Degradation of Concrete

Degradation processes of concrete can be classified (Figure below) as: physical (caused by natural thermal variations such as freeze-thaw cycles, or artificial ones, such as those produced by fires), mechanical (abrasion, erosion, impact, explosion), chemical (attack by acids, sulfates, ammonium and magnesium ions, pure water, or alkali aggregate reactions), biological (fouling, biogenic attack) and structural (overloading, settlement, cyclic loading). In practice these processes may occur simultaneously, frequently giving rise to synergistic action.

Alterations that occur in concrete before the structure has been completed, that is within the first hours to months after casting (among these are cracking due to plastic settlement, plastic or drying shrinkage, creep, thermal shrinkage), are traditionally not considered among the phenomena of deterioration, although they are important to the durability of the structure.

The processes of deterioration of concrete and corrosion of reinforcement are closely connected (Figure beside). The former provoke destruction of the concrete cover or cause microcracking that compromises its protective haracteristics. On the other hand, corrosion attack, because of the expansive action of corrosion products, produces cracking or delamination of the concrete and reduces its adhesion to the reinforcement.

Source: Bertolini, L. et. al, Corrosion of Steel in Concrete, Wiley-VCH, 2004