Electrochemical realkalisation for rehabilitation of reinforced concrete structures

In steel reinforced and prestressed concrete structures depassivation of the reinforcing steel can take place due to carbonation of the concrete cover. Depending on humidity and oxygen availability subsequent corrosion reactions will be initiated. Such conditions require measures to exclude corrosion induced damages during the designed lifetime of the structure. In the last few years an electrochemical realkalisation treatment has been proposed as adequate rehabilitation technique for carbonated concrete. This temporary treatment should increase the pH-value of the concrete pore water solution due to penetration of alkaline electrolyte from the concrete surface as well as repassivate the reinforcement due to electro chemical reactions at the steel surface. In order to clarify the different mechanisms taking place during electrochemical realkalisation laboratory tests have been carried out using carbonated reinforced mortar specimens. The investigations were aimed at checking the influence of various parameters, e.g. treatment time and current density, as well as the efficiency and long-term durability of this rehabilitation method.     

Electrochemical rehabilitation methods for reinforced concrete structures: advantages and pitfalls

Abstract

There are still many unresolved issues concerning the efficiency of electrochemical realkalisation and electrochemical chloride removal as electrochemical rehabilitation methods for corroding reinforced concrete structures. The present paper seeks to answer a number of questions which, though seemingly elementary, continue to arouse controversy in scientific, technical and economic communities, despite the vast amount of work that has been devoted to research on corrosion in concrete embedded steel, such as whether corrosion can be stopped once it has started, whether corroded reinforced concrete structures can be repassivated, and whether it is sufficient to remove the sources of corrosion in order to stop rusting. A discussion is conducted on the relationship between the prerusting grade of rebars and the possibility of their repassivation; on whether electrochemical rehabilitation methods treatments are efficient, and if so, when and on whether a simple potential measurement can determine the passive or active state of a rebar. For this purpose an analysis is made, using electrochemical, gravimetric and metallographic techniques, of the response of clean and previously corroded steel electrodes in a Ca(OH)₂ saturated solution with and without a sodium nitrite corrosion inhibitor and in cement mortar. The effectiveness of electrochemical realkalisation and electrochemical chloride removal for repassivating prerusted steel in concrete is found to depend heavily on the prerusting grade.     

Effect of the reinforcement bar arrangement on the efficiency of electrochemical chloride removal technique applied to reinforced concrete structures

This paper reports on the research done to find out the effect that different bar arrangements may have on the efficiency of the electrochemical chloride removal (ECR) technique when applied to a reinforced concrete structural member. Five different types of bar arrangements were considered, corresponding to typical structural members such as columns (with single and double bar reinforcing), slabs, beams and footings. ECR was applied in several steps. We observe that the extraction efficiency depends on the reinforcing bar arrangement. A uniform layer set-up favours chloride extraction. Electrochemical techniques were also used to estimate the reinforcing bar corrosion states, as well as measure the corrosion potential, and instant corrosion rate based on the polarization resistance technique. After ECR treatment, a reduction in the corrosion levels is observed falling short of the depassivation threshold.     

The electrochemical removal of chlorides from reinforced concrete

Results of a 2-year field study performed in Switzerland to evaluate the process and to obtain information on the efficiency of electrochemical chloride removal as a restoration technique for chloride contaminated concrete are presented. It is shown that about 50% of the initial chloride content is removed within 8 weeks (ca 5 × 10⁶ C m⁻²). In the treated zones of the structure the half-cell potentials became more positive by about 80–100 mV. A comparison with the theoretical efficiency for chloride removal, calculated from the transference number of chloride ions, show that mainly the free chlorides in the pore solution of the concrete are removed. Due to the slow chemical equilibrium between bound and free chlorides, a second treatment after several months also removes originally bound chlorides. For severe and inhomogeneous chloride contamination local zones with active rebars and high chloride content may remain after the treatment.     

