Burnt country tile
A
roofing tile is made of common clay into shape with a wooden mould and
fired in a kiln. The tiles can be used on a wooden or balli
understructure.
Country tiles are traditionally made by village potters in various ways
ranging from the wheel to wooden moulds.
Traditionally the tiles are held in
multiple layers over heavy wooden or bamboo understructure. The rising
cost of timber and bamboo and their competing use by contractors and
paper mills at subsidised rates have helped push tile roofs out of
existence. The tiles are prone to breakage under impact loads.
Appropriate for:
1.
All areas where pottery skills exist
traditionally and clay is available.
2. Areas where pitched roofs are common.
Improvements over
traditional techniques:
1. The
tiles are lighter and longer than traditional tiles and use less
understructure….
Extract from Appropriate Building Systems Instruction Manual.
Based on the experience of Shelter Group, Development Alternatives.
|
The
appropriateness of Earth as a building material is undisputed. However,
its acceptance as
pucca
construction is still limited, due to its susceptibility to water
erosion and resultant loss in compressive strength.
In regions with low or average rainfall, protection against rain can be
provided simply by taking suitable design precautions. However in areas
with high and driving rain, stabilisation of the wall material and/or
protective coverings on the walls become essential. Stabilised earth
blocks stand up better to moisture ingress but very often their cost
becomes a limiting factor. Plasters, on the other hand, being surface
treatments are cheaper in terms of initial capital investment.
Moreover, plasters provide a wide scope for surface treatment and
decoration and are a very popular form of self-expression.
Many traditional forms of plasters have been developed for the
protection of earth walls.
Traditional plasters of soil, straw and cowdung which are very effective
and popular even today, need recurring maintenance. This has never been
a problem in the rural areas where annual/biannual replastering (lipai)
of the house by the houseowners themselves is a rule rather than an
exception. However, lifestyle are changing fast and people are looking
more and more towards buildings which need less maintenance and care.
Therefore, longer lasting plasters which keep looking good while being
economical are required.
Plasters increase longevity and performance of earth wall by:
1.
Forming a weather barrier against moisture
ingress.
2. Increasing the resistance of the wall to wear and tear.
3. Improving appearance and decreasing maintenance.
The performance of a
plaster depends upon the type of the wall surface as well as the
materials used in the plaster. A difficulty with conventional cement
based plasters is that they are not compatible with earth walls. Earth
structures also react significantly to temperature changes and levels of
humidity. Alongwith thermal movements in the walls, they absorb and
release moisture. Conventional plasters of cement and sand are
relatively rigid and unable to respond to the movements of the wall
material. The bond between the two eventually weakens and the plaster
tends to crack and fall off.
Therefore, a plaster mix should perform its function of a good
protective cover while accommodating movements within the wall. In
essence, it should ‘allow the wall to breathe.’ It has been found with
experience that soil based plasters behave best on soil walls and are
inexpensive too.
A variety of plaster mixes and techniques have been developed to improve
the quality of soil based plasters. These soil based plasters work in
the following three ways:
1.
By reducing the absorption of the material
so that it prevents the moisture from reaching the wall underneath even
while getting wet itself. This is done by decreasing the surface pores
or by applying a thicker coat. A thick plaster may, however, slump off
the wall due to its own weight and in areas with continuous rainfall the
plaster still transmits moisture to the wall.
2.
Stabilisation of the plaster makes it more
water resistant. Stabilisers like cement, bitumen and lime are added to
soil in small quantities. Such plasters are longer lasting and more
flexible than conventional cement plasters. Limitations to the use of
these plasters are the availability and cost of the stabiliser and the
skill involved in its use. |
3.
Application of a surface coast over the
plaster or exposed mud wall surface to make the surface non-absorbent.
In this category are industrial paints and polymers, which tend to flake
and peel off over time due to ultraviolet radiation. Natural substances
like resins and cowdung-slurry need routine maintenance. Recently
developed silicon based materials get absorbed in the wall and reduce
its surface tension making it hydrophobic. These materials have shown
very promising results but their long term performance and life span is
still under test.
