Jeffrey Girard, P.E.

Competing with Mass-Market Products

2009-04-20T14:38:00.002-04:00

Question:
I’m worried about the solid-surface countertop people getting into the mass production of concrete countertops and ruining my market. How can I deal with this competition?

Answer:
It’s understandable to be wary of new competition, especially when that competition deals in mass-market products. And it’s easy to assume these new competitors will undercut prices and ultimately reduce the demand for concrete countertops. If you do nothing to change or adapt to the situation, then your worries will probably come true. But sometimes change is good, and you can use this to strengthen your market position by offering things the new competition can’t.

Concrete countertops are appealing to many people because they are custom, because of the artistry and creativity that goes into them, and because concrete itself has a different look and feel from other materials. You can better focus on these unique characteristics more than a mass-production shop can because you have the skills, time, creativity and business identity that caters to that level of customization. A mass-production shop will tend to be limited to a few standard colors, shapes and simple textures. Also, many solid-surface companies focus on commercial work. You could then focus on residential work, which often has more high-end clients wanting custom countertops.

If you’ve been in business for a while, you have built up a reputation with the builders and designers in your area. They trust you to continue to do good work, and this won’t change just because someone new comes along. Continue to strengthen these relationships and emphasize your experience and ability to meet the sophisticated tastes of their clients.

Having an experienced solid-surface company start making concrete countertops can actually be a good thing. In fact, it can help increase the demand for concrete countertops and allow you to keep your prices high. Most experienced solid-surface shops are profitable businesses that know what quality is and how to deliver on time. They understand the countertop process, the importance of scheduling and the need for production efficiencies. Their experience helps prevent trouble with their products and improves the quality of their installations. All of this increases the public’s confidence in concrete because they’ll be providing good examples of what concrete can be like, rather than delivering a poorly made sidewalk-grade countertop that degrades concrete’s image. This competition can also improve your business acumen because you must become more efficient and timely.

The more concrete is used in new applications, the greater the exposure and the more people’s comfort level will increase. And the more good examples of concrete countertops that are seen, the more people’s perception will improve. All this, in turn, will improve demand.

Proper Dosage of Pozzolans in Concrete

2009-04-06T14:28:00.001-04:00

Question:
When I’m using pozzolans in concrete countertops, what’s the best dosage rate?

Answer:
Generally pozzolans replace some of the cement in concrete. Because they are treated like cement, the total cement content in the concrete mix does not change. This keeps color formulas and admixture dosages consistent. However, simply adding pozzolans to a mix without reducing the total cement content can alter your colors and admixture proportions.

The pozzolan replacement dosage depends on which pozzolan you use. Fly ash and slag, for instance, often replace 20% to 40% of the cement, while silica fume generally only replaces 5% to 10%. The differences often have to do with the reactivity of the pozzolan and how it affects workability. Both fly ash and slag are not as reactive as silica fume, and both enhance workability. Silica fume, on the other hand, inhibits workability, sometimes described as “increasing water demand.” This is a troublesome description because adding water to increase workability often has other negative effects, such as altered color, lower strengths and higher porosity. Superplasticizers are the better choice and can help create a very workable concrete with a low water-cement ratio.

Here are some typical dosages for common pozzolans:

Fly ash, Class F: 15% to 25%
Fly ash, Class C: 15% to 40%
Slag: 20% to 50%
Silica fume: 5% to 10%
Metakaolin: 10% to 15%
VCAS (vitreous calcium aluminosilicate): 5% to 25%

Replacing large portions of the cement with pozzolans (especially the ones that are waste products) is a good way to make your concrete “greener,” or more sustainable, because the production of portland cement releases large amounts of carbon dioxide, a greenhouse gas. However, doing this sacrifices the early strength of concrete (usually that attained at 7 days or earlier). In general, the greater the pozzolan replacement, the greater the reduction in early strength. Most pozzolans only improve the concrete’s strength after a few weeks of curing.

To confirm this, I cast some concrete test cylinders using two identical concrete mixes. One contained gravel, sand, gray portland cement, water and a plasticizer. The other was identical, but 10% of the cement was replaced by VCAS, a popular white pozzolan used by many concrete countertop makers. Compression tests performed by an independent test lab showed that the 3- day compressive strength of the VCAS cylinders was 9% lower than the straight portland cement cylinders, and the 7-day compressive strength was 7% lower. I chose these times because early strengths are important for crack-free handling and processing of countertops. The 28-day strength of the VCAS concrete was undoubtedly higher than the plain portland cement concrete, but with countertops, long-term strength is not as important as early strength.

