An affordable GFRC concrete countertop backer mix sprayer

You know about spraying the mist coat for GFRC. That’s done with an inexpensive hopper gun.

But to spray the backer coat, most machines cost more than $16,000. That’s just not realistic for a concrete countertop shop, so most fabricators hand-place the backer. This is time-consuming, especially for large projects, and becomes a bottleneck for high production concrete countertop shops.

The $16,000 machines are also simply too large for countertop projects which are generally about 50 to 100 square feet each. Those machines are designed for mass production of huge building panels. Just the concrete required to fill the hose is more than a concrete countertop maker needs to mix in total.

Ball Consulting, in partnership with FRP Equipment, offers a solution in between: the RimSpray GFRC Mini. At approximately $7000, it offers high production concrete countertop shops a realistic alternative to hand-placing the backer.

RimCraft GFRC Mini

The high quality spray gun on the RimCraft:RimCraft GFRC Mini Spraygun

With any equipment purchase, be sure to consider return on investment. Will the machine allow you to reduce costs or increase revenue enough to justify its price? If you can’t keep up with demand for your GFRC concrete countertops, and you identify that placing the backer coat is the bottleneck in your production cycle, then the RimSpray GFRC Mini may be the answer.

Get more information and contact the manufacturer here.

Watch the video:

Artistic Concrete Expressions creates custom concrete countertops and more in Michigan

Brad Dykema of Artistic Concrete Expressions in Hamilton, MI, started out in tile. In 2006, he was doing tile projects in high end homes. Wanting to get into more kitchen-related projects, he became aware of concrete countertops and attended World of Concrete. There he met Jeff Girard and was inspired to try concrete countertops.

Brad experimented on his own for a while, then decided in April 2007 to get formal training in precast concrete countertops. Having 20 years of experience in high end tile work, he didn’t need artistic training. He recognized the importance of avoiding quality problems like staining and cracking, by learning the structure and theory behind concrete countertop construction.

After class, he got started right away, working in his 42′ by 40′ barn. In December 2008, Brad attended the GFRC class, and by July 2009 had expanded his business enough to move into a 4000 square foot shop. The new shop has a room for wet polishing and a room for spraying sealer, making it an ideal workspace for quickly producing high-quality concrete countertops.

Brad has carried his vision and passion for quality design and execution to the concrete form of art, and Artistic Concrete Expressions now fulfills that vision in the Grand Rapids and Lansing areas and throughout western Michigan.

Artistic Concrete Expressions has grown into a custom concrete fabricator of concrete countertops, furniture, fireplaces, shower bases and shower panels, serving both commercial and residential markets. Brad works closely with designers and homeowners to take their ideas and turn them into a real functional work of art, which not only will be unique but also very personal. As the Artistic Concrete Expressions motto goes, “Your imagination is our only limitation”.

www.artisticconcreteexpressions.com

Check out some of Brad’s creations:
Brad Dykema shower

Brad Dykema martini sink

Brad Dykema integral sink

Brad Dykema vanity

Brad Dykema bottles

Concrete countertops are not decorative concrete

But wait, you say. Concrete countertops are decorative and they are made of concrete, so that makes them decorative concrete, right? Well, not exactly. What is decorative concrete anyway, and how is it different from concrete countertops?

Decorative concrete encompasses a wide variety of surface finishes and treatments that are applied to or performed on a base of ordinary concrete. By and large, decorative concrete transforms or alters the appearance of plain concrete to enhance the look by adding color, patterns and texture. Other than stamped concrete, most forms of decorative concrete are applied to or performed on pre-existing plain concrete. These include acid staining, decorative overlays, scoring, etching, micro-toppings, sandblasting, dying and polishing. More often than not, decorative concrete is done on flatwork, such as floors, driveways, sidewalks and patios, and also includes vertical walls, faux stone and trees.

Concrete countertops often include elements used in decorative concrete. The obvious are integral pigments, acid stains and polishing. They also can be stamped, etched, scored and sandblasted. It is for these reasons that many people include concrete countertops in with the decorative concrete industry. However, the appearance of concrete countertops is only a part of what they are, whereas the appearance is essentially everything with regards to many forms of decorative concrete.

If they don’t fall completely within the decorative concrete industry, where do they belong? The answer is in the kitchen and bath industry. After all, concrete countertops are countertops that just happen to be made out of concrete. All of the factors, considerations and expectations associated with other high end countertop materials directly apply to concrete countertops.

Decorative concrete versus kitchen and bath

Concrete countertops are not floors merely reshaped and elevated to sit upon cabinets. They should not be treated like floors, because countertops in general are held to higher standards than floors. Client expectations of what a countertop should look like, how it should perform and the general design and functionality are all very different from floors.

The same standards clients hold granite and other natural stone countertops to are applied just as rigorously to concrete countertops. It is for these reasons that concrete countertops mainly belong in the kitchen and bath industry, because their primary purpose and functionality are as countertops. The decorative part, while important, is secondary. Concrete countertops are not decorative concrete. Concrete countertops are countertops.

