Corrosion Protection of Reinforcing Steels

Corrosion Protection of Reinforcing Steels

In reinforced concrete structures the concrete guarantees chemical and physical corrosion
protection of the unalloyed reinforcement. Thus, the alkaline electrolyte in the pores of the
concrete passivates the steel and prevents anodic dissolution of iron. Further the concrete – as
a more or less dense (fine porous) material - keeps corrosion-promoting substances away
from the reinforcement. That is, if a sufficient depth of concrete cover and a high concrete
quality are provided. In general, steel, in concrete is adequately protected against corrosion.
Loss of durability in reinforced concrete apart from problems caused by poor design and
construction only occurs if the passivating oxide layer is rendered unstable (if depassivation
occurs) due to carbonation of the concrete reducing the alkalinity of the pore solution in the
hardened cement paste around the steel or to the ingress of chlorides to the steel /concrete
interface [1,2]. Alkalinity can be lost by the ingress of carbon dioxide from the atmosphere
into the permeable concrete, to neutralise the alkaline hydroxides by forming carbonates, thus
lowering the pH. If the penetration front, which may also include moisture and oxygen
reaches the reinforcement, corrosion may occur. The probability of this event depends upon
the degree of permeability and porosity of the concrete.

Hardened concrete is an inorganic composite which consists of the binder and the mineral
aggregates. The constituents of the binder are cement and Type I [1] additions like limestone
or quartz powder and Type II additions which may be puzzolanic like fly ash, phonolite or
trass or maybe latent hydraulic like ground granulated blast furnace slag or silica fume. In the
fresh state, water is added to the concrete components in order to make the mix workable, and
to start the hydration reaction of the binder. Every cement is a mixture of Portland cement
clinker and other mineral materials. For the understanding of the corrosion-protection
capacity of concrete, it is necessary to focus first on the hydration process of the clinker.
Clinker consists of tricalcium silicate 3CaO·SiO2, dicalcium silicate 2CaO·SiO2, tricalcium
aluminate 3CaO·Al2O3 and tetracalcium aluminoferrite 4CaO·Al2O3·Fe2O3. Besides these
main compounds, there are oxides of the alkalis sodium (Na) and potassium (K), there is a
small percentage of MgO and free CaO, and there is SO3 for set control.


Share this

Related Posts

Next Post »