It’s important to realize that pozzolans work with portland cement but won’t work with CSA cement (calcium sulfo-aluminate). CSA does not generate calcium hydroxide, so there’s nothing for pozzolans to react with. (For more information, read The Pros and Cons of Using CSA Cements.)

What Are Pozzolans?

2009-03-23T14:26:00.001-04:00

Question:
What are pozzolans, and how do they work? What are the benefits of using them in concrete countertops?

Answer:
Pozzolans, also known as supplementary cementitious materials, are mineral admixtures added to concrete to improve its properties. Pozzolans react with the calcium hydroxide produced during cement hydration to form calcium silicate hydrate. In this way, the pozzolan is activated by the calcium hydroxide to behave like a cement.

When portland cement is added to water, a series of chemical reactions begin. One important reaction starts creating the “glue” that is responsible for the concrete’s early strength. Another starts later and is responsible for the concrete’s long-term strength. Other reactions happen too, and one of these is the generation of calcium hydroxide, or CH. CH provides no strength on its own. It simply takes up space in the concrete, and when dissolved with water can leach out and form efflorescence. Pozzolans react with the calcium hydroxide to form more of the beneficial glue that holds everything together. And because they consume the calcium hydroxide in the process, they fill the spaces the CH occupied, making the concrete less porous and less likely to effloresce.

The original pozzolan was pozzolana, a volcanic ash used by the Romans to make their concrete. Common pozzolans today are fly ash, silica fume, slag, metakaolin and VCAS (vitrified calcium aluminosilicate). Most pozzolans are waste products, and using them in concrete keeps them from ending up in a landfill. For example, fly ash comes from burning coal, silica fume from silicon production, and slag from steel refining. VCAS and metakaolin are manufactured pozzolans, but tend to be more consistent in performance because they are purposefully made. Note that the chemical makeup of the different pozzolans varies, and that can affect the color, reactivity and other characteristics of concrete, such as workability.

How Does Temperature Affect Concrete During Casting?

2009-03-09T14:00:00.003-04:00

Question:
How does temperature affect my concrete countertops during casting, and what are some measures I can take to help control temperature?

Answer:
Temperature plays a significant role during concrete casting and curing. Like most chemical reactions, the hydration rate of cement is affected by how hot or how cold it is. Warmer temperatures speed up the reaction, and cooler temperatures slow it down.

Once mixed, cement and water start reacting immediately. Depending upon the temperature, the initial set can occur within 30 minutes to as long as several hours. Cooler temperatures permit longer work times, but early strength suffers. Hot temperatures shorten work time but yield higher early strengths.

After the concrete sets and begins to gain strength, the rate at which its strength increases depends upon the concrete’s temperature. For example, concrete placed outdoors in the winter at 50° F gains strength very slowly, while in the summer, concrete placed at 90° F gains strength much quicker.

Similarly, when you can safely strip your concrete countertops and start handling them largely depends on how strong they are at a particular time. If you normally let your concrete cure in the mold for three days, but colder temperatures have retarded the strength, the concrete won’t be as strong as you expect. As a result, the concrete could crack from being handled while it’s still weak, and honing or polishing might not produce the desired results. For example, if the cement paste is weak, polishing may cause tear-out as the diamond discs grab the sand grains and pull them out of the weak paste.

Typically there are two ways to deal with hot or cold temperature extremes. One way is thermally, the other is through chemical admixtures. If curing conditions are too cold and you want to accelerate set time and strength gain, using hot water instead of cold water will speed up the initial reaction. The hot water will warm the ingredients, temporarily making them warmer than the ambient surrounding temperatures. As the concrete cools down, the effects of the hot water are reduced. But strength gain will still be higher than if cold water was used. Another method is to use insulation or even heating blankets (or steam) to warm the concrete after it’s cast.

Acelerating admixtures like calcium chloride, calcium nitrate and calcium diformate all speed up the set time, and some influence the early strength gain. However, you should never use calcium chloride in concrete countertops (or other decorative concrete) because the admixture can alter the color.

If it’s too hot and you want to slow down the set time, using cold water or even ice as part of the mix water can cool the ingredients, slowing set time. You can use the ice pound for pound as mix water, but be sure it’s fully melted before discharging the concrete from the mixer. Ice is much more effective at cooling than cold water because the phase change from solid to liquid absorbs 80 times more heat than simply warming an equal amount of cold water.

Retarding admixtures can help delay the set time of concrete too. While table sugar can be used (with extreme care), commercial retarders are much more reliable, easy to use and have far fewer negative consequences if accidentally overdosed.

How to Expose Aggregate Inside an Integral Sink

2009-01-01T11:37:00.000-05:00

Question:
I’ve had great success exposing aggregate on flat countertop surfaces, but how do I expose aggregate inside an integral sink? Are special tools or procedures required to work around curves and inside corners?

