Building with earth - Gernot MINKE (1)

dropped until the groove is closed over a
length of 10 mm.
5. The numbers of strokes are counted and
a sample of 5 cm 3 is taken from the centre
in order to determine the water content.
When the groove closes at 25 strokes, the
water content of the mixture is equal to the
liquid limit.
It is very time-consuming to change the
water content repeatedly until the groove
closes at exactly 25 strokes. A special
method described in the German standard
DIN 18122 allows the test to run with four
different water contents if the number of
strokes is between 15 and 40. Illustration
2.16 shows how the liquid limit is obtained
using these four tests. The four values
are noted in a diagram whose horizontal
co-ordinate shows the stroke numbers in
a logarithmic scale, and the vertical co-ordinate
shows the water content as a percentage.
The liquid limit is obtained by drawing
a line through the four values and reading
the interpolated value at the co-ordinate of
25 strokes.
Plastic limit
The plastic limit (PL) is the water content,
expressed as a percentage, at the boundary
between plastic and semisolid states. It is
determined by means of the following procedure:
the same mixture that was be used
to define the liquid limit is rolled by hand
onto a water-absorbent surface (cardboard,
soft wood or similar material) into small
threads of 3 mm diameter. Then the threads
are moulded into a ball and rolled again.
This procedure is repeated until the threads
begin to crumble at a diameter of 3 mm.
Ca. 5 g are removed from this mixture and
immediately weighed, then dried to obtain
the water content. This test is repeated
three times. The average value of three
samples that do not deviate by more than
2% is identical with the plastic limit.
As the liquid and the plastic limits have
been defined using a mixture containing
only particles smaller than 0.4 mm, the test
results must be corrected if larger grains
have been sieved out earlier. If that portion
is less than 25% of the dry weight of the
entire mixture, then the water content can
be calculated using the following formula:
W 0 =
L
1–A
where W 0 is the calculated water content,
L the determined water content LL or PL,
and A the weight of grains larger than
0.4 mm expressed as a percentage of the
dry weight of the total mixture.
Plasticity index
The difference between the liquid limit and
the plastic limit is called the plasticity index
(PI). The table in 2.17 gives some typical values
for LL, PL and PI.
Consistency number
The consistency number (C) can be calculated
for any existing water content (W) of the
plastic stage by using the following formula:
C =
LL – W
=
LL – PL
LL – W
PI
The consistency number is 0 at the liquid
limit and 1 at the plastic limit.
Standard stiffness
As the definition of the plastic limit in Atterberg
is not very exact, Niemeyer proposes
”standard stiffness“ as a basis for the comparison
of mixtures of equal consistency.
The method for obtaining this stiffness is
described on p. 24.
Slump
The workability of mortar mixtures is
defined by the slump. This can be specified
by a method described in the German
standards DIN 1060 (Part 3) or DIN 1048
(Part 1). Here, the mortar is poured through
a standard funnel onto a plate that is lifted
and dropped by a defined type and number
of strokes. The diameter of the cake thus
formed is measured in centimetres and is
called the slump.
Water content W
0.35
0.30
0.25
0.20
15 20 25 30 35 40
Strokes
2.16
2.16 Deriving the liquid
limit by the multi-point
method according
to the German standard
DIN 18122
2.17 Plasticity index of
loams (after Voth, 1978)
2.18 Test assembly to
obtain the ‘w’-values of
loam samples (Boemans,
1990)
Type of loam LL [%] PL [%] PI = LL–PL
sandy 10 – 23 5 – 23 < 5
silty 15 – 35 10 – 25 5 – 15
clayey 28 – 150 20 – 50 15 – 95
Bentonite 40 8 32
2.17
Acrylic glass plate
Polyurethene foam
Filter paper
Loam sample
Glass-fibre reinforced polyester layer
Water 2.18
26
Properties of earth