Introduction to GFRC (Glass Fiber Reinforced Concrete)
If you aren’t yet familiar with glass fiber reinforced concrete (GFRC) you should be.
GFRC is a specialized form of concrete with many applications. It can be effectively
used to create façade wall panels, fireplace surrounds, vanity tops and concrete
countertops due to its unique properties and tensile strength. One of the best ways to
truly understand the benefits of GFRC is to take a deeper look into this unique
compound.
What is GFRC?
GFRC is similar to chopped fiberglass (the kind used to form boat hulls and other
complex three-dimensional shapes), although much weaker. It’s made by combining a
mixture of fine sand, cement, polymer (usually an acrylic polymer), water, other
admixtures and alkali-resistant (AR) glass fibers. Many mix designs are available
online, but you’ll find that all share similarities in the ingredients and proportions
used.
Some of the many benefits of GFRC include:
Ability to Construct Lightweight Panels– Although the relative density is
similar to concrete, GFRC panels can be much thinner than traditional concrete
panels, making them lighter.
High Compressive, Flexural and Tensile Strength– The high dose of glass
fibers leads to high tensile strength while the high polymer content makes the
concrete flexible and resistant to cracking. Proper reinforcing using scrim will
further increase the strength of objects and is critical in projects where visible
cracks are not tolerable.
The Fibers in GFRC- How They Work
The glass fibers used in GFRC help give this unique compound its strength. Alkali
resistant fibers act as the principle tensile load carrying member while the polymer
and concrete matrix binds the fibers together and helps transfer loads from one fiber to
another. Without fibers GFRC would not possess its strength and would be more
prone to breakage and cracking.
Understanding the complex fiber network in GFRC is a topic in and of itself. Stay
tuned, I’ll post a more in-depth article on GFRC fibers next week.
Casting GFRC
Commercial GFRC commonly uses two different methods for casting GFRC: spray
up and premix. Let’s take a quick look at both as well as a more cost effective hybrid
method.
Spray-Up
The application process for Spray-up GFRC is very similar to shortcrete in that the
fluid concrete mixture is sprayed into the forms. The process uses a specialized spray
gun to apply the fluid concrete mixture and to cut and spray long glass fibers from a
continuous spool at the same time. Spray-up creates very strong GFRC due to the high
fiber load and long fiber length, but purchasing the equipment can be very expensive
($20,000 or more).
Premix
Premix mixes shorter fibers into the fluid concrete mixture which is then poured into
molds or sprayed. Spray guns for premix don’t need a fiber chopper, but they can still
be very costly. Premix also tends to possess less strength than spray-up since the
fibers and shorter and placed more randomly throughout the mix.
Hybrid
One final option for creating GFRC is using a hybrid method that uses an inexpensive
hopper gun to apply the face coat and a handpacked or poured backer mix. A thin face
(without fibers) is sprayed into the molds and the backer mix is then packed in by
hand or poured in much like ordinary concrete. This is an affordable way to get
started, but it is critical to carefully create both the face mix and backer mix to ensure
similar consistency and makeup. This is the method that most concrete countertop
makers use.
Spray-up GFRC Fibers
Coming soon: A more in depth look at GFRC mix designs, casting, thickness, curing
and processing.
Quick Facts About GFRC
GFRC was first created in the 1940s in Russia, but it wasn’t until the 1970’s
that the current form came into widespread use.
GFRC tends to run about $2.50-$3.00 per square foot for ¾” thick material.
The cost increases to about $3.50-$3.75 per square foot for 1” thick material when
accounting for the prices of sand, cement, admixtures, fibers and polymer.
Just like regular concrete, GFRC can accommodate a variety of artistic
embellishments including acid staining, dying, integral pigmentation, decorative
aggregates, veining and more. It can also be etched, polished, sandblasted and
stenciled. If you can imagine it, you can do it, making GFRC a great option for
creating concrete countertops and especially three-dimensional concrete elements.
