Abele and Gow (1976) found that snow temperature
product of recrystallization. The grain shape was
at the time of compaction exerts a considerable
remarkably equidimensional and the bubbles small and
influence on the microstructure of the resulting
uniformly distributed. Figure 14 is a photograph of a
material. At snow temperatures lying somewhere
typical thin section of CFG ice, and Figure 15 shows a
between 5 and 10C , the temperature range
graph of the grain size distribution.
including most of our snow samples, ice made by
Discussion
compaction at high stresses (around 6.87 MPa) is not
as quick to recrystallize as it is above 5 or below
Ebinuma and Maeno (1984), compacting ice
10.
spheres about 0.2 mm in diameter, found that the
spheres appeared as individual particles up to a density
of about 700 kg/m3. By the time the material reached
a density of 750 kg/m3, the individual particles had
APPLICATIONS
coalesced into a more or less uniform consistency.
Natural snow strength increases with density all the
The bricks fabricated in this study are too small to
way to ice of 910920. (See Kovacs 1993.) Up to this
be widely useful in most construction roles. A much
critical density, densification is achieved largely by
larger building unit would be needed for field appli-
rearrangement of the particles. At densities greater than
cations. The bricks should be as large as possible to
this, further densification results from plastic
minimize the time of construction but still be small
deformation, which leads to gradual extinction of the
enough so that one person can handle them easily. A
brick size about 0.15 to 0.2 m2 in surface area and 0.1
pores and creation of a uniform, fine-grained crystal
to 0.15 m thick, for example, weighing about 25 kg,
structure.
would be large enough that fairly extensive projects
In a similar study Yosida (1963) describes a series
like retaining walls and pavements could be constructed
of snow-compaction experiments in which low-density
rapidly with minimal manpower. In order to ensure
natural snow is compressed using a 10-ton press whose
uniform high density throughout the brick, the
speed range varied from 1.0 to 1.8 mm/min. to
thickness of the snow mass normal to the stroke should
investigate the effect of very high compaction
be the smallest dimension. Therefore, this brick would
pressures. In this study the original material was snow
with irregular grain sizes ranging from about 0.5 to
require a compressive force of about 130 tonnes
distributed evenly across the 30- 60-cm face. In
1.0 mm (see Fig. 16a), and the author used unconfined
columns of snow up to 15 cm high. Yosida made thin
theory, this is not a problem; however, in practice, such
a hydraulic press is very large, and seldom routinely
sections of the snow column at various stages in the
available.
compaction process. Photos of these samples are shown
on Figure 16. This snow, compacted to a density of
Such bricks should also be strong enough to endure
repeated traffic or other loads expected in their final
application. In areas where water is present, either salt
within the individual crystals. At a density of 780, at
or fresh, they should be impermeable for maximum
the low end of the density range of our CFG ice, the
useful life.
crystal structure was metamorphosed into a fabric with
A round shape would be best from the standpoint
generally regular grains about 0.2 to 0.5 mm in
of reducing edge effects, and the form would be simple
diameter. With additional pressure, these crystals are
to make, requiring little reinforcement. It would not,
further reduced in size.
however, be the best shape for a building block. On
This process appears to be at work in the CFG ice
the other hand, a square or rectangular shape, ideal for
since its grain size averaged about 0.2 mm (see Fig.
paving or building, would require a form that was very
15), roughly 25% of the grain size of our parent
strongly reinforced, and probably heavy and difficult
material. It is interesting to note that the fine-grained
to handle, and edge effects might introduce flaws and
structure of the final product is a result of the
inhomogeneities that we did not find in our round
subdivision of much larger grains in the original
bricks. A compromise such as a hexagonal or octagonal
material. There is great similarity in crystal shape and
size between the CFG ice shown in Figure 14 and
shape might prove the best choice.
Yosida's compacted snow at a density of 780 kg/m3
In terms of the energy cost of compaction to produce
(Fig. 16c). The much smaller crystals in his final section
ice as compared with melting snow and refreezing it,
at the density of ice were produced by a much higher
it is clear that compaction is by far the more efficient
load (not given) than we used. It is interesting to note
method. For example, to fabricate a block with a density
of 860 and size of 30 60 15 cm, weighing 23 kg,
that there are no bubbles visible.
Concerning why CFG ice is so slow to recrystallize,
the required compression pressure would be 130 tonnes
13
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