Answer:
Exposing aggregate in a concrete countertop, whether the aggregate is stone, glass or some other material, takes effort and time. Generally a very aggressive (low-grit) diamond pad or metal-bond turbo cup is needed to grind away the surface to expose the aggregate. Finer-grit pads are subsequently used to remove grinding marks and to smooth and refine the surface. As you’ve experienced, this process is fairly straightforward for flat countertop surfaces.

Working inside an integral sink is another story altogether. Often the sink’s shape is curved or complex, and access is tight. It may be physically impossible to get an electric polisher inside the sink. And using an aggressive turbo cup is often out of the question because of the sink’s curved surfaces.

A 3-inch-diameter backer pad with an extra-thick foam backer lets a 4- or 5-diamond polishing disc conform to the inside of a curved sink. While any thin, flexible diamond polishing disc will work, discs that have large diamonds, cut aggressively and have large spaces to flush the cuttings out from under the pad face work the fastest. I prefer DX Series Ultra Speed discs, sold by Granite City Tool. These pads last a long time and are particularly aggressive. While not extremely flexible, they will conform to the inside of a curved sink bowl.

An alternative to an electric polisher is to use a pneumatic polisher with a small flexible backer and large aggressive disc. I’ve already talked about the advantages of pneumatic polishers in a prior entry (see Pneumatic vs. Electric Polishers) so I won’t repeat that here. What matters in this case is that pneumatic polishers are much smaller and can often fit down inside a sink.
You can’t use rotary polishers inside all sinks, however. If your sink has corners, round discs won’t fit into them. So either you’re left sanding by hand, or you don’t expose the aggregate at all. Sometimes there are no easy answers for sinks like this.

It’s possible to expose aggregate inside a curved sink basin, but you’ll need some special tools.

A 3-inch-diameter backer pad with an extra-thick foam backer.

DX Series Ultra Speed discs.

A pneumatic polisher.


Combining the pneumatic polisher with the small flexible backer and 4- to 5-inch diamond disc will allow you to maneuver inside a sink basin.

If your sink has corners, round discs won’t work. To expose the aggregate, you’ll have to sand by hand.

UV-Activated Sealer Cures in Seconds

2008-12-25T19:09:00.000-05:00

Question:
I’ve heard about a new countertop sealer on the market that reacts with ultraviolet light. How does the sealer work and is it any good?

Answer:
The new sealer, called Counter-UV, manufactured by Kinloch and distributed by Surface 519, uses high-intensity ultraviolet (UV) light to cure the finish. It is formulated specifically for concrete countertops. While UV-curable finishes are common in the cabinet and flooring industries, a UV- curable sealer for concrete is a rarity. Historically, bonding issues have been the problem, but the manufacturer seems to have solved that with this product.

The finish itself is a specialized single-component, urethane-acrylic blend. It has no VOCs and offers excellent color enhancement, excellent stain resistance, and almost total acid resistance. Concrete Countertop Institute stain tests showed that only mustard left temporary discolorations in the sealer, and those could be easily and completely removed with household bleach. Very few sealers are completely resistant to mustard.

What makes this product so different from other high-performance sealers is how it cures. Most finishes, especially multicomponent products, can take hours to become tack-free and several days after that to reach full cure. Single-part finishes often merely need to dry out, so they become handleable faster. With the UV sealer, high-intensity, short-wavelength UV light initiates the cure. After exposure to the light for only a few seconds, the finish is ready for use. Thus you get the speed, convenience and ease of a single-part finish with the top-of-the-class performance offered by the best multipart finishes.

The UV sealer can be rolled on, sprayed on or wiped on. Rolling with a low nap (3/16- to 1/4-inch) lint-free roller is preferred, since the finish will flow out over time into a smooth film. Once applied, the finish does not cure and will not get hard, sticky or gummy until exposed to UV light. This is a great advantage because you can let the wet finish flow out or squeegee and back roll it repeatedly before curing.

You also have ability to adjust the sheen from high gloss to dead flat. If a glossy finish is not desired, you can sand the cured sealer to achieve whatever sheen level you’re after. But it’s best to use specialty sandpaper made for the solid-surface industry rather than ordinary woodworking sandpaper, which is not graded finely enough and can leave scratches in the finish. While these sanding discs are more expensive, they have very tightly controlled abrasive particle sizes that leave a uniform, scratch-free surface.

