rucial and the constituents should if possible be taken from constant sources.
Admix tures do not adversely affect staining, and the use of integral pigments may
give better colour control, minimize surface variations and aid workability.
Consideration of appearance and of overall durability impose even stricter limits
on workmanship, whilst curing, as later amplified, is of paramount importance.
Apart from isolated areas of formwork leakage or irregular absorption by the
lining, other causes of discoloration include concreting in cold weather, casting in
excessively thick layers and, above all, release of water to the surface during
placing and compaction. Another factor is mixing time, and longer periods may
reduce the extent to which water and cement separate, as may extended vibration.
The atmosphere may also contribute industrial effluents and traffic fumes to
produce chaotic patterns around corbels and ledges, which in any case
concentrate rainwater. The outer skin of concrete, however skilfully moulded,
takes the full force of attack and must of necessity be durable, whether airentrained or otherwise. Appearance is largely determined by the properties of the
cement paste, which in practice is regularly wetted so that dirt lodges in its pores;
the rich laitance is etched to change its colour towards that of the fine aggregate.
As the grime builds up it will give a measure of protection against further etching,
but may show more contrasting efflorescence if the salt source is still operative.
A more encrusted growth is produced by water percolating through cracks in a
wall bringing lime to the surface, but this to some extent protects reinforcement.
Efflorescence can be confused with streaking caused by rainwater which cleans
along irregular lines, and here again experience will indicate profiling or at least
diffusion.
Concrete, in the same way as any other material, becomes dirty, and with tall
buildings care must be taken to counteract the effects of wind turbulence.
Painting may have to be considered but this should always be seen as a second
line of defence. What is unforgiveable is the recurrent blemishing of an otherwise
welldesigned structure simply because of a careless attitude to long term
performance. Efflorescence is liable to occur on hardened concrete if, during the
pouring of successive lifts, the joint is not perfectly sealed, as free water can carry
a concentration of salts along myriad paths. Weather patterns are so often
determined by the configuration and efficiency of movement joints, so that their
location must be carefully established. Daywork joints must also be positioned
economically and intelligently.
Cracking of in situ walls or across a building unit allows moisture to
concentrate along channels which feed the fissures and highlight those already
evident at the end of sills and at badly formed joints between precast units.
Structural elements, such as the abutments of bridges, are prone to the migration
of groundwater into them, and retaining walls are targets for outpourings from
mortar

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