Benefits of Using a High-Shear Mixer

Question:

What is high-shear mixing, and why is it important in glass-fiber reinforced concrete (GFRC)?

Answer:

High-shear mixing, sometimes known as high-energy mixing, involves blending cement, water and other admixtures at high speeds to mechanically break up and disperse cement particles. Once these ingredients are thoroughly mixed, the remaining ingredients are added and mixed at lower speeds.

The benefits of high-shear mixing are increased workability, increased degree of hydration, earlier strength gains and decreased porosity. Additionally, less water reducer is needed because of the greater particle dispersion. The early strength gains are particularly important for GFRC, since GFRC precasters generally aim for rapid demolding.

So what determines if a mixer provides high-shear mixing? It has to do with the specific energy imparted on the cement paste by the rotating mixing blades. The more energy the blades impart on the mixture, the more effective the dispersion. Typically high-shear mixing is defined as having a specific energy of at least 5 kilojoules per kilogram of material being mixed. This works out to a blade tip speed of about 100 feet per second.

Almost all conventional concrete mixers, both large and small hand-held units, don’t impart such high specific energy levels. They mix, but they don’t disperse. So using a drill-type mixer won’t deliver the same energy or make the same concrete as a high-energy mixer will.

High-shear mixers are specialized machines designed specifically for the GFRC industry. Sources include Power-Sprays Ltd., RimCraft Technologies and Spray-Tech Inc. Prices range from a few thousand dollars for small units to over $15,000 for larger commercial units.

A high-shear mixer (top) vs. a conventional drill-type mixer.

Using Epoxy to Fill Seams

Question:
Can I use epoxy to fill the seams where two precast countertop slabs abut?

Answer:
Seams are often necessary in larger countertop slabs to make it possible to manufacture, transport and install them. Seams serve another important function as isolation joints, allowing movement at key areas where cracking would otherwise occur.

Some of your clients may object to the visual presence of a seam, even if it’s only 1/16 inch wide or less. An advantage of an epoxy filler is that it can be color matched to the concrete and finished flush with the surrounding surface. Epoxies can be tinted with finely powdered stone, mineral admixtures and dry concrete pigments, as well as pigments specifically for use in epoxy. While not completely invisible, epoxy seams are far less apparent than seams filled with flexible, sanded tile caulk.

Typically epoxy is the adhesive of choice in the granite industry because of its reliability, availability and the option of selecting a thick-bodied or flowable viscosity. However, epoxy is also a very powerful adhesive, and problems can arise if it’s used to fill a seam located in an area with significant movement, such as near a sink. While the epoxy itself is very strong, the concrete it is bonded to is not. Concrete has very low tensile strength. If there is movement or flexing, a crack will occur in the concrete, often just outside the epoxy seam. Essentially the crack is a new isolation joint that formed where the flexural stresses were highest. Although this doesn’t compromise the strength or integrity of the countertop, clients will often object to the appearance.

The key to eliminating these kinds of cracks is to locate flexible isolation joints in those areas, such as filling the seam with flexible tile caulk, rather than an epoxy.

How ‘Green’ Is GFRC?

Question:
Are countertops made of glass-fiber-reinforced concrete as environmentally friendly as standard concrete countertops?

Answer:
GFRC is roughly on par with other forms of concrete countertops in terms of “greenness.” GFRC tends to use about twice as much cement as ordinary concrete, but it has a higher strength-to-weight ratio so the countertop thickness can be less. In comparing a 1.5-inch-thick concrete countertop to a 3/4-inch GFRC countertop, the same amount of cement is used.

GFRC uses acrylic polymers, rather than ordinary water, as a key ingredient. The need to ship the acrylic makes GFRC less green. Both traditional cast concrete and GFRC can use recycled aggregates, such as crushed glass. Conventional concrete countertops use steel reinforcing, which is greener than the alkali-resistant glass fibers used in GFRC, since steel is a recyclable material.

How to Spray Apply GFRC

Question:
What sort of equipment do I need to spray apply glass-fiber-reinforced concrete into countertop forms? And how many coats of material should I apply?

Answer:
GFRC is a lightweight, high-strength cement mixture that contains alkali-resistant glass fibers. (Read “The Benefits of Using a GFRC Mix for Countertops.”) Typically the material is applied in two layers. The first layer, or face coat, usually has no fibers in it and is thin, often only about 1/8 inch. The second, or backer layer, contains the fibers and is applied to achieve an overall thickness of 3/4 to 1 inch.

The face mix and the backer mix are applied at different times, so it’s important to ensure that the general makeup of the two mixes is similar. Water-cement ratios and polymer contents should be the same to prevent curling.

A common and inexpensive method for spraying the face coat is to use an inexpensive hopper gun powered by an air compressor. Hopper guns are often used to spray cementitious overlays or other knock-down surfaces. The combination of a hopper gun and a 60-gallon air compressor can cost as little as $400 to $500. However, you won’t be able to use a hopper gun to apply the backer mix because of the heavy dose of fibers it contains. Hand placement or conventional pouring of a self-consolidating mixture will be required. To spray the backer mix, you would need a much more expensive premix spray gun or concentric chop gun, a pump and a compressor—a setup that can cost $10,000 or more.