As with any finish, this one has its cons to go with the aforementioned pros. The finish will only cure with a specialized, high-intensity UV light. Sunlight, tanning lamps or fluorescent UV bulbs will not do the job. This UV light is expensive—about the same price as a large, high-quality concrete mixer. Because of this, it is cost prohibitive to try a sample of the finish, because without the UV light, the finish will not cure.

A white GFRC desk sealed with Counter-UV.

Unless you want a high-gloss finish, the only way to get a lower sheen is to sand it. And because it’s a tough, scratch-resistant finish, sanding takes time. Curved surfaces and intricate molded edging can be a challenge, too, and will require effort and skill to achieve high-quality results.
While some of these disadvantages seem significant, the system as a whole saves you the time of waiting for a finish to cure. It also is one of the easiest finishes on the market to repair, since repairs blend in well.

How Do I Design My Own Concrete Mix?

2008-12-18T16:00:00.001-05:00

Question:
Currently I use a prepackaged concrete countertop mix, but I’m thinking about coming up with my own mix design. Is it worth the effort, and what’s involved?

Answer:
Designing your own concrete mix for concrete countertops is a rewarding endeavor. It gives you a mix that you understand and have significant control over. And because it’s made from ingredients you’ve chosen, are probably local to you and that you buy in bulk, it’s much less expensive than bagged mixes.

However, making a custom concrete countertop mix that performs well in the long run is not easy. It requires an understanding about the fundamentals of how concrete works, how all the ingredients interact and affect each other, and just as important, what kind of performance characteristics are essential for concrete countertops. There is not a universal, one-size-fits-all concrete mix.

Different mixes provide different looks and may require different casting methods. A mix tailored for a stiff, hand-packed casting method is not likely to be good for wet casting, which requires a highly fluid mix optimized for speed and use with complex forms. While the workability characteristics are the most obvious differences between mixes, their strength and shrinkage characteristics play a major role in how the concrete performs, both in the short and long term.

Concrete mixes are often described in a form of shorthand. The classic “3:2:1” mix (three parts aggregate, two parts sand and one part cement) is a good example. This shorthand is extensively used in the industry, but it fails to describe the subtle but important characteristics that make one mix great for concrete countertops and another mix a poor one. This shorthand is akin to describing a car by simply stating it has four doors, a 2-liter engine and is red. Not terribly descriptive, nor does it tell you anything about how the car will perform. It really tells you more about what it’s not than what it is.

Without getting into the details of the mix design procedure, it’s important for you to understand that there’s a lot more to creating a mix than simply collecting a bunch of ingredients and the most popular or readily available admixtures and combining them according to someone’s recommendations. What works for one person may not work for another because variations in local aggregates (size, shape, roughness, gradation, mineralogy, etc.) all play a significant role in how the concrete performs. You need to look at the background, experience and qualifications of the person giving the advice. If they don’t understand what’s necessary, how can they help you with what you need?

I’ve written a series of articles on mix design that can give you guidance and insight into what’s important and what’s not. These two articles, in particular, are a good place to start:

Bagged versus DIY Mixes

The Role of Aggregate in Concrete Countertop Mixes

Another great resource, although very technical and rather advanced, is “Design and Control of Concrete Mixtures,” a publication by the Portland Cement Association (http://www.cement.org/).

The Pros and Cons of Using CSA Cements

2008-12-11T16:00:00.000-05:00

Question:
I have been hearing a lot about the advantages of using CSA (calcium sulfoaluminate) cement as a replacement for portland cement in concrete countertops. What is CSA cement, and should I consider using it?

Answer:
Developed in China in the 1970s, calcium sulfoaluminate cements belong to a class of specialty rapid-hardening cements. The primary advantage of making concrete with CSA cement instead of portland cement is that you can often achieve compressive strengths exceeding 5000 psi in 24 hours, a strength that normally takes 28 days to achieve with conventional portland cement.

Another key advantage is that CSA cements are also quite green. Production of CSA cement generates 62% less CO2-equivalent emissions than does the production of ordinary portland cement. Concrete made with 100% CSA has two to six times less environmental impact than concrete made with a significant quantity of portland cement replaced with pozzolans—and that includes “green” pozzolans such as fly ash and slag.

The greatest benefits are realized when 100% of the portland cement is replaced with CSA cement and no pozzolans are used. Not only does this allow the full strength potential of the concrete to develop, it also minimizes cost and inventory by simplifying the mix design. Due to their lower alkalinity levels, pozzolans won’t work with CSA cement to boost strength as they do with portland cement. Pozzolans like fly ash and VCAS (vitreous calcium alumino-silicate) may be added to the mix as workability enhancers or microfine aggregates, but the pozzolans will not be activated, so they won’t behave like cement. In fact, replacing 20% of CSA cement with VCAS has been shown to lower the compressive strength of test cylinders by almost 30%!

