SCC Backer Mix for GFRC Concrete Countertops: Be Aware of Fiber Realignment

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The practice of making “self consolidating” GFRC backer mix for concrete countertops attempts to minimize the hand labor by making the backer very fluid and workable. The backer is simply poured in, and it self-levels, rather than hand placing multiple layers of a stiff backer mix.

While this technique does succeed in speeding things up, it defeats the effectiveness of the fibers, as I will explain below. Trading time and effort for structural performance can lead to breakage and lost profits.

Pouring the backer in a single thick layer reduces effectiveness of the fibers because it does not align the fibers in the preferred direction. This renders most of the fibers (up to 85% of them) ineffective because they’re oriented in the wrong direction – randomly 3-D instead of in a flat 2-D plane. When the backer mix is very fluid, it can’t be rolled to align the fibers 2-dimensionally. This article discusses the influence of fiber orientation and why it’s important to the structural performance of GFRC.

If you still want to use a self-consolidating backer, then instead of placing one thick layer, place it in thin lifts. By placing the concrete in thin sections, the act of spreading the concrete out into thin layers orients the fibers in a more 2D format. Even though this requires more work, it results in a stronger piece of concrete.

Random 3D Fibers

Random Fibers Resulting from Fluid Backer Mix

 

Properly Aligned Fibers Resulting from Thin Layer and Rolling

Properly Aligned Fibers Resulting from Thin Layers and Rolling

 

Exploring the Unlimited Possibilities of Concrete Countertops

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Andrew Lash doesn’t just run his concrete shop Lash Design Corp, he’s also a concrete ambassador of sorts helping to educate those in his area about the benefits and possibilities that concrete possesses. I recently had the opportunity to talk with him about his work and some of the challenges he faces as he perfects his craft and builds his business in an area where people think of concrete as nothing more than the gray stuff you use to make sidewalks.Lash Countertop

Educating About the Benefits of Concrete

Speaking about the challenge of attracting clients Andrew said, “In this area here in Florida it seems like most people aren’t aware of concrete as a potential solution for countertops or flooring… When they look at countertops they look at granite, quartz, Formica, laminate type things, not really considering concrete. So right now I feel like I really have to put money and effort into growing a market in this area and showing that concrete is a suitable surface for vanities and countertops. Once I do that, then I can make sure that people know I exist.

When people hear what I do, they so often think that I am referring to a sidewalk that they can put on their cabinets, but when they see pictures and samples that I have they are completely blown away with what concrete can look like.”

Lash Fossil CountertopLash Fossil CloseUp

Working with concrete is exciting and rewarding, but it is a business that often requires customer education in addition to making great pieces.

Unlimited Possibilities

One reason Andrew enjoys working with concrete is the wealth of possibilities this material provides. He often jokes with his customers that concrete’s versatility is one of its biggest advantages and biggest drawbacks. Customers can have literally almost anything they want. They aren’t limited to a few color choices or design possibilities. The difficult part however comes in helping his clients to figure out what they want so he can create a sample and make it happen.

“A lot of times I have customers that they love the idea that they can have what they want, but at the same time they can’t make a decision about what they want.”

Lash Bathroom Cups

Concrete Bathroom Cups

This drive to create unique pieces has pushed Andrew into exploring new possibilities using concrete. Recently he crafted a beautiful set of bathroom cups for his girlfriend and offers similar custom cups to interested clients. A local florist has also contacted Andrew hoping to have custom concrete vases created.

Custom Concrete Bathroom Cups- Side View

Custom Concrete Bathroom Cups- Side View

Taking His Training and Making It His Own

We love seeing our students take the skills they learn and put them into practical applications. Each student has a unique take on how to use concrete and each creative possibility inspires us. Andrew had the pleasure of joining us at Concrete Countertop Institute in October 2011 for our Ultimate Course. In fact one of his biggest pieces of advice for those just getting started in concrete is to find and take advantage of training. Even though he was already working with concrete before attending our training, he found that being able to ask questions and see processes demonstrated was a big help to his career.

One last piece of advice he offers to those new in concrete industry is to avoid the urge to oversell the product.

“I’ve heard a lot of people that try to sell a concrete countertop as… stain-proof or scratch-proof which is definitely not the case and it comes back to bite them when (customer) gets a small scratch. Set expectations for whatever sealer or process you’re going to use.”

What great advice! Marketing the benefits of concrete is essential, but it is important not to sell it as something it is not. When customers know what to expect they will be better able to enjoy their new concrete countertops and as we all know one happy customer can lead to many more.

Thanks again Andrew for showing us your work and sharing a few of your designs with us here on the Concrete Countertop Institute blog.

How to choose the best diamond pad for concrete countertops

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Selecting the right diamond polishing pad (or disc) is the key to getting a good finish for your concrete and getting the most bang for your buck.

There are many different sources, names, styles and prices for diamond pads on the market. This can be very confusing, and what often happens is selection comes down to price. This is unfortunate, because in many cases a cheap pad will cost you more in the long run. However, I’m not suggesting we all go out and spend $100 per pad either. There’s a place for those, and we don’t need to go there (granite folks might though).

Choosing the right pad starts with assessing what you plan on doing with it.

  • Are you using it for heavy stock removal?
  • Is it for general honing?
  • Are you polishing the concrete to a high gloss?

Each of these tasks will influence the choice in pad. I’ll outline some of the variables to consider and then create a general template for selecting the right pad for the right task.

Size

Larger diameter pads (e.g. 7″) are much more stable on large, flat areas than smaller diameter pads (3″ and 4″). However, larger pads become unstable on narrow sections of concrete. A 7″ diameter pad won’t stay flat or cut evenly on a 3″ wide strip of concrete. They are also difficult to use on the vertical edges of countertops.

Small diameter pads are less stable and more likely to gouge when processing large areas on a big polisher, but with a smaller polisher (espcially a pneumatic polisher) they work very well for processing edges and narrow sections.

A good all-around size is a 5″ diameter pad. Many low-cost polisher package deals come with 4″ pads. These can be difficult to control with hand held polishers on concrete. Smaller 4″ pads are really meant for use on hard stone, which is much less prone to gouging than concrete.

Thickness

Diamond pads come in a variety of thicknesses, from around 2mm thin to 8mm thick.

Thicker pads will last longer, but thicker pads are stiffer, and sometimes are prone to cupping when they dry.

Thin pads don’t last as long (especially when aggressively cutting), but they are more flexible. This is a big advantage when honing or polishing inside curved integral sinks.

Pattern

The pattern molded into the cutting surface plays a significant role in the lifespan and the cutting quality.

Often pads used for coarse honing (30 and 50 grit) have an aggressive, open pattern with wide and deep channels. Pads with large, open channels allow the abrasive cutting residue to be ejected quickly and effectively. This increases the lifespan of the pad when aggressive stock removal is performed. 

Pads that have many narrower channels are best only for polishing (400 to 3000 grit), where almost no stock is removed. Narrow channels clog more readily when aggressive cutting is performed and when insufficient water flows out from under the disc. 

