Page 82 - Handbook of Thermal Analysis of Construction Materials

Section 17.0 - Carbonation of Concrete 65

The deterioration of concrete depends on the exposure conditions.
Concrete not immersed but exposed to marine atmosphere will be subjected
to corrosion of reinforcement and frost action. Concrete in the tidal zone,
however, will be exposed to the additional problems of chemical decompo-
sition of hydrated products, mechanical erosion, and wetting and drying.
Parts of the structure permanently immersed are less vulnerable to frost
action and corrosion of the reinforcing steel.

16.0 CORROSION OF REINFORCEMENT

Corrosion of steel in concrete is probably the most serious durabil-
ity problem of reinforced concrete in modern times, and therefore, a clear
understanding of the phenomenon is of crucial importance. The phenom-
enon itself is an electro-chemical reaction. In its simplest form, corrosion
may be described as a current flow from anodic to cathodic sites in the
presence of oxygen and water. This is represented by the following
equations:
2+
At anode: Fe → Fe + 2e -
-
At cathode: ½O + H O + 2e → 2(OH) -
2
2
These reactions would result in the formation of oxide at cathodic
sites. The high alkalinity of cement paste, however, provides protection for
the steel reinforcement. Corrosion is increased in the presence of chloride
salts. It contributes, together with CO ingress, to the depression of the pH
2
of the pore fluid and increases the electrical conductance of the concrete,
allowing the corrosion current to increase.
Several methods of corrosion prevention have been tried over the
years. These include protective coatings, placement of impermeable con-
crete overlays, cathodic protection or the use of corrosion resistant steels,
and galvanized or epoxy coated bars. Recent work has shown that galva-
nized steel may be of benefit if used in low chloride bearing concrete.

17.0 CARBONATION OF CONCRETE

The corrosion of depassivated steel in reinforced concrete has
focused attention on the reactions of acidic gases such as carbon dioxide