CSA cements can be used as direct replacements for portland cement, and they are compatible with concrete pigments, superplasticizers and other admixtures such as viscosity modifiers. Decorative aggregates, metal and glass are all compatible, so the use of specialty embedments and exposed aggregate is possible. And like concrete made with ordinary portland cement, CSA concrete can be dyed and acid stained.

While the rapid strength gain, high “green” value and low shrinkage are valuable assets, such high performance does come at a price. On average, an 88-pound bag of white CSA cement can cost well over twice as much as a 94-pound bag of white portland cement. Since time is money in the business world, however, the days you can save waiting for the concrete to gain strength may be worthwhile, especially if shorter turnaround times are important. CSA concrete cast today can be stripped and processed tomorrow, and in many circumstances it can be cast and stripped all on the same day. This increases the production rate of your casting tables, and it minimizes the number of tables you need and thus the shop size required.

But if your production process is inefficient, you take a long time to get things done, or you are not experienced with from-scratch concrete mixes, then the benefits of CSA cements won’t be realized. Much like driving a fast sports car while being stuck in a traffic jam, having concrete that gains strength very rapidly is pointless if the whole production process is not optimized to take advantage of the rapid strength gains.

Finally, CSA cements are not, in my opinion, for the beginner. Everything about them is magnified and accelerated. If you don’t know what you’re doing, you can end up with a mixer full of very hard concrete. That said, using CSA cement requires a well-practiced, highly organized batching and casting process, and forces you to become efficient and organized. And that’s just plain good for business.

How to Make Concrete Countertops ‘Greener’

2008-12-04T16:37:00.000-05:00

Question:
My clients are asking how “green” concrete countertops are. What does it mean to be “green?”

Answer:
“Greenness” is the quality of being more environmentally sound, sensitive or responsible. Generally, products that incorporate recycled materials or are themselves recycled or diverted from the waste stream are considered green.

There are many ways to increase the greenness of concrete countertops. The most obvious is the use of recycled material in the concrete, especially as a replacement for virgin materials. A good example of this is using fly ash to replace a significant portion of the cement. Another example is the use of recycled glass cullet (crushed glass) in place of some or all of the aggregate. Generally, the more recycled material you use to replace virgin material, the greener the concrete becomes.

But being green means more than simply swapping one ingredient for another that’s labeled “recycled.” There are many other facets to consider. For example, is concrete made with 100% recycled glass purchased from a vendor that shipped it cross-country greener than concrete made with locally acquired natural aggregates? How does the impact of resource consumption (fuel, energy, water, etc.) for material transport and post-consumer recycling efforts impact a material’s greenness? Does the use of a solvent-based sealer negate the greenness of a high-fly-ash content concrete?

There are no easy or straightforward answers to these questions, but asking them and contemplating the implications of materials, sources and manufacturing techniques can affect how green your concrete really is. Creating green concrete also helps architects obtain LEED (Leadership in Energy and Environmental Design) credits, a topic I wrote about previously (see How Do Concrete Countertops Meet LEED Criteria?).

Pneumatic vs. Electric Polishers

2008-10-23T08:05:00.001-04:00

Question:
What are the benefits of a pneumatic (air-powered) polisher?

Answer:
Hand-held polishers for stone and concrete come in two versions: electric and pneumatic. Electric polishers are very popular with concrete countertop makers for a variety of reasons. There are many brands and sizes to choose from, they are relatively inexpensive, and they can be brought onto the jobsite if necessary.

Electric polishers come in a range of sizes, but they tend to be large and heavy due to the motor. The smallest weigh in at around 5 to 6 pounds and work well with diamond pads up to 5 inches in diameter. Generally, the coarser turbo-cup-style discs are too heavy for these smaller polishers, although small 4-inch cup wheels can be used for lighter grinding. The largest electric polishers can weigh two to three times as much as the smallest electric polishers, but these brutes can handle large cup wheels and the newer 7-inch-diameter diamond polishing pads. When it comes to safety, mixing electricity and water is always risky, so almost all electric polishers on the market come with a built-in ground fault circuit interrupter (GFCI).

Pneumatic, or air-powered polishers, are a mainstay of the granite industry but aren’t used much by concrete countertop manufacturers. An air polisher looks dainty compared to an equivalent electric polisher, often weighing about half as much (usually about 2 to 3 pounds) and about half the size.

An electric polisher (rear) and pneumatic polisher (front).

Air tools can be very powerful, despite the small size of their motor, which explains why almost all factories, auto repair shops and other heavy manufacturing industries primarily use air tools instead of electric tools. Air polishers are generally designed to use diamond pads up to 5 inches in diameter and smaller-diameter cup wheels. When supplied with the optimal pressure and airflow, air polishers can provide variable speed with plenty of torque.