A good balance of these factors is lots of channels that are large in proportion to the diamond-covered surface. This provides flexibility to the pad while still allowing for effective ejection of cuttings and a large proportion of diamond area. In contrast, pads with very large channels and very large “islands” of diamonds tend to be inflexible.

Diamond Quality

While it’s very easy to see the thickness and channel pattern in a diamond pad, it’s nearly impossible to assess the quality of the diamonds used in the pad, or for that matter, the grading and quantity of diamonds. Ultimately it’s the diamonds that do the cutting, and they are what makes a pad expensive. The better the quality of diamond, the better the grading and the more diamonds in the pad, the more expensive the pad will be.

It’s possible to find $1 pads and $100 pads that look almost identical. What’s not visible are the diamonds, and that’s where you need to trust your supplier to inform you of what you’re getting.

The most expensive pads use the best industrial diamonds. The diamond concentration is high (each manufacturer optimizes the quality and quantity of diamond in their pads), and the gradation is narrow. Like sand and gravel, diamonds come in a variety of sizes. A narrow gradation means almost all of the diamonds are about the same size, so the grit number of the disc is a more precise description of the diamond size gradation. Any variation tends toward smaller diamonds, which don’t affect the quality of the concrete surface.

Cheap pads use low grade diamonds. These tend to have more flaws and much poorer gradation. In addition, fewer diamonds may be used in the pad. Often the gradation is broader, so a 100 grit pad may actually have significant amounts of smaller diamonds and some coarser diamonds (like 70 grit). This results in a lower surface quality (more scratches) and slower cutting (because there are more smaller diamonds in the pad).

Binder

The binder that encapsulates the diamonds is just as important as the diamonds, and it has a profound effect on the performance and longevity of the pad. Binder materials range from ceramic to resin, and different materials are used for specific applications. Binder hardness is important. A binder that’s too soft will wear away quickly when processing an abrasive material like concrete.  

Nearly all wet polishing pads use a resin binder, and resins vary. It’s very rare that a pad distributor will describe the binder with any meaningful detail, so here personal observations and reliance on trusted recommendations are necessary to make a good choice. Too often the least expensive pads use soft resin binders that wear away quickly. If you end up using three times as many pads as you would with a pad that costs twice as much, you are not saving money in the end.

Ceramic binders tend to wear better and stand up to higher temperatures than resin pads. Many dry pads use a ceramic binder, which helps to prevent the smearing and glazing that can occur when a resin based dry pad is run at a speed that’s too high.

Recommended Diamond Pads for Concrete Countertops

The Concrete Countertop Institute has developed diamond pads specifically designed for concrete countertops, not granite and not concrete floors.

These pads have all the features you want in a diamond pad for concrete countertops:
- High concentration of diamonds for faster cutting
- Narrow gradation of diamonds for a swirl-free surface
- Thick enough for long life
- Thin enough to be flexible
- Ideal channel design to eject slurry and prevent clogging
- Ideal resin binder for long life without glazing

Click here for pricing.

diamond pads

Diamond Pads

Creating Concrete Countertops Using GFRC

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In the last two pieces about glass fiber reinforced concrete I’ve discussed the basics of glass fiber reinforced concrete and the importance of fibers. To finish off this series on GFRC I’d like to move into a few of the technical aspects you will encounter when creating GFRC countertops including mix designs, casting and processing.

GFRC Mix Designs

If you’ve worked much with concrete you know that finding the right mix can be difficult and often requires years of experience. Many different factors impact the ideal composition for concrete, and GFRC is no different. Mix design isn’t a concept that can be tackled in one blog post, but here are some of the basic components in a good GFRC mix:

  • Fine Sand- Sand used in GFRC should have an average size passing a #50 sieve to #30 sieve (0.3 mm to 0.6mm). Finer sand tends to inhibit flowability while coarser material tends to run off of vertical sections and bounce back when being sprayed.
  • Cement- Typical proportions use equal parts by weight of sand and cement.
  • Polymer- Acrylic polymer is typically preferred over EVA or SBR polymers for GFRC. Acrylic is non-rewettable, so once it dries out it won’t soften or dissolve, nor will it yellow from exposure to sunlight. Most acrylic polymers used in GFRC have solids content ranging from 46% to over 50%. Consider trying Smooth-On’s duoMatrix-C and Forton’s VF-774, two reliable acrylic polymer choices.
  • Water- Common water to cement ratios range from .3 to .35.  When determining how much water to use make sure to take the water content from your acrylic polymer into account. This can make calculating water to cement ratios difficult unless the solids content of the polymer is known. With a polymer solids content of 46%, 15 lbs of polymer plus 23 lbs of water are added for every 100 lbs of cement.
  • Alkali Resistant Glass Fibers- Fibers are an essential component of GFRC. If you’re using the spray-up method for casting the fibers will be cut and added to the mix automatically by your sprayer at the time of application. If you’re using premix or the hybrid method for casting you’ll mix the fibers in yourself. Fiber content varies but is typically between 5% to 7% of the overall cementitious weight. Higher fiber content increases strength but decreases workability.  
  • Other Admixtures- Some other elements you may choose to include in your GFRC mix include silica fume, metakaolin, VCAS and superplasticizers.

GFRC Casting

I talked a bit about casting in the Introduction to GFRC article. There are a few different methods you can use for casting. Since I’ve already discussed the basic concepts behind casting (if you need a refresher check out Introduction to GFRC) I’d like to take a minute to review the pros and cons associated with each casting method.

Spray-Up

A concrete mixture is sprayed into the forms using a special device that chops and sprays a separate stream of long fibers. The concrete and fibers mix when they hit the form surface.

  • Pros: Allows for very high fiber loads using long fibers resulting in greatest possible strength.
  • Cons: Requires expensive, specialized equipment (generally $20,000 or more).

Premix

Glass fibers are mixed directly into the fluid concrete. The mixture is then poured or sprayed into molds.

  • Pros: Less expensive than spray-up, although a special spray gun and pump is required.
  • Cons: Fiber orientation is more random than when using spray-up and fibers are shorter resulting in less strength.

Hybrid

The hybrid method for casting GFRC uses an inexpensive hopper gun (the same kind used with drywall) to spray a thin face coat into the forms. Once the face coat dries the fiber loaded backer mix is applied either by pouring or hand packing, just like ordinary concrete.

  • Pros: Affordable way to get started with GFRC. A hopper and air compressor run about $400-$500, much less than the spray guns used for spray-up or premix.
  • Cons: Since the face coat and backer mix are applied at different times careful attention is needed to ensure the mixes have a similar makeup to prevent curling.
Spraying GFRC

Spraying GFRC

GFRC Curing

The high polymer content of GFRC often means that long term moist curing is unnecessary. Cover a freshly cast piece with plastic overnight, but as soon as it has gained enough strength it can be uncovered and processed. Many GFRC pieces are stripped 16 to 24 hours after casting.

