Building with earth - Gernot MINKE (1)

Even after several months, small increases in
strength may be observed. A certain
amount of humidity is essential to this curing
process, so the loam or earth elements
have to be sheltered against direct sun and
wind.
4. The optimum water content is raised
with the addition of lime, while the density
at this new optimum level is less than that
without lime (4.12).
Results of experiments performed at the
BRL (4.13) show that the compressive
strength of a highly silty loam containing
12% clay, 74% silt and 14% sand, and having
a compressive strength of 50 kg/cm 2
without cement, decreases with the addition
of small quantities of cement. The original
compressive strength is reached again
with the addition of 2% cement.
As can be seen in 4.14, this original strength
is reached only at 4% when adding lime.
In this case, it decreases again after 6% of
lime stabilisation.
Even more significant is the reduction of
compressive strength while stabilising lean
mud mortars, as shown in 4.15 on the right.
The left side of the same figure shows the
corresponding changes in tensile bending
strength. The values of the dry and the wet
compressive strengths of handmade adobes
with varying percentages of cement content
are shown in 4.16.
Investigations at the ENTPE show that testing
pure Kaolinite with 4% cement increases
compressive strength, while with Montmorillonite,
the same amount of cement
shows a decrease in strength. With the
addition of 4% lime and 2% cement, the
compressive strength of both types of clay is
increased by nearly 100% (Oliver, Mesbah,
1985). It should be noted that these tests
were done with optimum water content
and with pure clay. However, in actual practice
this increase may not be so high, as
loam used in construction usually has a clay
content of 5% to 15% and may not be used
with optimum water content.
Results of tests conducted at the BRL with
handmade adobes are shown in 4.17 and
4.18. Here, four different mixtures of sand
and clay were tested with the addition of
6% cement and lime, respectively. It is interesting
to note that the results were nearly
the same in the case of sand for plastering
and sand with Bentonite. By adding lime to
these mixes, the compressive strength of
Kaolinite loam is even lower than that containing
sand (4.18).
From these investigations, we derive the
following guidelines:
1. Loam with high Kaolinite content should
be stabilised with cement (and not with
lime).
2. Loam with high Montmorillonite content
should be stabilised with lime or with a
mixture of lime and cement in the ratio 2:1
(and not with cement).
3. Strong compaction increases the compressive
strength of Montmorillonite significantly.
This effect is significant in Kaolinite.
CRATerre suggests appropriate stabilisers
Tensile bending strength (N/mm 2 )
Clayey loam plaster
Silty loam plaster
Sand
Cement added (%)
on the basis of liquid limit, plastic limit
and plasticity index (4.19), not taking into
account the type of clay minerals (CRATerre,
1979).
When adding cement to loam, the mixture
should be used immediately, since the
setting of cement starts at once. If the mix is
allowed to stand for several hours before
being pressed into soil blocks, the compressive
strength of these blocks may be
reduced by as much as 50%. However, if
lime is added, this time lag has no negative
influence on the final strength. If less than
5% cement is added, the drying process
affects the compressive strength. If the
5.0
4.0
3.0
2.0
1.0
0.0
Clayey
loam
Cement added (%)
Compressive strength (N/mm 2 )
Clayey loam plaster
Silty loam plaster
Sand
Silty
loam
Compressive strength (N/mm 2 )
Dry strength
Wet strength
Sandy
loam
Cement added (%)
4.13
4.15
4.16
46
Improving the earth