Civil Engineering Reference
In-Depth Information
are necessary for buildings more than two storeys high, whatever the quality
of the earth.
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To reduce water penetration.
Lime, Portland cement, pozzolana cement and
waterglass are examples. In areas where there is a great deal of driving rain it
is advisable to have one of these additives in the earth mix as well as external
cladding on the wall. In some case whey, casein, bull's blood, molasses and
bitumen have been used for the same reason.
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To avoid shrinkage.
This is mainly achieved by natural fibres, even though
cement and lime also reduce shrinkage.
Lime and cement
Lime is the stabilizer for argillaceous (clayey) earth. Both slaked and unslaked
lime can be used. The lime reacts with the clay as a binder. Lime can be used with
silt containing a lot of clay, sand or gravel and is usually mixed by sieving into
the proportion of 6-14 per cent by weight.
Portland cement is the stabilizer for earth rich in sand or containing very little
clay. The proportion of cement to earth is 4-10 per cent by weight. This can also
be used in foundation walls. The humus in the earth can attack the cement, so
this construction technique is assumed to have low durability.
Pozzolanic cement can be used in both types of earth, either lacking or con-
taining a lot of clay. It has about the same properties as Portland cement, but has
to be added in slightly larger quantities.
All lime and cement additions reduce or remove the possibility of recycling the
earth after demolition or decay.
Natural fibres
Natural fibres are best used in earth containing a lot of clay to increase thermal
insulation and reduce shrinkage. A mixture of 4 per cent by volume of natural
fibre will have a very positive effect on shrinkage and strength. The normal pro-
portions in the mixture are 10-20 per cent by volume. Larger amounts than this
will reduce its strength. In non-structural walls which are primarily for thermal
insulation, it is normal to increase the fibre content to 80 per cent, but this wall
will not hold nails.
Straw chopped into lengths of about 10 cm, preferably from oats or barley, is
normally used. Pine needles are also good binders; alternatively stalks from
corn, flax, dried roots, animal hair, twigs, sawdust, dried leaves and moss can
be used.
If large amounts of organic material are used, mould can begin growing only
a few days after erecting the wall. This is especially the case when blocks bound
with a thin loose mixture of clay are used. These walls must dry out properly and
cannot be covered until the moisture content has reduced to 18 per cent.
