Long-term effects of electrochemical realkalization on carbonated concrete

Abstract

The long-term effects of electrochemical realkalization on carbonated reinforced concrete with a W/C ratio of 0.65 were studied. Fourteen out of 16 carbonated specimens had been subjected to realkalization seven years ago, and the alkalinity of the concrete, the electrochemical characters (corrosion current density and potential) of the specimens and the corrosion conditions of the steel bars were examined. Results of different specimens and also at different time (4, 10, 13 months and 7 years after realkalization) were compared. According to the phenolphthalein and pH meter test, the alkalinity of the concrete had disappeared after seven years. Based on the potentiodynamic polarization test, various corrosion conditions had developed on the steel bars, which was verified by visual observation. All bars were in the depassivated state, and their corrosion current densities increased significantly after seven years. Cracks developed in some of the specimens, and the diverse compactness of concrete and excessive current of realkalization were considered to be possible causes. The effects of the realkalization treatment vanished after seven years.

Keywords

realkalization / concrete / carbonation / polarization curve / corrosion

Introduction

The durability failure of concrete induced by the corrosion that results from carbonation has created problems worldwide. To prolong the service life of carbonated concrete and achieve the sustainable development of concrete, an electrochemical realkalization technique was developed with the aim of reestablishing the initial alkaline environment of concrete and reducing the corrosion rate of the reinforcements [¹–³]. Studies on the long-term effects of this technique are quite necessary, as they determine the application prospects and future research directions of realkalization. Several experimental studies [⁴–⁶] were carried out to examine this issue, and these studies mainly concentrated on two aspects: the trends in restoring an alkaline environment in the concrete and the corrosion conditions of the reinforcements over time.

The pH value near a reinforcement decreased to 10.3 from the initial value of 12.0. 45 days after treatment and then tended to remain stable as time passed, as reported by Andrade et al. [⁷]. The author proposed that only a small amount of carbon dioxide participated in the second carbonation process and that the presence of the CO₃^2–/HCO₃^– buffer pair was also beneficial for maintaining the alkalinity of the concrete. Other experiments have also revealed the same trend; for instance, the pH decreased from over 14 to 12–13 after several months in an experiment by Mietz [⁸], from 15 to 11.1 after 180 days in a study by Qu et al. [⁹], and from 11 to above 9 after 900 days in an experiment by Tong et al. [¹⁰]. Based on the available results, the pH value near a reinforcement was proven to decrease gradually and eventually stabilize at a certain value, which is induced by the reverse diffusion of alkaline ions rather than a second carbonation process. The speed of alkalinity decay is related to the realkalization parameters, such as the electric charge, the water-cement ratio of the concrete and the hydroxide contents at the end of realkalization. Although a relatively constant value without drastic fluctuations might be ultimately obtained in a few days, this value is probably still not sufficient to provide stable protection of the rebar.

In terms of the corrosion current density, Ribeiro et al. [¹¹] observed that the corrosion potential varied linearly with time, with values of -250 mV/CSE one month after treatment and -200~–250 mV/CSE six months later with a further decrease 12 months later. In the studies of Andrade et al. [⁷,¹²] and Tong et al. [¹⁰], the current density decreased to over 0.2\mathrm{\mu A}/{\mathrm{cm}}^{\mathrm{2}} 60 days after treatment and 1.0\mathrm{\mu A}/{\mathrm{cm}}^{\mathrm{2}} 360 days after treatment, respectively, which meant that the steel bars were not passivated but instead remained in an active state. Meanwhile, Tong et al. [¹⁰] reported that 720 days later, the corrosion rate increased and then returned to the level before treatment after another 900 days. Mietz [³] drew a similar conclusion in which even though the corrosion current density decreased to a negligible value when realkalization was completed, just three months later, it increased again to 43 \mathrm{\mu A}/{\mathrm{cm}}^{\mathrm{2}}. Realkalization clearly reduces the corrosion current density of steel bars, but this property increases again just several months or years later whether the steel bars are repassivated after treatment or not. In addition, the corrosion severity before realkalization is a major factor that influences the durability of realkalization. According to the experiment of Redaelli and Bertolini [¹³], the corrosion current density of the non pre-corroded specimens exhibited a decreasing trend over the 400 days following realkalization, while that of the bars that had already corroded before treatment began to increase only 200 days after realkalization.

