Building with earth - Gernot MINKE (1)

2.32 Comparison of
indoor and outdoor air
temperature of a building
with adobe vaults (above)
with one using prefabricated
concrete slabs
(below) (Fathy, 1986)
2.33 Mould for preparing
test samples for the
binding strength test
according to the German
standard DIN 18952
2.34 Test apparatus
to measure the binding
force, developed at
the BRL
2.35 Relation of the
binding force to the permissible
compressive
stress in loam elements,
according to Niemeyer
2.38
compressive strength (N/mm 2 )
Compression
2.37
2.36
and rammed with a tool in a formwork in
three layers (see 2.33). At least three samples
have to be made from each mixture in
this way for immediate loading in the special
testing apparatus seen in 2.34. Here,
sand is poured into a container hanging on
the lower part of the sample at a rate of not
more than 750 g per minute. The pouring is
stopped when the sample breaks. The
weight under which the sample breaks,
divided by the section of the sample, which
is 5 cm 2 , gives the binding force. Then an
average is derived from the results of three
samples that do not differ by more than
10%. Typically, values vary from 25 to 500
g/cm 2 . Though in DIN 18952, soils with
binding forces below 50 g/cm 2 were not
recognised for building purposes, tests on
a variety of historic rammed earth walls in
Germany showed that some of these, in
fact, had much lower binding forces, and
one sample was even as low as 25 g/cm 2 .
Specific weight Compressive strength Allowable compressive force [kg/cm 2 ]
[kg/m 3 ] [kg/cm 2 ] wall column height/thickness
11 12 13 14 15
1600 20 3 3 2 1
1900 30 4 4 3 2 1
2200 40 5 5 4 3 2 1
Strength [N/mm 2 ]
Bending tension Tension
Green Brick A 3.5 1.1 0.4
Green Brick B 4.4 1.3 0.5
Green Brick C 6.1 1.6 0.6
Mortar D 2.02 0.69 0.21
Mortar E 2.63 0.85 0.35
2.36 Relation of binding
force to compressive
strength of various test
loams according to Gotthardt,
1949, and tests of
the BRL
2.37 Permissible compressive
stresses in loams
according to the German
standard DIN 18954
2.38 Strength of green
bricks and earth mortar
Compressive strength
The compressive strength of dry building
elements made of earth, such as earth
blocks and rammed earth walls, differ in
general from 5 to 50 kg/cm 2 . This depends
not only on the quantity and type of clay
involved, but also on the grain size distribution
of silt, sand and larger aggregates, as
well as on the method of preparation and
compaction.
The methods for treatment and additives for
increasing the compressive strength of loam
are discussed on p. 41. Niemeyer’s assertion
(1946) that the compressive strength is proportionate
to the binding force, and therefore
that loams with equal binding forces
should fall within the same range of permissible
stresses for use in buildings (see 2.35),
is disproved by Gotthardt (1949) and by the
BRL. By Niemeyer’s extrapolations,
a loam with a binding force of 60 g/cm 2
would have a permissible compression of
2 kg/cm 2 , and a loam with a binding force
of 360 g/cm 2 would have a permissible
compression of 5 kg/cm 2 . Experiments at
the BRL resulted in samples of a silty loam
with a binding force of 80 g/cm 2 but a compressive
strength of 66 kg/cm 2 , while they
also found samples of silty clay with a binding
force of 390 g/cm 2 which only displayed
a compressive strength of 25 kg/cm 2 . Some
of these results are shown in 2.36.
The permissible compressive strength of
earth building elements according to
DIN 18954 is between 3 and 5 kg/cm 2
(see 2.37). By this reasoning, the overall factor
of safety in earth components is about 7.
This implies that actual compressive strength
is seven times higher than the stress allowed
in the element. Going by the actual stresses
in the building illustrated in 1.11, built in
1828 and still in use, we have five-storeyhigh
solid rammed earth walls, and the
maximum compression at the bottom is
7.5 kg/cm 2 (Niemeyer, 1946), which would
not have been permissible as per DIN
18954.
In Yemen, there are examples of solid earth
houses as much as twice the height of the
one mentioned above. Obviously, it is possible
to build a ten-storey-high earth house,
but DIN 18954 permits only two storeys.
According to Indian standards for stabilised
soil blocks, the wet compressive strength
of the block has to be tested as well. Here,
the block has to be immersed to a depth
of 3 mm in water for 24 hours.
Tensile strength
The tensile strength or binding force of a
plastic loam was described on p. 32. For
earth construction, the direct tensile strength
of the dry material is of no relevance,
because earth structures must not be under
tension.
Table 2.38 shows that dry tensile strength is
about 10% of compressive strength with
blocks, and 11 to 13% with earth mortars.
33
Properties of earth