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Structural Concrete Frames 351
is often used. The mixing drum is rotated by a petrol or electric motor, the drum being
tilted by hand to fill and empty it. This type of mixer takes over a deal of the backbreaking
work of mixing, but does not control the quality of mixes as materials are measured by
volume.
A concrete batch mixer mechanically feeds the materials into the drum where they are
mixed and from which the wet concrete is poured. The materials are batched by either
weight or volume. For extensive works, plant is installed on site that stores cement (delivered
in bulk), measures the materials by weight and mechanically mixes them. Concrete
for high-strength reinforced concrete work can only be produced from batches (mixes) of
uniform quality. Such mixes are produced by plant capable of accurately measuring and
thoroughly mixing the materials.
Ready-mixed concrete
Ready-mixed concrete is extensively used today. It is prepared in mechanical, concrete
mixing depots where the materials are stored, weight batched and mixed, and the wet
concrete is transported to site in rotating drums mounted on lorries (cement mixers). The
action of the rotating drum prevents aggregates from segregating and the concrete from
setting and hardening for an hour or more. Once delivered it must be placed and compacted
quickly as it rapidly hardens.
Placing and compacting concrete
The initial set of Portland cement takes place from half an hour to one hour after it is mixed
with water. If a concrete mix is disturbed after the initial set has occurred, the strength of
the concrete may be adversely affected. It is usual to specify that concrete be placed as soon
after mixing as possible and not more than half an hour after mixing.
A concrete mix consists of particles varying in size from powder to coarse aggregate
graded to, say, 40 mm. If a wet mix of concrete is poured from some height and allowed to
fall freely, the larger particles tend to separate from the smaller. This action is termed ‘segregation
of particles’. Concrete should not, therefore, be tipped or poured into place from
too great a height. It is usual to specify that concrete be placed from a height not greater
than 1 m.
Once in place, concrete should be thoroughly consolidated or compacted. The purpose
of compaction is to cause entrapped bubbles of air to rise to the surface in order to produce
as dense and void-free concrete as possible. Compaction may be effected by agitating the
mix with a spade or heavy iron bar. If the mix is dry and stiff, this is a very laborious process
and not very effective. A more satisfactory method is to employ a pneumatically operated
poker vibrator, which is inserted into the concrete and, by vibration, liberates air bubbles
and compacts the concrete. As an alternative, the formwork of reinforced concrete may be
vibrated by means of a motor attached to it.
Construction joints
Because it is not possible to place concrete continuously (on the vast majority of construction
sites), it is necessary to form construction joints. A construction joint is the junction
of freshly placed concrete with concrete that has been placed and set, e.g. concrete poured
on the previous day. These construction joints are a potential source of weakness, because
there may not be a good bond between the two placings of concrete. When forming a
construction joint, the previously placed concrete needs to be clean, with a sound surface