This paper presents the results of pH tests of the concrete and corrosion condition tests of the steel bars in carbonated reinforced concrete specimens subjected to realkalization seven years ago with the aim of investigating the long-term effects of this technique.

Experimental program

Specimen preparation

Several concrete cylinder specimens with plain steel bars (diameter of 10 mm, height of 280 mm) placed in the center along the axis were fabricated, as shown in Fig. 1. The diameter of the specimens was 70 mm, and the height was 250 mm. Wires were welded to one side of the steel bar for the following treatment. Both ends of the specimen were isolated with epoxy resin. In view of that realkalization has a better effect on concrete materials with good permeability, a W/C ratio of 0.65 was chosen for the concrete mix design. The concrete mix consisted of ordinary Portland cement 32.5, river sand and coarse aggregate (gravel) with maximum size of 15 mm, in which cement, water, sand and coarse aggregate were mixed in a proportion of 1:0.65:2.25:3.68 by weight [¹⁴], as shown in Table 1. According to GB/T50081 [¹⁵], the 28-day compressive strength was determined to be 25.6 MPa based on the averages of three cube specimens.

The specimens were cured at 20°C±3°C and a relative humidity above 90% for 28 days. Then, the specimens were placed in a drying oven at a temperature of 60°C for 48 h. After drying, the specimens were placed in a carbonation chamber with a temperature of 20°C±3°C, humidity of 70%±5% and carbon dioxide concentration of 20%±3% until carbonation was complete, as monitored through phenolphthalein test.

Realkalization treatment

After carbonation was complete, the specimens were immersed in deionized water until weight measurement proved that there was no obvious change in the weight of the concrete, which indicated that there was no extra concrete pore space for deionized water and the specimens had achieved saturated surface dry condition. Then, the realkalization treatment was started. An electrical field was generated between the steel bar (the cathode) and the external steel mesh (the anode) using a DC power supply, and a 1 M sodium carbonate solution was adopted as the electrolyte [¹³]. After treatment, all the specimens were stored in the laboratory (natural indoor environment) at a temperature of 0-30°C and a humidity of 30%–80% for seven years.

The treatment parameters of the 14 realkalized specimens and 2 carbonated specimens can be obtained from their names, as shown in Table 2. In the specimen names, the first one or two letters (CR or C) before the digit indicate the treatment the specimens were subjected to, where CR represents carbonation and realkalization and C represents carbonation. The first two numbers in the CR specimen names are the current (A/m²) and duration time (days), respectively, used in the realkalization treatment, and the last number is the specimen number if multiple specimens were tested with the same parameters. The letter X in the names of the specimens (CRX-1 and CRX-2) indicates that the detailed realkalization parameters of these specimens are unavailable.

Measurement procedures

The measurements were conducted according to the procedures below:

1) Cracking condition observation

During the seven-year period, cracks developed in some specimens. Thus, the cracking conditions of 14 realkalized specimens were first recorded.

2) Electrochemical measurements

Twelve realkalized specimens out of 14 are applied electrochemical tests and the electrochemical characters of the other two specimens (CRX-1, CRX-2) were not tested due to their extensive cracking. The test specimens were immersed in a 0.5 M sodium carbonate solution for 3 days to increase the conductivity before the electrochemical measurements. The corrosion conditions of the rebars were studied by means of potentiodynamic polarization test, which was measured with a CHI660B electrochemical workstation. The saturated calomel electrode was used as reference electrode, and a platinum electrode was used as the auxiliary electrode. The scan rate was 2 mV/s, and the scan range was±0.25 V vs the open-circuit potential.

3) Alkalinity measurements

After the electrochemical measurements, four specimens CR3&14-2, CR3&14-1, CRX-1, and CRX-2 were cut, and a 1% phenolphthalein solution was sprayed onto the freshly cut surfaces as a pH indicator. The regions with a pH value of 8 or above appeared red, while the color of the regions with a pH value less than 8 remained unchanged, and thus, the alkaline regions of the concrete could be determined. To obtain a more accurate result, pH tests were carried out via a pH meter. Concrete powders (depth of 0–5 mm from the steel bar) were drilled and sieved through a 0.08 mm square hole sieve and dried to constant weight at a temperature of 105°C. A total of 4 g of the powder and 16 g of deionized water were mixed and oscillated at a frequency of 120 times/min for 24 h. Then, the pH of the supernatant was measured by the pH meter (Leici PHB-4) after filtration.