The small size, light weight and high power of an air tool make for a tough-to-beat combination. A small, lightweight air polisher is much more comfortable, less fatiguing and gives greater control over the polishing head than a large, heavy electric polisher. While suitable for general polishing work, an air polisher is perhaps the best tool to use when honing or polishing countertop edges, working inside integral sinks, or working in a hard-to-reach or critical area where lack of control can cause trouble. And using a non-electric polisher is much safer, especially when polishing edges and water to suppress dust is spraying everywhere, including all over the polisher itself.

The single greatest barrier to the widespread use of pneumatic polishers is that in order to run, these small powerhouses need very large air compressors. To run continuously, the polisher must be supplied with 90 psi air flowing at 16 cfm. To supply such a large volume of air, you’ll need a two-stage compressor driven by at least a 5-hp motor. Generally compressors of this size have an 80-gallon storage tank.

If the polisher is not going to be running continuously, a smaller (though still large) stationary compressor will work. I have found that my 60-gallon single-stage compressor works well for intermittent use. It supplies 10 cfm at 90 psi, so I can’t run it for more than a few minutes at wide open before the tank runs low and the compressor kicks in, but the polisher doesn’t stop working -- it just slows down. Most of the time this isn’t a problem, because when I stop to change positions, clean off the polishing residue or change pads, the compressor catches up.

While no one polisher can do everything well, an air polisher can supplement a larger electric one for jobs where finesse, control and maneuverability are important.

Clearing Up Misconceptions About Concrete Countertops

2008-10-16T08:00:00.000-04:00

Question:
I see and hear negative press about concrete countertops – how they stain and crack and are high maintenance. What can be done about this?

Answer:
It’s unfortunate that there often seems to be more negative information about concrete countertops than positive. I have tried for years to counter the general perception that concrete is high maintenance, stains easily and cracks, but it’s hard to overcome the perceived status quo with contrary information.

Concrete countertops don’t need to be high maintenance. They don’t need to stain, and they don’t need to crack. High-quality products should offer high performance, regardless of who makes them. The negative perceptions of concrete countertops often originate from people who see or hear about poor-quality work. Negative news seems to spread farther and faster than good news, so it’s the “elevated sidewalk” that has cracks and stains that people talk about. This is the root cause of the perception because it casts all concrete (pardon the pun) in the same lot.

A recent Consumer Reports review of kitchen countertop materials ranked concrete low, basically reiterating the general perception that concrete is undesirable because it stains and requires maintenance. In an unpublished letter to the editor of the magazine, I stated that their evaluation of the particular concrete they tested might have been accurate, but ranking all concrete the same was wrong. I likened it to them evaluating a single type of car, but ranking all cars based on that one test.

One way you can help to overcome this negative perception is to do high-quality work and prove that concrete can be a good choice. The technology exists to make very low maintenance, highly stain resistant concrete countertops that look good and perform well. Concurrent with this, you can direct people to articles written specifically about concrete countertops that address the issues regarding their performance. These articles, available in the “Questions & Answers” section of the Concrete Connections website, can help you clear up the misconceptions. There also is an article on the sanitary nature of concrete countertops (see Are Concrete Countertops Unsanitary?).

Will GFRC Make Conventional Concrete Obsolete?

2008-10-09T08:00:00.004-04:00

Question:
The use of GFRC (glass-fiber-reinforced concrete) to make countertops seems to be growing. Will it eventually replace all forms of concrete countertops?

Answer:
There are two basic methods for casting concrete: pouring in place and precasting. Within the precasting method there are three basic variations: wet casting, hand packing and spraying. GFRC can be sprayed, just like shotcrete, or wet cast (called premix in the GFRC industry) exactly the same as conventional concrete.

Sprayed GFRC, or spray-up in industry lingo, is the preferred method for placing large expanses of GFRC material in short amounts of time because it’s efficient and speedy. Both the veneer coat (called a mist or face coat) and the backer coat are sprayed through a specialized gun. The speed and efficiency are realized in large part because of the machinery employed, not necessarily because of the material used.

GFRC in the concrete countertop industry is generally not placed using the same large, expensive machinery used in conventional spray-up. Rather it is a hybrid of spray-up, hand packing and sometimes wet casting. The mist coat is often sprayed using an inexpensive hopper gun, and the backer is either hand packed for vertical edges or wet cast using a more fluid version of the GFRC mix on horizontal surfaces. This difference seems insignificant, but it does affect the structural properties and the economy of using GFRC. Time is money, after all.

