Comparative study on induced macrocell corrosion phenomenon in repaired ordinary reinforced and self-compacting concrete structures

Abstract

This paper aimed at an experimental investigation of the comparative corrosion phenomenon in the reinforcing steel of repaired chloride contaminated self-compacting concrete (SCRC) and ordinary concrete (OC) patches. This research is a continuation of a previous study in which macrocell specimens were prepared with OC having different chloride contaminations simulating the actual patch repair in the construction field. Here, in this research, the same specimens were prepared with SCRC to investigate more deeply the macrocell phenomenon in SCRC, which received limited attention in the past. Four prismatic specimens with two sets of 5 and 3% chloride contaminated SCRC at the two ends were prepared respectively, and the middle portion of these specimens was cast after 24 h with uncontaminated SCRC simulating the actual patch repair in the field. After 1 year of experimental observations of corrosion potentials and corrosion rates, interesting and novel results were obtained in comparison with OC and SCRC specimens.     

Mortars for encapsulating sacrificial zinc anodes in reinforced concrete

Expansion measurements, chemical analysis and petrography have been undertaken to assess a possible side‐effect of using strongly alkaline additives (lithium hydroxide or sodium hydroxide) in encapsulating mortars used to promote the activity of sacrificial zinc anodes employed for galvanic cathodic protection of steel in concrete. Very high concentrations of lithium hydroxide in the encapsulating mortars were found to cause no deleterious expansion associated with alkali‐silica reaction (ASR) in surrounding “model” concrete specimens even when the latter contained aggregate of known susceptibility to ASR and a near‐threshold level of intrinsic alkalinity. Encapsulating mortars formulated with an equivalent molarity of sodium hydroxide, however, were found to induce significant expansion due to ASR in similar specimens.     

Ten‐year results of galvanic sacrificial anodes in steel reinforced concrete

Zinc sacrificial anodes have been included in patch repairs to steel reinforced concrete structural elements suffering from corrosion since the mid‐1990s. A number of these anode‐containing repairs have been monitored with time. One of the first monitored sites was of a locally repaired cross beam of a bridge structure in Leicester, UK, which has now completed 10 years since its original repair and anode installation. This paper reviews the performance of the anodes installed at the Leicester site in terms of anode current output and steel reinforcement polarisation and corrosion rate over the period. It also presents results of analysis of recovered anodes exposed for 10 years which still show electrolyte continuity, uniform consumption of the zinc and coherent encasing mortar. The knowledge gained from the 10 year results has enabled the development of new, higher current output anodes, which are now trialled in this and other sites.     

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.     

Evaluation of deterioration in reinforced concrete structures by AE technique

Acoustic emission (AE) technique is gaining increasing interest for applications in civil engineering. AE indeed is a passive nonintrusive technique that can be applicable either in structural health monitoring (global evaluation) and in defect localization (local evaluation). The most promising application of such a technique seems to be however in the evaluation of growing defects. Defects such as cracks and delamination can be caused by reinforcement corrosion as well as by other concrete deterioration mechanisms (alkali silica reaction, sulphate attack, etc.) as well as originating from improper concrete casting, curing or structure setting. Damage severity can be evaluated by means of the intensity analysis (IA) through the use of different indexes such as ‘Load Ratio’, ‘Calm Ratio’, ‘Felicity Ratio’ or ‘Historical Index’. Damage evolution analysis along time could be however significantly improved by adopting some procedure already used in the field of geophysics and earth science. Among various parameters, the most significant one is the b‐value which is derived from the amplitude distribution data of AE following the methods used in seismology. Several application examples applied to prestressed concrete structures are reported in the paper.     

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.     

Present and future durability challenges for reinforced concrete structures

This paper sums up the International RILEM Workshop on Present and Future Durability Challenges for Reinforced Concrete Structures, held at ETH Zurich in Switzerland on 17–18 April 2012. Major focus is put on the discussions. During the latter it was revealed that one of the key durability challenges lies in predicting the performance of new materials, where the increasing diversity of cement and concrete plays a major role. As most current engineering models are not capable of predicting actual field performance adequately, a knowledge‐based approach to durability will become more important than ever. Only a scientific approach on a multi‐scale and interdisciplinary level will allow predicting the performance of new materials (where no long‐term experience is available for fitting purposes). This will facilitate the use of more performance‐based durability design that is urgently needed to promote innovative, long‐lasting solutions.     

