Concrete Carbonation Process Research Articles

A one dimensional free boundary problem for adsorption phenomena

In this paper we deal with a one-dimensional free boundary problem, which is a mathematical model for an adsorption phenomena appearing in concrete carbonation process. This model was proposed in line of previous studies of three dimensional concrete carbonation process. The main result in this paper is concerned with the existence and uniqueness of a time-local solution to the free boundary problem. This result will be obtained by means of the abstract theory of nonlinear evolution equations and Banach's fixed point theorem, and especially, the maximum principle applied to our problem will play a very important role to obtain the uniform estimate to approximate solutions.

A mathematical model of carbon dioxide transport in concrete carbonation process

In this paper we prove the existence of a solution for a mathematical model of carbon dioxide transport in concrete carbonation process. This model is a parabolic type equation with a nonlinear perturbation such that a coefficient of the time derivative contains a non-local term depending on the unknown function itself.

Effects of different mineral admixtures on carbonation resistance of lightweight aggregate concrete

Cement has been replaced respectively by equivalent grade II fly ash, pulverized fly ash, granulated blast furnace slag and steel slag, preparing the lightweight aggregate concrete containing mineral admixture. The carbonization characteristics of concrete were studied by the accelerated carbonation experiment and the test of concrete pore solution’s pH, afterwards the influence mechanism of different mineral admixtures on carbonation resistance of lightweight aggregate concrete was analyzed. The results indicate that the carbonation depth of the samples containing grade II fly ash and steel slag powder are higher than that of the reference sample, while their pore solution’s pH at different depths are also lower than those of the reference sample. Moreover, the later-stage carbonation resistance of the samples containing 20% of pulverized fly ash and granulated blast furnace slag are better than that of the reference sample, and the 28days carbonation depth of which are only 77.6% and 86.2% of the reference sample respectively, while their pore solution’s pH are very close. Therefore, it is concluded that the neutralized velocity of the lightweight aggregate concrete could be delayed to some extent because of the higher fineness of pulverized fly ash and granulated blast furnace slag. Moreover, when cement is replaced by large amount of admixture, the carbonation process of lightweight aggregate concrete has an accelerating tendency with the increase of admixture content.

Mesoscopic simulation method of concrete carbonation process

Concrete structures in the atmosphere may suffer from the problem of steel corrosion induced by the carbonation process. Most of previous research on concrete carbonation problems has been performed on the macro-scale. These studies did not consider the influence of the aggregates, which should be involved on meso-scale. In order to extend the present studies on the problems of concrete carbonation from macro-scale to meso-scale, an improved method to simulate the random aggregate structure (RAS) in concrete has been proposed. This method can be divided into two steps: the process of RAS in a three-dimensional model, and the process of numerical analysis in a cross-section. In this method, the effect of the randomness on the results of RAS is analysed. Based on the mass conservation law and kinetic model of chemical reactions, a simplified numerical model of concrete carbonation is developed. The effect of the key parameters in this model such as effective diffusion coefficient of CO2 and carbonation reaction constant is discussed. The comparison of the simulations of concrete carbonation process on macro-scale and meso-scale is discussed as well. Finally, an application of the method to determine the proper composition of concrete in the practical engineering is presented.

Mathematical model for hysteresis phenomenon in moisture transport of concrete carbonation process

From civil engineering point of view it is very important to construct and analyze a mathematical model for a mechanism of concrete carbonation process. On this subject there are several mathematical results concerned with a one-dimensional model, in which hysteresis effects are neglected. Our aim is to give a model with hysteresis effects appearing in carbonation process. In this paper, as the first step of this research we focus only on moisture transport in the process and propose an initial boundary value problem for a system of partial differential equations as a mathematical model. Also, we give results on the existence of a solution to the problem, globally in time and the uniqueness in only one-dimensional case without proofs.

Effect of concrete carbonation process on the passivating products of galvanized steel reinforcements

AbstractThe composition of passivating products on galvanized steel reinforcement in concrete during carbonation was studied. Cube‐shaped concrete specimens were manufactured with Portland 52.5 R cement and reinforced with hot‐dip galvanized steel sheets obtained from pure Zn and ZnSnBiNi alloy bath. The concrete specimens were exposed to air curing for 28 days and then to the carbonation chamber. Corrosion rate and potential measurements were performed both during the curing in air and exposure in a carbonation chamber. At defined periods of time, some concrete specimens were broken and the galvanized steel sheets were submitted to XRD, SEM observations and EDX analysis. The growth of the passivation products was evaluated by integrating the diffraction peaks. XRD analysis and SEM observations show that the layer of calcium hydroxyzincate (CaHZn), formed on the galvanized sheets during the air curing, is destroyed by the concrete carbonation process. However, potential measurements indicate that the galvanized steel always remains in the passive state. X‐ray diffractometry was not able to identify the new passivating product; EDX maps suggest the presence of zinc carbonates.