4) Corrosion condition observation of the steel bars

Finally, 6 realkalized and 2 carbonated steel bars were removed from the concrete, and the corrosion conditions were observed directly.

Results and discussions

Cracking of the specimens

New cracks that were not present in the previous tests developed in some specimens seven years after realkalization. Generally, for most realkalized specimens (9 out of 14), no cracks developed on the surface. However, longitudinal cracks developed in specimens CR3&14-1, CR10&14-2, CR10&28-2, while extensive cracks developed in specimens CRX-1 and CRX-2 and the concrete was easily crushed when tapped with a hammer, as shown in Table 3.

The cracking of the specimens after the realkalization treatment had not been reported before and was unexpected. First, no rust was observed inside the cracks or on the surfaces of the specimens, and the amount of corrosion products was not sufficient to cause cracking. In addition, all specimens were stored in the same environment, and the corrosion degree of the rebars with same parameters was assumed to be the same. However, only a small part of the samples cracked. Therefore, the possible causes of cracking are the occurrence of an alkali-aggregate reaction (AAR), the diverse compactness of the concrete and the excessive current of realkalization.

Some researchers doubt that the realkalization treatment could cause an AAR to occur, as sodium carbonate was used as the electrolyte [¹⁶], but no cases were reported until now. In this experiment, no alkali-silicate gels were found in the cracks, and the specimens were kept in the laboratory during the entire seven-year period and were not exposed to the natural environment, which decreased the potential for the reaction to occur [¹⁷]. The specimens were stored in the room with a humidity that was always less than 80%, and the amount of moisture was not sufficient to initiate the AAR. Additionally, the restored alkalinity in the concrete induced by realkalization was not as high as the initial pH developed during curing. No AAR and crack formation occurred during curing, carbonation and realkalization, which proved that no reactive aggregates were present, even in a more alkaline and moist environment. Initiation of the AAR became less likely as the alkalinity and moisture decreased over time [¹⁸].

Another possible cause of the crack formation is the diverse compactness of the concrete. The long and thin shape of the specimens and the presence of a central bar increased the difficulty of the casting process, and parts of the specimens were not sufficiently vibrated during casting, which led to the poor compactness of the concrete. The compactness of concrete is closely connected to the concrete strength, and the compressive strength of cubic specimens of the concrete was only 25.6 MPa (described in Section 2.1). Dry shrinkage would occur when the moisture of the concrete decreased, which might produce a tensile force. For the specimens with poor compactness, this tensile force might exceed their tensile strength and cause cracking. In addition to this process, during the process of realkalization, chemical reactions and ion and water exchange have an influence on the microstructure and may increase the quantity of fine pores in the vicinity of the steel bar [¹⁹], which may increase the possibility of cracking.

The number of alkaline ions and the efficiency in reducing corrosion strongly depend on the charge of the realkalization treatment. To achieve a better realkalization effect, higher charges (1008–6720 Ah/m²) and electric currents (3, 5, and 10 A/m²) were adopted in this experiment, and no side effects (excessive charge and current of realkalization) were reported at the beginning of the experiment. However, CEN/TS 14038-1 [²⁰] and NACE SP0107 [²¹] suggested that the electric charge during realkalization should exceed 200 Ah/m² but did not specify a maximum amount of charge. COST [²²] recommended a charge between 200 and 450 Ah/m². Furthermore, the current used was also larger than that recommended. A current between 0.8 and 2 A/m² and less than 4 A/m² was recommended by Mietz [³] and CEN/TS 14038-1 [²⁰], respectively. Yeih and Chang [²] reported that the compressive strength and modulus of elasticity of realkalized concrete decreased with the increase of electric charge, which made the concrete with excessive charge easier to crack.

During realkalization, cathodic electrolysis is conducted in two ways, as described in Reactions 1 and 2.

