Figure 108 shows how a good workable concrete mix turns stiff over time. When this happens more water must be added to render the concrete workable again, and this reduces both its strength and durability.
A workable mix will stiffen more rapidly when:
As well as using good materials, as described in Chapter 4,
concreting should be carried out early in the morning, before the
sun has had time to warm up the aggregate stockpiles.
The formwork should always be prepared the previous day. It should then be oiled just prior to concreting, making sure that no oil gets on to the steel reinforcing bars.
When concrete has to be poured underwater a tremie should be set up as shown in Figure 110a. This prevents the water washing the large aggregate out of the concrete mix (Figure 110b). The tremie is gradually pulled up as the pipe fills with concrete. The cement content for an underwater mix should be double that of the 1:2:4 mix described in Chapter 4. In other words, it should have one part cement for every one part of sand and two parts of aggregate.
All concrete that is not laid underwater should be vibrated
using a poker vibrator like the one shown in Figure 74c. The
vibration compacts the fresh concrete by eliminating the air
pockets trapped in the mix during the mixing. This generally
allows the use of a drier mix (i.e. less water is required for
compaction), which enhances the strength and durability of the
finished concrete. Concrete that is over-vibrated, on the other
hand, causes separation or segregation of the aggregate.
To achieve the right standard of compaction the poker should be worked into the concrete at regular intervals in an orderly fashion (Figure 110b). Each successive layer should be no deeper than 300 mm. In Figure 110a, the poker has been operated in a haphazard manner producing a substandard cast.
As soon as new concrete has been laid, the exposed surface
should lie covered with jute or reed mats and water sprinkled on
it at regular intervals for at least three days. This prevents
the concrete from drying out too rapidly. If left exposed to the
sun the surface will dry very quickly, shrink and crack. Once
shrinkage cracks develop, the surface cannot be repaired and the
concrete casting will deteriorate very rapidly.
Freshly cast concrete will only reach maturity after 28 days, but precast elements can usually be handled after seven days.