Steel reinforcement protection

In the case of higher protection current densities and protection currents, interference can occur on nearby installations not covered by the protection. The danger of anodic interference must be investigated by making measurements and prevented by taking appropriate measures [7] (see Section 9.2). For the same reasons, anode systems should not be installed near steel-reinforced concrete foundations. 

The danger of corrosion is in general greater for pipelines in industrial installations than in long-distance pipelines because in most cases cell formation occurs with steel-reinforced concrete foundations (see Section 4.3). This danger of corrosion can be overcome by local cathodic protection in areas of distinct industrial installations. The method resembles that of local cathodic protection [1]. The protected area is not limited, i.e., the pipelines are not electrically isolated from continuing and branching pipelines. 

Pumping or compressor stations are necessary for the transport of material in pipelines. These stations are usually electrically separated from the cathodically protected long-distance pipeline. The concrete foundations are much smaller than in power stations and refineries. Since the station piping is endangered by cell formation with the steel-reinforced concrete foundations, local cathodic protection is recommended. 

Structures or pits for water lines are mostly of steel-reinforced concrete. At the wall entrance, contact can easily arise between the pipeline and the reinforcement. In the immediate vicinity of the pit, insufficient lowering of the potential occurs despite the cathodic protection of the pipeline. Figure 12-7 shows that voltage cones caused by equalizing currents are present up to a few meters from the shaft. With protection current densities of 5 mA mr for the concrete surfaces, even for a small pit of 150 m surface area, 0.75 A is necessary. A larger distribution pit of 500 m requires 2.5 A. Such large protection currents can only be obtained with additional impressed current anodes which are installed in the immediate vicinity of the pipe entry into the concrete. The local cathodic protection is a necessary completion of the conventional protection of the pipeline, which would otherwise be lacking in the pit. 

Very often steel sheet pilings exist in conjunction with steel-reinforced concrete structures in harbors or locks. If cathodic protection is not necessary for the reinforced concrete structure, there is no hindrance to the ingress of the protection current due to the connection with the steel surfaces to be protected. The concrete surface has to be partly considered at the design stage. An example is the base of the ferry harbor at Puttgarden, which consists of reinforced concrete and is electrically connected to the uncoated steel sheet piling. 

Cathodic protection of reinforcing steel with impressed current is a relatively new protection method. It was used experimentally at the end of the 1950s [21,22] for renovating steel-reinforced concrete structures damaged by corrosion, but not pursued further because of a lack of suitable anode materials so that driving voltages of 15 to 200 V had to be applied. Also, from previous experience [23-26], loss of adhesion between the steel and concrete due to cathodic alkalinity [see Eqs. (2-17) and (2-19)] was feared, which discouraged further technical development. 

Cathodic protection can be used as a renovation measure for steel-reinforced concrete structures (see Chapter 19). Although material costs of from 100 DM m" (particularly with preparation, erection, and spray coating costs) up to 300 DM m are quite high, they do not compare with the costs of demolition or partial replacement. ... 

Since stray current corrosion damage can occur after only a few years, the economy of stray current protection measures is obviously not questionable [12], In Fig. 22-3 the effect of stray currents is shown by curve 2 [14]. Without there being firm evidence, it is apparent that the shape of the corrosion damage curve in steel-reinforced concrete (see Sections 10.3.6 and 4.3) is similar to that for stray current corrosion [15]. 

Considerable alterations have been made in the chapters concerned with technical applications which are the result of advances in electrochemical corrosion protection in general practice. Here also, abbreviation and omission of less relevant parts of the older editions have had to be made to create space for more recent information. Recent applications in the chemical industry have necessitated a complete rewriting of the industrial chapter. A new chapter is included on the cathodic protection of steel reinforcement in concrete. 

A conductive polymer electrode has been designed specifically for the cathodic protection of steel reinforcing bars in concrete and is marketed under the trade name Ferex . The anode consists of a 16 AWG stranded copper conductor surrounded by a carbon-loaded polymeric coating similar to that used on the Anodeflex system ) to provide a nominal anode diameter of 8 mm The manufacturer claims that at the maximum recommended current density of 0 08 Am the anode life in concrete will be 32 years with a proportionately longer life at lower current densities. 

Conductive paints (resins) have recently been used for the cathodic protection of steel reinforcing bars in concrete, but they are always used in conjunction with a primary anode material, e.g. platinised-niobium or platinised-titanium wire or a conductive polymer rod. 

Finally, it is necessary to point out that although a particular method of corrosion control may be quite effective for the structure under consideration it can introduce unforeseen corrosion hazards elsewhere. Perhaps the best example is provided by cathodic protection in which stray currents (interaction) result in the corrosion of an adjacent unprotected structure or of steel-reinforcement bars embedded in concrete a further hazard is when the cathodically protected steel is fastened with high-strength steel bolts, since cathodic protection of the tatter could result in hydrogen absorption and hydrogen cracking. 

Concretes made with Portland cement have a specific weight of 140 to 150 Ib/ft (2,242 to 2,400 kg/m ). Concrete absorbs the heat of a fire when chemically bound water is released from a crystalline structure and is reduced to lime. Dense concretes can be formed in place, or pneumatically sprayed to the required thickness using steel reinforcement. The corrosive effect of chlorides on the steel surface in moist saline environments (coastal or other chloride environments) dictates the use of protective primers and topcoat sealers. Major advantages of dense concrete are ... 