(1)\mathrm{1}/{{\displaystyle \mathrm{2}\mathrm{O}}}_{\mathrm{2}}+{{\displaystyle \mathrm{H}}}_{\mathrm{2}}\mathrm{O}+{\mathrm{2}\mathrm{e}}^{-}\to {\mathrm{2}\mathrm{OH}}^{-},

(2)\mathrm{2}{{\displaystyle \mathrm{H}}}_{\mathrm{2}}\mathrm{O}+{\mathrm{2}\mathrm{e}}^{-}\to {{\displaystyle \mathrm{H}}}_{\mathrm{2}}+{\mathrm{2}\mathrm{OH}}^{-}.

Reaction 1 dominates when sufficient oxygen is present; otherwise, Reaction 2 dominates. All the specimens were immersed in deionized water until they reached the saturated surface dry condition, after which they were immersed in a sodium carbonate solution during realkalization, which guaranteed a sufficient supply of water but a shortage of oxygen. Thus, Reaction 2 occurred. The cathode gained a large number of electrons, as the current was excessively high in this study. A large amount of hydrogen was produced and may not have diffused immediately. The pressure caused by the accumulation of hydrogen may have caused the concrete to crack. A current of 10 A/m² was applied to two of the cracked specimens in this study. However, no cracking had occurred immediately after realkalization was completed. Since concrete is a porous material, its content of hydrogen will decrease over time, which means that the formation of cracks in the concrete becomes increasingly more difficult. Cracks induced by hydrogen may have developed in the vicinity of the rebars first and then propagated to the surface very slowly over seven years. Further researches on the reasons for cracking are planned to be performed because it may be potentially a limiting factor for the use of the technique.

Corrosion current densities and potentials of the steel bars determined by electrochemical measurements

The corrosion current densities and potentials of 12 steel bars measured 7 years after realkalization were calculated with the software CView, as shown in Fig. 2 and the polarization curves are presented in Fig. 3.

The corrosion potentials of the various bars ranged between -560.4 and -268.5 mV with corrosion current densities ranging between 2.17 and 48.36\mathrm{\mu A}/{\mathrm{cm}}^{\mathrm{2}} 7 years after realkalization. According to ASTM C876 [²³] and Chinese standard GB/T50344 [²⁴], the steel bar is depassivated when the corrosion current density is greater than 0.2\mathrm{\mu A}/{\mathrm{cm}}^{\mathrm{2}}.

The results of the corrosion current density and potential measurements show that the maximum current is much higher than the minimum. The corrosion current density is dispersive even between specimens treated under the same realkalization conditions, such as CR3&14-1–CR3&14-3 and CR10&14-1–CR10&14-3. The occurrence of cracking is proposed to be one of the causes of the large variations, as shown in Table 3. Several cracks developed in CR3&14-1 and CR10&14-2 over 7 years, and the current densities of these specimens were dramatically higher than those of the other specimens. The diverse distribution and width of the cracks in the different specimens can cause variations in the diffusion coefficients of oxygen and vapor and thus different corrosion rates [²⁵,²⁶]. Even in complete specimens, the conditions of crack propagation vary, although this variation cannot be observed directly. Furthermore, the presence of different pore structures and propagated cracks cause differences in the water content in the concrete following immersion in the sodium carbonate solution before measurement, which may influence the results of the electrochemical measurements [²⁷,²⁸].

In addition, excluding the cracked specimens, the corrosion current densities of CR3&14, CR3&28 and CR5&14 were similar and of the same order of magnitude, while those of CR10&14 were much higher, and those of CR10&28 were the highest. Thus, excessive current and charge can be inferred to have the opposite effect and increase the corrosion.

For comparison, the polarization curves measured previously at 0, 4, 10, 13 months were present in Fig. 3. The polarization curve test was destructive, and each specimen could be tested only once. Therefore, the polarization curves measured at 0, 4, 10, 13 months and 7 years after realkalization were obtained from different specimens subjected to the same realkalization parameters, and the tests were conducted with the same parameters and data processing method.

