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The technical side of Granite
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 Granite
is one of the oldest building materials known
to man. The use of granite adds both permanence
and style to any project. Granite will enhance
your project with an aesthetically pleasing image
while providing a façade of high performance
with low maintenance.
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By
geological definition, there are hundreds of stone
types commonly used as dimension stone products.
The commercial  definitions
of stones are much broader, allowing stones with
similar mineralogy, workability, performance,
and behavior to be combined into one classification,
discounting the fact that they may be scientifically
classified as different stone types. For example,
stones such as gabbro, diabase, diorite, anorthosite,
etc., are marketed commercially as granites because
their properties are similar, even though they
are not true granites by geological definition.
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Most
stones used in dimension stone applications will
fall under one of five commercial definitions:
Granite, Marble, Limestone, Quartz-Based, or Slate.
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The
term granite comes from the Latin root word granum,
meaning "grain". The geological definition
of granite is "any plutonic rock in which
the mineral quartz makes up 10 to 50 per cent
of the felsic components, and the ratio of alkali
to total feldspar is between 65 and 95 per cent."
Commercially, any holocrystalline quartz-bearing
plutonic rock is generally included in the granite
group
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The
granite group is one of the most versatile stone
types available. Granite, and granite-like materials,
are capable of taking a wide variety of finishes
which allow the designer to custom-tailor the
stone to the aesthetic or performance requirements
of a specific application.
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 Resistance
to scratching and durability in foot traffic areas
are largely dependent upon the hardness of the
minerals that make up the stone. In most granites,
the primary minerals are quartz and feldspars,
accounting for approximately 90% of the stone.
The hardness of a mineral is oftentimes defined
by use of Moh's Scale of Relative Hardness, developed
in 1822 by the Austrian Mineralogist Friedrich
Moh. This scale lists 10 minerals in ascending
order of scratch resistance:
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Feldspar
and quartz are the minerals that give granite
its exceptional abrasion resistance. This abrasion
resistance contributes to its long service life
in high traffic areas of public buildings.
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The
dimensional stability of granite is very good,
so good in fact, that granite is the material
of choice for high precision applications such
as surface plates, machine mounts and press rolls,
where tolerances can be measured in micro-inches
(millionths of an inch).
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Granite,
like any solid, will expand and contract with
changes in temperature. This change is relatively
small. The coefficient of linear thermal expansion
of granite is typically in the neighborhood of
4.4 x 10-6 inches per inch per degree Fahrenheit.
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Granite
will typically return to its original dimension
when the original temperature is reestablished.
Permanent strain, or failure to return to its
original dimension will not normally occur unless
the material has been heated to excessive temperatures
(above 480°F [250°C]).
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Granite
has a natural resistance to caustic chemicals.
This level of chemical resistance contributes
to the ability of granite to resist attack from
airborne pollutants associated with acid rain
and/or snow-melting chemicals. Certainly there
are chemicals that will attack granite, but exposure
to them in a typical building environment would
be extremely rare.
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Flexural
strength, or the ability to resist bending force,
is a factor that determines the allowable span
of a dimension stone panel in a given thickness
subjected to given loads. Flexural strength varies
amongst different types of granite, and typically
is between 1,000 and 2,000 lbs/in². This
allows the use of "thin" (30 mm) panels
for many applications, minimizing both curtainwall
cost and dead load for the building frame. Thicker
granite panels (15/8" [40 mm], 2" [50
mm] or greater) are available where spans or loads
necessitate their use.
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For
applications that are below grade or in  contact
with soil, water absorption is an important property.
Absorption rates of granites range from 0.10%
and 0.40% by weight. Furthermore, most granite
materials will effectively allow water to evacuate
during freezing cycles to prevent surface damage
from the freezing water. Repetitive freeze/thaw
cycles, particularly saturated cycles, will result
in a reduction of strength in the granite panel.
This loss can be significant, perhaps 20%. Laboratory
experiments have shown that the strength loss
occurs most aggressively in the first 100 cycles,
after which the strength loss is much slower paced.
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| Measurement
of Physical Properties |
The
various physical properties of dimension stones
are tested by means of the procedures documented
by ASTM. ASTM also publishes standards for the
major stone types, listing the minimum/maximum
values to be expected from a particular stone
type in a particular test. It should be noted
that there are many stones that do not meet these
values, yet have demonstrated satisfactory performance
in a variety of applications. This table should
then be considered to be more of a general guide
than an absolute pass/fail gauge.
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Copies
of the ASTM Standards and test procedures are
available from ASTM, 100 Barr Harbor Drive, West
Conshohocken, PA 19428-2959, Tel: (610) 832 9500,
or visit www.astm.org.
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From: http://www.coldspringgranite.com/comparing_granite_to_other_build.htm
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