Building with earth - Gernot MINKE (1)

4.11
P d t/m 3
1.83
1.80
t’Su
0.13 t/m 3
1.70 1.70
+ 6%
Kalk
1.60
+ 3.5%
13.0 16.5
1.50
7 10 15 20 25
4.12
the German standard DIN 18127, the optimum
water content is said to be the one at
which a maximum dry density is achieved.
The compaction is to be done with a Proctor
hammer. In order to obtain this optimum
water content, samples with varying water
contents are compacted in this way and
their densities determined. The water content
which gives the highest density is called
the optimum water content. The curve
obtained by connecting these points is
called the “Proctor Curve” (4.11).
In earth construction, however, the maximum
density or compaction, and therefore,
the so-called optimum water content, do
not necessarily lead to maximum density or
compaction. Therefore the so-called optimum
water content does not necessarily
lead to the maximum compressive strength,
nor is it the most decisive parameter. On the
contrary, the decisive parameters are workability
and binding force; hence it is recommended
that loam should not be used with
optimum water content as per DIN 18127,
but instead with a water content somewhat
higher than the optimum so derived. In fact,
this so-called optimum water content may
be treated, in practice, as a minimum water
content. With compressed soil blocks, it has
been shown that a water content 10%
higher than the optimum gives better results
than the so-called optimum. Boemans also
stated that the optimum water content
does not usually result in maximum compressive
strength. He also discovered that if
there is lesser compaction and higher water,
then the same compressive strength may
be achieved by using higher compaction
and less water (Boemans, 1989, p. 60 ff.).
At the Labor Géomatériaux of the Ecole
Nationale des Travaux Publics de l’Etat
(ENTPE) in Vaulx-en-Velin, France, it was
found that the type of clay minerals involved
also influence the compressive strength
after compaction. For instance, by raising the
static pressure from 2 to 8 MPa when producing
soil blocks using a press, the compressive
strength rose by about 50% with
Kaolinite, and by about 100% with Montmorillonite
(Oliver, Mesbah, 1985).
Mineral additives
Lean clayey loam can reach a higher compressive
strength with the addition of Montmorillonite
clay. At the BRL, tests were conducted
with sand enriched with 17% by
weight of Kaolinite and Bentonite respectively.
(Bentonite contains about 70% Montmorillonite).
With Kaolinite, the compressive
strength reached was 5 kg/cm 2 , and with
Bentonite, 12 kg/cm 2 .
The addition of lime and cement, usually
intended to increase the weather resistance
of loam, also generally increases compressive
strength. As described here, however,
compressive strength may also be
decreased by these additives, especially in
amounts lower than 5%. This is because
lime and cement interfere with the binding
force of clay minerals. The greater the clay
content, the higher must be the amount of
lime or cement added.
Tests have shown that as a rule, lime offers
better stabilisation with rich clayey loams,
while cement gives better results with
leaner loams. Furthermore, cement is more
effective with Kaolinite and lime with Montmorillonite.
In practice, it is always recommended
that relevant tests be conducted.
When doing so, the following points are to
be kept in mind:
1. When loam is stabilised with cement or
lime, some pores should remain. Only the
points of contact of the larger particles
should be cemented together, but fewer
pores should be filled than with concrete.
2. When the cement hydrates, free lime is
formed. This reacts with the silicate acids of
the clay minerals so that in addition to the
early stabilisation caused by cement, a
longer lasting hardening also occurs. Unlike
cement concrete, therefore, the strength of
cement-stabilised loam increases a little
even after 28 days.
3. When adding hydraulic lime, an ion
exchange between the clay minerals and
the added calcium ions takes place, lasting
between four and eight hours. The additional
hardening process caused by the
reaction of the hydrated lime with the carbon
dioxide from the air occurs very slowly.
45
Improving the earth