According to the polarization curves, there was little difference between the Tafel slope of anodic curves and cathodic curves before and 0 months after realkalization while the Tafel slope of anodic curves of 4 months increased and the Tafel slope of cathodic curves decreased slightly. The polarization curves of 4, 10, 13 months showed similar trend and the increased Tafel slope of anodic curves indicated that the anodic reaction was restrained. Reactions in the process of the corrosion of the steel bar are present below.

(3)\mathrm{Anodic}\hspace{0.17em}\mathrm{reaction}:\mathrm{Fe}\to {\mathrm{Fe}}^{\mathrm{2}+}+{\mathrm{2}\mathrm{e}}^{-}

After realkalization, the pH of concrete around the steel bar increased, which caused the formation of the passivation film on the surface of the steel bar and decreased the rate of anodic reaction. As for the results of 7 years, the corrosion current densities of CR3&14-2,3, CR3&28-1,2, CR5&14-1,2, and CR 10&14-2 were relatively lower and the Tafel slope of their anodic curves decreased, which indicated the steel bars surface became active and the corrosion was enhanced. The effects of realkalization were weakened. However, the anodic curves of CR3&14-1, CR10&14-1,3 and CR10&28-1,2 with higher corrosion rates still showed high slope, which may result from the high corrosion degree of the steel bars. The rebars had depassivated and been corroded for a period of time and most of the surface was covered with corrosion products (iron oxides). The existence of rusts reduced the dissolution rate of iron and restrained the anodic reaction.

The changes in the corrosion current density and potential over seven years are shown in Fig. 4.

Based on Figs. 3 and 4, both the corrosion current density and corrosion potential of all the specimens increased after realkalization due to polarization and then gradually decreased over time. Little difference was observed between the data from 4, 10, and 13 months, and the values became stable 13 months after realkalization. However, after 7 years, the variations in the data from the different treatment conditions are larger than those in the data measured at earlier time points. Based on just these data, no direct relationship existed between the corrosion current density, corrosion potential and realkalization parameters.

Compared with the results after 13 months, considerable increases in the corrosion current density and potential of varying degrees were observed after 7 years. The corrosion current density of the carbonated specimen before realkalization was 1.86 \mathrm{\mu A}/{\mathrm{cm}}^{\mathrm{2}}. Seven years later, the corrosion current densities of all the specimens increased and became even higher than those of the samples before treatment, which was probably due to the excessive current and occurrence of cracking.

An excessive electric current may influence the effects of realkalization. A certain amount of hydrogen may be produced during realkalization under oxygen-deficient conditions, as described before. Wang et al. [¹⁴] found that the corrosion potential was maintained at a high level in the presence of hydrogen, which was not beneficial for the passivation of the bars.

The longer period in the active state caused by the higher electric current may have accelerated the corrosion of the steel bars. The potential takes even longer to decrease, and therefore, the potentials of the specimens after realkalization were higher than those not subjected to realkalization and those treated by realkalization at a lower electric current.

In addition, the formation of cracks in the concrete will accelerate the diffusion of oxygen, vapor and other negative substance into the concrete, which is unfavorable for the protection of the steel bar. Thus, corrosion will be accelerated to some extent.

Alkalinity measurements of the concrete

Since some of the specimens were cracked, we were afraid that the complete specimens would also crack during cutting or drilling, which would prevent us from conducting the electrochemical measurements. We preferred to examine the corrosion rates and test the alkalinity after realkalization because under normal circumstances, 3 days of immersion will not influence the pH around the steel bar. Specimens CR3&14-2, CR3&14-1, CRX-1, and CRX-2 were chosen as representatives for the alkalinity measurements due to their different surface conditions. The results of phenolphthalein test and pH meter test are presented in Fig. 5 and red regions can be visually observed in the images.

For specimen CR3&14-2 with an unbroken surface and CR3&14-1 with longitudinal cracks, the external concrete but not the concrete around the rebar appeared red, as shown in Figs. 5(a) and 5(b). The pH of concrete powders around the rebar were 7.94 and 7.86 which were not high enough for the passivation of the steel bars. The specimens were immersed in a sodium carbonate solution before the electrochemical measurements, and the carbonate ions penetrated the concrete by diffusion and capillary forces, leading to an increase in the pH of the external concrete. Therefore, the red color did not result from the realkalization treatment performed seven years ago. The same phenomenon was observed for specimen CRX-1 and CRX-2, as shown in Figs. 5(c) and 5(d). These two specimens were not immersed in the alkaline solution, and the electrochemical measurements were not performed due to the extensive cracking of the samples. The color of the cut surface did not appear red and the pH were 7.78 and 7.42, which also indicated that the alkalinity produced in the realkalization treatment had disappeared completely over seven years. Since the complete and broken samples exhibited similar results, the results for the other samples can be inferred to be the same.