GFRC’s advantages are that it is fairly easy to execute, it produces robust pieces, and because it’s only a shell, it’s lighter. It can be used to cast any shape piece and is perhaps the best way to cast complex, three-dimensional pieces. But these features, though very attractive, do not make GFRC the best choice for all applications.

The flatter and more two-dimensional the piece (like a simple countertop), the less advantageous GFRC becomes. This has more to do with the time it takes to mix and place the material than the material itself. Plus, GFRC costs about three times the price of a from-scratch conventional concrete mix for equal volumes of material.

For some simple applications, especially smaller pieces like bathroom vanities, it can be cheaper and faster to wet cast conventional concrete rather than use GFRC, and the savings in weight of a few dozen pounds may not make much difference.

Visually, GFRC can have many looks, but it’s predominantly monolithic and monotextural in appearance. Some have criticized it for being too perfect, but certain techniques can be used to give it variation and mottling similar to that seen in wet casting. Troweling is one of the few techniques that I’ve not seen reproduced well in GFRC.

Concrete is an emotionally involving material for clients. Its customization, character and sheer essence draw them to it. Its look, variety and tactile qualities are unique, and something as simple as the sound a concrete countertop makes when a cup, glass or plate are placed on it or when knuckles are rapped against it has a profound influence on how some people react to the material. Solid concrete has a different feel and a different sound from GFRC, and that can make a difference to a client.

Each type of concrete and each casting technique has strengths and weaknesses, and some are more versatile than others. GFRC is a versatile tool. It can be used for a wide variety of applications, but choosing it for all applications may not be the best choice. Other factors such as time, cost, appearance and tactile sensation all play a part in which casting method is right for a particular project. Will GFRC replace all other forms of concrete? I doubt it, but it does remain to be seen how widespread its use will become.

For more details on basic GFRC casting techniques, read my article “Introduction to GFRC,” on the Concrete Connections website.

Using Overlay Pigments in Countertops

2008-09-11T18:43:00.001-04:00

Question:
Is it all right to use overlay pigments in concrete countertops?

Answer:
Most pigments made for decorative concrete overlays can be used in concrete countertops, especially dry pigments. But they should always be used with caution. Many pigments are liquids, and liquid pigments designed for use in overlays have additives that can cause problems with concrete.

I recently observed a situation illustrating why. A contractor used liquid overlay pigments to tint a bagged concrete countertop mix. During mixing, he noted that the concrete had a strange, foamy consistency similar to chocolate mousse. (It didn’t help that the pigment was brown!) The day after casting, the concrete was not nearly as hard as it should have been. In fact, a fingernail could still leave an impression in the backs of the countertop slabs.

I have personally used that particular concrete countertop mix before, and I know that it normally gets very hard 24 hours after casting. I strongly suspected that some of the additives in the liquid pigment were responsible for the foamy texture and the low strength. I recommended that the concrete be wet cured for an additional two days before stripping, otherwise the risk of cracking from demolding would be too great.

Even after curing for four days in the mold, the concrete still was not as strong as it should have been, but it hadn’t cracked. Eventually the strength caught up with where it should have been and the countertops turned out all right, but he got lucky.

Using a product made for another type of material is risky. It’s always safest to stay with an additive (like a pigment) that’s intended for concrete, not some other product. The chemical interactions between the pigment, the concrete and whatever admixtures you are using may not “play” well together, resulting in weak concrete that doesn’t perform as it should. Saving a few dollars up front by using some leftover overlay pigment could end up costing you hundreds or thousands of dollars if the countertops have to be redone. Not to mention the time lost and the cost to your reputation.

If you must use an overlay pigment, it’s very important to test it out on a realistic test pour. Make a 2 x 2-foot concrete slab. Cure it, and process it as you would a real countertop. If the overlay pigment has a negative influence on the concrete, you’ll find out on the test piece, not on a paying client’s project.

Using Curing Blankets vs. Plastic

2008-09-04T06:40:00.003-04:00

Question:
What are curing blankets, and are they better than using plain plastic for curing concrete countertops?

Answer:
Curing blankets are sheets of plastic with a synthetic felt-like material bonded to one face. They are used primarily with flatwork to help cure the concrete once it’s troweled.

Curing blankets work by trapping moisture under them and holding the moisture in the felt layer. The felt wicks pooled water away and moves the moisture laterally through the felt layer. This helps redistribute moisture to drier areas, and it helps reduce the marks that plain plastic leaves on a troweled finish. Often the felt is prewetted with water so it acts like a reservoir.

