Building with earth - Gernot MINKE (1)

Addition of fibres
The rule of thumb says that cement and
Linear shrinkage (%)
The shrinkage ratio of loam can be reduced
by the addition of fibres such as animal or
human hair, fibres from coconuts, sisal,
bitumen as stabilisers are good for loam
with less clay, and lime for clayey loams. This
rule, however, does not take into considera-
Bentonite
Kaolinite
agave or bamboo, needles from needle
tion the type of clay. For instance, Montmo-
trees and cut straw. This is attributable to
rillonite and Kaolinite clay react quite differ-
the fact that relative clay content is reduced
ently, as described in chapter 4, p. 45. The
and a certain amount of water is absorbed
stabilisers cover the clay minerals and pre-
into the pores of the fibres. Because the
vent water from reaching them and causing
fibre increases the binding force of the mix-
swelling. In this chapter, common stabilisers,
ture, moreover, the appearance of cracks is
reduced. Some results of tests conducted at
used traditionally and up to the present,
are described. Other stabilisers that mainly
4.4
the BRL are shown in 4.5.
increase the compressive strength are mentioned
in this chapter, p. 45 and 47.
Structural measures
Water resistance can also be raised by
The simplest method for reducing shrink-
changing the grain distribution of silt and
age cracks in earth building elements is to
sand, as this author has demonstrated using
reduce their length and enhance drying
three mud bricks (shown in 4.6) onto which
time. While producing mud bricks, for
ten litres of water were poured for a period
instance, it is important to turn them upright
of two minutes. The brick in the middle,
and to shelter them from direct sunlight
with high silt content, showed extreme ero-
and wind to guarantee a slow, even drying
sion up to 5 mm depth. The brick on the
process.
right, with a higher clay content (ca. 30%)
Another sensible method is to design
showed erosion up to 3 mm depth; the
shrinkage joints that can be closed separately,
and which avoid uncontrolled shrink-
brick on the left, with the same clay content,
but less fine and more coarse sand, exhibit-
Fibre added (%)
age cracks (see chapters 5, p. 56; 8, p. 76;
ed very little erosion.
Coir
Flax straw
Rye straw
and 14, p. 113).
Mineral stabilisers (binders)
Silty loam mortar
Sandy loam mortar
4.5
Stabilisation against water erosion
In general, it is unnecessary to raise the
water resistance of building elements made
from earth. If, for instance, an earth wall is
sheltered against rain by overhangs or shingles,
and against rising humidity from the
Cement
Cement acts as a stabiliser against water,
especially in soils with low clay content. The
higher the clay content, the more cement is
needed to produce the same stabilising
effect.
Cement interferes with the binding force of
4.4 Reduction of shrinkage
by adding sand to
Kaolinite and Bentonite
4.5 Shrinkage ratio of
loam mortars with addition
of fibres
4.6 Erosion test on green
bricks
soil through the foundation by a horizontal
the clay and therefore it is possible that the
damp-proof course (which is necessary
compressive strength of cement-stabilised
even for brick walls), it is unnecessary to add
soil is less than that of the same soil without
stabilisers. But for mud plaster that is
cement, as shown in this chapter, p. 45.
exposed to rain, and for building elements
left unsheltered during construction, the
addition of stabilisers may be necessary.
Theoretically, a weather-resistant coat of
paint is sufficient as protection, but in practice,
cracks often appear on the surface or
are created by mechanical action. Further,
there is the danger of rainwater penetrating
the loam, causing swelling and erosion.
4.6
40
Improving the earth