Corrosion of Concrete in the Chlorides Solutions

The corrosion process consists in migration of chloride ions into the concrete with simultaneous OH ions diffusion in an opposite direction, from the leached cement hydrates. Therefore the porosity, and consequently the permeability of concrete is of fundamental importance. Cement matrix is a part of concrete which is subjected to the destructive action as a first one. The attack of chloride ions on cement paste will be then discussed first. However, some information concerning the effect of aggregate on the chloride ions diftusion is needed for comprehensive presentation of phenomena occurring in concrete. 

The chloride ions are highly mobile (the highest effective diffusion coefficient) and that is why the corrosion of concrete, due to the attack of chloride containing solution, occurs so rapidly. The rate of diffusion is of significant importance when the durability of concrete is considering. Therefore this property is often exarttined. 

In the first one, the measuring stand consists of the horizontal cylinder divided in two chambers by separation with plate paste specimen, in which the diffusion is to be determined. One chamber is filled with NaCl or CaCl2, the second one with deionized water or Ca(OH)2 saturated solution. For these solution the following requirements are recommended by Marchand et al. [197]  

In these conditions the chemical activity gradient of chlorides in the first chamber should be decreased to minimum, and consequently the disarrangement of pores structure in the specimen should be negligible. However, the eonditions of experiments do not comply generally with these requirements and in the most oases the pure chloride and calcium hydroxide solutions are used in the first and in the seeond chamber or their mixture in the first one (e.g. [198], [199]). 

The effective diffusion coefficient is calculated generally from the eqnation derived by Page etal. [191] from the first Pick law  

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