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.

Eliminating Flex in Supporting Steel

Question:
I used 2-inch-wide, ½-inch-thick steel flat bar as supports for a raised concrete bartop, but it’s flexing too much. What should I have used?

Answer:
The use of flat steel barstock as a bartop support is common, mainly because thin steel (1/4 to ½ inch) is easy to hide under the countertop. Steel is strong, but strength is not the issue here, stiffness is. And the stiffness of steel depends on the shape and orientation. Flat steel barstock is too flexible to provide a secure, rigid support for a heavy concrete countertop.

A piece of steel flatbar oriented horizontally seems fairly stiff, but it really acts like a spring. So if use a series of steel bars fastened to the top of a pony wall as the supports for a raised concrete bartop, the steel bars behave structurally as a cantilever and the weight of the concrete bears on the steel. But any additional weight from someone leaning on the bartop will cause the steel bars to flex.

If you orient the same steel flatbar vertically, so it’s now 2 inches tall and ½ inch wide, that increases its stiffness by a factor of 64! You may think that increasing the width of the bar would have similar benefits, but actually increasing the bar’s width from 2 inches to 4 inches increases stiffness only by a factor of 2. So you can see that a vertical orientation is much stronger. This is just like floor joists or roof rafters. If you’ve ever seen a stick-framed house, you know that joists and rafters are oriented vertically.

If you simply can’t orient the bars vertically because of aesthetic concerns, using wider bars and/or increasing the number of bars will help. But that’s not nearly as effective as turning them on edge.

Another thing you can do to accommodate vertically oriented steel bars is design the bartop with a thick, dropped edge. The bartop will appear to be very thick, but in fact will be only 1 ½ or 2 inches thick, and the steel will be concealed behind the dropped edge.

I did this with a round bartop that extended 2 feet out, as shown in the photos. The steel in this case is square in cross section, and it is lagged to a stud in the wall behind the cabinet. The vertical sides of the steel provide stiffness, and lagging the steel to the stud prevents tipping.

A steel bar oriented vertically will support the weight of a countertop without flexing. A dropped edge conceals the steel without adding to the overall thickness of the countertop.

Avoiding Streaks When Applying Water-Based Sealers

Question:
How do I prevent streaking when wiping on a water-based sealer?

Answer:
The wipe-on technique is commonly used to apply water-based sealers. Typically it involves using a microfiber cloth to apply a very thin, even film of sealer. Done right, it’s a fast and easy way to lay down a beautiful finish.

However, water-based finishes can be tricky to apply because they get sticky quickly as the finish loses water. Achieving a smooth, streak-free finish is possible only if the wet sealer has time to flow out before it dries. If the wet film of sealer isn’t thick enough, or if the finish dries too soon, the wiping marks won’t disappear and streaks result.

Water loss can happen in two ways: through evaporation and through absorption by the concrete. If the finish is applied very thin, then it flash dries before it flows out. Applying a thicker wet film helps, plus the added material flows out more readily than a very thin film. A hot, dry environment accelerates evaporation, so a light spray of water on the sealer as it’s being spread will also help it stay fluid. Be careful not to overapply the water. Just give the sealer a quick spritz, not a thorough soaking.

Dry concrete often sucks water out of the sealer, dehydrating it. Prewetting the concrete with water helps eliminate this tendency. I soak the concrete with water for several minutes, then wipe up excess water with a microfiber cloth. This leaves the surface slightly damp and won’t draw moisture out of the finish.

The final trick to achieving a flawless wipe-on finish is keeping the cloth properly saturated with sealer during the final wipe-down. If you wring out the cloth too much, you’ll remove too much sealer from the concrete surface. The cloth should be very wet with sealer but not dripping. The sealer in the “wake” of the cloth should flow out very quickly, leaving a smooth film. If the concrete looks barely wet with sealer, too much is being removed.

The key to achieving streak-free results when applying any sealer is practice, whether you are using a quick-and-easy wipe-on sealer or a complex, multipart, spray-applied system. I see too many people trying to apply sealers to their clients’ countertops before they have adequately practiced the technique. Practice is the only thing that makes perfect!

Grinding Flush Over Metal Embedments

Question:
I can’t keep the surface of my countertops flat after polishing over metal embedments. The embedments end up sticking above the concrete. Why does this happen and how can I prevent it?

Answer:
The problem occurs because concrete wears away faster than metal when polished with diamond abrasives. Because the embedded metal is “gummier,” grinding wears it away at a slower rate and leaves the metal slightly raised above the concrete. I call this “pillowing.”

Pillowing is least pronounced when very little honing is done to the concrete. Concrete with a lightly polished cement finish will show almost no pillowing because very little concrete is removed. More heavily honed concrete, where the cream is completely removed and some of the finer aggregates are exposed, will tend to show more pillowing. And concrete that has exposed aggregate, where aggressive grinding and extensive honing are performed, will tend to show the most pillowing.

