Hi David, The physical resistance of concrete to chloride penetration is influenced by factors relating to the concrete itself, such as the porosity of concrete and interconnectivity of the pore system; and to factors relating to the concrete structure such as the stress conditions and the integrity of the cover.

The porosity of concrete is highly sensitive to water-cement ratio. The connectivity of the pore system depends on the amount of original mixing–water filled space and the degree to which it has been filled with hydration products. Capillary pores are those voids remaining that were originally filled with mixing water: ie pores with diameters in the range of 3.2 to 3,000 nm. These capillary pores will cease to be connected at different times in the age of the concrete as a function of w/c and curing condition.

A number of absorption-related properties are used to indicate the porosity and durability potential of concrete. Absorption, initial surface absorption (ISAT), sorptivity and apparent volume of permeable void (AVPV) are each fundamentally a measure of capillary absorption or absorption rate of concrete.

External chlorides penetrate into the interconnecting pores in concrete as bulk liquid by convection, and chloride ions diffuse further into the saturated pore system. Diffusion is controlled by concentration gradients of the free chlorides; thus the capacity of the concrete to physically adsorb and to chemically react with chloride ions affects the free chloride ions concentration in the concrete.

The chloride resistance of concrete is thus highly dependent on the porosity of concrete in terms of pore size, pore distribution and interconnectivity of the pore system. The porosity of concrete is determined by:

(1) the type of cement and other mix constituents;

(2) concrete mix proportions;

(3) compaction and curing.

The type of cement influences both the porosity of the concrete and its reaction with chlorides. The porosity of concrete is highly dependent on the water-cement and aggregate-cement ratios whereas the type and amount of cement affect the pore size distribution and chemical binding capacity of the concrete. The porosity or permeability of insitu concrete is highly dependent on the degrees of compaction and curing during placing and the early life of concrete respectively. Curing greatly affects the porosity of the concrete cover which protects the steel from chloride induced corrosion.

For further information please check the CCAA website:

http://www.ccaa.com.au/imis_prod/documents/Library%20Documents/CCAA%20Reports/Report%202009%20ChlorideResistance.pdf

Thank you very much, i´ve just read the artilce, it has been very helpful

Dear David Flores Vidriales,

     For concrete, there are some factors that interfere with simple interpretation of diffusion data. First of all, the chloride ions are not diffusing through a homogeneous solution. Concrete is a porous matrix that has both solid and liquid components. The diffusion through the solid portion of the matrix is negligible when compared to the rate of diffusion through the pore structure. The rate of diffusion is thus controlled not only by the diffusion coefficient through the pore solution but by the physical characteristics of the capillary pore structure. This effect is normally considered implicitly, however, and the effective diffusion coefficient of the chlorides into the concrete as a whole is considered, called here Deff.

The rate of ingress of chlorides into concrete depends on the pore structure of the concrete

RCPT depends upon the conductivity of the concrete being in some way related to the chloride ion penetrability. Thus, any conducting material present in the concrete sample will bias the results, causing them to be too high. This would be the case if any reinforcing steel is present, if conductive fibers are used (e.g. carbon or steel), or if a highly ionic conductive pore solution is present [ASTM C1202].

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