Concrete Neutralization In The Atmospheric Environment Based On Multi-factor Effects

There are a great number of concrete structures existing in the atmospheric environment.The process of industrialization has led to the severe air pollution.The CO₂ concentration increases and acid rain areas expand so that the neutralization of concrete is increasingly concerned about.The neutralization of concrete and the steel corrosion are the main aspects in the durability of concrete structures,and the neutralization of concrete is the prerequisite for steel corrosion.Moreover,the durability of concrete is greatly influenced by the stress state as the actual structures are under external loads.Therefore,the neutralization of fly ash concrete under load subjected to the simultaneous attack of acid rain and carbonation should be investigated,which is significant for the durability design and assessment and for the further investigation of reinforcement corrosion and service life prediction of concrete structures.In this paper,the multi-factor durability program and how to carry out the tests were designed based on the combination of the mechanical(bending load) and environmental factors(CO₂ and acid rain).Then the effects of single,double and multiple influencing factors on the neutralization of fly ash concrete were investigated, and the action mechanism and the rule of performance degradation were revealed. Finally,a predicting model of neutralization depth of fly ash concrete in the atmospheric environment was proposed.The main contents of this paper are as follows:Firstly,the neutralization process of CO₂ and acid rain was analyzed through the theory of diffusion and mass transfer and the mechanism of carbonation and acid rain attack together was revealed.The method to simulating acid rain solution in laboratory was proposed and the cyclic soaking-drying to accelerate the acid rain attack was introduced on the basis of the survey and analysis on the components of acid rain around China.The advantages and disadvantages of the load devices for durability test were analyzed,and the devices suitable for the multi-factor durability tests were adopted.The above works would be helpful to carry out the neutralization tests and establish the predicting model of neutralization depth for fly ash concrete.Secondly,laboratory accelerated carbonation tests were carried out.The two dimensional diffusion of CO₂ on the carbonation depth of the corner concrete was investigated,and the carbonation rule of fly ash concrete was obtained considering the influencing factors(water-binder ratio,the amount of fly ash,casting surface).At the same time,the accelerated acid rain tests were carried out in the laboratory.Parameters such as surface condition,mass,cube compressive strength and neutralization depth of concrete were used to evaluate the acid rain resistance of fly ash concrete.Then,the degree of the different influencing factors on the evaluating index was analyzed.The acid rain attack not only made concrete surface desquamate,aggregates denudate,but also made concrete mass decrease,compressive strength reduce and neutralization depth increase.Thirdly,test modes of acid rain and carbonation working together were built up on the basis of the mechanism and the characteristics of acid rain and carbonation separately.Then,durability tests of acid rain and carbonation working together on fly ash concrete were carried out.It is the first time that acid rain and CO₂ in the atmospheric environment as two important corrosive media were considered.The coupling effects were further analyzed and the mechanism of acid rain and carbonation working together was obtained.Fourthly,accelerated carbonation test,acid rain attack test,and simultaneous attack test of acid rain and carbonation of concrete under load were carried out.The neutralization and corrosion mechanism of fly ash concrete under load was investigated in different corrosive media.The calculation formula of the bending stress coefficient of concrete neutralization was suggested.By virtue of the bending stress coefficient of concrete neutralization,the influence of stress levels on concrete neutralization was analyzed.Fifthly,a predicting model of carbonation depth of fly ash concrete was established on the basis of analysis of carbonation mechanism and main influencing factors of fly ash concrete.The calculation of parameter m₀,that is the amount of CO₂ absorbed by unit volume of concrete completely carbonated,was proposed by the analysis of concrete hydration process and carbonation mechanism,and the simplification formula of fly ash concrete applied in practice was suggested.The calculation of another important parameter D_e,that is the effective diffusion coefficient,was also proposed on the basis of the mechanism gas diffusion in concrete.Finally,a neutralization model for fly ash concrete subjected to acid rain attack was proposed for the first time on the basis of the theory of diffusion and mass transfer.The values of the model parameters were studied,and the calculation formula of effective diffusion coefficient DA was established considering both material and environmental aspects.A neutralization predicting model of fly ash concrete in the atmospheric environment emphasizing the role of carbonation was established for the first time on the basis of carbonation model.The effects of acid rain and bending loads were taken into account in this model.The model is reasonable as the results calculated by the model agree well with test results,and it can be used as a reference for the predicting of service life of fly ash concrete structures.