Civil Engineering Reference
In-Depth Information
The combination of these factors can cause an unusual reaction between
the cement, the carbonate and the sulfate, to form
thaumasite
, a sulfate
mineral. The effect is to cause serious damage and softening of the exposed
outer surface of the concrete (assuming an external source of sulfate).
Figure
1.14
shows one of the affected foundations.
It should be noted that sulfate-resisting cement has not proved to be any
more resistant than normal portland cement (CEM1) in resisting this type
of attack.
An international conference to discuss thaumasite attack was organised
by the Building Research Establishment (BRE). The proceedings of the
conference are available (BRE, 2002).
Acid attack
Acid attack is typified by 'raised' aggregate, or in extreme cases disintegration.
It can sometimes be difficult to diagnose since the acid is neutralised by
the cement paste and may be washed away. The most common sources
are spillage from acid tanks, acidic groundwater and oxidation of sewage
effluents. The latter can cause quite dramatic damage to concrete unless
appropriate precautions are taken.
Other contaminants
In addition to those specifically mentioned above, many potentially corrosive
substances may come into contact with reinforced concrete. The extent of
their effect may depend on the type of cement used.
Examples of such materials are certain alkalis, beer and wine (carbonic
acid, lactic acid and acetic acid), vegetable and fish oils, milk (lactic acid),
lime, sugar, sulphides, manure and silage.
Fire damage
Damage and repair of concrete due to fire is summarised in Technical Report
No.68 (Concrete Society, 2008) of the Concrete Society. Three principal
types of alteration are usually responsible.
1
Cracking and microcracking in the surface zone: This is usually sub-
parallel to the external surface and leads to flaking and breaking away of
surface layers. Cracks also commonly develop along aggregate surfaces
- presumably reflecting the differences in coefficient of linear expansion
between cement paste and aggregate. Larger cracks can occur, particularly
where reinforcement is affected by the increase in temperature.
2
Alteration of the phases in aggregate and paste: The main changes
occurring in aggregate and paste relate to oxidation and dehydration.
Loss of moisture can be rapid and probably influences crack development.
The paste generally changes colour and various colour zones can
