New model for chloride ingress in concrete

At the Expert Centre for Infrastructure Materials we have investigated the ingress of chloride into (1) a number of Danish concrete bridges in marine environments and (2) concrete blocks placed at the marine exposure sites in Rødbyhavn (Denmark) and Träslövsläge (Sweden). This has led to the suggestion of a relatively simple model for chloride ingress into concrete, which is based on a linear correlation between the ingress depth (x(cr)) of a given reference chloride concentration (cr) and the square root of exposure time:

where a(cr) is a factor of proportionality and b(cr) is the intercept with the y-axis in a plot of x(cr) against the square root of exposure time (t). The value of a(cr) is an expression of the rate of chloride ingress of the reference chloride concentration (cr), while the value of b(cr) is interpreted as a result of a "fast" initial ingress of chloride during the first few months (or years) after the first exposure to the marine environment.

Hydration reaction will typically still occur during this early period, which means that the permeability of concrete will be somewhat higher compared to the same concrete in a more mature state. The initial penetration depth (b(cr)) could also partly be a consequence of initial capillary suction of seawater at the time of the first exposure to a submerged marine environment. Therefore, b(cr) will most likely be different for the same concrete depending of the maturity, as well as the moisture content in the concrete, at the time of the first exposure to seawater.

The equation above can be rearranged for t:

In principle, this equation can be utilized to estimate the time until initiation of reinforcement corrosion in a concrete structure, i.e. the duration of the initiation phase, which is sometimes used as a definition of the service life of a concrete structure. Such an estimation can be obtained by equating the value of x(cr) with the thickness of the concrete cover above the reinforcing steel and by setting the reference chloride concentration (cr) at a level equal to the threshold value for initiation of chloride-induced reinforcement corrosion for the given concrete.

The research leading to the suggested chloride ingress model has been carried out with financial support from the Danish Agency for Institutions and Educational Grants.