Introduction to concrete building products
Practical Action
Both methods of prestressing rely on the use of expensive
equipment and are not suitable for small-scale operation. It
should also be noted that it is more difficult and less safe to
demolish a building containing pre-stressed concrete elements
than one incorporating conventional reinforced concrete.
Decorative Products
These comprise garden ornaments and statuettes, benches
and seats and balustrades for staircases, for example. They
are usually made with a mix rich in cement that is made
flowing by the addition of excess water or the use of
superplasticizers. The maximum aggregate size may be
limited, e.g. to one centimetre across for larger products and
half a centimetre, or less for smaller and more decorative
products.
Colouring or staining of the concrete may be used to achieve
the decorative effects. They can also be painted, although
most paints do not hold fast well on concrete, especially
outdoors.
The concrete is usually poured into plastic or stiff rubber
moulds that have been specially fabricated for the products.
These can incorporate quite intricate shapes. When the
concrete has hardened the plastic moulds are simply prised
open while rubber moulds can be peeled off. It is especially
important to keep these moulds clean between uses and to
discard them when they start to deform with usage.
Large-scale Components
Large prefabricated concrete products are usually made at
large factories that use large specialised moulding, handling
and transport equipment. Products include wall panels,
staircases and even whole sections of buildings that are
subsequently bolted together at the building site.
In Europe and North America prefabrication was common for
building, especially for mass housing, several decades ago.
However, technical problems have been found with significant
amounts of this construction leading to the need for
demolition in some cases. Quality control at large-scale
prefabrication works then needs to be exceptionally high.
Steel reinforcement is used in nearly all large-scale
prefabricated components. This may also be pre-stressed.
Experimental and practical studies have also been
undertaken on prefabricated building components of
ferrocement. Ferrocement is a type of reinforced concrete
that uses one or more layers of thin steel wire mesh, e.g.
chicken wire, as reinforcement instead of conventional steel
rods, ties and cages. It is therefore cheaper than
conventional reinforced concrete and more orientated
towards the conventional builder rather than more highly
engineered construction.
Figure 2: Selection of Concrete
Products (page 30) – No. 1 Simple
air brick (as made in Kenya), No. 2
More complex example of air brick,
No. 3 Floor tile, No. 4 Hollow
block, No. 5 Wall coping.
Illustration from Vibrated Concrete Products
Production Manual (Draft), by Amon
Ng’ang’a, Building Materials and Shelter
Programme of IT Kenya, October 1994.
Prefabricated ferrocement wall and roof panels have been
7