Civil Engineering Reference
In-Depth Information
chloride is limited, however, and depends on the type of cement. Sulfate-
resisting cement, for example, has a low C3A content and is therefore less able
to complex the chlorides. In any case, experience suggests that if the chloride
exceeds about 0.4% by mass of cement, the risk of corrosion increases. This
does not automatically mean that concretes with chloride levels higher than
this are likely to suffer severe reinforcement corrosion: this depends on the
permeability of the concrete and on the depth of carbonation in relation to
the cover provided to the steel reinforcement.
When the concrete carbonates, by reaction with atmospheric carbon
dioxide, the bound chlorides are released. In effect this provides a higher
concentration of soluble chloride immediately in front of the carbonation
zone. Normal diffusion processes then cause the chloride to migrate into the
concrete. This process, and normal transport of chlorides caused by water
soaking into the concrete surface, is responsible for the effect sometimes
observed where the chloride level is lower at the surface, but increases to a
peak a short distance into the concrete (usually just in front of the carbonation
zone). The increase in unbound chloride means that more is available to
take part in corrosion reactions, so the combined effects of carbonation and
chloride are worse than either effect alone.
Passivation of the steel reinforcement in concrete normally occurs due
to a two-component system comprising a portlandite layer and a thin pH
stabilised iron oxide/hydroxide film on the metal surface (Leek and Poole,
1990). When chloride ions are present, the passivity of the system is lost by
dissolution of the portlandite layer, followed by debonding of the passive
film. Physical processes operating inside the passive film may also contribute
to its disruption.
The critical chloride content required to initiate corrosion depends on
whether the chloride was present at the time of mixing, or has ingressed
post-hardening, as discussed above. Clearly this also depends on the
microclimate of the concrete (temperature and humidity) and also whether
the concrete has carbonated. Typical contents are about 0.2% by mass of
cement where chlorides are added at the time of mixing and slightly higher
(0.4-0.5%) of cement for chloride ingressing post-hardening of cement.
Good quality concrete can often show a remarkable tolerance for chloride
without significant damage, however, at chloride contents up to about 1% by
mass of cement (usually for chloride added at the time of mixing: reinforced
concrete is much less tolerant of ingressed chloride).
When chlorides have ingressed from an external source particularly,
in conditions of saturation and low oxygen availability, insidious pitting
corrosion can occur, causing massive localised loss of cross-section. This can
occur in the early stages without disruption of the overlying concrete.
