Chloride reinforced concrete

Materials acceptable for underground piping use include ductile iron, fiberglass-reinforced epoxy plastic, polyethylene, polyvinyl chloride, reinforced concrete, and carbon steel. Plastic pipes are not acceptable in areas subject to solvent exposure. 

Unfortunately, the protection provided by concrete can be overcome by contamination of the concrete by chloride. Chloride, when entering the concrete as a contaminant of the mix constituents, is to a large extent (about 90%) complexed within the cement matrix and only a small percentage is free in the pore solutions. The present codes of practice ban the use of chloride-bearing additives and restrict the amount of chloride present in concrete. For normally reinforced concrete made with ordinary Portland cement it should be not more than 0.4% chloride ion with respect to the cement content weight/weight. 

Concrete exposed to deicer salts, or to a marine environment is subjected to chloride and sodium loading. The ability of concrete to resist the penetration of chlorides and sodium is a primary design consideration in marine or cold environments. The ingress of chlorides into concrete is a major problem due to chloride-induced corrosion of the reinforcing steel and deicer salt scaling [a process by which a thin layer (< 1 mm) of concrete deteriorates from the surface of the concrete]. The penetration of sodium from sea water or deicer salts is generally... 

There has been controversy over the use of calcium chloride in concrete containing embedded metal in view of the possibility of corrosion, particularly where the concrete is of a porous nature. Many countries have made provision in the relevant codes of practice to prevent or limit its use where steel reinforcement is present. This has renewed interest in chloride-free accelerators as replacements for calcium chloride in reinforced concrete. However, calcium chloride remains a most effective material for use in unreinforced concrete for economic production under winter conditions and its effects on concrete, whether beneficial or undesirable, are well researched and quantified. In some areas the newer non-chloride materials, although shown to reduce the likelihood of reinforcement corrosion, have not been widely studied and their other effects on concrete are less known. 

Latex-modified mortars and concretes have become promising materials for preventing chloride-induced corrosion and for repairing damaged reinforced concrete structures. In Japan and the USA, latex-modified mortar is widely used as a construction material in bridge deck overlays and patching compounds, and for finishing and repairs [99]. Polymer-cement hydrate-... 

Chloride contents must be kept low to avoid corrosion of steel in reinforced concrete (Section 12.3) and formation of kiln rings and preheater deposits. Contents below 0.02% are preferred, though higher ones can be acceptable if a sufficient proportion of the kiln gases is bypassed or in less energy-efficient (e.g. wet process) plants. 

Acrylic-modified cement coatings protect atmospherically exposed reinforced concrete structures from attack by chloride ions, oxygen and water. 

Chlorides, generally in the form of the sodium salt are found in sedimentary deposits, particularly in marine and coastal areas. In reinforced concrete, they can increase the corrosion rate of the steel. Chlorides can also adversely affect the performance of sulfate-resisting Portland cements. BS 5328 [8.9] specifies chloride contents in concrete for various types and uses. BS 882 (Appendix C) [8.2] provides guidance on limits for chloride in aggregates when it is required to limit the chloride ion content , ranging from 0.01 to 0.05 %. 

While calcium chloride tends to decrease permeability of concrete, its corrosive influence and ability to increase shrinkage make it a potential hazard to long-term durability in reinforced concrete. 

The service life of reinforced-concrete structures can be divided in two distinct phases (Figure 4.1). The first phase is the initiation of corrosion, in which the reinforcement is passive but phenomena that can lead to loss of passivity, e.g. carbonation or chloride penetration in the concrete cover, take place. The second phase is propagation of corrosion that begins when the steel is depassivated and finishes when a limiting state is reached beyond which consequences of corrosion cannot be further tolerated [6, 7]. 

Submerged structures. When reinforced-concrete structures are submerged in water, or in any case the moisture content of concrete is near the saturation level, the transport of oxygen to the steel is low and the reinforcement reaches very negative potentials, for example between -400 and -600 mV SCE [1]. In this case, the chloride threshold is much higher, sometimes even reaching values... 

P. R. W. Vassie, Investigation of reinforcement corrosion 2. Electrochemical monitoring of steel in chloride-contaminated concrete . Materials and Structures, 1991, 24, 351 358. 

J. Van der Bijen, Reinforced concrete an assessment of the allowable chloride content , Proc. of Canmet/ ACl Int. Conf on Durability of Concrete, Nice, 1994. 

The most frequent type of macrocell in reinforced-concrete structures exposed to the atmosphere is that established between more superficial rebars that have been depassivated by carbonation or chloride penetration, and internal passive rebars. Another example may be walls where chloride penetrates from one side and oxygen penetrates from the other side, which may occur in hoUow structures Hke tunnels and offshore platform legs or with ground retaining walls. 

Coated reinforcement MacroceU formation may be important in the case of epoxy-coated rebars (Section 15.4) in chloride-contaminated concrete if there are defects in the coating and the coated bars are electricaUy connected with uncoated passive steel bars in deeper parts of the structure. Small anodic areas are created at the defect points of coated rebars in contact with chloride-contaminated concrete, while the uncoated passive rebars provide a cathodic surface of much greater size. In these situations the macroceU can result in considerable anodic current densities and can significantly accelerate the attack on corroding sites. This is why coated rebars should be electrically isolated from uncoated bars. 

Consequences of DC stray current in reinforced concrete change according to the properties of the concrete (alkahne, carbonated or contaminated by chlorides), to the duration of the current circulation and to the current density. It is therefore necessary to distinguish concrete structures not contaminated by chlorides and not carbonated from those contaminated by chlorides in quantities insufficient to initiate corrosion and, finally, from those that already have corroding rebars because of chlorides or carbonation. 

For passive reinforcement in non-carbonated and chloride-free concrete, current can flow only if there is a great enough increase in the potential of the anodic area to exceed the threshold for oxygen evolution (Figure 9.3). It was shown in Chapter 7 that at potentials below +600 mV SCE no iron dissolution or any other anodic process takes place, and thus it is impossible for the current to leave the metal. 

Figure 9.3 Schematic representation of electrochemical conditions in the cathodic and anodic zones of reinforcement in non-carbonated and chloride-free concrete that is subject to stray current...

DC stray currents may have more serious consequences in chloride-contaminated concrete. On passive reinforcement in concrete containing chloride in a quantity below the critical content and thus in itself insufficient to initiate localized corrosion, the driving voltage AE required for current to flow through the reinforcement is lower than in chloride-free concrete and decreases as the chloride content increases (Figure 9.7). This is a consequence of less perfect passivity, and in particular a lower pitting potential. 

Protection that concrete offers to steel against stray current ceases when corrosion of the reinforcement has initiated, e. g. due to carbonation, chloride contamination, or the stray current itself In this case, any current flowing through the steel will increase the corrosion rate at the anodic site, similarly as in buried steel structures. Figure 9.8 shows that even small driving voltages can lead to an increase in the corrosion rate on the anodic area (from to Furthermore, it has been observed that if steel is subjected to pitting corrosion in chloride-contaminated concrete, the anodic current increases the size of the attacked area [5]. 

For steel embedded in concrete, it was observed that current densities up to 50 A/m applied for 5 months to passive steel in concrete with up to 0.4% chlorides did not lead to corrosion initiation [5]. Since steel in reinforced-concrete structures is not coated, it is not actually possible to reach such high current densities. It can be assumed, therefore, that interference from AC current cannot induce corrosion on passive steel in concrete. 

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