Evaluating effect of chloride attack and concrete cover on the probability of corrosion

Abstract

Corrosion of reinforced concrete (RC) structures is one of the significant causes of deterioration of reinforced concrete (RC) structures. Chlorination is a major process governing the initiation and advancement of the injurious corrosion of steel bars. Now, several researches on the chlorination of concrete structures have been ongoing around the world. Present article reviews several recently performed chlorination studies, and from results of a field survey evaluates the effect of chloride content on the probability of corrosion and the influence of concrete compressive strength on the chloride content and penetration, also evaluates the effect of concrete cover over the chloride content of the RC structures at rebar depth and on the probability of corrosion.

Keywords

concrete / chloride / reinforcement / corrosion / deterioration / cover

Introduction

Performance degradation of RC structures due to corrosion initiated by chloride attack is a significant problem especially in marine regions. The chloride ions penetrates in the concrete and results in initiation of the corrosion. Corrosion of reinforcement leads to the formation of cracks on concrete cover, which, in the end results in decrease of residual life of RC structures. Wardhana and Hadlipriono [1] defined failure of RC structure as “incapacity of a constructed facility or its components to perform as specified in the design and construction requirements.”

Chloride ion can damage the protective passive film and lead to corrosion causing loss of area of steel, reduction in load carrying capacity of structure and loss of durability [2]. Durability of concrete structures exposed to chloride environment depends mainly on the resistance of concrete against chloride ingress (Costa and Appleton) [3].

The chloride content in concrete structures at depth with respect to age of concrete has been evaluated by most of the researchers by relationship based on Fick’s second law of diffusion shown in Eq. (1), and found that chloride ingress in the concrete structures depends on the chloride diffusion coefficient of concrete.{\displaystyle \frac{\partial C}{\partial t}}={\displaystyle \frac{\partial }{\partial x}}\left[D{\displaystyle \frac{\partial C}{\partial x}}\right],where C is the chloride content at depth x and at time t; D is the chloride diffusion coefficient.

For improving the tensile strength of cement concrete, reinforcing steels have been added to form a new material reinforced cement concrete (RCC). During hydration of cement in reinforced concrete structures a highly alkaline pore solution due to high pH (12.5-13.5) forms a passive oxide film on the steel to protect it from corrosion. This protective film is destroyed when sufficient amount of chloride ions penetrates into concrete, and after the removal of protective film reinforcing steel bars gets in contact with oxygen and water corrosion, thereafter, corrosion in the form of rust formation and loss in cross section occurs. Hence, it has been recognized that the presence of chloride ion is a significant factor causing the removal of passive oxide coating. Small quantity of chloride will normally present in concrete structures, but high ingress of chloride ions form deicers or seawater can destroy the protective film. Several researchers considered diffusion as the main transport mechanism of chloride ion into concrete.

Chloride ion penetration in the concrete structures is of the main concern, for the durability of concrete structures. So, studies related to chloride attack on concrete structures have been the subject of interest for the researchers in the last few decades. In the present article several recently performed chlorination studies have been reviewed, and from results of a field survey evaluates the effect of chloride content on the probability of and the influence of concrete compressive strength on the chloride content and penetration, also effect of concrete cover over the chloride content of the RC structures at rebar depth and on the probability of corrosion has been evaluated.

Determination of chloride content

Chloride content in concrete as the percentage of concrete weight can be measured on concrete powder drilled out of structures on site, this method is in conform with ASTM C114. This concrete sample is then mixed into a 10ml of extraction liquid and shaken for five minutes. The extraction liquid removes disturbing ions, such as sulfide ions, and extracts the chloride ions in the sample, and then sample is tested with electrode to evaluate chloride content as the percentage of weight of concrete. Table 1 presents various NDT methods for the evaluation of chloride content

Modeling chloride ingress in concrete structures

The significance of corrosion in service life of structures has led to extensive researches to the deterioration of RC structures. Deterioration of concrete structure is mainly governed by corrosion of steel, which is the result of chlorination governed by diffusion coefficient and surface chloride content (Eq. (2)). Most of the researchers considered chlorination for modeling life of RC structures by dividing it in different phases. This method has been first adopted by Tutti (1982), these models divide service life of RC structures in two periods. First period is initiation (t_i) which is the time required for chloride ion concentration to reach the threshold value at the rebar level and initiate corrosion, and second is propagation (t_p) which is the time between initiation and cracking, hence total service life of structures is t_i + t_p,. The two phase service life concept is shown in Fig. 1, it has been observed that with the increase in age of the structures the deterioration level of structures increases and when deterioration level reaches critical deterioration level it reflects end of the service life.