GFRC Processing

Your skill level, the composition of your mix and the method used will determine how much processing is needed once your GFRC countertop is removed from its molds. Grouting may be needed to fill in bug holes or surface imperfections. Any blowback (sand and concrete that doesn’t stick to the forms) needs to be cleaned or the concrete’s surface will be open and granular. Achieving a perfect piece right out of the mold is very difficult and requires great skill.

Common Questions About GFRC

  • How Thick is a Typical GFRC Countertop?- Typical concrete countertops made with GFRC range from ¾” to 1” in thickness. This is the minimum thickness that a long, flat countertop can be made so it doesn’t break when handled or transported. Smaller wall tiles can be much thinner.
  • Is GFRC Green?- GFRC is roughly on par with other forms of concrete countertops in terms of the “green-ness”. In comparing 1.5” thick concrete countertops to ¾” GFRC countertops, the same amount of cement is used, since GFRC tends to use about twice as much cement as ordinary concrete. This sets them equal to each other. The use of polymers and the need to truck them does make GFRC less green than using ordinary water, which could be recycled from shop use. Both traditional cast and GFRC can use recycled aggregates, and steel reinforcing is more green than AR glass fibers, since steel is the most recycled material, so its use in concrete of all forms boosts the concrete’s green-ness.

I hope you enjoyed this three-part series on GFRC. Before you go check out this short 7 ½ minute video featuring excerpts from my Comprehensive GFRC Self Study Course. Watching an actual GFRC countertop being constructed will help you better understand many of the topics I’ve covered in this series.

Glass Fibers- An Essential Component of GFRC Concrete Countertops

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If you’re wondering about the importance of glass fibers in GFRC just think about the name for a minute.  Glass fiber reinforced concrete- without the fibers all you have is concrete. These alkali resistant glass fibers give GFRC its strength and make it an ideal choice for a variety of applications including concrete countertops.

According to Wikipedia.com, “[g]lass fiber reinforced composite materials consist of high strength glass fiber embedded in a cementitious matrix. In this form, both fibers and matrix retain their physical and chemical identities, yet they produce a combination of properties that can not be achieved with either of the components acting alone. In general fibers are the principal load-carrying members, while the surrounding matrix keeps them in the desired locations and orientation, acting as a load transfer medium between them, and protects them from environmental damage.”

GFRC utilizes both concrete and strong AR glass fibers. Both possess benefits on their own, but when combined they become something amazing. Let’s take a look at the important role fibers play in GFRC.

Why Fibers?

One of the benefits of GFRC is its flexural and tensile strength. The tensile strength helps GFRC to resist pulling apart forces while the flexural strength helps it to resist bending. The glass fibers and the high polymer content of GFRC provide these unique properties that are essential to a long lasting concrete countertop. Rather than using steel for reinforcement, GFRC relies on these glass fibers to prevent cracking and breakage. Reinforcement is essential any time you create a concrete countertop, and GFRC uses fibers to create this reinforcement.  

This nine minute video, while it addresses steel reinforcing, will help you better understand the importance of reinforcement in general when constructing a concrete countertop:

Tips for Using Fibers in GFRC

Making GFRC isn’t as simple as just adding some fibers to your concrete mix design. There are many important considerations to remember. Here are a few:

  • Amount of Fiber Present- GFRC relies on a high load of glass fibers. Without sufficient fiber the concrete will be unable to resist cracking and breakage when faced with a high tensile load. Fiber content varies, but is usually between 5% and 7% of the cementitious weight. Some mixes go as high as 10% fiber content. The more fiber present the stronger the GFRC, but increased fiber does lead to decreased workability.
  • Orientation of Fibers- Orientation of the fibers in the mix is also important. Truly random fiber orientation means more fiber is needed since many of the fibers will be pointing in the wrong direction.
  • Method of Reinforcement Used- There are three different levels of reinforcement used in general concrete and GFRC. Each type carries different benefits.

Level 1: Random 3-D Reinforcing

This type of reinforcement occurs when fibers are mixed into the concrete and the concrete is poured into forms. The fibers are evenly distributed throughout the concrete and point in every direction. Typically only 15% of the fibers are oriented in the proper direction requiring very high fiber loads. This level of reinforcing is very inefficient requiring large amounts of fiber for lower levels of reinforcement.

Random 3-D Fibers

Fiber Orientation with Random 3-D Reinforcing

Level 2: Random 2-D Reinforcing

In this level of reinforcing concrete is sprayed onto a form using special equipment that chops and adds the fiber during the spraying process. Spray-Up GFRC is an excellent example of this type of reinforcing. Typically 30% to 50% of the fibers are optimally oriented. This method is more effective than 3-D reinforcing, but not the most effective method available.

Spray-up GFRC

Spray-up GFRC

Level 3: 1-D Reinforcing

The final level of reinforcing, one-dimensional reinforcing, is the most effective method available because it uses the least amount of reinforcing material to resist tensile loads. All reinforcing is placed in the tensile zone, or the area that needs the extra strength, reducing the overall amount of reinforcement needed. This method is used to create structural concrete beams with steel reinforcing. When creating a concrete countertop slab, the bottom of the slab is the tensile zone, as you saw in the video. Scrim in GFRC or steel in precast concrete are examples of 1-D reinforcing. Scrim is a glass fiber mesh used to give extra strength to GFRC, in addition to the fibers.

Reinforced Beam

1-D Reinforcing

When it comes to GFRC glass fibers are essential, but as this article clearly illustrates there is more than one way to add those fibers in. The method you select will determine how much fiber is needed and how strong your finished concrete countertop will be.  

Introduction to GFRC (Glass Fiber Reinforced Concrete)

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If you aren’t yet familiar with glass fiber reinforced concrete (GFRC) you should be. GFRC is a specialized form of concrete with many applications. It can be effectively used to create façade wall panels, fireplace surrounds, vanity tops and concrete countertops due to its unique properties and tensile strength. One of the best ways to truly understand the benefits of GFRC is to take a deeper look into this unique compound.

What is GFRC?

GFRC is similar to chopped fiberglass (the kind used to form boat hulls and other complex three-dimensional shapes), although much weaker. It’s made by combining a mixture of fine sand, cement, polymer (usually an acrylic polymer), water, other admixtures and alkali-resistant (AR) glass fibers. Many mix designs are available online, but you’ll find that all share similarities in the ingredients and proportions used.

Some of the many benefits of GFRC include:

  • Ability to Construct Lightweight Panels- Although the relative density is similar to concrete, GFRC panels can be much thinner than traditional concrete panels, making them lighter.
  • High Compressive, Flexural and Tensile Strength- The high dose of glass fibers leads to high tensile strength while the high polymer content makes the concrete flexible and resistant to cracking. Proper reinforcing using scrim will further increase the strength of objects and is critical in projects where visible cracks are not tolerable.

GFRC is strong. Check out this YouTube video to see just how strong it can be:

 

The Fibers in GFRC- How They Work

The glass fibers used in GFRC help give this unique compound its strength. Alkali resistant fibers act as the principle tensile load carrying member while the polymer and concrete matrix binds the fibers together and helps transfer loads from one fiber to another. Without fibers GFRC would not possess its strength and would be more prone to breakage and cracking.

