marine environments.The electrochemical analysis method has the advantages of simple test,high sensitivity and high accuracy,and it is easy to monitor the evolution of concrete performance in the medium and long term.Therefore,electrochemical method is used to study the durability of concrete and concrete structure.In this paper,electrochemical impedance spectroscopy(EIS)was used to analyze the hydration process of concrete cement,chloride ion corrosion and steel bar corrosion.In addition,corrosion potential method,linear polarization method,cyclic voltammetry and Motto-Schottky curve were used to analyze the corrosion of steel bar in concrete,and the results were compared with EIS.The main research contents and conclusions of this paper are as follows:(1)The hydration processes of ordinary concrete and fly ash concrete were studied by electrochemical impedance spectroscopy.Through the analysis of electrochemical parameters,the change of concrete microstructure during the hydration process is discussed.The results show that with the decrease of water-cement ratio,pore solution electrolyte resistance Rs,charge transfer reaction resistance Rct and impedance coefficient ? all show an increasing trend,and the smaller the porosity of concrete,the denser the structure.For fly ash concrete,at the same age,the higher the fly ash content,the lower the growth rate of cement hydration matrix resistance R1,indicating that the cement hydration rate decreases with the increase of fly ash content.(2)The diffusion process of chloride ions in ordinary concrete,fly ash concrete,slag concrete and fly ash/slag concrete in simulated marine environment was studied.The results show that with the increase of mineral admixture content,the electrolyte resistance Rs,charge transfer reaction resistance Rct and impedance diffusion coefficient ? increase,and the concentration of free chlorine ion also increases at the same depth.Compared with fly ash or slag containing only 30%,the peak value of chloride ion concentration in fly ash/slag concrete is lower under the same erosion time.Based on the experimental results,the relationship between chloride ion concentration in concrete and electrochemical impedance spectroscopy parameters(Rs,Rct,?),admixture proportion,water-cement ratio and time can be used to predict chloride ion concentration in concrete.(3)The electrochemical characteristics of the steel in the simulated pore solution,such as passivation process,debonding process and corrosion process,are studied.At the same time,the minimum pH range needed for the formation of passivation film and the threshold value of chloride ion in the pore simulation solution with different pH value were analyzed.The results show that the minimum pH value range of passivation film is 11.6?12,and the time required for passivation decreases with the increase of pH value.In the initiation stage of corrosion,the passivation film resistance Rc of the reinforcement decreases continuously,and the capacitive arc radius of the passivation film Qc decreases,and pitting gradually occurs.In the stage of corrosion stabilization,the passivation film resistance Rc and charge transfer resistance Rdl of the reinforcement continued to decrease,and the double-layer capacitance Qdl of the surface of the reinforcement increased,and the corrosion developed from pitting to local corrosion.In the stage of corrosion deterioration,both the passivation film resistance Rc and the charge transfer resistance Rdl continue to decrease with a large drop,and the double-layer capacitance Qdl on the surface of the steel bar continues to increase.The resistance reduction capacitance of the diffusion tail part increases,and the corrosion develops from local corrosion to comprehensive diffusion corrosion.(4)The corrosion process of chromium alloy alloy steel bars in simulated pore solution of concrete was studied by electrochemical method.The threshold value of chloride ion and the redox reaction of cyclic voltammetric process were analyzed under different pH values.The results show that the chromium alloy steel bar has a two-layer passivated film structure,and the corrosion conditions are more severe than that of carbon steel steel bar.The lower the pH value of the simulation liquid,the higher the complexity of the equivalent circuit.The Nyquist impedance spectra of chromium alloy steel bar are divided into two parts:the impedance spectra at the passivation stage is nearly parallel to the horizontal axis,and the impedance spectra at the failure stage of the passivation film is semicircle.