Introduction to concrete building products
Practical Action
forward to cast the next set. The blocks are consolidated in the moulds by vibration and with some
machines a head also applies static pressure before the block is ejected. This latter type of
machine has the advantage that a drier mix can be used, which responds to pressure as well as
vibration.
Small machines are driven by one person standing behind them. Some types are fitted with a
control device to ensure that the machine moves on the appropriate distance after each drop. The
moulds used are normally gang moulds holding up to 10 individual compartments. Production
rates vary from about 60 to about 500 blocks per hour depending on size of block and size of
machine.
Most types of egg-laying machines are made in Europe or in North America. The cost of the
smallest new machines on the market would be between US$10,000 and US$50,000 depending
on capacity.
Static Machines
Most static machines can make a range of different products just by changing the moulds.
However machines have also been designed to make only one product, concrete pipes for example.
They are often larger than egg-laying machines and, because of their high production rate, the
production yard may need to be mechanised, relying on an overhead hopper to feed concrete to the
machine and a conveyor belt to take cast products away from it.
The machine vibrates the concrete as it moulds it and sometimes compacts it by pressure as well.
Machines are usually driven by a three phase electricity power supply, although there may also be
an option with the smaller machines to drive them with a petrol or diesel engine.
Concrete products are normally cast on plastic or wooden pallets, and these pallets can be taken
away and stacked, so space requirements for a static machine can be considerably less than with
an egg-laying machine.
Prices of static machines start at around US$20,000. Production rates are 100 products per hour,
upwards.
Vibration Units
Vibration of wet mix concrete is very beneficial. It ensures that the concrete completely fills the
mould in which it is put and helps to remove air bubbles from the mix. The result is a stronger,
denser, more durable and more dimensionally accurate product. There is an optimal vibration time
for each type of product, which is usually around 20 seconds. Excessive vibration will lead to
segregation of the cement and aggregate and the development of weak areas in the product.
Vibration units are usually cheaper than egg-laying machines or static machines and ideally suited
to labour-intensive production in a small yard.
Three types of vibration unit are used in concrete work. They are the poker vibrator, the clamp-on
vibrator and the vibrating table.
Poker vibrators are placed inside the concrete. They are little used for concrete products except for
very large precast sections.
Clamp-on vibrators are attached to the side of the mould and are suitable for use on a variety of
different mould designs.
A vibrating table is essentially a flat metal table standing on legs. The table itself is isolated from
the legs by dampers which prevent the vibration of the table being transmitted through the legs and
into the ground. These dampers can be stiff springs, rubber studs or of some other elastomeric
material. An eccentrically weighted camshaft is attached below the table and turned by an
electric, petrol or diesel motor using a belt. Alternatively a commercially available vibrator unit
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