Introduction to concrete building products
Practical Action
incorporating an eccentric rotor can be attached to the table, avoiding the need for a belt. The
frequency of the vibration can be changed by running the motor faster or slower, and the
amplitude, or size, of the vibration can be altered by increasing the eccentricity of the weights
below the table. Products of different size and weight will have different optimum amplitudes and
frequencies of vibration, although in practice a wide range of frequencies and amplitudes will give
satisfactory vibration performance. In the 1990s Practical Action helped to develop a vibrating
table in Kenya which includes a small lever type press for pressing down on concrete blocks as they
are vibrated - because blocks are usually made with somewhat drier mixes than other products and
such a mix consolidates under pressure as well as vibration.
Vibrating tables offer the benefits of allowing a wide range of products to be vibrated and,
additionally, they are cheaper than conventional concrete products machines (the F.O.B. price of a
table starts at about US$3,000), and, where suitable workshop facilities exist, they are not too
difficult to fabricate locally. Their main drawback is that production rates are low. A person needs
to mind the concrete mould while it is being vibrated to avoid it falling off the table, so effectively
only one product can be vibrated at any one time. Alternatively, if the moulds are clamped onto
the table while they are being vibrated, extra time is needed to fasten and unfasten the clamps.
The Concrete Blockmaking Yard
Before considering layout of a concrete products yard it is important to note that the location of the
production site is very important. It should be located close to mains electricity and also close to a
water supply. The position in relation to transport of raw materials and finished products should
also be looked at and, in addition, if sales are carried out from the yard, this should be located
reasonably conveniently for customers, such as close to a main road.
Concrete products should ideally be made on a level concrete surface, although in drier areas
concrete products could be left on compacted ground. In wet areas, or in areas subject to seasonal
heavy rains, the casting surface should be set on a gentle slope, at least 1 in 100, to facilitate
drainage of the site.
Additionally a small one-room office and a shed to store the cement would also be required. The
yard should be close to a mains water supply for washing tools, hands and so on. The aggregate
should be stored in bunkers and separated into different size fractions. The sand should ideally be
kept under cover because when it rains the sand will tend to absorb a lot of water and increase in
bulk.
A small yard running a mechanical mixer and either an egglaying machine or a vibrating table could
be run by three people. The operation would run much more smoothly and productivity would be
higher if each person was not tied to a specific job and was prepared to work in a flexible manner.
The main operations required at the site would be :-
(i) Buying and, possibly, delivering cement and aggregate for the site.
(ii) Transporting cement and aggregate around the site.
(iii) Mixing concrete.
(iv) Loading the hopper of the egglaying machine, or filling moulds with concrete.
(v) Operating the egglaying machine or vibrating table.
(vi) Demoulding.
(vii) Stacking the products once they have hardened sufficiently.
(viii) Inspecting the products while curing and watering if necessary.
(ix) Quality control and testing.
(x) Stock control.
(xi) Maintaining production records.
(xii) Maintaining financial records.
(xiii) Selling and, possibly, delivering the products.
(xiv) Cleaning of tools and equipment.
(xv) Security.
A minimum size for the concrete products yard is 100 m2, or at the very minimum 90 m2, even for
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