Understanding the complex fiber network in GFRC is a topic in and of itself. Stay tuned, I’ll post a more in-depth article on GFRC fibers next week.

Casting GFRC

Commercial GFRC commonly uses two different methods for casting GFRC: spray up and premix. Let’s take a quick look at both as well as a more cost effective hybrid method.

Spray-Up

The application process for Spray-up GFRC is very similar to shortcrete in that the fluid concrete mixture is sprayed into the forms. The process uses a specialized spray gun to apply the fluid concrete mixture and to cut and spray long glass fibers from a continuous spool at the same time. Spray-up creates very strong GFRC due to the high fiber load and long fiber length, but purchasing the equipment can be very expensive ($20,000 or more).

Premix

Premix mixes shorter fibers into the fluid concrete mixture which is then poured into molds or sprayed. Spray guns for premix don’t need a fiber chopper, but they can still be very costly. Premix also tends to possess less strength than spray-up since the fibers and shorter and placed more randomly throughout the mix.

Hybrid

One final option for creating GFRC is using a hybrid method that uses an inexpensive hopper gun to apply the face coat and a handpacked or poured backer mix. A thin face (without fibers) is sprayed into the molds and the backer mix is then packed in by hand or poured in much like ordinary concrete. This is an affordable way to get started, but it is critical to carefully create both the face mix and backer mix to ensure similar consistency and makeup. This is the method that most concrete countertop makers use.

Spray-up GFRC Fibers

Coming soon: A more in depth look at GFRC mix designs, casting, thickness, curing and processing.

Quick Facts About GFRC

  • GFRC was first created in the 1940s in Russia, but it wasn’t until the 1970’s that the current form came into widespread use.
  • GFRC tends to run about $2.50-$3.00 per square foot for ¾” thick material. The cost increases to about $3.50-$3.75 per square foot for 1” thick material when accounting for the prices of sand, cement, admixtures, fibers and polymer.
  • Just like regular concrete, GFRC can accommodate a variety of artistic embellishments including acid staining, dying, integral pigmentation, decorative aggregates, veining and more. It can also be etched, polished, sandblasted and stenciled. If you can imagine it, you can do it, making GFRC a great option for creating concrete countertops and especially three-dimensional concrete elements.

To learn more about GFRC check out these great articles on our blog:

Types of portland cement used in concrete countertop mixes

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Portland cement comes in a variety of different types. In the United States, these types are classified as Type I, II, III, IV and V. Only Types I and III are necessary for consideration by concrete countertop fabricators; the benefits of Type II cement are generally irrelevant to the concrete countertop industry.

Type I is ordinary portland cement, and it is available in white or gray.

Type II is a moderate sulfate resistant cement, important when concrete is cast against soil that has moderate sulfate levels.

Type III is a high early strength cement. It is ground finer and reacts faster than Type I, so the early strength gains are greater. However the ultimate strength is not higher than Type I. Concrete made with Type III will have slightly higher 28 day strengths than concrete made with Type I, all else being equal. Type III is available in white or gray, but white Type III is difficult to find in small (less than pallet) quantities; it often has to be special ordered.

Type IV and V are often used in special construction applications where high sulfate resistance is required or a low heat of hydration is important. Neither of these types are practical choices for countertops.

What’s the difference between Type III cement and CSA cement?

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Type III cement is a form of portland cement. This article explains Type III cement, but basically, Type III is a high-early-strength cement. It is ground finer and reacts faster than Type I cement, so the early strength gains are greater. Note the word “early”.

Generally Type I cement based concrete reaches about 60% of its 28 day strength in the first 3 days;

Type III cement achieves about 70% of its 28 day strength after 3 days. That is indeed a little faster than Type I.

With either Type I or Type III portland cement, continued strength gain requires continuous wet curing for weeks. Concrete that dries out prematurely never reaches its full potential. (Note however that with concrete countertop mixes that achieve over 8000 PSI compressive strength well before 28 days, it is really not necessary for them to reach their full potential. They are strong enough after a few days, so I generally cure regular portland cement mixes for only 3 days.)

CSA cement typically achieves 80% or more of its 28 day strength in the first 24 hours, and usually close to 100% of its strength within the first 3-7 days. Because of the very rapid reaction, wet curing is necessary only for the first few hours, not continuously for weeks like with Portland cement based concrete.

 

In the graph below, note the steeper curve for Type III, meaning faster strength gain.

Strengths of type I vs type III

Strengths of type I vs type III cement

Figures taken from PCA, Design and Control of Concrete Mixtures, 2003.

How to use set retarders with CSA cements in concrete countertop mixes

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All CSA products are inherently fast setting. In order to boost the work time, chemical retarders are often used. Unlike portland cements, CSA cements can use ordinary citric acid, often at doses of 0.2% to 0.4%.

Straight CSA cements such as Rapid Set Cement actually benefit from moderate retardation. The structural silicate crystal growth occurs more slowly, and like old-growth wood, results in better concrete with higher strengths.

Another way to increase work time when using CSA cement is to substitute ice for some of the mix water. Generally 1/4 to 1/3rd of the mix water is ice, and the melting ice absorbs heat from the other ingredients. The advantage of thermally retarding the mix with ice is that once the mixture warms up to ambient temperatures, its normal setting rate return, so early strength is far less affected than when citric acid is used.

Both ice and citric acid can be used together for very long work times. I have achieved over an hour of working time with Rapid Set Cement in 90 degree weather when citric acid was used along with ice. Ice can also be used with “plain” portland cement based concretes, as this is a time tested technique used in the construction industry for hot weather concreting.

Ice to cool and retard the concrete

Ice to cool and retard the concrete

Note: Qwix is a CSA additive sold by Ultimax. Qwix, since it’s blended with portland cement, may use citric acid to extend working time, but that may also reduce early strength development of the portland cement it’s blended with. Dosages vary depending upon Qwix amounts and upon the amount of working time desired.

CSA cement vs. accelerators in concrete countertop mixes

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A student asked me a question recently:

What’s the difference between CSA cements and accelerators? Don’t they both make the concrete cure faster?

The answer is no.

Accelerators don’t speed up strength gain, only set time. This means that concrete made with or without an accelerator will have developed the same 28-day strength at the same rate. The concrete with the accelerator just sets up faster so that it can be troweled or finished earlier.

CSA cements, on the other hand, actually speed up strength gain. They speed up strength gain so much that the concrete is as strong after 28 hours as it would have been in 28 days with portland cement! Plus CSA cements have a host of other benefits, such as elimination of efflorescence and reduction of carbon emissions.

Do pozzolans and CSA cements work together in concrete countertop mixes?

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In short, no.

There is still a lot of confusion, even several years after CSA cements became popular in concrete countertops. The simple, chemical fact is that CSA cements do not produce calcium hydroxide, so pozzolans have nothing alkaline to activate them.