The role of aggregate in concrete countertop mix formulas

Aggregate in concrete is a structural filler, but its role is more important than what that simple statement implies. Aggregate occupies most of the volume of the concrete. It is the stuff that the cement paste coats and binds together. The composition, shape, and size of the aggregate all have significant impact on the workability, durability, strength, weight, and shrinkage of the concrete. Aggregate can also influence the appearance of the cast surface, which is an especially important consideration in concrete countertop mixes.

When selecting the most appropriate aggregate for a particular concrete mix, here are the key factors to consider:

Material

Most natural stones and crushed rock are appropriate for use in concrete. Commonly used stones are quartz, basalt, granite, marble, and limestone. If a concrete countertop is going to be ground with diamond tooling, the aggregate will show, so aesthetics also affect the choice of aggregates.

Problems arise with soft, reactive or weak stone or rock. Lightweight aggregates, a topic for another discussion, are also used in concrete.

Size

Aggregate size and gradation are the most important factors when selecting aggregate. Aggregate can be large or small, from fist-sized rocks to fine sand. Aggregates larger than ¼ inch are classified as coarse aggregate, while anything smaller than ¼ inch is termed fine aggregate. As a general rule, the largest aggregate should be no greater in diameter than one-third the depth of the slab, or one-fifth the smallest dimension of the form. For example, the largest piece of aggregate allowed for a 1 ½-inch-thick countertop slab is ½ inch. Generally coarse aggregate is blended with finer aggregates (such as sand) to fill in the spaces left between the large pieces and to “lock” the larger pieces together. This reduces the amount of cement paste required and decreases the amount of shrinkage that could occur.

Shape

Aggregate shape influences strength, but has more of an immediate impact on the workability of the plastic concrete. Rough, angular particles pack tighter, have more surface area, and have greater interparticle friction than smooth, rounded particles, which reduces workability. Angular particles also require a bit more cement paste to coat them than rounded particles. Therefore, mixes containing them will require a slightly higher cementitious content.

Gradation

In general, coarse aggregates tend to be about 10 times larger than the fine aggregates in concrete, but the range of sizes could be greater than that in certain circumstances. As shown in the figure, there are three typical range categories:

  • Well-graded aggregate has a gradation of particle sizes that fairly evenly spans the size from the finest to the coarsest. A slice of a core of well-graded aggregate concrete shows a packed field of many different particle sizes.
  • Poorly graded aggregate is characterized by small variations in size. This means that the particles pack together, leaving relatively large voids in the concrete.
  • Gap-graded aggregate consists of coarse aggregate particles that are similar in size but significantly different in size from the fine aggregate. A core slice of gap-graded concrete shows a field of fine aggregate interspersed with slightly isolated, large aggregate pieces embedded in the fine aggregate.  

Typical aggregate gradations are shown in the drawing below:

aggregate gradation

Poorly graded concretes generally require excessive amounts of cement paste to fill the voids, making them uneconomical. Gap-graded concretes fall in between well-graded and poorly graded in terms of performance and economy. Gap-graded concrete is a viable gradation, but not optimal.

Well-graded aggregates are tricky to proportion. The goal of aggregate proportioning and sizing is to maximize the volume of aggregate in the concrete (and thus minimize the volume of cement paste) while preserving strength, workability, and aesthetics. This balances the proportions of each so there are just enough of each size to fill all the voids, while preserving workability and cast-surface quality.

Note that aggregate gradation is particularly important in cast in place concrete countertop mixes. This blog entry explains further about cast in place mixes.

Mortar Concrete

Concrete made with just fine aggregate (or sand) is known as mortar concrete. Like the mortar used for brick and concrete block construction, which is simply made with mortar cement and sand, mortar concrete has no coarse aggregate in it, so a ground finish will have a fine-grained appearance. Mortar concrete is commonly used in concrete countertop mixes, since the surface finish is so important.

Even with an all-sand mix, aggregate gradation is still an important factor to consider and affects strength, workability, and aesthetics. It is always preferable to have some particle size variation rather than absolute uniformity because the interparticle void volume will be lower than with uniform particle sizes. While it is possible to blend different sands of different sizes together in a fashion similar to graded aggregates, generally only one type of sand is used. Most sand, especially bulk or bank-run sand, already has a particle size distribution that has some variation to it.

In order to achieve adequate workability, the cement paste volume must be high enough to encapsulate all of the aggregate particles and to provide some workability while the concrete is fresh. Therefore, mortar concrete tends to have a high cement content.

Conclusion

Aggregate gradation, whether in a mortar concrete or a traditional concrete mix, involves tradeoffs between strength and workability and is always a delicate balance. Understanding the implications of aggregate gradation is especially important when creating a from-scratch mix and will ultimately help you produce a better concrete countertop.

Adding decorative aggregate to concrete countertops

Decorative aggregates, whether they are colored glass, crushed stone or other materials, can create interest and add pizzazz to a concrete countertop. This is one of the easiest and most fun ways to create an exciting custom look for your client. You can use recycled glass, elements of the room such as glass tile pieces, or even the client’s own mementos such as a broken vase.