Whatever be the type of
plaster, general rules of plaster application have to be followed.
These include:
1.
Preparation of the wall surface to receive
the plaster coat. This may be done by grooving all horizontal mortar
joints to a depth of 1cm. to provide keys for a greater wall to plaster
bond and allow direct load transfer of the plasters’ self weight as
opposed to simple adhesion between the two vertical surfaces. The same
could also be achieved by facing the wall with bits of broken pottery,
tiles etc. which project from the wall surface.
2.
Brushing the surface clean to remove loose
pieces, dirt etc.
3.
Filling cracks with mud slurry and allowing
the surface to dry.
4.
Applying a thin coat of the slurry of the
binding agent (in case of stabilised plasters) prior to the application
of the plaster.
5.
Moistening the surface before plastering.
6.
The plaster
is then applied with force and spread smooth by the mason’s
phanti.
Horizontal
joints are normally avoided and vertical joints are never made at the
corners. Two thin layers of soil based plasters work better than one
thick layer. The plaster must be allowed to dry slowly. Therefore, it
is best to apply the plaster when the wall is shaded from direct
sunlight and to avoid plastering on very hot or windy days. It is clear
that workmanship and quality of the raw material are as important a
determinant of plaster quality and its performance as the compatibility
of the mix to the wall. Improved plaster mixes and techniques can
significantly upgrade the overall performance of an earth wall, but
cannot completely replace the need for maintenance.
A variety of plasters have
been developed by R&D institutions and individuals active in this
field. Some of these recipes are:
1.
Bitumen cutback plaster (CBRI): The bitumen
cutback is prepared by mixing hot bitumen (having 80/100 penetration)
with kerosene in a ratio of 5:1. One part of this cutback is mixed with
20 parts by volume of a previously fermented (for 10 days or so) soil
and bhusa mix (1:1 in ratio). Water is added and it is well pugged by
feet. It is applied in two layers, the second after the first has
dried. The plaster has shown very good results with a life of at least
7 years.
2.
Lime-soil plaster (ASTRA,IIS,Bangalore): One
part of hydrated lime is mixed to 2 parts of soil and to 3-6 parts of
sand depending on the clay content of the soil. Before application the
surface has to be well prepared and a base coat of lime wash applied.
After plastering the surface must be rendered with lime. The
performance of this plaster depends on the quality of lime available and
the type of soil. It works best with red soils.
3.
Traditional lime-sand-surkhi plasters in
1:2:4 ratio also work very well on earth walls. Their use is however
limited by the non-availability of good quality lime, the skill required
to use it and the cost of the plaster.
4.
Surface
coats:
Most soil based plasters absorb moisture to a greater or lesser degree.
When a suitable time lag is available between two consecutive rain
showers the wet plaster gets time to dry out. But, during continuous
and long duration rains, even good plasters moisture upto the wall
underneath. In such a case a surface coat applied over the plasters
proves helpful. The function of the surface coat towards the plaster is
the same as that of the plaster to the wall. This is a thinner coat and
is cheap and easy to maintain; it also helps avoid frequent plaster
replacements.
Traditionally a
lipai
of cowdung and soil in a ratio of 1:1 has been the most popular surface
coat used over soil walls in rural areas. Besides the strong belief in
the rural areas on its hygienic qualities, it is easy to apply and
readily available. It is redone as frequently as every month. Other
natural surface coats include the resin from the
Accacia arabica
tree (very commonly found in India), certain gums milk products like
cassien (and even blood and urine of some animals) and mixed in the
soil.
The in-situ calcium soap coating developed by ASTRA is also very
effective and economical.
Industrial paints and polymers too, are gaining acceptance; but their
use will be limited because of cost. The silicon based compound
Repellin S-101 has shown promising results. It is not too expensive and
being transparent does not spoil the natural colour and character of
earth walls. Its long term performance and life are under test.q |