CSA additives are different. Materials such as the brand Qwix® must be blended with portland cement. Therefore, the basic chemistry and the side effects that stem from using portland cement are inherited. Pozzolans can be used, and are dosed based on the portion of portland cement used in the concrete.

While it’s possible to make strong, rapid setting concrete using Qwix® , portland cement and pozzolan, this requires three ingredients, ingredients that each need to be sourced, shipped, dosed and weighed. I prefer simply to use a true CSA cement such as CTS Rapid Set® .

To test the interaction of a true CSA cement and pozzolans, in 2008 I performed compressive strength tests on a variety of concrete mixes that used Ultimax brand CSA cement and VCAS pozzolan. The cylinders were prepared according to ASTM C192 standards and tested by an independent concrete testing laboratory. Three different concrete mixes were made with Ultimax cement where 20% of the cement was replaced by VCAS.

Test results showed that all of the mixes had a 30% loss in 1- and 7-day compressive strengths versus mixes that used only Ultimax CSA cement. Rather than getting the strength increase seen with pozzolans and portland cement, you get a strength decrease.

Bottom line: Don’t use pozzolans with CSA cements.

VCAS strength tests

Various CSA cement brands and manufacturers

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Currently there are three manufacturers of calcium alumino silicate (CSA) cement products in the United States: CTS Cement, Buzzi Unicem and Ultimax. Though they are all called CSA cement, the products are not the same, nor do they yield similar strength concrete for a given age.

CTS manufactures Rapid Set brand cement, which is a CSA cement that is used alone and is not made with nor blended with Portland cement.

Buzzi Unicem makes a CSA cement simply called “CSA”, but on their website it’s clear that it should be used with Portland cement:

“CSA® is a hydraulic, cementitious binder (per ASTM C219), high in Calcium Aluminum Sulfate Crystals. When used with Portland cement concrete, CSA generates a strong cementitious matrix that enhances the physical and chemical properties of the mix.” 

In this regard this “cement” is really a cement additive.

Ultimax sells a CSA cement additive called Qwix that also is meant to be blended with ordinary Portland cement.

“Qwix® is a mineral additive used with Portland cement concrete mixes to enhance the physical properties and the chemical durability related properties.”

According to Ultimax’s website, Buzzi Unicem manufactures Qwix under license. Qwix is a CSA cement additive that must be used with Portland Cement.

Ultimax also makes rapid hardening cements that use their “Ultimax cement technology”. It is not clear whether these rapid hardening cements are blended with Portland cement, but compression test data (shown below) clearly show a gradual strength gain similar to Portland cement, while CTS Rapid Set CSA cement does not.

In April 2009, The Concrete Countertop Institute conducted standard concrete compression tests to compare the early compression strength of concrete cylinders made using CTS Rapid Set cement, Ultimax cement and Portland cement. (Qwix was not tested, nor was Buzzi Unicem CSA, since both are CSA additives, not CSA cements.)

Cylinders made using Rapid Set and Ultimax Gray cement used only those products; cylinders made using ordinary Type 1 Portland cement used a blend of 90% (by weight) gray Portland cement and 10% VCAS pozzolan. All cylinders were prepared (C192) and tested (C39) in accordance with ASTM guidelines. All concrete cylinders used the same aggregate-based concrete mix design with water-to- cementitious ratios of 0.35 and a CSA retarder (Ultimax Delay) dose of 0.5%. Concrete compression tests were performed at 1, 3 and 7 days after casting.

CCI compression strengths are similar to or even a bit higher than strengths advertised by Rapid Set and Ultimax on their websites. Data for Portland cement and VCAS are shown for comparison purposes.

csa_compressive_graph

 

Differences between CSA cements and CSA additives in concrete countertops

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CSA-based cements, which I refer to as CSA cements, such as Ultimax’s UC Precast rapid hardening cement and CTS’ Rapid Set cement, are true cements. They don’t need anything else, besides water, to work. CSA-based cements are added to sand, gravel and water to make concrete, exactly the same way portland cement is added to those ingredients to make concrete.

CSA cements are made using pure CSA clinker that is blended with portland clinker and fired in a kiln, similar to the way portland cement is fired in a kiln. Because the CSA and portland clinker are fired together at high temperatures, they combine to form CSA-based cements. There is no portland cement left after firing. It is chemically transformed into a rapid hardening CSA-based cement. This is NOT the same as simply dry blending portland cement and CSA-based cement together.

Rapid Set and Ultimax CSA cement products

Rapid Set and Ultimax CSA cement products

Another CSA-based product, Qwix, is NOT a cement. Qwix is made from pure CSA clinker, the same precursor to CSA-based cements. It is ground like portland cement and then bagged. This does not produce the same end product as the process I described above.

Qwix is an additive, not a cement. I refer to it as a CSA additive. It is designed to be, and MUST be, blended with portland cement. In this way it is similar to pozzolans like VCAS or metakaolin. Pozzolans must be blended with portland cement. They won’t do anything alone.

QWIX by Ultimax

QWIX by Ultimax

Keep it simple: A recommendation for 100% CSA usage in concrete countertops

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I have encountered many different approaches among concrete countertop professionals in using CSA cements. Some use portland cement, CSA cement and pozzolan blends to achieve high-quality concrete. I recommend against this approach, as it adds complication and negates the benefits conveyed by CSA cements.

Pozzolans are substituted for portland cement to address a host of problems caused by a by-product of the reaction of portland cement with water: calcium hydroxide. Calcium hydroxide makes concrete weak and causes effloresence. Pozzolans consume calcium hydroxide, thus improving the quality of the concrete.

When CSA cements react with water, they do not produce calcium hydroxide. Therefore, simply substituting CSA cement for 100% of the portland cement eliminates all of the problems listed above and obviates the need for pozzolans.

Let me say that again: Simply using 100% CSA cement (and 0% portland cement) completely eliminates all the problems that portland cement caused and pozzolans were trying to solve. Why in the world would you substitute CSA cement for only part of the portland cement, leaving all the problems that then need to be remedied with pozzolans? In my opinion this merely adds complication and expense without providing much benefit. It causes people to spend endless hours tweaking their mix proportions when they should be focusing on selling countertops. I recommend the simple “100% replacement” approach that saves time and money and maximizes benefits.

CSA cements in concrete countertops: Rapid strength with a low carbon footprint

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This article begins a series on CSA cements. Bear with me – this article is highly technical, but it gives you a background on CSA cements. In subsequent articles, I will explain practical issues such as how to set retard CSA cements and whether to use pozzolans with them.

Developed in China in the 1970’s, calcium sulfoaluminate (CSA) cements are a class of specialty cements that are included in the family of rapid-setting cements. Rapid-setting cement is used in many applications such as bridge decks, airport runways, patching roadways, sidewalks, etc. where rapid strength development is necessary. Additionally, CSA cements are sometimes used in shrinkage compensated concrete by mixing with portland cement and for controlled low-strength materials (CLSM) used for diggable back-filling of utility trenches.

CSA cements have yet not seen widespread use in concrete countertop manufacture, but they should – they offer tremendous advantages over portland cement in terms of strength, speed and greeness.