Following equations based on Fick’s second law of diffusion (Eq. (1)) have been used by many researchers to predict corrosion initiation period by considering the following boundary conditions

C(x>0, t = 0) = 0 initial chloride content is zero at any depth.

C(x = 0, t>0) = C_s surface chloride content.

C(x = ∞, t>0) = 0 chloride content at large depths is zero.

Where C_s is the chloride content on the surface of the concrete structures.

The solution of Eq. (1) is

(2)C(x,t)=C_{\mathrm{s}}[\mathrm{1}-\mathrm{e}\mathrm{r}\mathrm{f}({\displaystyle \frac{x}{\mathrm{2}\sqrt{Dt}}}\left)\right],

where erf() is the error function

(3)\mathrm{e}\mathrm{r}\mathrm{f}(u)={\displaystyle \frac{\mathrm{2}}{\sqrt{\mathrm{\pi }}}}{\displaystyle {\int }_{\mathrm{0}}^u{e}^{-t^{\mathrm{2}}}}\mathrm{d}\mathit{t}\mathit{.}

So, corrosion initiation period (t_i) which is the time required by chloride content at rebar depth to reach threshold value, is given by

(4)t_i={\displaystyle \frac{C^{\mathrm{2}}}{\mathrm{4}D}}{\left[\mathrm{e}\mathrm{r}{\mathrm{f}}^{-\mathrm{1}}\right(\mathrm{1}-{\displaystyle \frac{C_{\mathrm{t}\mathrm{h}}}{C_{\mathrm{s}}}}\left)\right]}^{-\mathrm{2}},

where C_th is the threshold chloride content; C = concrete cover. From Eq. (4) it has been observed that in RC structures with high concrete cover the corrosion initiation period is longer than that for structures with low concrete cover.

Review of few recent chloride studies

The deterioration of RC structures is mainly associated with the reinforcing steel corrosion. One of the main agents causing corrosion of rebars is the chloride ion, which diffuses into concrete and causes chlorination of concrete. When chloride content at rebar depth reaches threshold value, it initiates the corrosion of steel reinforcement. Fick’s laws of diffusion have been used many authors and researchers for modeling the chloride ingress in concrete structures. Several recent chlorination studies conducted have been reviewed in Table 2.

Effect of concrete cover and chloride content on corrosion of steel bars

Form the data obtained from a field survey conducted on several RC structures situated around the City of Bhopal India, effect of concrete cover over the chloride content at rebar depth has been evaluated. Almost hundred structures are identified and classified according to age of the structures. Cover-meter, Rebound hammer, Ag/AgCl half cell and rapid chloride tests have been performed to evaluate concrete cover, compressive strength, half cell potential and chloride content respectively to perform studies presented in sections 5.1, 5.2 and 5.3.

Corrosion probability is indicated by the half cell potential values, decrease in half cell values (increase in negative value) indicates higher probability of corrosion. Figure 2 presents the values of chloride content and concrete cover of the surveyed structures. As the values of chloride content are smaller in comparison with concrete cover, so for plotting those in the same graph values of chloride content are multiplied 100 times. Hence, threshold value of chloride content (0.2%) becomes 20. Fall in value of chloride content has been identified with the rise in concrete cover also it has been observed that for most of the structures with concrete cover more than 40mm chloride content is lower than threshold value.

Effect of chloride content on probability of corrosion

Chloride attack is a major cause of corrosion in RC structures. Significance of chloride attack in the corrosion process has evolved the threshold chloride content concept. Threshold value of chloride content is defined as the chloride content at rebar depth required to destroy the passive protective film and initiates the corrosion. Effect of chloride content at rebar depth on the half cell potential has been presented in Fig. 3, which indicates that increase in chloride content increases the probability of corrosion. It has been also observed from Fig. 3 that when value of chloride content at rebar depth is more than threshold value which is 0.2% of the weight of concrete, than the probability of corrosion is more than 90%.

