| Glass aggregate
is typically graded by color and size. Sizes can range from
sixinch rocks to gravel-sized pieces to a fine talc-like powder.
Polishing, grinding or other exposed aggregate techniques are
employed to reveal the glass. Or glass can be seeded on the
surface and then exposed.
Colored glass can be coordinated with the matrix of integrally
colored concrete. In general, lighter colors of glass are
used in darker matrixes, and vice versa. But not always.
A dark brown glass in a dark brown matrix can have an appeal
all its own. Mixing light and dark colors of glass will give
you a terrazzo effect. If you use clear glass aggregate,
it will take on the color of the matrix, and it will add
the most depth. Since glass is acid resistant, acid staining
will color the surrounding matrix without affecting the color
of the aggregate.
Finely ground glass can add background colors to the matrix.
Using finely ground clear glass in place of sand can make
for purer colors of concrete. Finely ground glass also lends
itself to highly polished finishes. A marble or granite took
can be attained by putting a high polish on concrete made
with finely ground, earth-toned glass aggregates.
As for strength, glass aggregate can match, exceed or fall
short of traditional aggregates, depending on size. Studies
have found that very finely ground glass aggregate used in
place of sand actually increases the strength of the concrete,
whereas gravel-sized glass aggregate decreases strength.
Mixing fine and coarse glass aggregates can have a net effect
of zero, rendering concrete no stronger or weaker than that
mixed with traditional sand and gravel.
Buy it or bash it
Glass aggregate can be obtained from a variety of sources.
Locally, recycling centers may have cutlet - crushed bottles
and other glass - cleaned and sorted by size and color. Nationally,
specialty glass manufacturers melt down bottles and window
glass to produce glass aggregate for terrazzo floor contractors,
landscapers and decorative concrete artisans.
And then there's always your personal stash of empty beer
bottles. With a small investment in a high-impact glasscrushing
machine, you can make your own cutlet, though some contractors
opt to do it the hard way - with the help of a scrub brush
to remove labels, safety goggles to protect eyes, and a hammer
to do the dirty work, The use of a good crushing machine
is preferred, though, because it doesn't produce the sharp,
dangerous edges of the hammer-the-bottle method.
Tom Ralston, of Tom Ralston Concrete in California, has
glass aggregate connections that include a hard-core beachcomber
who collects heaps of beach glass, and a glassblowing shop,
which always has an interesting mix of aggregate just waiting
to be raked off the floor. He has also mined his own stash
of empties, the down-and-dirty way. "I had my crew sitting
around like Cro-Magnon creatures, breaking beer bottles with
hammers into a cardboard box," says Ralston. "It
was like an ancient ritual."
Specialty glass aggregates made from recycled glass that
is melted down and re-formed give you a different look than
plain old crushed glass. Crushed bottles and window glass
tend to be flat, with parallel sides, whereas specialty glass
aggregates can have fuller, more irregular shapes, like crushed
gravel..
"Crushed bottles don't have as much depth and sparkle," says
Ken Thornley, general manager of Utah-based Heritage Glass,
one of the country's largest specialty glass manufacturers. "But
with the more irregular shapes, light bounces off it like
it does off the facets of a diamond."
Sparkle notwithstanding,
some decorative concrete artisans prefer cutlet for the
simple reason that it takes less polishing
or grinding to bring out flat pieces of aggregate than
it does to reveal more multidimensional aggregate.
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