Rapid Strength Gain

The primary advantage of CSA cements is that concrete made with CSA instead of portland cement often achieves compressive strengths of in excess of 5000 psi in 24 hours; with CSA’s, it’s possible to achieve 28 day strength in 24 hours. This is the main reason CSA’s are used in place of ordinary portland cement (OPC) for certain applications. Rapid strength gain is critical in situations where an airport runway, a bridge repair or a damaged freeway must be returned to service in a very short amount of time.

Low Carbon Footprint

Another key advantage is that CSA cements are also significantly greener. Portland cement is fired in kilns at temperatures of around 1500°C (2700°F), whereas CSA cements only need to be fired at temperatures of around 1250°C (2250°F). The resulting CSA clinker is softer than OPC clinker, requiring less energy to grind.

The cement industry represents a small yet significant proportion of total global carbon dioxide emissions. The chemical conversion of limestone to calcium oxide reveals the inherent production of carbon dioxide. For every 1000 kg of calcium trisilicate (C3S) produced from limestone a resulting 579 kg of CO2 gas is emitted solely from the chemical reaction, regardless of the process used or the fuel efficiency. Green Cities Competition. “Green Cement: Finding a solution for a sustainable cement industry”, Department of Civil and Environmental Engineering, University of California at Berkeley. April 22th, 2007. John Anderson. 

Calcium trisilicate (C3S) is the compound responsible for early strength gain in portland cement. The other compound, calcium disilicate (C2S), forms more slowly and is responsible for longer term strength. C3S makes up about 50-60% of portland cement composition, while C2S makes up a smaller fraction of OPC, generally around 18-20%.

As is evident in the breakdown of CO2 emission sources, the chemical conversion of limestone to calcium oxide contributes to about 48% of the CO2 emissions generated in the production of ordinary portland cement. Burning fossil fuels to achieve the high kiln temperatures accounts for an additional 42%. Combined, 90% of the CO2 emissions are directly associated with the chemical conversion of limestone into cement.

In contrast, producing 1000 kg of CSA results in only 216 kg of CO2, a reduction of about 62% relative to OPC. This reduction is far greater than that achieve by using industrial waste derived pozzolans as OPC replacements, such as fly ash and blast furnace slag, which are often used to replace only about 10% to 30% of the portland cement. Concrete made with 100% CSA is 2 to 6 times greener than OPC that has had a significant quantity of cement replaced with pozzolans, and that includes “green” pozzolans like fly ash and slag. In fact, CSA cements had the lowest carbon emissions out of nine alternative cements, including magnesia (Sorel cements), sodium metasilicate (water glass) and calcium aluminate cements.

Lower Alkalinity

The main mineral components in CSA cement are anhydrous calcium sulfoaluminate (4CaO·3Al2O3·CaSO4), dicalcium silicate (2CaO·SiO2) and gypsum (CaSO4·2H2O). The lime in CSA cement is bonded and not free so its alkali is lower. The pH value is only 10.5-11; the pH of ordinary portland cement (OPC) is around 13, which is 100 to 300 times more alkaline than CSA cement. The low alkalinity naturally minimizes the chance for alkali aggregate reaction. This is important when glass is used in the concrete and the concrete is exposed to moisture.

CSA cements do not work like portland cement. Because of the much lower alkalinity, they don’t work with pozzolans, so using a pozzolans like silica fume, metakaolin and VCAS as a cement replacement to boost strength or reduce cement content (and thus restore or even improve the strength relative to 100% OPC) just won’t work. Compression tests performed by CCI showed a 30% loss of strength at both 1 day and 7 days when 20% of the CSA cement was replaced with VCAS.

Lower Shrinkage

CSA cements get stronger, faster than OPC, and CSA cements demonstrate very low shrinkage characteristics. This due in part for two reasons. The first is that CSA’s require about 50% more water than portland cement for proper hydration. The minimum recommended water to cement ratio (w/c) is 0.35, whereas with OPC it’s around 0.22-0.25. Because of the higher water of hydration requirements, most of the mix water is consumed for hydration and less excess water is available to cause problems with shrinkage. The second reason is that the very rapid strength gain can prevent shrinkage cracks because the concrete strength increases more rapidly than do the concrete’s shrinkage stresses.

However, if w/c ratios below 0.35 are used significant shrinkage can occur. This not only can mean curling but also large cracks and discoloration. CSA cements have a strict minimum water requirement that should not be ignored.

Shorter Curing Time

Curing with CSA is important, but wet curing durations are often measured in hours, not days or weeks. Optimal hydration and slab stability are achieved when the CSA concrete is kept wet for at least 3 to 4 hours after casting. During the initial hydration phase, the concrete demands moisture and the rapid reaction generates significant heat. If sufficient moisture is not provided during curing cracking and curling are possible. When moisture is provided through ponding or repeated wetting during the first few critical hours, long term stability and strength are preserved and ensured.

Direct Portland Cement Replacement

CSA cements can and should be used as direct, 100% replacements for portland cement.

Because CSA’s don’t react with pozzolans, none are needed to achieve high strengths and eco-friendly concrete. This simplifies mix design and minimizes inventory. All you need to do is replace 100% of the cementitous material in your current mix design, eliminating the pozzolans. Using pozzolans with CSA cements can actually weaken the concrete, so it’s best not to use them at all.

Superplasticizers, especially polycarboxylates, and viscosity modifiers work the same with CSA’s as with portland cement. Other exotic admixtures like liquid silicates or acclerating agents are not necessary, and won’t work or are not compatible with CSA cements. Conventional cement retarders are not compatible. Only special citric acid based retarding admixtures made for CSA cements will work.

Color Considerations

There are white and gray versions of CSA’s, as well as a light tan/buff color. The gray color is lighter and more brownish/greenish than most portland, so blending white and gray with some iron oxide black pigment can create a convincing “natural” portland cement color. Be sure to experiment with your color formulas when you make the switch to CSA cements.

CSA cements are compatible with concrete pigments, and they can be dyed and acid stained just like OPC. Decorative aggregates, metal and glass are all compatible, so specialty embedments and exposed aggregate looks are possible.

Cost

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 lb bag of white CSA can cost well over twice as much as a 94 lb bag of white portland cement. Since time is money in the business world, saving days spent waiting for the concrete to gain strength may be worthwhile, especially if shorter turn around times is needed. 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, if you take a long time to get things done, or if you are not experienced with from-scratch concrete mixes, then the benefits of CSA cements won’t be realized. Much like driving a very fast sports car while being in a traffic jam, making concrete that gains strength very rapidly is pointless if the whole production process is not optimized to take advantage of its rapid strength gains.

Special Considerations

Finally, CSA cements are not, in my opinion, for the beginner. Everything about them is magnified and accelerated. They are far more sensitive to temperature, w/c ratios, pozzolan replacements and the like. Everything happens faster, so if it’s hot out and you aren’t using the enough of the right retarder, most of the concrete you just made could very well become a solid mass before you’re able to place it. At summer temperatures (above 80-85°F), non-retarded CSA concrete made with a w/c of 0.35 can set in as little as 5 minutes. With the right retarder that can be extended to 15 to 20 minutes of working time, with a few more minutes before setting takes place. While this seems very short as compared to OPC based concrete, using CSA’s requires a well-practiced, highly organized mixing, cleaning and casting process. It forces you to become efficient and organized. And that’s just plain good for business.

