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Axel Gray
Axel Gray

Cathodic Protection Of Steel In Concrete || |LINK|

The salt seeps into the concrete and erodes the steel reinforcing bar (rebar) causing cracks and spalling in the concrete and eventually the potential for failure of the structure. One very effective, long-term solution is metal or thermal spraying the concrete with zinc or a variety of zinc alloys. This is a technology that protects or extends the life of a wide variety of products in the most hostile environments.

cathodic protection of steel in concrete ||

The majority of metallised zinc cathodic protection systems are operated in galvanic or sacrificial mode. However, metallised zinc cathodic protection systems can be, and are in many instances, operated in impressed current mode. In impressed current cathodic protection systems, the anode is made of material with an unpolarised potential that may be equal, higher or greater than that present initially in the steel to be protected. An external power supply is then connected between the anode and the steel with the appropriate polarity voltage to deliver the required amount of electronic current to the steel. The anode is usually embedded near the concrete surface and an external power supply installed nearby. The sprayed coating, a high purity zinc or a zinc alloy, is then connected to one pole of a DC power supply and the steel rebars are connected to the other pole. The electrical circuit is completed between the rebar and the zinc by the presence of moisture in the concrete. The action of the corrosion cell causes the zinc to corrode in preference to the steel rebars, therefore protecting the rebars from corrosion. It is important with impressed current that the anode and the reinforcing bars must not short out. As mentioned, many systems are used in galvanic mode where contact between the anode and the rebar is not a problem.

The costs to apply metal sprayed coatings to large concrete structures is not insignificant, particularly when many structures are difficult to access, such as bridges. In many cases, special enclosures need to be fabricated to offer containment of the blast and metal spray area. These enclosures are used to provide environmental protection but also to provide suitable environmental conditions for spraying. However, the long-term benefits can make the process extremely commercially attractive. If performed correctly and depending on the coating applied, the process can offer corrosion protection for up to 20 years before the next significant maintenance is required. The protection offered can greatly prolong the life of the structure and also prevent costly accidents from cracked sections falling from the structure. Once applied, the coating requires minimal maintenance. If required for aesthetic purposes, zinc coatings can also be painted.

An example of corrosion protection using this alloy has been trialled by Aeroports de Paris at Charles de Gaulle (Roissy) airport. Aeroports de Paris, responsible for the maintenance of most of the Roissy airport infrastructure, recognised deterioration in some of the concrete panels at the airport, and sought a long-term corrosion protection solution. The precast concrete panels, which are 2.6 x 2.8m of lightly reinforced 8cm thick units, form the underside of the concrete viaducts carrying road traffic to and from a busy terminal complex. Run-off from de-icing salt has led to an important level of chloride in the panel concrete. Although the panels are not structurally significant, spalling could present a hazard to passing traffic.

Oregon Department of Transportation (ODOT) demonstrates another success story for cathodic protection on concrete. In a bid to reduce the high costs of bridge reconstruction, ODOT has applied a system of metallised zinc anodes and impressed current cathodic protection. This process has been used to protect its Cape Creek Bridge from corrosion and subsequent reconstruction. The bridge is exposed to a coastal environment and is subject to attack by chloride from the salty air. Prior to the cathodic protection project on the bridge, it had suffered substantial concrete spalling on its columns and underdeck.

Cathodic protection (CP) is being increasingly used on reinforced concrete structures to protect steel reinforcing bars from corrosion in aggressive conditions. Due to the complexity of environmental conditions, the design specifications in national and international standards are still open to discussion to achieve both sufficient and efficient protection for reinforced concrete structures in engineering practices. This paper reports an experimental research to investigate the influence of chloride content on concrete resistivity, rebar corrosion rate and the performance of CP operation using different current densities. It aims to understand the correlation between the chloride content and concrete resistivity together with the CP current requirement, and to investigate the precision of the CP design criteria in standards.

