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.     

Einflüsse von Werkstoff und Verarbeitung auf die Spannungsrißkorrosion von Spannstählen

Influence of material and processing on stress corrosion cracking of prestressing steel In prestressed concrete constructions the highstrength prestressing steels perform essential bearing effects. The alkaline layer of concrete or mortar protects the steels against corrosion and guarantees a permanent load capacity. If the corrosion protection as a result of poor workmanship is not guaranteed from the beginning, or is lost because of lacks of construction in the course of time, or the steels are predamaged during handling, stress corrosion cracking and failure of steel and construction may occur. Also an application of unsuitable materials (prestressing steel, injection mortar, concrete) can alone or in combination with the before mentioned influences favour stress corrosion cracking. In the contribution the correlations and typical failures are discussed.     

Corrosion‐technical properties of high‐strength stainless steels for the application in prestressed concrete structures

Experience with prestressed concrete over about half a century has indicated that the corrosion resistance of conventional prestressing steel does not always satisfy, especially the prestressing steels are susceptible to chloride attack (de‐icing salts) and hydrogen (hydrogen‐induced stress corrosion cracking). On the other hand corrosion agents, such as chloride, condensation water, can penetrate in the concrete and arrive at the surface of steels. Hence, corrosion damage of prestressing steels can happen and, in the extreme cases, the prestressed concrete structure collapsed resulting from the failure of the tendon. In this paper, consideration is made to use high‐strength stainless steels as prestressing tendon with bond in concrete. The high‐strength stainless steels of qualities 1.4301 (X5CrNi18‐10), 1.4401 (X5CrNiMo17‐12‐2), 1.4436 (X3CrNiMo17‐13‐3) and 1.4439 (X3CrNiMoN17‐13‐5) with sequence of increasing austenite stability were investigated. For application in prestressing tendon with bond in concrete the cold‐drawn high‐strength stainless steel of quality 1.4401 is an optimal proposition regarding its satisfactory resistance against pitting corrosion and stress corrosion cracking (SCC) in structure‐related corrosive conditions. The lower alloyed steel 1.4301 has an insufficient resistance against the chloride‐induced corrosion because of the lack of molybdenum and the content of deformation martensite due to the strong cold‐drawing of its unstable austenitic structure.     

Zum rückseitigen kathodischen Korrosionsschutz einseitig korrodierender Stahlbetonbauteile – Modellversuche und numerische Berechnungen

Cathodic corrosion protection of corroding reinforced concrete structures from the opposite side – Model experiments and numerical calculations Cathodic protection is a worldwide approved method to protect the reinforcement of concrete structures against corrosion. For this purpose a voltage is applied between the reinforcement an inert anode or a sacrificial anode which is placed on the concrete surface to shift the electrochemical potential to a more negative range where only cathodic reactions on the surface of the reinforcement take place. In case of concrete structures, that are only accessible from one side, it is often of interest to have only one anode at the opposite side of the endangered reinforcement layer (retaining walls, tunnels, etc.). In this regard investigations at the institute of building materials research Aachen, ibac, are currently carried out to clarify, whether a protection of the reinforcement of the opposite layer is possible or it is prohibited by the nearer layer. First results of these investigations show, that the protection of the opposite reinforcment layer is possible under certain conditions. Furthermore the first attempts to predict the current densities and the potential distributions under variable boundary conditions by the use of numerical calculations are presented.     

Epoxy‐coated bars as corrosion control in cracked reinforced concrete

One of the most common corrosion protection methods in reinforcing concrete bars is the application of fusion‐bonded epoxy coatings. Although considerable research has been carried out on the performance of epoxy‐coated bars (ECR), there are still many uncertainties about their performance in cracked concrete. In this experimental program, reinforcing steel bars with six types of epoxy coatings embedded in concrete slabs with a 0.4 mm wide preformed crack intersecting the reinforcing steel at right angles were tested. Results of corrosion potentials, corrosion current density, coating adhesion tests, chloride content, and visual examination after 68 months of exposure to a simulated marine environment are reported. Results revealed that under the studied conditions the ECR did not provide total protection of steel reinforcement in cracked concrete. Their use however, tended to reduce significantly the damage caused by the chloride‐induced corrosion when compared with the uncoated bars embedded in concrete with similar characteristics.     