Once you spray apply the thin face mix into the forms, allow it to stiffen before applying the backer mix. This prevents the backer mix from being pushed through the thin face mix. When applying the backer layer by hand, place it in layers 3/8 to 1/2 inch thick and compact each layer to remove air voids and ensure good bonding with the face mix. Specialized spring rollers are the tool of choice for this job.

Using Metakaolin vs. VCAS

Question:
What’s the difference between VCAS and metakaolin, and can they be used interchangeably in concrete countertop mixes?

Answer:
Both VCAS and metakaolin are manufactured mineral admixtures. VCAS is a trademarked name for vitreous calcium alumino-silicate, and metakaolin is an amorphous alumino-silicate. VCAS is manufactured using post-industrial ingredients, while metakaolin is manufactured using mined kaolin clay.

VCAS and metakaolin are pozzolans, meaning they react with the calcium hydroxide generated during cement hydration. This increases the strength of the concrete while decreasing efflorescence, porosity and concrete alkalinity (read Benefits of Metakaolin As a Cement Substitute).

Both pozzolans are often used as cement replacements, although they could be used as cement additions. When used as a cement replacement, VCAS tends to have a more positive effect on workability, allowing for better flow and even a slightly reduced water-cement ratio. Metakaolin, on the other hand, tends to have a higher water demand, which means it decreases workability slightly when compared to a pure portland-cement-based mix. Typically more superplasticizer is needed to maintain workability with metakaolin, while VCAS usually allows for a slight decrease in superplasticizer.

Both metakaolin and VCAS increase the strength of concrete relative to plain cement-based concrete. However, very early strengths (after 3 days) are somewhat higher with metakaolin than VCAS. After 7 days, the strength of both metakaolin and VCAS mixtures begins to surpass the strength of plain cement-based concrete. In summary, metakaolin and VCAS can be used interchangeably (or even together) to boost longer-term strength. VCAS gives the edge in workability while metakaolin provides greater early strength. Both are white (or nearly white), so pigmented concrete remains true to the desired color.

The Benefits of Using a GFRC Mix for Countertops

Question:
What is GFRC? Does the material offer any advantages when used for concrete countertops?

Answer:
GFRC, or glass-fiber-reinforced concrete, is typically sprayed, much like shotcrete, but the mix design is different. Typically a GFRC mix consists of a large amount of cement, fine sand, and a very high loading of alkaline-resistant (AR) glass fibers. GFRC often has a low water-cement ratio (in the range of 0.33 to 0.38) and relies on acrylic polymer for early strength and high flexural strength.

GFRC can be spray cast into structural shells as thin as 3/4 to 1 inch thick. This cuts weight, one of the prime reasons to use GFRC. Some people use GFRC for concrete countertops because the material makes it much easier to design, make and install large, complicated elements by significantly reducing weight and by allowing for much simpler mold designs (the molds need only be one-sided rather than two-sided, as for wet-cast techniques).

Unlike conventional concrete that relies on steel reinforcing, GFRC relies on the high dosage of structural glass fibers and the polymer to create a strong composite. Although it is made with portland cement, it behaves differently from ordinary concrete. And some people think that GFRC looks and feels different than ordinary concrete, so be sure to show clients what the material looks like.

In addition to requiring AR glass fibers and other expensive materials, GFRC also requires special equipment for spray application, such as a hopper gun. So for simple, flat countertop slabs, the benefits of GFRC may not outweigh the costs. But I feel that for large, complicated designs, GFRC offers tremendous advantages. It is worth having in your “bag of tricks,” especially as you take your business beyond plain countertops.

Slurry Won’t Stick

Question:
My slurry won’t stick to my concrete countertops, and it flakes off when I polish it. What can I do to make it stick?

Answer:
Homemade slurry, or grout, is usually a mix of cement, pigment and water. It may also include a fine filler (such as cenospheres), metakaolin or other pozzolan, superplasticizer and even an accelerator. However, adding an acrylic polymer is the key to getting the grout to stick.

Just like cementitious overlays and microtoppings, grout depends on the polymer additive to provide bond strength. Without polymer, you are depending on the cement bond alone. Since grout applied to a concrete countertop is often honed off within a day of application, the cement bond has not strengthened to the point that it can withstand the rigors of polishing. Polymer solves that.

Almost any polymer additive made for concrete can work, but you need to use the right amount. If you don’t use enough, you’ll get flaking. Add too much polymer, and the grout will be gummy when polishing it off.

To get the right consistency for your grout, dilute the polymer with water until it looks like whole milk. It’s better to err on the less-dilute side, but you don’t want it to be so thick that it’s like white glue. Whole-milk consistency is just right.