For comparison, the pH values measured at 4, 10, and 13 months after realkalization are presented in Fig. 6.

Whether the concrete still has an alkaline environment depends on the reverse diffusion rate of the alkaline ions, which is related to the compactness of the concrete. The lower the compactness of the concrete is, the higher the reverse diffusion rate is. Realkalization is most effective for concrete materials with good permeability, so a water-cement ratio of 0.65 was utilized for the specimens in this experiment to guarantee a high porosity. According to this design, the compactness of the concrete was low at the beginning. In addition, crack propagation may aggravate diffusion, even for those specimens without obvious cracks on the surface.

Corrosion conditions of the steel bars determined by visual observation

After all the tests, some of the steel bars were removed from the concrete specimens, and their surface corrosion conditions were observed, as shown in Fig. 7. The above six specimens are realkalized specimens CR3&14-2, CR10&14-3, CR3&14-1, CR10&28-2, CRX-1, and CRX-2 and the other two are carbonated specimens C-1 and C-2.

Most of the rebar surface in specimens CR3&14-2 and CR10&14-3 maintained a metallic luster, and a small region was covered with yellowish-brown corrosion products. In contrast, for the rebars in specimens CR3&14-1 and CR10&28-2, most surface of the bar lost its metallic luster and was covered by sepia rust. As for CRX-1 and CRX-2, almost the whole surfaces were covered by corrosion products. The corrosion current densities of specimens CR3&14-2, CR10&14-3, CR3&14-1 and CR10&28-2 were 3.47, 15.34, 35.37 and 43.15 \mathrm{\mu A}/{\mathrm{cm}}^{\mathrm{2}}, respectively. Thus, the electrochemical results were consistent with the actual corrosion conditions.

The degree of crack propagation in specimens CR3&14-2 (CR10&14-3), CR3&14-1(CR10&28-2), and CRX-1(CRX-2) increased in the given order, and the observed corrosion also became more extensive. This result is consistent with the finding by Montes [²⁹] that cracking plays an important role in the corrosion process. Specimen C-1 and C-2 are carbonated specimens that were not subjected to realkalization. No obvious cracks were observed on their surfaces, and the bars retained a metallic luster. In other words, when cracks were not sufficiently developed, minimal amounts of rust were observed in the specimens seven years later, even in the carbonated concrete.

Conclusions

1) Realkalization had vanished completely seven years after the treatment for a concrete with a W/C ratio of 0.65.

2) Multiple cracks developed in some of the specimens, and the crack propagation differed between specimens. The possible causes of cracking may be the diverse compactness of the concrete and the excessive electric current of the realkalization treatment.

3) The differences between the corrosion conditions of the different steel bars seven years after treatment were significant, as indicated by the potentiodynamic polarization measurement. The differences probably resulted from the diversity in the extent of cracking. The corrosion potential of the steel bars was between -560.4 and -268.5 mV, while the corrosion current density was in the range of 2.17to 48.36\mathrm{\mu A}/{\mathrm{cm}}^{\mathrm{2}}. Based on both the potential and the current, the steel bars were concluded to have returned to the depassivated state, and the corrosion current was even higher than that of the carbonated concrete before realkalization.

4) Based on the above, the effects of the realkalization treatment were concluded to have disappeared after seven years, and the long-term effects of this technique were unsatisfactory. The protection provided by the realkalization treatment was temporary, and a simple sodium carbonate solution or alkaline solution was not sufficiently effective to spontaneously form a stable passivation film on the rebars. However, the data in this study are limited and discrete. More data and investigations of the durability of the realkalization technique are required. Because the protection provided by the realkalization treatment for high W/C ratio concretes is not durable, further research to improve the existing realkalization technique (such as to decrease the permeability of concrete cover immediately after realkalization) should be performed.

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