Building with earth - Gernot MINKE (1)

in turn enlarges cracks and even causes
plaster to flake off.
During repairs undertaken in 1992, the oldest
German rammed earth house, built in
1795 (1.10), was found to have massive frost
erosion, which had destroyed the loam up
to a depth of 20 cm, because water had
penetrated through cement plaster applied
some decades before. A similar phenomenon
was reported from New Mexico, USA
by Bourgeois (1991). During a restoration
carried out in 1967, the church in Ranchos
de Taos (12.5), constructed of adobes in
1815, was covered with cement plaster.
Eleven years later, the cement plaster had
to be dismantled when the loam below
showed heavy moisture damage.
In cold climates, quick drying of the wall is
necessary if rain penetrates from the outside
or if vapour condensation from the inside
occurs. Therefore, the vapour diffusion
resistance of the outer layer should be
lower than that of the inside.
The German standard DIN 18550 (Part 3)
states that water-repellent external plasters
should fulfil the following conditions: water
absorption coefficient w ≤ 0.5 kg/m 2 · h 0.5 ,
the specific vapour diffusion resistance s d
must be ≤ 2.0 m and the product w · s d ≤
0.2 kg/m · h 0.5 .
The following sections describe the composition
and application of non-loam containing
plasters.
Preparation of ground
To provide a good bond, loam surfaces
that are to be plastered should be dry and
rough. Smooth surfaces should be sprayed
with water, so that their outer layers will
moisten and swell, after which they can be
grooved diagonally 2 to 3 mm deep, as
shown in 11.2. While the surface so prepared
is still moist, it should be primed with
thin lime milk, which should penetrate the
ground up to a depth of several millimetres.
A mix of 0.5 to 1 part of fat-free white
cheese, 2 parts hydraulic lime and 30 parts
water has also proved successful. If the lime
plaster is exposed to severe thermal forces,
if the unbroken area of the plaster surface is
very large, or if the bond is poor, expanded
metal meshes or reed mats fixed to the
ground may be required to take the plaster.
When using reed mats, it is advisable to dip
them in lime milk to prevent rotting.
Reinforcement
Larger unbroken panels subject to strong
thermal forces may require reinforcement.
For this purpose, a galvanised steel net with
hexagonal meshes (rabbit or chicken wire
mesh) or similar nets are commonly used.
Workmen often prefer using plastic covered
glass-fibre nets because they do not corrode
and are more pliable.
Composition
Normal lime plaster usually consists of
1 part hydraulic lime and 3 to 4 parts sand.
Since it is commonly used in construction
worldwide, it is not discussed further in this
book. However, lime-casein plasters are less
common, and are therefore described
below.
Old recipes often prescribe that animal hair
and casein be added to a normal plaster
to improve its behaviour. In former times,
casein was added in the form of whey or
buttermilk. Casein and lime react chemically
to form calcium albuminate, a wash-resistant
compound. The addition of casein
reduces the water absorption of lime plaster,
but at the same time hinders vapour
diffusion.
At the BRL, a lime-casein plaster for exterior
work was successfully tested. The mix consisted
of fat-free cheese, hydraulic lime and
sand in a ratio of 1:10:40. The lime has to
be first intensively mixed into the cheese to
form a creamy paste without adding any
water. After allowing the mix to rest for a
while, water and sand should be added.
For a thinner plaster that can be brushed on,
a slightly different mixture might be adequate,
with the proportion 1:6:25 of the
same ingredients respectively. In warm climates,
some kitchen salt should be added
to keep the lime plaster moist for a longer
period, which improves curing.
12.6 µ-values of
lime plasters (figures
referred to as volumetric
parts)
12.7 Loam wall with
additional exterior
insulation and wooden
planks forming air
cavity
12.8 Plinth designs
made incorrectly and
correctly
12.7
102
Weather protection