Report 1 (of 6): Bricking
it - Zimbabwe
Introduction
The building industry in Zimbabwe is growing fast
and new brick houses are being built everywhere.
Traditional homes which are round and built using
wattle and daub (pole and dagga) are being replaced
by these brick houses. There are standard specifications
for bricks in Zimbabwe which effectively prohibit
the use of farm bricks in urban construction. The
lowest quality of brick required for general building
purposes in towns and cities is called the common
brick. Many other countries have similar standards.
Farm Bricks
Farm bricks are made using the traditional method
of slop moulding' brick clay. Slop moulding involves
digging up clay and mixing it with water before leaving
it overnight to make it ready for moulding. The site
should be close to a source where clay can be easily
found, for example, large anthills, and near to a
supply of water. Once moulded, the bricks are laid
on the ground to dry in the sun and afterwards they
are stacked into a clamp which is a rectangular pile
of bricks with firing tunnels built into it. The
clamp is plastered with mud to provide insulation
and burnt by lighting wood fires for 2 or 3 days.
Farm house bricks are generally of poor quality,
misshapen, underfired, and relatively weak with high
water absorption. However, they are perfectly adequate
for farm or single storey building purposes.
Common Bricks
A slop moulded brick will satisfy the minimum standards
required for common bricks' providing the soil used
is of exceptional quality and has been carefully
prepared. Common bricks must have an average compression
strength of 7 Mega-Pascals; they must have a water
absorption of less than 15% by weight; and they must
resist a specified water spraying test for erodability.
Their shape must be regular, their faces smooth and
the dimensions uniform. Common bricks are the type
most in demand by house builders.
[1 Mega-Pascal = 1 million Newtons per square metre
= 145 pounds per square inch]
Soil Preparation
Crushing
The hammer hoe is a hand tool used for digging clay
and provides some initial crushing of the soil before
it is taken to the manufacturing site. At the site,
the soil can be crushed using "punners" which are
heavy metal sections on a vertical shaft used by
one person to stamp the soil.
Sieving and Weathering
The partly crushed soil is sieved through a screen
with a mesh size of 5 to 6 mm. The soil that does
not pass through the mesh is crushed again with the
punners and if after this, it does not pass through
the sieve it is left in weathering heaps for the
elements to break down.
Soaking, Mixing and Tempering
The crushed soil is placed in soaking pits, along
with any other additional materials which make it
suitable for brick making, for example, sand etc.
Water is added and allowed to saturate the soil.
The soil must be thoroughly mixed and the cheapest
and most effective way of doing this is treading
it with the feet, known in Zimbabwe as "dancing".
The soil is then left to "temper" in the pits until
the water has been completely absorbed and the consistency
of the mixture is uniform. Tempering times vary between
soils but generally the longer the better.
Moulding
When the clay has the right moisture content it
can be moulded. A good brick can be made even in
a simple hand mould. The production rate can be increased
and the uniformity of the bricks can be guaranteed
if a table moulding system (figure 1) is used.
Lumps of clay (clots) are prepared on the table and
coated in sand. The mould is placed on the table over
a plate with an indent and the clots are placed into
the mould. The excess clay is trimmed off and returned
to the clot forming stage, leaving a smooth and consistent
finish to the brick. The mould is lifted off the plate
and pressed down over an ejecting piston where the
brick is left until it is taken to the drying area.
The mould is dipped in water, the bottom coated with
sand and the process continues.
Drying
Stacking bricks too high or while they are still
wet or handling them excessively will result in deformation
and damage. The unfired bricks need to be stacked
with sufficient air flow between them on a smooth
and clean drying surface. They also need to be shaded
from the sun to prevent cracking.
Firing
Bricks are fired in clamps after they have been
dried. The clamps are flat topped pyramid shaped
heaps with steep sides and an insulating layer of
mud. The kiln takes approximately a week to build,
two weeks to fire and a week to cool although this
is to some extent dependent on the size of the kiln
and the weather. Boiler waste is used as fuel for
the kiln and is poured into channels between the
bricks and smoothed down until it is evenly spread.
Clamp kilns - one being fired, the other dismantled.