The protection of steel reinforcements. Concrete produces a layer of passivity at the steel/concrete interface and any breakdown of this can increase the chance of reinforcement corrosion. In addition, it is important that concrete be maintained in a state of low permeability to minimize the passage of moisture and air to the steel. 

In order to study the effect that water-reducing admixtures may have on the role that concrete plays in protecting steel reinforcement, it is necessary to consider the following aspects. 

The formation of the passive layer at the concrete/reinforcement interface referred to earlier (Section 1.4) is due to the alkaline nature of the concrete. The alkalinity is due to calcium, sodium and potassium hydroxides which, over a period of time, react with atmospheric carbon dioxide to form carbonates. This reduction in alkalinity in reflected in a diminished protective capacity towards the steel reinforcement. 

Steel reinforcement rods in concrete are only practicable when the iron is deeply embedded in the concrete and therefore protected for decades against corrosion by the very durable alkaline environment of the concrete, since concrete is only slowly carbonated by the carbon dioxide (CO2) in the environment, resulting in a neutralization of its pH value. The reinforcement rods in the ceiling of the morgue in question lie directly on the surface, where the pH value would fall very quickly (i.e., would become less alkaline), particularly when rain water containing CO2 penetrated the concrete see the crack in Fig. 25 which would quickly allow the entry of rain water. 

The final ph value of this material lies within the neutral range. Since this medium no longer provides sufficient protection for steel reinforcement rods and offers only slight environmental resistance, it is usually used for the plastering of interior walls and for interior brick walls only, in the latter case often mixed with cement.400 The specific surface of lime mortar lies considerably beneath that of cement mortar (up to one order of magnitude) 404 The water content is similar to cement mortar. 

The effect that accelerators have on the role of concrete in providing protection against the corrosion of steel reinforcement has been the subject of several investigations and considerable controversy. Many studies have shown that the factors below are relevant to the discussion ... 

Monitoring of the electrochemical potential of steel reinforcement in concrete is a well established technique for assessing the severity of corrosion and for controlling cathodic protection systems. A reference electrode is the electrochemical device used for measuring these potentials. The reference electrode is either placed on the concrete surface during the measurements or permanently embedded in the concrete in close proximity to the steel. The latter technique enables remote long-term monitoring. 

McDonald, D.B., Sherman, M.R., Pfeifer, D.W., and Virmani, Y.P., Stainless Steel Reinforcing as Corrosion Protection, Nickel Development Institute, Reprint Series 14034, August 1995. 

Concrete is a highly alkaline environment because of the high concentration of calcium oxide or calcium hydroxide. The pH is greater than 12, which protects the steel-reinforcing bars used in the building from corrosion. Carbonation occurs when carbon dioxide reacts with the calcium carbonate and the pH of the environment drops to around 9. In this situation, carbonation becomes rapid, especially in industrialised areas and towns where there is a lot of exhaust emission from traffic. Acrylic coatings have become a versatile choice to overcome the problem of carbonation. 

Carbon dioxide converts calcium hydroxide to calcium carbonate, gradually reducing the alkalinity of the concrete. The concrete then loses its ability to protect the steel reinforcing rods from corrosion and therefore these begin to rust. 

Metal or steel reinforced concrete structures which are not in themselves inert to chemical attack (corrosion) from the environment in which they are designed to serve can very rarely be protected by a metallic surfacing. The normal protection under such conditions will be supplied by a nonmetal, often a coating. Each nonmetal so used has its own limitations—chemical or thermal—which must be considered. Therefore, in many cases, a combination of two or more nonmetals is required to provide the necessary ultimate protection to the steel or concrete. 

Woods (32) states "The reaction between atmospheric carbon dioxide and dense hardened concrete is very slow, and even after a considerable number of years, may affect only a thin layer nearest the exposed surfaces. A principal product of the reaction is calcium carbonate, the presence of which may enhance the early resistance of concrete to attack by some chemicals in solution, such as sulfates. In practice, however, any beneficial effect that may exist appears to be of relatively small moment." The harmful effect of carbonation arises when the carbonated layer created on the surface of reinforced concrete over the years reaches the steel reinforcement. The alkaline protective layer is then considerably less alkaline, and the steel bars may start to rust. 

Composite structures Application of two-layer RubCon-conventional concrete structures (with RubCon in the tension region) protects steel reinforcement from the effects of corrosive environments and creates a structure that operates under load without cracks. Research [30] proved that two-layer RubCon-concrete bending structure elements can replace prestressed concrete elements. 

In principle, stainless-steel reinforcement can be a viable solution for preventing corrosion in a large number of applications. The chloride threshold is much higher than the chloride content that is normally found in the vicinity of the steel even in structures exposed to marine environment or de-icing salts. There is no objection to using stainless steel only where its improved protection is necessary, combined with normal steel at other areas. Hence, stainless-steel bars can be used in the more vulnerable parts of structures exposed to chloride environments, such as joints of bridges or the splash zone of marine structures. Similarly, they can be used when the thickness of the concrete cover has to be reduced, such as in slender elements. 

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