Electrochemical noise from oxygen reduction on stainless steel surfaces

Oxygen reduction occurring at the passive layer is probably the most important cathodic reaction involved in corrosion processes on stainless steel. Furthermore, the influence of the surface state on the oxygen reduction reaction is a key point for the understanding of the mechanism of localized corrosion on stainless steel. In this study, electrochemical noise measurements under cathodic polarization were carried out to obtain new information about this influence. It has been confirmed that the surface state of stainless steel plays a very important role in the kinetic of this cathodic reaction. Oxygen reduction kinetics was significantly reduced on passivated surfaces and improved on pre-reduced and ground surfaces. In addition, electrochemical current noise measurements allowed to differentiate between the electrochemical activity produced by the oxygen reduction reaction and that due to the reduction of the passive layer, in direct dependence on the characteristics of the different surface states investigated.     

Cathodic protection of new and old reinforced concrete structures

Current and potential distributions measured on concrete slabs and simulated by computer modelling are discussed in relation with the application of cathodic protection to new reinforced structures as a corrosion preventive method. The results show high penetration of cathodic protection over the depth of uncontaminated concrete, so that even a rebar relatively distant from the anode can be polarized, whereas the penetration is limited to the rebars near to the anode in chloride contaminated slabs with corroding rebars. The great throwing power and the wide range of protection potentials for uncorroding concrete structure enable safe application of the cathodic protection even to new prestressed structures.     

Finite element modelling of chloride removal from concrete by an electrochemical method

This paper presents a finite element model for simulating the transport behaviour of ions in a porous medium when a constant DC current density is applied through electrode pairs emplaced in the medium. The model is then applied to predict the mass transport associated with electrochemical chloride extraction from hydrated cement paste. Numerical results are compared with those obtained in experiments of various duration. Effects of externally applied current density, treatment period, diffusion coefficients, ionic binding, boundary conditions and medium porosity on the efficiency of chloride extraction are discussed.     

Application of radar technology to reinforced concrete structures: a case study

The radar technology, used to perform investigations on the civil buildings, derives from that used for investigations of the ground known with the name of Georadar. This is diffusing rapidly among the investigation methodology not destructive in the field of the structural engineering. It is based on the sending of electromagnetic waves of very short length and the recording of the time of arrival and of the breadth of any signals reflected on the interface between materials with a different dielectric constant. The aim of this paper is to present the operating methodologies and the results achieved by the application in the field of radar methodologies to map utilities, and for applications to civil building with special regard to the determination of the intern morphology, to the lack of homogeneity research and defectiveness and to the determination of the location of the steel reinforcements.Specifically, the system used, made up of one apparatus of field acquisition and another of delayed processing, seems to be able to provide good planimetric and three-dimensional restitution with regard to location and placement. In this paper, special attention has been paid to the processing of the acquired data and on the interpretation of experimental tests conducted on a civil building.     

Electrochemical extraction of chlorides from reinforced concrete using a conductive cement paste as the anode

The results of this research show the viability of using a conductive cement paste anode for the electrochemical extraction of chlorides from reinforced concrete with an efficiency similar to that obtained with a classic Ti-RuO₂ anode. The obtained efficiencies are within the typical range of values of overall efficiencies found for such treatments.The thickness of the conductive cement paste anode has a great influence on the capacity of the anode to retain an important part of the extracted chlorides after finishing the electrochemical treatments.     

Electrochemical studies of steel in cement mortar containing chloride and micro-silica

Steel electrodes have been exposed for 3.5 years in cement mortar with and without chloride and ELKEM micro-silica. Corrosion potentials in mortar without Cl⁻ varied between −0.1 and +0.06 V(SCE), indicating passivity. One percentage of cement weight of Cl⁻ reduced the corrosion potentials to about −0.2 V(SCE). Micro-silica in chloride containing mortar increased the severity of corrosion the first year. However, after 2 years exposure reduction of interconnected porosity and increased resistivity caused by 15% micro-silica, raised the corrosion potentials and counteracted chloride-attack.The polarisation resistance was undefined, the apparent values varying with the technique and parameters used.     