Preventing pillowing is not difficult, but the challenge is to anticipate how much concrete you’ll need to remove when grinding, honing or polishing so that the metal and the concrete are flush. The more concrete that needs to be removed, the deeper the metal embedment needs to be cast into the concrete to compensate for the concrete thickness that gets ground away. With polished cement finishes, very little material is removed so little or no depth compensation is needed.

Typically metal embedments are glued to the forms with 100% silicone caulk. Applying a thick layer of caulk, without squeezing out the excess, holds the face of the embedment away from the form so the surface will be sunken relative to the cast concrete surface. By adjusting the thickness of the caulk, you can adjust how deep the metal is cast. Some trial and error is involved, although it is possible to measure the thickness of the caulk. First measure the thickness of the embedment in several areas. Then glue it down and measure the height of the back of the embedment relative to the casting surface. Subtract the embedment thickness and this results in the caulk thickness.

Also be careful about the grit level you use when grinding. Using a coarser diamond grit (such as 50 or 100) will wear the concrete away much faster and show more pillowing than using a medium grit (200 or 400). Fine grits (600 +) will provide the most balanced results, with the metal and the concrete wearing away at about the same rate.

Sources of Recycled Glass

Question:
I want to use recycled glass in my concrete countertops, to make them more “green,” or environmentally friendly. Where can I find it?

Answer:
Crushed glass, also known as “glass cullet,” comes from used bottles and jars, broken windows and windshields, or waste glass produced during the manufacturing process of bottles, windows and mirrors. Generally glass bottles from bars, restaurants and homes get collected and sent off for recycling. Usually little or no processing or screening is done to the glass. The glass is often sorted by color, but not always, and sometimes the bottles are crushed to reduce the bulk volume for storage and shipment.

Glass collected in this way can be very inexpensive, with prices in some areas as low as a few dollars per ton for minimally processed, mixed-colored glass. However, the glass will not be clean, and some paper labels will still be stuck to the glass. In many cities, you can purchase the unwashed cullet either from the city recycling department or from a commercial collection and recycling company. Sometimes the cullet is available for free. Alternatively, you can get used bottles directly from the source: restaurants and bars. Processing the used bottles yourself takes time, energy, water and effort. Cleaning, crushing and screening can be a daunting task if large quantities of glass cullet are required.

Decontaminating recycled glass and crushing it to the right size is very important to making sound concrete countertops. Residual sugars from beverages can cause delayed setting and lowered strengths, and contamination from paper, adhesive and plastic can further degrade the performance and appearance of the concrete. For this reason, it’s vital to thoroughly wash the glass prior to use.

Breaking the glass into small enough pieces is also important. Because concrete countertops are thin, large pieces of glass can weaken the slab. I recommend the largest piece of glass be no larger than about ¾ inch in diameter. Exceptions to this are possible, but larger pieces reduce workability, can lead to internal voids, and can crack or chip if a struck by a heavy object.

Several specialty glass cullet suppliers, terrazzo supply companies and concrete countertop supply companies offer crushed, cleaned and screened glass in a variety of colors, sizes and degrees of processing. Typically glass of a single color that’s different from the typical bottle green or brown starts with remelted plate glass tinted to create new colors. While the original cullet may be recycled, the remelting process consumes energy, making this option less “green” than glass diverted from the waste stream and used more or less as is. However, the advantages are greater design flexibility and greater control of size and color.

Never Do Business Without a Contract

Question:
I don’t ask my customers to sign contracts, and now a customer has changed his mind about the color of his countertops and won’t pay for them. What can I do?

Answer:
While you can attempt to collect payment yourself, it’s doubtful you have much legal leverage to get paid. Often verbal agreements don’t hold as much weight as a written contract, and details of what’s to be done and how much it will cost often get misconstrued or even forgotten. It’s unfortunate that this happened because there’s little you can do other than to try to reach a compromise with the customer or move on.

Doing business without a contract is risky, and this example highlights the importance of having a well-written contract. In my opinion, a contract is an essential part of running a successful business. In some parts of the country, it’s a legal requirement for home improvement projects over a certain dollar value. A well-written contract is simply a document that clearly defines what the client and the countertop manufacturer have discussed and agreed to. It sets expectations about the product, and it outlines the payment terms and consequences of not satisfying the contract’s terms.

Contracts protect the buyer as well as the countertop maker. It should clearly define expectations, such as potential color variations, seams and maintenance requirements. Good contracts also have homeowner care and use guidelines in them, as well as a warranty (if one is offered) and the precise terms.

A contract won’t guarantee payment, but it does give you a strong legal leg to stand on should a problem arise. Having a contract shows that you are a professional and not some fly-by-night operation, it clearly defines what’s to be done and how much it will cost, and it shows that you want to make sure your customers get what they want and that they will be happy.