Concrete is an ingenious building material, also because in combination with reinforcing steel it exhibits tremendous load-bearing capacity. The combination of steel and concrete has the advantage that under normal conditions the high pH value of concrete creates a passivating layer of iron hydroxides on the steel surface, which protects it from corrosion. Particularly steel, however, can be compromised in its durability of performance by the presence of moisture and salt.
Chlorides are displaced at the steel surface by Sika FerroGard. It forms a protective film which moves the corrosion potential and reduces the current densities to a very low level.
Standard construction practices ensure that corrosion of steel reinforcements is limited.
These practices include observance of minimum concrete quality (w/b-ratio, cement content, minimum strength) and minimum concrete cover of re-bars. However, in many cases, especially in environments with high levels of chlorides (de-icing salts, seawater or even contaminated concrete mix components), these basic protection procedures prove insufficient.
In order to prevent corrosion or delay it’s onset start and thereby extend the life of a structure, four additional steps can be taken to protect the steel from corrosion: increase concrete quality, increase the concrete cover, utilise corrosion inhibitors and application of protective coatings.Increasing concrete quality means reduction of the number and size of capillary pores. This increases the density in the concrete matrix and as a result hinders the transport of chlorides or CO2 into the concrete.
Reduction of the w/c-ratio through application of high range water reducers or use of supplementary cementitious materials like fly ash, silica fume or natural pozzolans represent opportunities in concrete technology to improve the mix design even more.
When choosing improved concrete quality for protection against corrosion, extra attention must be given to proper placement, curing of concrete and shrinkage potential of the concrete mix, as small cracks can allow chlorides or CO2 to penetrate to the reinforcing steel regardless of the density of the concrete mix.
Corrosion inhibitors are added to the concrete mix during the batching process. Inhibitors do not significantly influence the density of concrete or impact the ingress of chlorides or CO2, but act directly on the corrosion process.
Corrosion inhibitors are defined in a number of ways. On one hand either as an admixture which will extend the time before corrosion initiates, or as one which reduces the corrosion rate of the embedded steel, or both, in concrete containing chlorides.
By another definition a corrosion inhibitor must reduce the corrosion rate and the corroded area of re-bars in concrete containing chlorides.
The main products used as corrosion inhibitors today are either calcium nitrite based products or aminoester organic corrosion inhibitors.
Protective coatings are used to reduce the ingress of chlorides or carbon dioxide. Coatings can be applied according to two basic options, either to the surface of the concrete or to the steel re-bars themselves before they are embedded in the concrete.