The corrosion of steel reinforcements has been recognised as the major cause for the premature deterioration of reinforced concrete structures worldwide [1]. Extensive researches on the deterioration mechanisms have concluded that the combined presence of chloride and the decrease in pH due to carbonation plays the most significant role in the corrosion of concrete reinforcements [2, 3]. So far, many technologies using chemical, mechanical, and electrochemical methods have been developed to address the problem [4, 5]. Among those, cathodic protection (CP), has been widely recognised and become the most popular technique implemented in civil engineering practices for its reliable long term protection [6,7,8].

Adequate protection provided by CP for the steel reinforcement in concrete depends on many factors. In addition to the steel composition and the nature of concrete components, the physical conditions, such as concrete porosity, degree of carbonation, water and chloride contents, and environmental temperature, play the important roles affecting the effectiveness of CP operation. CP arrangement and the applied current densities are all related to the above conditions [9, 10]. Additionally, the service life of the anode is another factor to be taken in consideration [11, 12]. Traditionally, titanium mesh sheet with noble metal oxides coating, such as iridium, ruthenium and cobalt, have been the most common type of anodes [12]. Other materials, offering ease of installation and cost efficiency, have also been employed [13]. In recent years, due to its good chemical stability, carbon fibre has been successfully used as anode material in CP implementation for concrete structures [13,14,15].

Based on the discussion above, it is noted that some uncertainty still exist on the topic of defining the current specification for CP design for reinforced concrete structures for varied and complex application conditions. As an effort to obtain more detailed specific information for the CP design for chloride contaminated reinforced concrete structures, this paper reports an experimental study on the effect of concrete chloride contamination degree on the corrosion evaluation parameters that are employed for reinforcement cathodic protection assessment. Specifically, this work investigates the correlation between the chloride content and concrete resistivity, and the relationship of these two parameters with the rebar corrosion rate. These studies enable identification of more precise characteristic relationships between concrete chloride content, the applied current density and the instant-off potential. Thus, the experimental results provide a direct guidance for the specification of the CP current density requirements for atmospherically exposed concrete structure at different levels of chloride contamination.

Reinforced concrete is universally recognized as an outstanding artificial construction material, with excellent mechanical performance and workability. However, its durability is strongly associated with the steel corrosion accompanied by electrochemical reactions. Corrosion in reinforced concrete is often induced by the chloride permeation and exacerbated by temperature gradients, humidity changes and potential differences1. Normally, the embedded steel bar is protected by the surrounding concrete thanks to the passive film formed on its surface to keep it from corroding by providing a highly alkaline environment. However, if the alkalinity locally compromises, for example in the case of chloride permeation, carbonation or sulfation, a high corrosion risk may be initiated, which will be further enhanced by the presence of water (usually entraining harmful ions), oxygen and potential difference. Such processes may significantly shorten the service life of reinforced concrete structures, causing a widespread concern in construction engineering.

How to cite this article: Wang, Y. et al. Self-immunity microcapsules for corrosion protection of steel bar in reinforced concrete. Sci. Rep. 5, 18484; doi: 10.1038/srep18484 (2015).

This article summarises the international development of cathodic protection of steel in concrete. The technology was developed in Europe and the USA for applications to buried prestressed concrete water pipelines (Refs. 1 & 2) and in California to deal with deicing salt attack of reinforced concrete bridge decks, and has been widely applied throughout North America for that purpose. It has been used and further developed in the UK to deal with a variety of problems ranging from buildings with cast in chlorides to bridge substructures contaminated with deicing salts and to marine structures and tunnels. It is also widely used on buildings and car parks in UK and Northern Europe. In the Middle East, severe corrosion problems caused by high levels of salinity in soils as well as marine conditions have lead to many large projects being carried out. It has also been used extensively in the Far East including Australia, Japan and Hong Kong.

Stratfull modified this by adding the coke breeze to asphalt. He applied silicon iron anodes at l2ft (3.66m) centres on the concrete bridge deck and then overlaid the anodes with a 3 inch (76mm) conductive asphalt wearing course. That system was energised in 1972 and operated for over 20 years. When surveyed in 1983 the system was working well even though some cracks and delaminations had been repaired before the cathodic protection system installation with insulating polymers that prevented current flow to all areas. Two similar systems installed in the mid 1970s are believed to be still operating.


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