Detection and identification of concrete cracking during corrosion of reinforced concrete by acoustic emission coupled to the electrochemical techniques

The tenacious oxide passive film, which is formed on the surface of embedded reinforcing steel under high alkaline condition of concrete, protects the steel against corrosion. However, the condition of passivity may be destroyed, due to processes such as leaking out of fluids from concrete, atmospheric carbonation or through the uptake of chloride ions. Passive steel reinforcing corrosion induced by chloride is a well-known problem, especially where chloride-containing admixtures or chloride contaminated aggregate are incorporated into the concrete. The objective of this work is on one hand to study the effect of chloride ions on passivity breakdown of steel, respectively, in simulated concrete pore solution (SCP) and in concrete reinforcement, and on the other hand to reproduce the carbonation phenomena by applying to the concrete samples a heating–cooling cycles. In this context, the acoustic emission coupled to the electrochemical techniques (potentiodynamic and electrochemical impedance spectroscopy (EIS)) are used.

The results show clearly that [Cl⁻]/[OH⁻] ratio of 0.6 is the critical threshold where the depassivation set-up can be initiated. In addition, the carbonation process is very aggressive with chloride ions and shows a perfect correlation with acoustic emission evolution.

A physical model of the reinforcement/electrolyte interface is proposed to describe the behavior of the reinforcement against corrosion in chloride solution.     

钢筋混凝土构筑物电化学保护的新进展

综述了钢筋混凝土构筑物电化学保护的发展和现状,包括牺牲阳极系统和外加电流系统。内容涉及对不同类型、不同龄期构筑物的保护标准、保护参数选择、保护效果的评价以及检测、监测等诸方面。认为有必要加强电化学驱氯、电化学再碱化等新技术的研究;根据构筑物的不同使用环境,采用多种有效的、综合的保护措施。     

NO_2~-和Cl~-对模拟混凝土孔隙液中钢筋腐蚀行为的影响

应用极化曲线法和电化学阻抗技术研究了NO_2~-和Cl~-对钢筋在不同pH值的模拟混凝土孔隙液中的腐蚀行为,结果表明,钢筋耐蚀性与溶液的pH值,以及NO_2~-和Cl~-的浓度相关,pH值的降低和Cl~-浓度的增高都会使钢筋的耐蚀性降低。在含Cl~-的模拟液中,随着NO_2~-浓度升高,钢筋腐蚀速率降低,在pH值为12.50和10.50的溶液中,当[NO_2~-]/[Cl~-]≥0.4时,NO_2~-对钢筋具有良好的阻锈作用。     

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.     

环氧涂层钢筋及其应用

钢筋的腐蚀，尤其当有氯离子存在时，是混凝土结构腐蚀破坏的主要原因。在防止钢筋腐蚀的各种措施中，采用涂层既简便又有效。熔融结合环氧粉末涂层的综合防护性能优良，已经大量应用于钢质管道和钢管桩，也很适合于钢筋的保护。钢筋表面涂装环氧涂层可用静电喷涂方法，工艺基本成熟。环氧涂层钢筋的研究开发在美国始于20世纪的70年代，主要应用于撒布化冰盐的路桥和各类海工结构。我国在1997年就制订了环氧树脂涂层钢筋的行业标准。目前，国内已能生产环氧涂层钢筋，并开始实际应用。随着我国国民经济的发展，越来越多的工程要求使用环氧涂层钢筋，市场前景肯定十分广阔。     

大管径供水管线腐蚀与防护

对比混凝土结构中钢筋的各种防腐措施,其中以阴极保护为最有效最经济的方法。以津滨供水工程预应力钢筒混凝土管（PCCP）和钢管共同实施阴极保护为例,提出设计、施工及检测方面的几个问题供大家共同商榷。     

Influence of chloride ions and hot weather on isolated rusting steel bar in concrete based on NDT and electro-chemical model evaluation

The most prominent durability concern for reinforced concrete (RC) structures is the corrosion of steel reinforcement in concrete. RC buildings exposed to chloride and high temperature environments like sea and deserts suffer from accelerated corrosion of rebars. The chloride attack and increase in the electro-chemical reaction rate of corrosion due to high temperature is a thermodynamic phenomenon influenced by several parameters and some of them are being neglected in the past research works. The purpose of this present paper is therefore, to model and verify by NDT the coupled effects of chloride and temperature on corrosion of reinforcement throughout the life of concrete buildings by incorporating realistic thermodynamic model evaluations and actual field condition NDT. The model evaluation has been accomplished by the use of concrete durability model as a computational platform on which the corrosion based reinforced concrete building performance and quality at early age and throughout the life of concrete structure is examined in both space and time domains under environmental actions of chloride and temperature. On this line, the thermodynamic modeling evaluation of concrete forms the fundamental core of the theoretical approach to achieve both the scientific knowledge and engineering simulations of altering materials. The NDT results for the effect of chloride and temperature on corrosion have been compared with the DuCOM electro-chemical thermodynamic corrosion model evaluation and are found to be in close agreement with each other.     