Boiler Waste
Coal is produced in Zimbabwe and is readily available
in the urban areas. It is used extensively in power
stations, as well as in processing plants that use
steam or oil for conducting heat, for example, sugar
refineries and food processing industries. The efficiency
of operation of these plants determines to a large
extent how much energy is being extracted from the
coal, and how much is being left behind as boiler waste.
The ash from inefficient boilers retains a high energy
value and burns very cleanly. Boiler ash will always
vary in terms of energy value, depending on the efficiency
of the power station or processing plant.
Boiler waste is available free from certain industrial
and manufacturing plants - particularly Harare's
coal-fired power station which has a problem with
disposal and brickmakers need only pay for transport.
Consequently, it is a viable energy option for firing
clay bricks on a small-scale basis.
The fine particles of the boiler ash are normally
sieved out and used as a blending material before
moulding, especially in soils with high clay content.
This ensures that the brick will burn uniformly,
with lower levels of external energy required. The
bricks are lighter, as the burnt ash leaves small
cavities which gives the bricks effective heat insulating
properties. Only the coarse particles are used for
firing the clamp. These permit an efficient air flow
within the clamp, achieving uniformity and higher
temperatures all over.
A clamp kiln showing boiler waste in the channels.
The quality of the brick is influenced by the firing
temperatures. By using sieved boiler waste as the
sole source of energy and the clamp arrangement described
above, temperatures of between 950 and 1150 degrees
centigrade can be achieved. A rich boiler waste can
reach vitrification temperatures, which for most
soils are around 1100 degrees centigrade and result
in much stronger bricks.
Small-scale brickmakers will build clamps of 20,000
to 30,000 bricks. The larger the clamp the more energy
efficient the firing process. The clamp needs no
permanent structure and therefore, the only investment
costs are hand tools and the land to build the kiln
on. The savings derived from using boiler ash are
vast compared to commercially sourced coal because
only the transport costs needs to be paid for and
a better quality of brick is produced. The use of
boiler waste rather than wood helps reduce the problems
of deforestation and subsequent soil degradation.
For further information, please contact:
Intermediate Technology Development Group PO Box
1744 Harare Zimbabwe
Tel: 263 1 1402896 Fax: 263 4 49041
Micro Concrete Roofing Tiles
Corrugated iron sheets and asbestos are still used
regularly as roofing materials. Corrugated iron sheets
are unbearably hot during a tropical summer and extremely
noisy in the rain, while asbestos is fundamentally
a dangerous material. Micro concrete roofing (MCR)
tiles are being produced which are straight forward
to manufacture and are made from locally available
raw materials, for example, sand, fine gravel, and
cement. These tiles produce a high quality, low cost
roof which is cheaper, cooler and quieter than corrugated
iron sheets or asbestos. It is much cooler to live
in, even in tropical temperatures, because the tiles
transmit much less heat than corrugated iron and
they absorb the noise of raindrops hitting the roof
making it much quieter during rainstorms. The roof
is extremely durable because the tiles are unaffected
by corrosion which means water can be collected from
it and will be clean enough to drink. MCR tiles are
produced using a self contained vibrating device
which is available at a reasonable price from about
30 international dealers. A plastic sheet is clamped
onto the vibrating table and a measured scoop of
mortar mix is placed into the frame. The vibrator
is switched on and the mortar is trowelled flat.
The frame is removed and the wet screed is carefully
drawn onto a plastic mould. It is left to harden
overnight and then placed in solar curing areas for
3 to 4 days or it is cured underwater for up to two
weeks.
The vibrating table operates from mains electricity
through a transformer, or direct from car batteries
or solar power, or it can be hand powered. The scale
of production depends upon the number of moulds supplied
with each tile making machine and the number of workers
employed. The JPM Parry & Associates: making
roofing tiles tiles can be produced with direct labour
and material costs as low as 10 US cents each.
For further information, please contact:
Roofing Advisor Service SKAT Vadianstrasse 42 CH-9000
St. Gallen Switzerland
Tel: +41 71 228 5454 Fax: +41 71 228 5455 JPM Parry & Associates
Ltd Overend Road Cradley Heath West Midlands B64
7DD England
Tel: +44 (0) 1384 569171/2 Fax: +44 (0) 1384 637753
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