Chloride transport through concrete

Ion An atom or molecule with electrons added or subtracted. Ionic compounds like salt (sodium chloride) are composed of balanced ions (NaCl=Na>Cl ). Some iorks are soluble (e.g. Na% Cl, Fe ) which can be important for transport through concrete. 

The chloride ingress into concrete follows Pick s law of difiusion for cured concretes. Concrete mixtures and admixtures affect the diffusivity of chloride ions. Figure 12.7 shows the transport of chloride ions through the concrete deck under moderate corrosion environment. The surface chloride concentration is assumed to be equal to 3.56 kg/m, which corresponds to moderate corrosive conditions. Chloride diffusion follows an exponential decay into the concrete. Chloride concentration at the rebar surface increases with time. 

Bazant developed a complete physical-mathematical model which describes the corrosion process in submerged concrete exposed to sea water (Bazant, 1979). A complete set of equations has been derived for the transport of oxygen and chloride ions through the concrete cover, the mass... 

J. F. Lopez, E. Viqueira, C. Andrade, Transport and binding of chlorides through non-saturated concrete after an initial limited chloride supply ,... 

The influence of temperature and humidity affect the corrosion rate through their influence on the electrochemical reactions at the steel/concrete interface and through their influence on ion transport between anodes and cathodes. Although the mechanisms are not fully understood, it appears that the concrete resistivity (or conductivity) is strongly related to the corrosion rate at moderate or low temperature [35-38]. Variation in resistivity due to variation of humidity (at constant temperature) caused an inversely proportional variation of corrosion rate in carbonated mortar and concrete with low amounts of chloride or without chloride. Variation of temperature (at constant humidity) caused a similarly varying corrosion rate. 

Chlorides can be cast into concrete or can be transported in from the environment. The chloride ion attacks the passive layer even through there is no significant, generalized, drop in pH (puts in the steel can become very acidic, see Section 2.3). Chlorides act as catalysts to corrosion. They are not consumed in the process but help to break down the passive layer of oxide on the steel and allow the corrosion process to proceed quickly. 

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