Introduction to concrete building products
Practical Action
mechanical mixer.
Mechanical mixers can be powered by electricity, or a petrol or diesel engine. Small mixers which
run off electricity usually use a single phase mains supply or generator while larger mixers usually
use a three phase mains supply. There are also simple manual mixers on the market which require
no power supply. One design of such a mixer consists of a specially shaped plastic drum which
can be rolled about on the floor. Mixing concrete in this way is said to be easier than mixing with a
shovel.
There are essentially two types of mixer used in concrete products yards and building sites - the
drum and the pan. Drum mixers are typically used in small yards. Concrete is tumbled in a
rotating inclined drum with an opening at one end. Usually the drum can be tilted downwards to
discharge the concrete from the open end at after mixing. Pan mixers are usually larger (and also
more expensive) than drum mixers. Most types consist of a large bowl with a flat bottom, with this
bowl being rotated. Various paddles and scrapers are inserted into the concrete in the bowl and
held stationary while the bowl rotates thus churning the concrete. In general it is claimed that pan
type mixers are more efficient in mixing concrete than drum types.
The dry ingredients for concrete are usually mixed first in the mixer for a minute or more before the
water is added gradually over about a minute. Mixing is continued for another minute, or longer,
depending on the type of mixer and the quantities being mixed, until a homogenous concrete mass
is achieved. It is good practice in small yards to mix concrete in batches rather than to
continuously remove concrete and add more raw materials, because it is easier to keep the mixer
clean and also this avoids the inclusion of old concrete, more than an hour old, which has started
to set. Note, however, that continuous mixing of concrete will delay setting slightly and this is one
extra advantage of having a mechanical mixer.
To prolong the life of concrete mixers it is important that they are thoroughly cleaned after each
use. Although most mixers have been designed to minimise concrete getting into the motors,
bearings and drive shaft, they are not protected against continual misuse, which may break the
mixer after only a few weeks and result in a large repair bill.
Moulds and Moulding Machines
Before considering the different types of moulds and moulding machines used to shape concrete
products it is worthwhile to mention the different states in which concrete is used because the type
of moulding technique to use will depend strongly on the state in which the concrete is in.
(i) Dry Mix
In this state the concrete cannot be said to be workable in the conventional sense. In the hand it
will barely feel moist. It contains very little water above the quantity required for the cement
hydration reaction to proceed. This type of concrete could not be used by the small producer who
would not have the equipment to mould it. It is usually moulded in presses exerting very high
pressures and cured in a high humidity chamber or heated steam chest. Because of the low water
content in the mix, the cured concrete can have very high strength.
(ii) Semi-dry Mix
A semi-dry mix is not easily workable and will just feel moist rather than wet in the hand. Normally
the concrete would have zero to 40mm slump. With semi-dry concrete, consolidation by vibration
may not be effective and pressure, or a period of continuous tamping may be preferable. This is
the best type of mix to use for products with simple shapes, such as blocks or slabs, which are cast
manually into moulds rather than by machine. Normally the moulds are filled in several layers and
after each layer the concrete in the mould is pressed down with repeated application of a tamping
tool, such as a thick metal plate, about half the size of a hand (about 7 x 5 cm), welded onto a
metal handle.
When making blocks with a semi-dry mix the mould can be stripped from the block immediately
after casting. The cast block will be strong enough to remain standing without slumping and can
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