Suppliers

Currently two companies in the U.S. are suppliers of CSA cements: Ultimax and CTS Cement Manufacturing. Ultimax produces white, gray and a light gray version, and has a specialized retarder that provides extended working time. (Do not confuse the Ultimax product “Quix” with a pure CSA cement. Quix is a CSA additive. Read more here.) CTS manufactures a range of blended rapid-setting products, one of which is called “Rapid Set” cement. At this time only a gray version is commercially available from CTS.

 

Concrete fireplace is the result of collaboration and creativity

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One of our French-Canadian students, Eric D’Amours of Gatineau, Quebec, sent us some pictures of a great project he’d recently completed, and I talked with him to get the full scoop. It’s a great example of working with the client to achieve success.

Eric: The client for this project is my wife’s co-worker; she saw some pictures of work I had done previously and asked if I could do a fireplace mantel for her. She did not really have any idea of what she wanted.

I set up a meeting and showed her different colored samples. I had one that was from an integrated sink done with the pressed technique. She really liked it. After talking for a while, she thought she would want the base gray color a little lighter than the sample. As for the color in the veining, I suggested that we match the surrounding decor, and she thought that a dark brown was a wonderful idea.

Jeff: It’s always important that the customer is aware of what is possible, and that even though you might not have a sample immediately ready, that you can create a custom look that will fit their home. If Eric hadn’t communicated to the customer that he could design a color for her, she might not have accepted the offer. 

Eric: I went and templated the fireplace which gave me a good idea of what it would look like. On the existing fireplace there was a metal piece that was at an angle. I decided that I would make an angled piece also to put more punch to the look and also to better finish the fit from the mantel to the existing fireplace.

Jeff: Always work with what you’ve got whenever possible. If there’s a way to incorporate an existing bit of character, do it! The customer will be happy that you haven’t torn out more than you have to, and the end result will be unique.

Eric: The challenge was to have an even thickness of 1 inch all around in a pressed technique with a 3D piece  (and I want to take the 3D course at one point to help me with having more ideas and techniques for future projects). The little angled piece was also a challenge, mostly with making the mold, and finding a way to have finished edges all around that portion.

Jeff: Once you have a solid foundation in the basics, you can go a really long way. Extra training is never a bad thing, but don’t limit yourself just because you’ve never tried something before. Practice, experiment, and practice some more!

Eric: Definitely when you de-mold the piece it is the most exciting part. You are so anxious to see if what your mind has created will turn out like you thought it would.

I also wanted to see how the integrated logo in the piece  would look. What better advertisement to have your logo in the piece  (the client had agreed to this before).

Jeff: If the customer agrees, that’s a great way to put your stamp on a project. Word of mouth referrals are great.

In the end, the client was pleased, and the project turned out great. The client has a beautiful, functional piece of art that truly compliments her home.

Closeup of pattern and angle

 Eric attended The Ultimate Concrete Countertop Training in March 2012.

 

Understanding Self-Consolidating Concrete

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Self-consolidating concrete (SCC) is a highly flowable concrete that can spread through and around dense reinforcing under its own weight to adequately fill voids without segregation or excessive bleeding and without the need for significant vibration. In the decorative concrete arena it has been used to produce castings with a high surface quality, often with few or no pinholes. Less labor, quicker casting times, better surface finish and increased concrete densities are common reasons for choosing SCC.

The flowability of SCC is measured in terms of spread instead of slump, because the material flows so readily. The slump flow test (ASTM C 1611) is similar to a standard slump test (ASTM C 143), but instead of measuring a vertical height change, a horizontal spread measurement is made instead; typical spread values range from about 20” to 30”. The spread in the photo below is over 29”. Note how there is almost no segregation, bleeding or variation.

SCC photo

Photo of SCC

 Other tests that are approved or are under consideration are the J-Ring (ASTM C 1621) and the Column Segregation test (ASTM C 1610).

How it’s made

SCC is achieved by designing a mix that has a low yield stress and an increased plastic viscosity (see Figure 1). In other words, the mix should require minimal force to initiate flow, yet have adequate cohesion to resist aggregate segregation and excess bleeding. The yield stress is reduced by using an advanced synthetic high-range water-reducing admixture (HRWR), while the viscosity of the paste is increased by using a viscosity-modifying admixture (VMA) or by increasing the percentage of fines incorporated into the SCC mix design.

scc diagram

SCC diagram

The preferred admixture for reducing yield stress in self-consolidating concrete is a polycarboxylate-based admixture due to its superior water-reduction capabilities and high early-strength gains at low dosing rates. This new generation of synthetic admixture has been specially designed to increase the dispersion of the cement particles, which aids in plasticity, strength and can also help with pigment dispersion.

Avoiding segregation through aggregate gradation and by increasing the amount of fines is possible, but it must be done carefully to preserve the SCC properties. A well graded aggregate distribution minimizes cement paste content as well as minimizes admixture dosage.

Other considerations

Excess moisture in the ingredients, especially the fine aggregates, can have a profound influence on the consistency of the mix. Small fluctuations in moisture content may lead to segregation or affect the mix’s ability to flow. And variations in the aggregate gradation from batch to batch can also cause consistency problems. VMA’s help even variations in aggregate gradations and can account for moderate variations in moisture content. A more robust mix is ultimately created with an enhanced capacity to absorb fluctuations in aggregate gradations and moisture contents.

DIY or Bagged Concrete Countertop Mix?

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All concrete countertops have a basic requirement: a concrete mix that provides the structural, physical, and aesthetic characteristics necessary to make a high-quality countertop that meets the client’s needs and wants. Aside from ordering concrete from a ready-mix supplier, there are two basic ways to obtain a concrete mix. One is by using a commercially available bagged concrete countertop mix, and the other is to do it yourself, making a from-scratch mix with basic ingredients. There are pros and cons to both approaches. Which one you use is ultimately a personal preference.

Bagged Mixes 

Bagged mixes offer simplicity and convenience as their key feature. Generally all of the necessary ingredients, except pigment, are pre-blended; all that is required is to add the proper amount of water. Implicit in the offering is that the concrete mix is consistent from bag to bag, that the resulting concrete meets the performance specifications stated, and, most importantly, that the mix itself is appropriate for concrete countertops in general, and specifically for the casting method (cast-in-place versus precast) used.

There are several different concrete countertop mixes on the market. Some come in a single color (e.g. gray cement), while others have gray or white cement bases. Some even come preblended with pigment. Aggregate size, shape, color, and gradation can vary widely. Some bagged mixes have large amounts of coarse aggregate, while others are all-sand mixes with no large aggregate. Some require the addition of polymer admixtures, which are sold along with the dry ingredients.

