Building with earth - Gernot MINKE (1)

11.4
11.1 Cutting joints
with the use of a
trowel
11.2 Scratching a
moistened loam surface
with a small rake
11.3 Tools for scratching
moistened loam
surfaces
11.4 Loam mortar test
• Animal or human hair, coconut or sisal
fibres, cut straw or hay should be added
(however, too much of these additives
reduce the ability of the plaster to adhere
to the ground).
• For interior plastering, sawdust, cellulose
fibres, chaff of cereal or similar particles can
also be used as additives.
• In order to develop enough binding force,
the adhesive forces of the clay minerals
should be sufficiently activated by an adequate
amount of water and by movement.
• When the plaster sticks to a sliding metal
trowel held vertically, yet is easily flicked
away, the correct consistency has been
achieved.
In order to test the characteristics of a loam
plaster, a simple adhesion test can be carried
out. The plaster to be tested is applied
2 cm thick to the flat surface of an upright
baked brick. The plaster should stick to the
brick until it is totally dry, which might take
two to four days.
If the plaster falls off in one piece by itself,
as seen in the left sample in 11.4, then it is
too clayey, and should be thinned with
coarse sand. If it falls off in portions after the
sample is hammered on the floor, like the
second sample in 11.4, then it possesses
insufficient binding force, and should be
enriched with clay. If the plaster sticks to the
brick but displays shrinkage cracks like the
third sample in 11.4, it is too clayey and
should be slightly thinned with coarse sand.
However, it can be used without thinning as
the first layer of a two-layer plaster. If the
surface shows no cracks and the plaster
does not come off when hammered, as in
the fourth sample seen in 11.4 (right), then
the sample might be adequate. In this case,
it is advisable to make a larger test, about
1 m wide and 2 m high, on the actual wall.
If shrinkage cracks now occur, then the mixture
needs to be either thinned with coarse
sand or mixed with fibres.
Exposed exterior loam plasters
Exposed exterior plasters must either be
seasonably weather-resistant, or else must
be given perfect weatherproof coating.
In cold climates, it is important that the
plasters, together with their coatings, have
a low vapour diffusion resistance, so that
water condensed in the wall can be easily
transported to the exterior. In order to meet
thermic and hygric influences without cracking,
the exterior plastering must be more
elastic than its ground. For cold climates, in
general, an external loam plaster is not recommended,
unless sufficient roof overhang,
plinth protection and good surface coating
can be assured.
Since plastered wall edges are very easily
damaged, they should either be rounded or
lipped with a rigid element. In extreme climates,
when the elasticity of large expanses
of flat plaster is insufficient to cope with the
effects of weather, vertical and horizontal
grooves filled with elastic sealants are recommended.
Chapter 4 discusses the overall
possibilities of reducing shrinkage and
enhancing weather resistance and surface
hardness.
Interior loam plasters
Interior plasters are less problematic. As a
rule, fine shrinkage cracks cause no problems
because they can be covered with
coats of paint. Dry loam plaster surfaces can
be easily smoothed by wetting and worked
with a brush or felt trowel.
93
Loam plasters