Effect of compressive strength on chloride ingress of concrete

It has been observed by many authors that chloride ingress is slower in the high strength concrete. To evaluate the variation of chloride content with the change in compressive strength, values of chloride content and compressive strength obtained from field survey have been plotted and fitted in Fig. 4, and weak correlation is obtained because of scattered values. It has been observed from Fig. 4, that increase in compressive strength reduces the chloride content at rebar depth, and for the concrete structures with low compressive strength the chloride content at rebar depth is more than threshold value which indicates higher probability of corrosion.

Effect of concrete cover on chloride ingress and probability of corrosion

A graph (Fig. 5) is plotted between the evaluated values of chloride content and concrete cover, then using Microsoft excel curve fitting tool an exponential relation is obtained between the plotted data shown in Eq. (5).

(5)C=\mathrm{9.521}{\mathrm{e}}^{-\mathrm{0.009}{C}_{\mathrm{c}}},

where C = chloride content at rebar depth (% wt. of concrete), and C_c = concrete cover (mm). Negative sign indicates inverse relationship between concrete cover and chloride content. It has been observed from Fig. 5, that chloride content at rebar depth decreases with the increase in concrete cover of RC structures resulting in less probability of corrosion and further increase in the service life of the structure. And the value of minimum concrete cover required to maintain chloride content lower than threshold value has been obtained from above relation is almost 40mm. Also from Fig. 6 it has been concluded that with the in concrete cover half cell value increases (decrease in negative value), which indicates lesser probability of corrosion.

Data obtained from field survey has been classified considering chloride penetration period or age of structure and considering compressive strength of structure, classification of structures in different groups has been shown in Tables 3 and 4. Now, considering the following classifications effect of concrete cover on the chloride content has been analyzed through Fig. 7 to 13.

From Fig. 7 to 10 effect of concrete cover over on the chloride content has been evaluated, and it has been observed from Fig. 7 that for the structures of group A1 even with low concrete cover chloride content is lower than threshold value (0.2%). Figures 8 and 9 indicate that with the increase in chloride penetration period most of the structures with low concrete cover are having chloride content more than threshold value. Also, from Fig. 10 it has been observed that after the age of 50 years structures with high concrete cover are having chloride content more than threshold value. Therefore, it has been interpreted that both concrete cover and age of structures are affecting chloride content of the RC structures and probability of the corrosion.

Figures 11-13 represent the effect of concrete cover on the chloride content over the structures of different compressive strengths. Figure 11 indicates that in the structures with low compressive strength usually chloride ingress is more than threshold limit, and from Figs. 12 and 13 it has been observed that with the increase in compressive strength chloride content remains under the threshold limit for most of the structures.

Conclusions

In RC structures corrosion takes place when passive protective layer is destroyed by chloride ingress. Chloride penetration in concrete is affected by variation in temperature, wet-dry cycles and change in other exposure conditions. There are numerous experimental and mathematical studies performed related to chloride ingress in concrete structures.

Several recent chloride studies have been reviewed and it has been observed that most of the researchers used Fick’s laws of diffusion to model the chloride ingress. Chloride ingress in concrete structures mainly depends on the diffusion coefficient. Increase in diffusion coefficient increases the ingress of chloride ion [27] in concrete structures, and significantly reduces the service life of RC structures. Increasing concrete cover depth is the most appropriate technique for increasing chloride resistance and further improving the service life of structures [16].

Replacing cement by supplementary materials increases the chloride resistance of concrete structures, which results in improvement in estimated service life of structures [8,31]. And it has been found that use of pozzolanic materials is more effective than decreasing the W/C ratio for improving the chloride resistance of concrete structures.

Effect of chloride content on the probability of corrosion has been evaluated, and it has been observed from Fig. 3 that increase in chloride content increases the probability of corrosion. Effect of compressive strength on the chloride content has been investigated in Fig. 4, and it has been observed that they are having inverse relationship with each other. Concrete structures with low compressive strength have higher probability of corrosion.

It has been observed from Fig. 5 that concrete cover and chloride content at rebar depth are having inverse relationship. And a minimum concrete cover of 40mm is required to keep the chloride content below the threshold limit. And Fig. 6 indicates decrease in probability of corrosion with the increase in concrete cover. Also, Fig. 7 to 13 indicates the significance of age and compressive strength on the chloride content of the structure.

Several methods are available for measuring chloride content and chloride ion resistance of concrete structures. However, additional research is required to develop more reliable and rapid methods.

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