A bag of dry concrete countertop mix contains a variety of ingredients that the manufacturer has chosen for a specific reason. There might a wide range of factors that influence a particular blend, such as the desired compressive strength, economics, the availability or cost of a particular ingredient, or something more esoteric, such as the concrete’s in-hand feel and workability or satisfying certain textural criteria.

Regardless of whether the bagged concrete mix is originally designed for the do-it-yourselfer or a professional concrete countertop maker, all bagged mixes share a common characteristic: You don’t really know what’s in the bag, and you have to trust the manufacturer’s instructions. Ideally, the mix should always yield the same results, but external variables such as temperature can significantly affect your concrete. So having some control over the mix can be important. If you do need to alter the mix – say by adding accelerator on a very cold day – you don’t know how much cementitious material it contains, so you can’t dose properly.

From-Scratch Mixes 

Control, therefore, is one of the main reasons for using a from-scratch mix. Since all of the ingredients are known exactly, accelerators, superplasticizers, pozzolans, pigments, and decorative aggregates can all be used to tweak the performance and appearance of the mix. However, from-scratch mixes are less user-friendly than bagged products and require an understanding of mix design. Myriad factors such as mineralogy and aggregate particle shape, size, and gradation can have powerful influences on the fresh and hardened properties of the mix. With so many variables it can be difficult to strike a balance between aesthetics, workability, and physical performance.

Making your own concrete also requires you to source, obtain, and batch all of the ingredients. Variations in ingredients, such as color, moisture content, and availability, all come into play and must be considered. Means for precise batching is essential for consistency, and storage of raw materials requires space. 

For more of my articles about from-scratch mixes, click here.

Making from-scratch mix Making from-scratch mix Continue reading →

From Solid Surface Countertops to Concrete: What you need to know

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Many countertop fabricators, whether they offer natural stone, engineered stone, solid surface or a combination, are recognizing the consumer demand for greener, more custom products such as concrete. As a countertop fabricator considers adding custom concrete countertops to his or her offerings, there are a number of practical considerations such as startup costs, learning curve, profit margin and the logistics of incorporating concrete into shop space and production process flow.

Please note that this article addresses only custom-made concrete countertops, not pre-made concrete slabs such as IceStone and Vetrazzo.

Startup Costs

Adding concrete countertops to your offerings incurs low startup costs. There is very little equipment to buy. The largest equipment purchase is generally a concrete mixer, at about $2000 – $3000. Most of your existing grinding and polishing equipment can be used. Typically concrete countertops are hand processed using hand held one-head or three-head grinders, but a radial arm polisher can also be used.

It is important to understand, however, that the diamond pads used for grinding and polishing concrete are different from those used for granite. Unlike stone, which is uniformly hard, concrete consists of hard sand and aggregate bonded with relatively soft cement. This is extremely abrasive to diamond pads, and will tear the diamonds out of the binder on typical diamond pads, resulting in much faster wear. It is more economical to purchase diamond pads specifically designed for concrete.

The materials needed for custom concrete countertops are very inexpensive. From-scratch mix ingredients, along with all the consumables such as diamond pads, forming materials and sealers, generally cost only about $8 to $10 per sq ft of finished 1.5″ thick concrete. If you use bagged mixes, material costs will be significantly higher.

Learning Curve

Although templating and installation are almost exactly the same for concrete as for stone, concrete is cast, not cut. For shop employees who are accustomed to cutting shapes out of slabs, creating forms and molds can be challenging because it is a different way of thinking. Add to this that precast concrete countertops are typically cast upside down, and you have the potential for mistakes.

Another steep learning curve involves working with the concrete mix, whether you use a from-scratch or bagged mix. Precision is extremely important, with differences as small as a gram sometimes having significant consequences. It is also important to understand technical aspects of concrete such as water/cement ratio, water reducers and admixtures, because the behavior of concrete depends on many factors such as humidity and temperature.

Profit Margin

Custom concrete countertops command a high price, generally about $80 to $120 per square foot or higher, depending on your market area. Due to the low startup costs and low material costs, the profit margins can be very good.

However, you need to be acutely aware of your labor costs. Custom concrete countertop generally require a high amount of hand labor, although there are ways to optimize and automate this, which will be discussed later in the article. As with other materials, the biggest barrier to high profit margins in concrete countertop manufacturing is mistakes and re-dos. Avoiding re-dos in concrete countertops requires all of the normal quality control procedures you have in place, plus a good understanding of concrete as explained above.

Shop Layout

Even the fastest casting and curing methods for concrete can require that a concrete countertop project take up space on casting tables for at least half a day. Casting table times are more typically around two days. You will need to allocate space in your shop for casting tables to accommodate the concrete slabs during forming, casting and curing. You will also need space for the mixer and mix ingredients.
It is possible to install a racking system to store concrete slabs while they are curing, however moving newly cast concrete slabs must be done with great care.

Process Flow

Templating and installation of concrete are almost identical to stone procedures.

Most concrete countertop shops create physical templates manually, then hand build forms around the templates. They use materials such as melamine and foam to create the forms along with sink and faucet knockouts.

Digital templating is rarely used with concrete countertops because most concrete countertop-only shops simply do not have the equipment.
If you do have a digital templating process that feeds into your CNC machine, you can take advantage of this to greatly streamline the templating and forming process. Use your existing digital templating process, but then instead of using your CNC machine to cut countertop shapes out of a slab of stone, use it to cut a hole out of a slab of foam. This will create your form quickly and automatically.

Installation of concrete countertops is very similar to stone. The countertop slabs are transported vertically, and can usually be carried using the same equipment as stone. The biggest difference in installation is the seam filling. Sanded acrylic tile caulk is used instead of epoxy, and cleanup must be done with a sponge, not a razor, especially if a topical sealer was used on the concrete. Also, the underside of an undermount sink opening in concrete must be sealed to prevent contact of the concrete with the silicone used to fill the gap between the countertop and the sink. If silicone comes into contact with bare concrete, it will react and form an oozing, oatmeal-like consistency.

The time in-between templating and installation is very different. As mentioned before, concrete is a high touch process requiring a lot of hand work. It requires forming, mixing, casting and curing time that do not come into play with stone and solid surface. As mentioned above, it is possible to streamline forming by cutting forms out of foam on your CNC machine, and to use radial arm polishers instead of hand held polishers.

Conclusion

By understanding all of these considerations and focusing on employee education, quality control and carefully thought-out procedures, you can profitably add concrete to your business. The benefits go beyond pure profit to building competitive advantage through diversifying and differentiating your business with this beautiful, unique product.

Concrete countertop professionals around the world

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I was working on the November newsletter and thinking about the things I am thankful for, in preparation for the Thanksgiving holiday. One of those things is that I’m part of the exciting, growing concrete countertop industry that is rapidly expanding not just in the U.S. and Canada, but all over the world.

This inspired me to make a map of where all my students have come from. Here is the result. It’s amazing where this concrete countertop phenomenon has traveled!

Concrete countertop students around the world

CCI student map