Investigations on the protection effect of filling materials for post‐tensioning systems under construction conditions

About the long‐term protection behaviour of corrosion protection materials (filling materials) for prestressed systems under critical environmental conditions is only little known. The corrosion protection effect is usually based on theoretical considerations and is proven by short‐term tests. The selection of the different products is mainly made according to economical or workability criteria. In a research project the barrier effect of different commercial corrosion protection materials (waxes as well as oil‐based greases) against water, their tendency for undercutting as well as their additional corrosion protection effect were investigated. The exposure tests were carried out with non‐stressed as well as stressed prestressing steel specimens which were subjected to critical conditions (condensed water, artificial soil solution, direct soil contact). Parallel to these long‐term exposure tests the applicability of different electrochemical techniques and their significance with respect to testing the corrosion protection ability and water absorption was evaluated. Within the project a suitable method for simple testing the performance of corrosion protection materials under real conditions was developed. By means of a small compact cell submicroscopical reactions of the used sensors could be measured. The high sensitivity of this measuring technique enables the detection of degradation processes at thin protection layers.     

Lateral and radial corrosion propagation behavior of 9–21% Cr and 18% Cr + 2.8% Mo stainless steel reinforcing materials in simulated concrete environments

The life of a concrete structure exposed to deicing compounds or seawater is often been limited by chloride induced corrosion of the steel reinforcement. A complete assessment of the potential benefits afforded by new candidate rebar alloys must address both the lateral and radial corrosion propagation behavior in comparison to conventional steel as well as other factors that might affect the risk of corrosion‐induced concrete cracking. The radial (depth) and lateral (length) corrosion propagation behavior of 18% Cr + 2.8% Mo (S31653) stainless steel, 21% Cr (S32101) duplex stainless steel, and 9% Cr steel compared to plain ASTM A615 carbon steel were characterized in saturated Ca(OH)₂ solution. Radial pit growth was found to be Ohmically controlled for all materials but repassivation occurred more readily at high applied potentials for 18% Cr + 2.8% Mo and 21% Cr stainless steels. Conversely, pit growth on plain steel propagated at all applied anodic potentials and did not repassivate until deactivation by cathodic polarization. Stainless steel also showed the highest resistance to lateral corrosion propagation from an active site during microelectrode array testing. 21% Cr duplex stainless and 9% Cr steel showed similar radial propagation behavior and corrosion morphology, which was intermediate to that of plain steel and S31653 stainless steel. Based on an existing concrete cracking model, it is expected that 9–21% Cr and 18% Cr + 2.8% Mo corrosion resistant rebar materials would require a greater depth of corrosion attack than carbon steel before damaging concrete via corrosion product formation.     

Zum Einfluss der kathodischen Reaktion auf die chloridinduzierte Bewehrungskorrosion

Investigations on cathodic control of chloride‐induced reinforcement corrosion Regarding the mechanisms of reinforcement corrosion there are still contradictions with respect to the controlling/rate‐determining factors of the corrosion process. It is often discussed, that the electrolytic resistance of the concrete is the controlling factor and that the corrosion rates can subsequently be calculated from concrete resistivity. However, extensive research carried out by the authors has clearly demonstrated, that instead of concrete resistivity the resistance to cathodic polarisation normally is the controlling factor in the case of chloride‐induced macrocell‐corrosion. Generally, cathodic control can be related to restricted oxygen diffusion or activation control. In the present paper, these relationships are discussed in detail by results of numerous tests on the cathodic polarisation behaviour of passive reinforcement. For simple defined geometrical conditions simulating practical cases it is shown by a numerical analysis that the resistance to activation is normally the controlling factor for the corrosion rate and that oxygen diffusion has only to be taken into account, when the concrete is permanently water saturated or extremely dense. To verify, whether it is correct to estimate corrosion rates from resistivity data, tests should be carried out to check the influencing parameters on concrete resistivity and cathodic activation of passive steel surface areas.     

Development of a new combined corrosion protection system for chloride‐contaminated reinforced concrete structures

Chloride‐induced damage of reinforcing steel is especially for parking garages and bridges often very severe as large amounts of chlorides act on horizontal surfaces which could lead to fast ingress of these detrimental ions up to the level of the reinforcement and subsequently to high rates of corrosion. In order to avoid the disadvantages of conventional rehabilitation (unreliable prognosis with patch repair or high costs and regular maintenance with cathodic protection using impressed current) a new combined protection system was developed where the principle of drying out the concrete (by means of a surface protection system) is combined with a temporary cathodic protection (by means of a sprayed zinc layer) during the transitional period of the drying out process. This new system was tested both in the laboratory and on‐site at a pilot application. Based on the results obtained the possibilities and limitations of the new system are discussed in this paper.     

