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GFRC applications, for the most
part, never exceed 1/2 to 3/4 inch thick. With the incorporation of high-strength
glass fiber, the tensile and flexural strengths of this composite material
are tapped. Due to the particular production processes and physical properties
required of the finished GFRC product, the specifications and quality
of the input raw materials are critical. GFRC is a cast product, meaning
it takes the form, shape, and texture of whatever it is cast against.
It is a moldmaker's delight, and at the same time it can be his biggest
nightmare.
Due to the material's incredible capacity to
replicate detail, it is important that the mold it is cast into be made
to high quality standards. When training a start-up company, I stress
that they should not short-change the tooling and moldmaking department
with poor materials or equipment, and marginal talent. There is no magic
that can make a piece suddenly perfect if it has been cast in a poor mold
taken from a bad pattern. It is very expensive to remedy the problems.
Most GFRC molds are created using flexible mold compounds made from liquid
rubbers like Polytek®'s Poly 74-30 and the new Polygel®
Spray 50 urethanes. These materials allow complex shapes and high detail
to be demolded easily.
The chameleon characteristics of GFRC come to
the fore when developing the composition of the face mix. The face mix
is defined as the mix that goes into the mold first. The mixture has the
consistency of a thick milk shake, and is not reinforced with glass fibers.
It normally is applied 1/8 inch thick. The face mix is sprayed into the
mold in several passes. It is brushed lightly between each pass to make
sure that it is in all the details of the mold, and to remove entrapped
air from the mixing process.
Through the choice of aggregates, pigments and
mold texture, keeping in mind any post-demolding finishing treatments
such as sand-blasting or acid-washing, the molded GFRC part can be made
to look like anything. What you see is not necessarily what you think
it is. For example, the volcano in front of the Mirage Hotel in Las Vegas
is all 1/2-inch GFRC. The same is true for the rock formations next door
at Treasure Island. The new exterior signage at the National Museum in
Washington, DC, is GFRC made to look like limestone. Replication of terra
cotta is a major application. The ongoing restoration of Shepherd Hall
in New York City, with new pieces of GFRC made to look like the old, is
a standout project. There are many similar ones.
The largest market for GFRC is architectural
cladding panels and architectural ornamentation. Because of its high flexural
strength, 1/2-inch thickness weighing 5 pounds per square foot, with the
capability of being panelized on a light-gage structural-steel framing
system, it is the cladding of choice for fast-track, mid- to high-rise
construction projects. All these features, plus being a cement-based material,
make it popular in seismic areas such as San Francisco and Los Angeles.
Then What Is GRG?
Answer: Glass Reinforced Gypsum—a strong, thin,
lightweight, and fire-resistant material with unlimited architectural
possibilities.
How does GRG differ from GFRC? Answer: GFRC is
based on Portland cement, but GRG is based on gypsum.
Plaster is gypsum, but not all gypsum is plaster.
Fiber reinforcement will strengthen all gypsums. For example, the common
use of hemp, burlap, or gauze strengthens plaster for mold shells or body
casts. The plaster for those uses generally has high water demand per
pound of dry plaster; thus the strength is less than if less water is
required. Just adding less water to common plasters will not work, because
it will not result in a workable compound but in a dry mess of lumps.
GRG is possible with gypsum cements that require
less water. They are based on alpha-hemihydrate gypsum, which is different
from ordinary plaster. US Gypsum's FGR-95 grade (FGR = Fiber Glass Reinforced)
is a popular example. It cures to a strong, dense material yet is a low
viscosity fluid which wets fiber reinforcements well before it sets. GRG
is not recommended for exterior use without special treatments. GFRC is;
it usually utilizes a polymer additive.
With polymer additives some kinds of GRG can
be used outdoors with proper care. An example is Forton Modified Gypsum,
which utilizes FGR-95 in exterior applications. The primary uses are architectural
ornamentation, e.g., column covers, window molding, and the like—very
similar to the interior GRG market.
Because of the chemistry in the Forton system,
some interesting effects can be created. The product takes pigments very
uniformly; aggregates can be added and the cured piece sandblasted to
look like stone. Metal powders can be added, patinaed, and polished. All
this in a water-based system that is very lightweight and easy to install.
The Forton polymer modified GRG is often used for interior applications
as well, because it exhibits a toughness and resilience far superior to
regular GRG.
More information on GRG and GFRC can be obtained
by calling Ball Consulting at (800) 225-2673 |