Cost effectiveness and application of online monitoring in reinforced concrete structures

The annual costs for the maintenance of the Swiss national road networks amount to SFr. 560–640 millions 1 . Due to the limited means and the large number of engineering structures at the critical age between 30 and 45 years cost‐effective solutions for the monitoring and the maintenance of these structures have to be found 2 . In this paper the life cycle costs of an engineering structure using a sensor network (online‐monitoring system) were investigated. These costs were compared to the costs of classical inspection techniques.     

Embeddable reference electrodes for corrosion monitoring of reinforced concrete structures

Mixed‐metal oxide (MMO), graphite and laboratory‐made Ag/AgCl electrodes were electrochemically characterized to be used as reference electrodes embedded in concrete structures. Electrodes were studied in both, aqueous solutions of pH ranging from 7 to 13.5 and embedded into cement mortars; and the electrochemical studies were carried out in the absence and presence of chloride ions. Potential evolution, polarisation behaviour, galvanostatic pulse response and impedance characteristics of the electrodes were carried out in aqueous solutions. Besides, the electrochemical stability of the electrodes embedded in mortar was studied for an exposure period of two years. It was found that the MMO pseudo‐reference electrode is pH‐sensitive, the graphite pseudo‐reference electrode is oxygen sensitive and the Ag/AgCl pseudo‐reference electrode is chloride sensitive. Then, regarding the corrosion monitoring of steel rebars, care should be taken to avoid misleading interpretations of the corrosion potential measurements. However, any of them can be used to measure the corrosion rate of the rebars by means of traditional electrochemical techniques.     

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.

     

On the importance of execution for obtaining the designed durability of reinforced concrete structures

The design and execution of reinforced concrete structures shall together with planned maintenance lead to the intended level of safety and serviceability during the entire service life of the structure. The possible impact of execution on the performance of the finished concrete structure is not always recognized; e.g., is pre‐testing of alternative mixes often undertaken at laboratory conditions. Mix design should include an assessment of the stability and property development of the concrete at the anticipated execution conditions as well as an evaluation of the robustness of the concrete to natural variations in mix composition and execution. Through selected examples the paper addresses the importance of execution for obtaining the designed performance and durability of reinforced concrete structures.     

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.     

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.     

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.     

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.     

On the effectiveness of realkalisation as a rehabilitation method for corroded reinforced concrete structures

Based on available experience with the use of electrochemical realkalisation (ERA), a fairly recent method for rehabilitating reinforced concrete structures (RCS), carbonated concrete can undoubtedly be realkalised. To the authors' minds, however, the following questions remain unanswered: (a) does ERA always effectively repassivates reinforcements?; (b) can ERA be considered an end or just a means to ensuring RCS durability?; and (c) what is the use of ERA if it cannot stop corrosion? This work was aimed at answering the previous, controversial questions. To this aim, the behaviour in a Ca(OH)₂ saturated solution, and in a sound uncarbonated mortar consisting of 1:3:0.5 cement, sand and water, of reinforcing bars from an RCS that failed through carbonation after 29 years of service life was examined.     

Modelling the corrosion‐induced cracking of reinforced concrete structures exposed to the atmosphere

The prediction of concrete cracking due to corrosion in atmospheric/carbonated conditions is a major issue for the evaluation of the durability of structures and the choice of maintenance policies. Because of the complexity of the phenomenon, a fully predictive approach is still missing. The proposed work can be considered as one step in this direction. It deals with a modelling study achieved at the Commissariat à l'Energie Atomique (CEA) with the CAST3M finite elements software. Model is constituted of three components: (1) concrete hydric behaviour, (2) rebar corrosion and (3) mechanical consequences on concrete (mainly concrete cracking). Actual developments consider analogies between rebar corrosion mechanisms and atmospheric corrosion ones, assuming that corrosion processes are influenced by the relative humidity evolution of atmosphere and/or of concrete.     

Reinforcement corrosion-induced cover cracking and its time prediction for reinforced concrete structures

A relation between the radial pressure induced by expansion corrosion products and weight loss percentage of corroded steel was developed and simplified by investigating the influence of the mechanical properties of corrosion layer on cover cracking. By taking account of the penetration of the rusts into open cracks, a mathematical model that predicts the cover cracking time was proposed based on Faraday’s law. Some experimental observations were compared with the predictions made by the proposed model and two existing models, which indicates that the presented model is more suitable for predicting cracking time for both accelerated and long-term corrosion conditions.