Application of EIS to cathodically protected steel: Tests in sodium chloride solution and in chloride contaminated concrete

The efficiency of cathodic protection is usually checked by switching off the current and either by measuring the polarized potential or by recording the evolution of the potential decay. These methods although based on a long experience are not indicating directly on the state of the metal, and the interpretation of the results is not always evident. It remains as an interesting goal for the research on alternative methods to verify the efficiency of the cathodic protection. In this paper, a study made by using linear polarization and EIS is presented based on the behaviour of the two cathodically protected systems. The results indicate that the impedance diagrams show important and consistent changes in shape and associated parameters when the steel is protected or unprotected. Although changes have been noticed before by other authors in the EIS diagrams, this study assumes that the system is a galvanic couple.     

Several questions about electrochemical rehabilitation methods for reinforced concrete structures

Electrochemical realkalisation (ERA) and electrochemical chloride removal (ECR) have shown their capacity to eliminate, in just a few weeks, the causes of corrosion in reinforced concrete structures (RCS), and for this reason are currently receiving very special attention. Nevertheless, the present state of the art has not progressed enough to dissipate doubts about the efficiency of ERA and ECR as electrochemical rehabilitation methods (ERM) for corroding RCS. A series of highly important questions persist, and continue to be cause of controversy among specialists in the construction sector, which can be summed up in one all-encompassing question, namely:

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Is removing the sources of corrosion in RCS sufficient to stop rusting?To obtain a response to this capital question, electrochemical corrosion analysis techniques are used to study the responses of clean and precorroded steel electrodes embedded in small mortar specimens without chloride additions and large mortar slabs with and without chloride contamination. According to the results obtained, ERM are efficient in delaying the start of corrosion if used preventively. However, if applied too late they do not assure the repassivation of rebars with high precorrosion levels and consequently are not useful.

     

Experimental and probabilistic analysis of time to corrosion-induced cover cracking for marine reinforced concrete structures

Corrosion-induced concrete cover cracking is an important indication of durability limit state for marine reinforced concrete structures. In this paper, two analytical models predicting the time from corrosion initiation to cover cracking and their main differences were introduced. Based on an accelerated corrosion test, two models’ applicability and variability were compared and discussed with experimental data. Considering the random nature of influencing factors, a probabilistic model was developed by using Monte Carlo simulation technique. The results showed that the cracking time could be modelled by the Weibull distribution. Finally, the probability analytical technology was applied to a marine reinforced concrete pier with four different durability design levels. It is found that both the mean and 90% confidence interval of the cracking time increase when the durability design specifications change from low level to extreme level, which indicates that the accurate prediction of cracking time with a deterministic model will become more difficult accordingly.     

Damage analysis and cracking model of reinforced concrete structures with rebar corrosion

The damage of concrete cover in reinforced concrete structures induced by reinforcing steel corrosion is investigated in this study. The damage process of the concrete cover can be divided into two distinct stages: the non-cracking stage and the partial cracking stage. An analytical model based on damage mechanics and elastic mechanics is developed to predict the concrete cracking due to steel corrosion. Based on this model, the expansive pressure and the radial loss of steel bar are discussed. Parametric studies are carried out to examine the effects of the correlative factors on the expansive pressure and the steel loss.     

