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Casting Concrete Furniture with GFRC

Time:2016-10-28 share:

Concrete furniture has been around for as long as concrete countertops have. In those early days, concrete furniture was primarily made out of precast concrete. Since it needed to be reinforced with steel, the concrete tended to be thick, massive and heavy. This made large, thin and delicate pieces impractical or impossible.

The current trend towards using glass-fiber reinforced concrete (GFRC) has changed that. GFRC is a form of concrete that has inherently high flexural strength. It can be easily shaped into complex, three-dimensional shapes that don’t need to be thick to be strong.

Furniture-making in and of itself can be challenging, since the object being made often has to be functional, ergonomic, durable, lightweight and portable. Plus, it has to be aesthetically pleasing. These challenges can be daunting when it comes to using concrete as the material of choice. Even GFRC, with its higher flexural strength and ease of molding, has limitations.

GFRC has a density almost four times greater than wood, yet wood’s flexural strength is roughly four times greater than GFRC’s. This means that your design must accommodate GFRC’s strength-to-weight disadvantage. This doesn’t mean GFRC is inadequate. It simply means that it may not be possible to replicate the thin, delicate lines of a particular design based on wood.

Still, with a bit of care, understanding and adaptation, concrete best known for flat two-dimensional slabs can be crafted into stunning three-dimensional pieces.  

Why GFRC is the right stuff

GFRC is concrete, but it’s the fiber reinforcement that creates the high flexural strength necessary for thin, lightweight shapes.

Flexural strength is also called bending strength, and it is this characteristic that is the most important when it comes to making a durable, high-strength material.

In the past I’ve discussed steel reinforcing principles in great detail. But to appreciate the benefits of GFRC I think it’s worth summarizing the significant differences (and advantages) that GFRC has over steel-reinforced concrete.

Conventional steel-reinforced concrete has two important and very different components: the concrete and the steel reinforcing material. The concrete is often an aggregate-based mix, but it also can be a mortar or all-sand mix.

Either way, the unreinforced concrete component can be described as a highly brittle material with high compressive strength and low flexural strength. This means the unreinforced concrete portions of the piece are very strong in compression but will crack fairly easily if they are bent or flexed the wrong way. The steel reinforcing is structural steel very high in tensile strength. Its job is to resist all of the tension forces that are developed in the concrete when the concrete object is flexed.

With the right design and construction the steel completely resists the tension forces and doesn’t stretch to the point where the concrete cracks and those cracks become visible. Good industry practice (and the laws of physics) dictate that steel reinforcing in this application should be placed close to the area with maximum developed tension. In all beams, that area is near the face that stretches the most when it’s flexed.

Greater bending strength is obtained when the distance between the compression face and the tension face is large. This is the main reason floor joists are oriented so they are tall and skinny, not wide and short.

Because the steel must be embedded inside the concrete and the concrete piece still has to be easily constructed, strong, steel-reinforced concrete beams tend to be relatively thick and therefore heavy.

Further complicating things, furniture tends to be moved and handled much more frequently than countertop slabs, so the concrete gets flexed much more often and it is frequently flexed in different directions. To resist the flexing, the steel reinforcing must be placed everywhere tension is anticipated, which is sometimes challenging to predict and often very difficult to achieve. These two factors make steel-reinforced concrete less than ideal for most furniture applications.

In contrast, GFRC is essentially a single material that encompasses the concrete and its reinforcement. Yes, there is a thin, decorative, nonstructural face coat that hides the fibers, but the principal strength element is the GFRC backer coat, the bulk of the material. Because the glass fibers are mixed into the concrete, the strength is more or less the same everywhere. Not only is the material much easier to cast, but it behaves more uniformly, a great benefit to furniture that gets pushed and pulled and moved often.

That said, GFRC must be made correctly. The right kinds of fibers should be used in the right amounts and the concrete should be cast in the correct way. Too often GFRC is made incorrectly, and the result is disappointment and failure.

Use the right fibers

GFRC gets its strength from a high volume of alkaline-resistant (AR) glass fibers. The glass is treated with zirconia to resist the highly alkaline environment inside concrete. Alkalinity will weaken ordinary glass fibers (those used in fiberglass applications like boats and spas), so the first key to success is to use the right kind of fiber.

Use the right amount of fibers

Because AR glass fibers account for about a third of the material cost of GFRC, there is a disturbing tendency to use fewer fibers in order to save money. This is not a good idea, because using less fiber creates GFRC that is weaker and more brittle.

GFRC gets its strength from the fibers, and lower fiber contents dramatically reduce the flexural strength. Generally a minimum dose of 3 percent fiber provides useful flexural strengths. Reducing this to 2 percent or less can drop the flexural strength so much that the result is little better than unreinforced concrete. In contrast, increasing the fiber content to 4 percent or even 5 percent can boost the flexural strength significantly, provided it is cast properly.

Cast the right way

Another disturbing tendency is to make “self-consolidating” GFRC backer coat. The idea is that soupy GFRC is easier and faster to cast. This also creates GFRC that is weaker and more brittle.

GFRC that is made to be highly fluid and then simply poured into a mold has its fibers randomly oriented, so only about 5 percent to 10 percent of the fibers are able to provide any meaningful strength. GFRC’s whole purpose is to have high flexural strength, and that can only be achieved by applying the GFRC in thin layers and then compacting each layer with special rollers. This compaction process orients the fibers into a more two-dimensional configuration, dramatically boosting the fibers’ effectiveness because 30 percent to 50 percent of the fibers are now optimally oriented. In addition, AR glass mesh, also called scrim, can be laminated into the GFRC to boost the flexural strength even more.

The concrete used to make thin, lightweight and functional pieces of furniture requires a material with a high flexural strength. Correctly made GFRC is formulated to provide these high strengths, so to ensure your furniture is strong and durable, you must make the GFRC correctly.

GFRC is an ideal material for creating concrete furniture, such as the examples included below.

 If you do interesting in, you can see Freeson Furniture.

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