For concrete countertops, curing blankets work very well, especially for troweled finishes. But for regular precast concrete countertops, there’s no need to buy expensive curing blankets when regular 4- or 6-mil polyethylene plastic does the job just as well. Precast concrete countertops are often cast upside down, so the underside of the concrete is what touches the plastic, not the finished face. Discoloration marks left by the plastic never get seen.

Both curing blankets and regular plastic sheeting trap moisture under them, preventing it from evaporating. In essence, the sheeting acts like a greenhouse, maintaining a high humidity level. The concrete under the plastic may look dry, but if water droplets are on the plastic and damp areas are visible, then the humidity level is high enough for curing to continue.

The felt moisture reservoir in curing blankets serves to keep the humidity levels high. So it does offer an important benefit. But the same benefit can be had by tightly wrapping the concrete in plastic that doesn’t have tears or holes in it. (Loose plastic or loose curing blankets let moisture escape, defeating the purpose of using them.) If a moisture reservoir is needed and you don’t have a curing blanket on hand, simply use burlap, old towels, blankets or cloth under the plastic to serve as a moisture reservoir.

Keep in mind that concrete needs moisture to cure, but it doesn’t need to be wet. Adding extra water to the concrete, flooding the concrete or even submerging it doesn’t make it cure any faster. According to the Portland Cement Association, as long as the internal relative humidity level within the concrete stays above 80%, the cement will continue to hydrate. It’s not the moisture on the concrete that matters, it’s what’s inside. Moisture loss is what must be prevented.

What to Consider When Choosing a Casting Surface

2008-08-28T18:21:00.008-04:00

Question:
Does it make any difference what material I cast my concrete countertops on, especially in terms of appearance and economy?

Answer:
In general, yes. Any surface you cast concrete upon should not be porous or affected by water. So materials like bare wood are obviously not appropriate. However, that still leaves many different materials to choose from.

When choosing a casting surface, you need to weigh several factors because every material has advantages and disadvantages. Surface smoothness, scratch resistance, durability, cost, reusability, water resistance, repairability and availability are just some of the things that will influence your choice.

Another is surface texture. The smoother the casting surface, the less finishing that will be required once the piece is stripped. However, many materials with a high surface quality are prone to scratching, which rapidly erodes the quality level. Surfaces that don’t have such a high- quality surface will require you to hone or grind the concrete after casting. However, if you plan to hone or grind the concrete anyway, then the quality of the casting surface won’t matter that much.

One of the most common casting surfaces is melamine-coated particle board. It’s cheap, readily available and can be cut and shaped with common woodworking tools. It’s often used to build whole molds, rather than a dedicated casting-table surface. Melamine has a subtle texture to it, so it leaves a fairly smooth cast surface. And because melamine doesn’t stick well to concrete, a form-release agent is often unnecessary.

The disadvantages of melamine are that it’s fairly weak, it’s easily scratched (it can only be used a few times before it scratches) and water-induced swelling degrade it. Because you can use it only a few times at most, melamine can get expensive. And a material that’s only used once or twice is not as environmentally sound as one that can be used dozens or hundreds of times.

A less wasteful and less expensive option than using melamine for ground surfaces is steel. Steel casting surfaces are very durable and highly reusable, and they are widely used in commercial precast operations. Because steel is durable, it can be reused year after year. The drawbacks: Because concrete bonds well with steel, an appropriate form-release agent must be used. Steel also must be cleaned of rust after use. Still, I prefer to use steel casting tables and steel angle iron for forming because steel is so reusable and rigid, and I almost always grind and slurry my countertop surfaces.

If a mirror-smooth cast surface is desired, consider using materials such as glossy laminate, plastic sheets or even glass as casting surfaces. Their ultra-high surface quality means that the concrete is shiny when it’s stripped. The downside is that often ultra-smooth casting surfaces can be used only once, since any scratch will degrade the concrete surface. And with perfect cast surfaces, there’s no way to repair damage to the concrete or to selectively remove scratches without honing. If you need to hone the concrete surface to remove casting blemishes, then the entire reason for using such a smooth, polished casting surface is moot.

One of the most economical casting materials is
steel because it’s highly durable and reusable.

Benefits of Using a High-Shear Mixer

2008-08-21T13:14:00.002-04:00

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

2008-07-31T16:08:00.001-04:00

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?

2008-07-24T15:54:00.001-04:00

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

2008-07-17T08:00:00.001-04:00

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

2008-07-10T15:36:00.002-04:00

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

2008-06-13T08:00:00.001-04:00

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

2008-05-30T08:00:00.001-04:00

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

2008-05-16T08:00:00.001-04:00

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

2008-05-02T14:20:00.000-04:00

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

2008-04-18T08:00:00.002-04:00

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.