There are two basic ways of getting the decorative aggregate into the concrete countertop:

  • Method 1: Mix all of the decorative aggregate in with the concrete
  • Method 2: “Seeding” or place the aggregate into the empty forms before the concrete is placed.

Both methods can be used together, and each has its pros and cons.

Mixing the aggregate into the concrete before casting is an easy way to achieve consistency, control and uniformity.

Since the decorative aggregate is mixed into the concrete, all surfaces will look the same. In addition, the average spacing of the aggregates is more uniform.

Because the total amount of ingredients is known, duplication of the concrete is easy. Varying degrees of exposure are controlled by altering the amounts (weights) of decorative aggregates used and the sizes of the aggregate particles. Larger aggregates have fewer particles per pound, so the overall coverage looks lower than with smaller aggregates.

The biggest disadvantage to this method lies in the amount of aggregate required. Only a small fraction of the decorative aggregate is actually exposed. Most of the aggregate never is seen, and this can significantly increase material costs.

Additionally, mix design adjustments may be necessary depending on the size and amount of decorative aggregate used. Large amounts of aggregate might call for substituting some of the other ordinary aggregate for some or all of the decorative aggregate, and it also might require a change in cement paste content or consistency.

However, if cost is not an issue and the client wants a random but uniform look, this is the easiest method.

To summarize, advantages are:

  • More uniform spacing
  • Easier to duplicate a random look
  • Easy to do

Disadvantages:

  • May be costly because more aggregate must be used
  • May require mix design adjustment

Pre-placing the decorative aggregate into the forms is an alternative method that uses much less material. This method is very useful when the decorative aggregate is very expensive, only a small amount is available or specific patterns or locations of aggregates are desired. Sometimes aggregate is glued to the forms to prevent shifting, although this works best with larger pieces that have a distinct flat gluing surface.

preplacing aggregate

Disadvantages include the tendency for scattered aggregates to shift, move or get lost during casting. Also, edges and vertical surfaces are difficult to seed so that they match the surface.

Replicating the look of random, scattered aggregates can be tricky too, since the effects of concrete placement and consistency can have a strong influence on the final appearance.

Advantages:

  • Uses less aggregate
  • Allows for specific patterns or locations of aggregate

Disadvantages:

  • Aggregate can shift or get lost during casting
  • Hard to match edges
  • Random look hard to replicate

Either method will require some degree of grinding to expose the embedded decorative aggregate. Larger, more rounded aggregates require the most grinding in order to expose some significant portion of their cross section. Small, angular particles take the least amount of grinding to expose.

Would thinset and fiberglass cloth make a good reinforced concrete countertop?

You’ve probably seen my reinforcing video on YouTube. Recently I got a question about whether one could use thinset with fiberglass cloth on the bottom as a countertop construction method, and possibly reduce thickness that way. I can see how this idea might have appeal, because it takes a cement-based material and attempts to boost flexural strength by adding a form of reinforcement reminiscent of the glass fibers in glass fiber reinforced concrete (GFRC) to the bottom.

However, the answer is no.

Thinset is not a self-supporting structural material. Yes it gets hard, but its design is to adhere and support tiles to a substrate. That’s not the same function as structural concrete. Concrete needs both a high flexural strength and a substantial compressive strength (plus shrinkage resistance, good aesthetics, hardness, etc) in order to be used for making top-quality concrete countertops. Not even the sidewalk and fence-post grade concrete sold in home centers is good enough.

Fiberglass cloth for resin-based fiberglass structures is typically made from E-glass. GFRC glass fiber is made from alkali-resistant (AR) glass. E-glass (and any other glass that’s not AR glass) will, over time, be weakened from the cement in concrete.

As for thickness, the best way to reduce that is to use GFRC. Traditional precast concrete countertops are often 3/4″ to 1″ thick. GFRC that uses AR glass as reinforcement is typically 1/2″ to 1″ thick depending upon what is actually being made. It’s this reduction in thickness, and the resulting reduction in weight, that make GFRC a very popular form of concrete in the concrete countertop industry.

Conventional precast concrete that uses structural steel reinforcing is often 1.5″ thick or greater. This is because reinforced concrete is a composite that relies on the concrete for the compressive strength and the steel for the tensile strength. As the total concrete thickness decreases, the compression and tensile faces move closer together. This greatly increases the stresses in those faces, so the amount of steel (and the strength of the concrete) needed to resist those forces increases proportionally. At some point it becomes physically impractical to create very thin precast countertops with enough steel in them to resist the loads imposed from moving and using the countertops. That thickness is around 1.5″.

It is possible to make thinner pieces of concrete that use less or no steel. But those pieces of concrete must be smaller, often shorter, under 4 feet in length, and the actual strength of the concrete is quite low. Think of a wall panel or backsplash versus a free-spanning countertop. The wall panel can be made thin because it only has to support itself, and in use it’s stuck to a wall. The countertop could be sat or stood upon (a realistic possibility some time after installation), and breakage is very likely.

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