Chlorides in concrete

Accelerator in Ready-Mix Concrete. Calcium chloride accelerates the set time of concrete giving it a high early strength development. It is not an antifreeze, but by using it duriag cold weather protection can proceed in a timely manner (31—34). In Russia, calcium chloride forms a component of several antifreeze admixtures (33). Reviews of the concerns and possible remedies of calcium chloride corrosion problems in concrete are available (21,35). There is no consensus on what the safe levels of calcium chloride in concrete are. 

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. 

V.S. Ramachandran, Calcium Chloride in Concrete, Applied Science Publishers, London, 1976. Environmental Protection Service, Sodium Hydroxide. Technical Services Branch, Ottawa, 1984. 

The other main source of chloride in concrete is penetration from the environment. This occurs, for instance, in marine environments or in road structures in regions where chloride-bearing de-icing salts are used in wintertime. 

Figure 6.6 Examples of profiles of diffusion for chlorides in concrete for different values of the apparent diffusion coefficient (Djpp) after 10 y of exposure...

Chlorides in concrete structures can be present as cast-in chlorides (in cases where chloridemixing water was used) or can be transported into concrete from the environment (seawater, de-icing salts). To distinguish between these two cases and to be able to get information on the future service life of a structure, a chloride depth profile should be determined. This is essential because the actual chloride level at the reinforcement determines the present probability of corrosion, but the profile determines the future development of corrosion. 

M. Collepardi, Quick method to determine free and bound chlorides in concrete , Proc. Int. Rilem Workshop Chloride Penetration into... 

The diffusion coefficient of chloride in concrete varies with time and the water to cement ratio. The exact rate of chloride diffusion through concrete cannot be determined due to concrete heterogeneity and concrete material differences. The relationship given by Lin [7] was used to estimate the diffusion coefficient in the model ... 

As stated in Chapter 2, corrosion proceeds by the formation of anodes and cathodes (Figures 2.1 and 2.2). In the case of chloride attack they are often well separated with areas of rusting separated by areas of clean steel. This is known as the macrocell phenomenon. Chloride induced corrosion is particularly prone to macrocell formation as a high level of water is usually present to carry the chloride into the concrete and because chlorides in concrete are hygroscopic (i.e. they absorb and retain moisture). The presence of water in the pores increases the electrical conductivity of the concrete. The higher conductivity allows the separation of anode and cathode as the ions can move through the water filled (or water lined) pores. 

The electrical resistivity is an indication of the amount of moisture in the pores, and the size and tortuosity of the pore system. Resistivity is strongly affected by concrete quality, that is, cement content, water/cement ratio, curing and additives used. The chloride level does not strongly affect resistivity as there are plenty of ions dissolved in the pore water already and a few more chloride ions here or there does not make a big difference. However, chlorides in concrete can be hygroscopic, that is, they will encourage the concrete to retain water. This is why chlorides are often accused of reducing concrete resistivity. 

It is important to recognize that diffusion is not the only transport mechanism for chlorides in concrete, particularly in the first few millimetres of cover. There may be several mechanisms moving the chlorides including capillary action and absorption as well as diffusion. Rapid initial absorption occurs when chloride laden water hits very dry concrete. In many circumstances these will only affect the first few millimetres of concrete. If so then the expedient of ignoring the first few millimetres of drillings and then calculating diffusion profiles will work. If the cover is low, the concrete cycles between very dry and wet or the concrete quality is low then the alternative transport mechanisms may overwhelm diffusion, at least to rebar depth. 

Ramachandian, V.S. Calcium chloride in concrete. Appl. Science Publ., London (1976)... 

It is important to recognise that diffusion is not the only transport mechanism for chlorides in concrete, particularly in the first few millimetres of cover. There may be several mechanisms moving the... 

Hansson, C. M. and Sorensen, B., "Threshold Concentration of Chloride in Concrete for Initiation of Reinforcement Corrosion, Corrosion Rates of Steel in Concrete, ASTM STP 1065, N. S. Berke, V. Chaker, and D, Whiting, Eds., ASTM International, West Conshohocken, PA, 1988, pp. 3-16. 

Chlorides in concrete and critical chloride levels. Chlorides in concrete exist in two basic forms, so-called free chlorides and boimd chlorides. The former are mobile chlorides dissolved in the pore solution, whereas the latter type represents relatively immobile chloride ions that interact (by chemical binding and/or adsorption) with the cement paste. At first glance, it may appear that only the free chlorides should be considered for corrosion reactions. However, Glass and Buenfeld have recently reviewed the role of both bound and free chlorides in corrosion processes in detail and have concluded that both types may be important. Boimd chloride may essentially buffer the chloride ion activity at a high value, and localized acidification at anodic sites may release some bound chloride. 

Work at the Federal Highway Administration laboratories showed that for hardened concrete subject to externally applied chlorides, the corrosion threshold was 0.20 percent acid-soluble chlorides. The average content of water-soluble chloride in concrete was found to be 75 percent to 80 percent of the content of acid-soluble chloride in the same concrete. ... 

Lackey, H. B. (1992). Factors Affecting Use of Calcium Chloride in Concrete. Cement, Concrete Aggregates, CCAGDP 14(2), 97-100. 

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