Building with earth - Gernot MINKE (1)

Bending tensile strength
The bending tensile strength of dry loam is
of little importance for earth construction.
Still, it has a certain significance when judging
the quality of mud mortar and the edge
rigidity of mud bricks.
Bending tensile strength depends mainly
on the clay content and the type of the clay
minerals involved. Montmorillonite clay has
a much higher bending tensile strength
than Kaolinite. The lowest value investigated
by Hofmann, Schembra, et. al. (1967) with
Kaolinite reached 1.7 kg/cm 2 , the highest
with Montmorillonite clay 223 kg/cm 2 .
Clays without Montmorillonite tested by
Hofmann, Schembra et. al. (1967) showed
tensile bending strengths between17 and
918 N/cm 2 .
Bond strength
Adhesive or bond strength is important only
with mud mortars. It depends on the roughness
of the base and the bending tensile
strength of the mortar. While the German
standard DIN 18555 (Part 6) gives a complex
standard testing method to obtain this,
a very simple test to check the bond
strength is shown in 2.39: two baked bricks
are joined by a 2-cm-thick mortar, the upper
skewed at 90° to the lower. After the mortar
is dry, the upper brick is laid on brick
supports at both ends, while the lower is
loaded with a sand-filled container. When
the mortar breaks, the weight of the lower
brick and the sand-filled container divided
by the mortar area gives the adhesive
strength. However, this is relevant only if
failure occurs at the joint. If it occurs within
the mortar, then this represents the direct
tensile strength of the mortar, which is less
than that of the bond.
Resistance to abrasion
Loam surfaces like mud mortar and mud
floors are sensitive to abrasion. One simple
test for abrasion is to use a metal brush,
loaded by a weight of about 5 kg, and
move it over the loam sample from side
to side. The material that comes off after a
certain number of cycles is weighed and
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2.39
compared with that of other samples. A
plate covered with sand paper can also be
used in place of a metal brush.
At the BRL, a special test for loam surfaces
was developed: a strong plastic brush of
7 cm diameter is rotated on the surface
under a pressure of 2 kg. After 20 cycles,
the amount of abrasion is weighed. Illustration
2.40 shows the apparatus and 2.41
the results with different earth plasters available
on the German market.
Modulus of elasticity
The dynamic modulus of elasticity of loam
usually lies between 600 and 850 kg/mm 2 .
Impact strength of corners
Due to mechanical impacts, corners often
break during the handling of mud bricks. In
practise, therefore, this kind of strength is
more important than either compressive or
bending strength. At the BRL, a special test
was developed to measure this kind of
strength against shocks (see 2.42): a weight
is dropped onto the surface at a 60° angle,
10 mm distant from the corner. Its bottom
is formed by a semi-spherical steel ball
30 mm in diameter.
Properties of earth
2.40
2.42