In a message dated 5/10/2007 10:15:13 A.M. Eastern Daylight Time,
[log in to unmask] writes:
what is the binding agent for "cast stone"? what the hell is even in cast
stone? is it dolphin-safe? is it just sand (which I guess is long-ago stone)
or do they crush up stone directly for the purpose?
Christopher:
One of best online discussions of cast stone is by Cary Concrete Products
_Click here: Cary Concrete Products Inc. - Materials_
(http://www.caryconcrete.com/index.php?page=materials#cstone)
I think that there is a lot of confusion in the recognition of cast stone.
I've had architects, architectural conservators, allied professionals,
masons, salesmen, and private citizens point at nearly every material, including
natural stone, and call it cast stone. Most are ignorant; some have an agenda.
Cast stone is first and foremost, a product intended to be installed by
masons in a similar method to natural stone blocks. This definition, if you
will, limits the product to the restoration market. Very little new
construction, except monuments, uses this technique. However, a variety of precast
concrete panels has the surface appearance of some cast stone, but is installed by
non-masons.
Cast stone is expected to appear to the eye as a variety of stone. To
achieve this appearance, the material incorporates small sizes (<3/8" and
generally 1/8" to 3/16" size) of crushed rock, usually pink or white granite, marble,
and/or dark rock (basalt or diabase) in a binder of usually white or buff
Portland cement. Only rarely does one see a natural sand (as opposed to a
manufactured (i.e. crushed sand); then the natural sand is not tan but white such
as that from parts of Ohio and South Carolina. Sometimes there is a pigment
intermixed with the cement binder. The aggregate at the surface(s) to be
seen is usually exposed by acid washing, although sometimes the material is
sandblasted. Sometimes the product is manufactured with more aggregate at the
surface than in the interior. As far as the binder is concerned, I've not seen
any cast stone with a binder other than Portland cement, although I don't see
why other cementitious binders couldn't be used as long as expected
production times were not extended.
Cast stone is manufactured using molds in a factory. They tend to be solid
blocks, as opposed to hollow blocks. There are two methods of manufacture:
dry-tamp and wet-cast. The dry-tamp process compacts a loose granular mixture
into a mold; the wet-cast process places a wet flowing mixture into a mold.
If optimized, dry-tamp has a different proportion of aggregate to cement
than wet-cast. It always has a lower-water-cement ratio; that improves the
durability and strength. The dry-tamp process uses fewer molds than wet-cast and
the factories tend to have a higher throughput for the factory size. The
dry-tamp process is similar to that used to make most hollow concrete blocks,
although cast stone units are usually not hollow. The wet-cast process is
essentially the same as used for precast-concrete, although precast-concrete
plants tend to focus on large structural units such as double T's, an only
rarely make exposed aggregate products. Also, the use of small aggregate in
cast-stone products requires the units to be smaller in size than that in precast
plants so that shrinkage cracks do not cause product rejection.
The durability of cast stone is similar to that of historic but not modern
concrete. The claims of superior durability are based upon apple-orange
comparisons. Cast stone tends to be used in building construction in areas with
relatively benign weathering. Cast stone is susceptible to ASR
(Alkali-silica-reactivity) and freeze-thaw damage. It tends to not have ASR because the
fine-grading of the aggregate disperses the reaction and because the buildings
loose so much heat that the internal relative humidity of the product is
below 70%. There is more freeze-thaw damage than with stone or cast-in-place
concrete when the installation is in a saturated freeze-thaw environment, such
as exterior stairwells or foundations, or when deferred or non-existent
maintenance results in gutter or downspout discharges.. The dry-tamp product
cannot be air-entrained for durability. The wet-cast product could be
air-entrained, although cast stone manufacturers do not air-entrain their product for
indeterminate reasons related to cost, specifications, marketing, and
stubbornness. It is possible that cast stone could deteriorate due to sulfate attack,
but I've never seen an installation in a suitable weathering environment,
such as a sewer.
Most cast stone units do not contain reinforcing steel. Some lintels and
sills are reinforced because they perform in tension.. They almost always fail
after 50 years because the steel is unprotected and has insufficient cover
concrete for the service life. Cast stone lintels are still made without
corrosion-resistant steel such as green epoxy-coated rebar or stainless rebar. I
guess that they are looking to future sales to the restoration market.
S. J. Stokowski, Jr.
Stone Products Consultants
1058 Sodom Rd., Bldg. 2
Westport, MA 02790
508-881-6364
www.members.aol.com/crushstone/
************************************** See what's free at http://www.aol.com.
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