Electrochemical and SEM study on Type 254 SMO stainless steel in chloride solutions

An investigation was conducted to examine the critical crevice potential (E_crev) and the critical protection potential (E_prot) for Type 254 SMO stainless steel in 4% NaCl solution by using potentiodynamic cyclic anodic polarization (PCAP) technique at temperature ranging from 30 to 90 °C. The critical crevice temperature (CCT) and the critical crevice protection temperature (T_prot) were determined by plotting the values of breakdown potential and E_prot versus solution temperature, respectively. The values of CCT and T_prot were recorded at the abrupt transition with increasing the temperature from transpassive corrosion to crevice corrosion and were found to be at 55 and 52 °C, respectively. Above CCT (70 °C) the following points were recorded. The E_crev and E_prot decreased linearly with log [Cl⁻]. The addition of bromide ions to chloride ions at a fixed halide content of 4% increased both E_crev and E_prot. The E_crev value in 4% NaCl increased linearly with increasing pH in the range 1-10. The addition of 0.5 M bicarbonate ions to 4% NaCl completely removed the crevices effect while increasing the addition of sulphate ions to 4% NaCl increased both of E_crev and E_prot. The morphology of the crevice corrosion produced on the steel surface was examined by scanning electron microscope (SEM) after PCAP treatment under different test conditions.     

Electrochemical chloride extraction (ECE) from steel-reinforced concrete specimens contaminated by “artificial” sea-water

Electrochemical chloride extraction (ECE) was studied in the laboratory using cylindrical concrete specimens containing chlorides from “artificial” sea-water. The ECE was carried out for 21 and 90 days, using current densities of about 1 A/m² of steel surface. About 60% to 50% of the initial chloride was removed from the concrete on average. Around 1% chloride by mass of cement remained around the steel after treatment. No influence on the chloride removal efficiency was found from anolyte refreshment after treatment. The quantities of chloride removed correlated well with those observed in practice. Due to the negative polarity of steel in an ECE treatment, Na⁺, K⁺ and Ca⁺⁺ ions from the concrete pore solutions accumulate near the steel surface. K⁺ ions move towards the steel rebar more quickly than the other cations analyzed in this work. Although both the chloride content and the dissolution of the steel were reduced, the repassivation of steel rebar cannot be guaranteed.     

Potentiostatic study of reinforcing steel in chloride contaminated concrete powder solution extracts

This paper presents the findings of an experimental investigation of potentiostatic study on reinforcing steel in chloride contaminated concrete powder solution extracts. Various zones of corrosion for the steel reinforcement at various chloride levels have been identified. In addition the chloride concentration and pH value of these solutions were also measured. The major test variables include steel type, w/c ratio, cement content, and admixed chloride content.     

Oxidising type inhibitors for protection of aluminium and steel surfaces in sodium chloride solutions

Abstract

Photoelectron spectroscopy was used to study the composition of surface films formed on aluminium and steel when protected from corrosion in aqueous chloride media by inorganic inhibitors of the MeO^2?₄ oxidising type and by mixtures of these and monoethanolamine (MEA). The surface condition plays a significant role in the protection of aluminium. In the adsorption of molybdate on to an etched surface reduction of Ma(VI) to Mo(IV) occurs whereas on an unetched surface, as also for a steel surface, no reduction occurs. In the case of chromate, reduction to Cr( III) took place on steel and on aluminium. Synergism was established in the inhibition of corrosion of steel by a combination of inorganic oxidisers and MEA and a study made of this phenomenon.     

Electrochemical processes on steel tube outer surfaces of cathode protected underground gas pipelines

An electrochemical model of steel corrosion of a cathode protected underground gas supply system and methods of determination of the location and dimensions of corrosive metal damage and the spots of the breaching of hydroinsulation have been suggested.     

Mechanical properties of an iron oxide formed by corrosion in reinforced concrete structures

The paper deals with elastic properties of iron oxides formed in reinforced concrete structures. Due to the difficulty to perform mechanical tests on the real oxides presented in the form of (multiple) laminated stratums, the elastic modulus of iron oxides remains unknown. Young’s moduli of porous compact “synthetic oxides” in powder form, obtained in laboratory conditions, were measured from both acoustic measurements and oedometric tests. The elastic modulus of the compact polycrystalline iron oxide is deduced with respect to two models, a micromechanical one and a Hertz’ theory. The full method is validated on a well-known material, the alumina.