The Owner Built Home & Homestead
(Page 4 of 15)
Granites are the least affected by weathering: Limestone
and sandstone the most. They are commonly destroyed by
surface erosion (from sea salts, for instance) and
atmospheric pollution. Rain will leach the cementitious
materials found in some sandstone to the surface, where
they become brittle, weak and finally flake off.
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A number of stone preservatives are available, designed to
protect rock from the aforementioned frost and moisture
penetration hazards. A waterproofing agent prevents the
penetration of moisture but the moisture that does
gain access into the wall is not permitted to escape. This
is bad. The wall should "breathe", whatever material is
used. Moreover, the outer waterproofing layer is a thin
skin which differs in physical properties from the
underlying material. This difference causes certain
stresses to be set up which inevitably force the outside
skin to flake off.
One may reason that strength should be the
foremost requisite of rock for building purposes. Rock that
is sound and suitable in other respects, however, is almost
invariably strong enough for use in a wall. Recent tests at
the U.S. Bureau of Standards on samples of Montana
quartzite indicated a compressive strength of 63,000 pounds
per square inch (a rather typical rock strength). A
structure of such material would have to be over 10
miles high before failure would occur from crushing
the lower courses!
Another good example of structural strength is illustrated
in the 555-foot high Washington Monument. Pressure at the
base course is 700 pounds per square inch; but marble will
sustain a crushing load of 25,000 pounds.
The appearance of your dwelling should not be
underestimated when choosing a building stone. Every rock
has its unique color and rock of different color can be
mixed in a wall. Every rock also has its unique lustre, be
it vitreous, pearly, resinous, dull, metallic or whatever.
Rock containing much iron should be avoided, since stains
caused by oxidation of iron under atmospheric influence
will discolor the mortar.
Some rock can be "worked" better than others. Angular,
square-edged, quarried rock "lays up" better than roundish
cobblestone boulders. The last are sometimes called
"rolling stones", because they are loosened and weathered
from the parent ledge by natural processes.
Workability depends as much upon the correct mortar mix as
it does upon the type of rock laid. A proper mortar is
weather resistant and has adequate bond strength and
compressive strength. The proportion of sand, cement,
fire-clay—and especially—water must be
controlled to within a narrow margin. The optimum
proportion is 12 shovels of clean, washed concrete sand, 4
shovels common cement and 2 shovels fireclay.
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