Comparison of concrete carbonation process under natural condition and high CO2 concentration environments

To compare the results obtained under both natural and accelerated environments, the pH values of carbonated concrete were measured, the variation of pH values was determined, and the variations of Ca(OH)2 and CaCO3 contents in the carbonated concrete under natural condition and high CO2 concentration accelerated climate environments were determined by microcosmic test methods such as DTA and X-ray diffraction. The experimental results showed that the overall variation trend of pH values and phase component of carbonation layer of concrete under accelerated climate environments with high CO2 concentrations were the same as those under natural conditions. Therefore, the carbonation processes of concrete were considered consistent under both conditions. However there was a difference in the length of semi-carbonation zones. The one measured under high CO2 concentration accelerated climate environments was shorter than that under natural condition. Experimental investigation showed that it was caused by the differences in climate condition (temperature and relative humidity) as well as the properties of the concrete. The concentration of CO2 and the duration of the carbonation process have no effect on the length of semi-carbonation zone. Thus, it is acceptable to simulate the natural condition by applying the high CO2 concentration artificial accelerated carbonation technique for the purpose of the study of carbonation process of concrete.

On the interplay between fast reaction and slow diffusion in the concrete carbonation process: a matched-asymptotics approach

A matched-asymptotics approach is proposed to show the occurrence of two distinct characteristic length scales in the carbonation process. The separation of these scales arises due to the strong competition between reaction and diffusion effects. We show that for sufficiently large times τ the width of the carbonated region is proportional to $\sqrt{\tau}$ , while the width of the reaction front is proportional to $\tau^{\frac{p-1}{2(p+1)}}$ for carbonation-reaction rates with a power law structure like k[CO2] p [Ca(OH)2] q , where k>0 and p,q>1 and identify the proportionality coefficient asymptotically. We emphasize the occurrence of a water barrier in the reaction zone which may hinder the penetration of CO2 by locally filling with water air parts of the pores. This non-linear effect may be one of the causes why a purely linear extrapolation of accelerated carbonation test results to natural carbonation settings is (even theoretically) not reasonable. Finally, we compare our asymptotic penetration law against measured penetration depths from Bune (Zum Karbonatisierungsbedingten Verlust der Dauerhaftigkeit von Außenbauteilen aus Stahlbeton, 1994). The novelty consists in the fact that the factor multiplying $\sqrt{\tau}$ is now identified asymptotically by solving a non-linear system of ordinary differential equations, and hence, fitting arguments are not necessary to estimate its size. We offer an alternative to the (asymptotic) $\sqrt{\tau}$ expression of the carbonation-front position obtained in Papadakis et al. (AIChE J. 35:1639, 1989).

Mathematical Modeling and Applications for Concrete Carbonation

A new concept, carbon dioxide (CO2) transport in concrete, is proposed in this paper to describe solute-transport processes. Using this concept, a new one-dimensional mathematical model was developed to describe the transport phenomena of carbon dioxide in concrete structures. By treating transport phenomena as a concrete carbonation process, a one-dimensional linear partial differential equation was derived based on the principle of mass balance and convective-dispersive equation and was found the analytical solution by the separation of variables and the Laplace transform methods combined with some substitution approaches. The concrete carbonation numerical results were determined using parameters, such as the diffusion coefficient, CO2 concentration, carbonation depth and time, occurring in concrete structures. The numerical results are presented to illustrate the practical applications of this model. These results are shown clearly that the proposed model may actually describe the concrete carbonation process chemically and physically. It is suggested that experiment should be performed to select suitable parameters for numerical simulations.

Mathematical Modeling and Prediction Method of Concrete Carbonation and its Applications

The carbonation process of concrete is principally a diffusion phenomenon. The penetration rate of carbon dioxide depends mainly on the concrete quality and the exposure condition. Based on both Fick’s first and second laws of linear diffusion equations, the threedimensional equation of conservation of mass is expressed. This equation can be reduced to two- and one- dimensional equations of conservation of mass which can predict the carbonation depth beneath corners and the general surface of concrete structures, respectively. The objectives of this investigation are to measure the depth of carbonation from the free surface of a concrete member and to predict the carbonation depth using a statistical method. The result of the present study indicates that the maximum carbonation depth of concrete at corners is larger than 2 times that of the general surface under the conditions of homogeneous, isotropic and uncracked concrete subjected to static load. The carbonation depths of ten existing reinforced concrete (RC) bridges in Taiwan were predicted by the statistical method. The predicted results can be offered as a reference basis of repair, reinforcement or demolition for the existing concrete structures.