Effect of cathodic protection on macrocell currents: A viable method to monitor and control the efficiency of CP?

Corrosion protection of steel reinforcement in concrete structures by cathodic protection (CP) is a cost effective, reliable, and widely accepted method to stop and prevent the corrosion of the steel reinforcement. The efficiency of CP is usually monitored by the “24 h, 100 mV depolarization criterion,” a purely empirical criterion whose implementation is cost and labor intensive and that does not allow online control of CP. Within an extended research project on CP applied to concrete members of a highway bridge exposed to penetrating moisture, three sets of macrocells (MC), each composed of five MC sensors, were installed in conjunction with concrete resistance sensors and silver/silver chloride reference cells. Chloride profiles were determined from the cored or drilled powder originating from the installation of sensors and from drilling cores. Corrosion currents, steel potentials, and concrete resistance were monitored over a period of 1 year before, after installing the CP systems (which remained switched off for half a year for evaluating the effect of the conductive coating), and after start-up of the CP systems. The CP systems applied consist of a moisture resistant conductive coating. As expected CP has a pronounced effect on local MC currents: Anodic MC currents were reduced or changed into cathodic currents, whereas cathodic currents were only weakly influenced. By adjusting the applied protection current all anodic MC currents may be changed into cathodic MC currents. Results indicate that the 100 mV depolarization criterion is a conservative criterion in atmospherically exposed concrete; it is not reliable in strongly wetted concrete. There was no consistent correlation between the CP induced changes in the local MC currents and 24 h depolarization values indicating that large potential shifts induced by CP do not necessarily imply overprotection. Results show that monitoring MC currents before, during, and after CP operation allows to demonstrate in a transparent way the effect of CP on the corrosion of the steel reinforcement. Online monitoring of MC currents is proposed as a viable and comprehensible method to monitor and control the efficiency of CP.     

Corrosion induced failure mechanisms of prestressing steel

Rarely occurring fractures of prestressing steel in prestressed concrete structure can, as a rule, be attributed to corrosion induced influences. The mechanism of these failures often is not well understood. In this connection it is difficult to establish the necessary recommendation not only for design and execution but also for building materials and prestressing systems in order to avoid future problems.This paper gives a survey about corrosion induced failure mechanisms of prestressing steels with a particular emphasis on post‐tensioning tendons. Depending on the prevailing corrosion situation and the load conditions as well as the prestressing steel properties the following possibilities of fracturing must be distinguished: • Brittle fracture due to exceeding the residual load capacity. Brittle fracture is particularly promoted by local corrosion attack and hydrogen embrittlement. • Fracture as a result of hydrogen induced stress‐corrosion cracking. • Fracture as a result of fatigue and corrosion influences, distinguishing between corrosion fatigue cracking and fretting corrosion/fretting fatigue.     

Embedded sensor system to detect chloride permeation in concrete: an overview

Exposure of concrete structures to chloride-bearing solutions such as sea water or de-icing salts brine can affect their durability to stand against deterioration caused by corrosion of steel reinforcement. In order to monitor the chloride ingress into concrete, embedded chloride sensors can be used in concrete structures. Through the sensors, they monitor the initiation of corrosion of steel reinforcement with time, and measure the open circuit potential and polarisation of steel reinforcement, humidity inside the concrete, chloride content and many more. This paper provides an overview of the latest development of chloride permeation measurement in concrete by evaluating the current techniques being used in industry and highlighting a new approach to monitor chloride ion progress in concrete structures wirelessly.     

Monitoring of corrosion processes in chloride contaminated mortar by electrochemical measurements and X‐ray tomography

Corrosion of steel reinforcement in concrete exposed to chloride containing environments is a serious problem in civil engineering practice. Electrochemical methods, e.g., potential mapping, provide information whether the steel reinforcement is still passive or depassivation has been initiated. By applying such techniques no information on the type of corrosion, its extent and distribution of corrosion products is available. Particular the corrosion progress is a significant problem. Especially in the case of macrocell corrosion in reinforced concrete structures, the development at the anode cannot be separated into corrosion damage resulting from macrocell corrosion or self‐corrosion. Until now also in laboratory tests it is impossible to collect such information without destroying specimens after electrochemical testing was performed. To overcome this problem it was tried to study the steel surface within the mortar specimens by X‐ray tomography (CT). Within the scope of these investigations it could be shown, that X‐ray tomography is suitable to make corrosion pits and their development visible which are embedded in a mortar with a cover thickness of about 35 mm. In this publication the time‐dependent corrosion damage of reinforced steel is documented by X‐ray tomography.