Designing the Structure of a Concrete Countertop
Here is a recent inquiry we have received, which is reflective of many questions we get-
"I am looking to do something different and am looking for advice. I am thinking about precasting my countertops, but doing it with the finished side up and a layer of plywood and backer board underneath. I'm thinking about making the tops 1/2" thick and forming an edge return that is 1-1/2" thick. I want to add glass aggregate and polish to expose it. Is this doable? Which product should I use for this?"
Let's start with the method of casting- "Can I precast a countertop right side up?
Yes. This is a popular technique, particularly for an artisan that wants a troweled finish but doesn't want to cast in place for one reason or another. One of the things that this approach changes, compared to typical precast (cast upside down), is that you are far more responsible for the ultimate finish during the casting process. When you cast into a mold you are allowing the mold to do the work of ensuring that the surface is flat, and that the texture is whatever the mold dictates ( read more about the mold material choices, and how that effects finish here) . When casting right side up, you are taking control of the flatness and finish of the surface during the casting process. Whether you cast right side up, or upside down, is a question of where "control" is most important in the finished casting.
Can I use a plywood backer?
Probably not necessary. It is commonly confused that a countertop's strength is improved with a wood backer of some sort. This is not true in most cases, and in many cases can cause problems.
- The first thing to understand is how a concrete countertop gains tensile strength. You can read about that in this blog post about reinforcement.
- The next thing to understand is one of the more passed over realities of concrete- concrete shrinks. In spite of the most advanced mix designs being able to perform amazing feats, concrete still shrinks, the only question being a question of how much. When you embed a fixed dimensional material into a countertop, you are risking the concrete moving, and the embed not moving (or moving at a different rate).
- The reason there is a backer material used in Cast in Place applications is to provide the concrete with a place to be during the period of placing and curing. Beyond that point, it doesn't serve any substantial function. The concern with using wood for this type of application is that wood can absorb water and distort. If you combat that by sealing the wood, you risk trapping moisture in between the substrate and the concrete, which can cause issues. This is the reason HardiBoard is used in these applications, it allows moisture to move back and forth, without the risk of swelling and moving in the substrate.
- If you are using a rigid backer of any sort in the countertop, whether cast in place or precast, it is prudent to allow for some type of movement, using a compressible material like foam as a border to your rigid substrate.
How thick does the counter need to be?
One common rule of thumb is 5x the thickness of the largest aggregate in the mix. While this is not true in all circumstances, it is a good rule of thumb. Much of what we do in the process of designing a mix is to allow that we are able to overcome this rule, meaning that the admixtures, fibers, and ratios that we employ allow that the threshold is pushed in this area. That said, if you want to be safe, then this is the measure you use, if you need to push the envelope then you must consider a whole host of other variables.
The Craftsman Mix contains a 1/4" aggregate, meaning that 1-1/4" is a good target thickness. The GFRC and ECC Mixes utilize only fine sands, so 3/4" is a common menial thickness. That said, we produce sample tiles in the Craftsman Mix that are 24" x 24" and only 1/2" thick. The smaller tile size, coupled with the lack of strain and stress that the object will see, means that we can make the tiles much thinner than the rule. Would we make a large countertop in this same thickness? Probably not, but if we did it would be a most cautious endeavor with a large inherent risk.
What about when using decorative aggregates? You can build a countertop with decorative glass or other aggregates that are a larger size. If the decorative aggregate is only used in the finished surface, and not throughout the object, then you can cheat from the rule a bit. If a large aggregate is used throughout the mix, then stick to the rule for safety's sake.
What should be done in this case?
As with all things concrete, context drives the ultimate decision. If I were casting this piece and I wanted to have a 1-1/2" overall thickness with a hollowed out core to save on weight and materials, and I wanted to employ a decorative aggregate...
I would precast the piece upside down. I would build a 1/2"- 3/4" thick 'backer' mold that was removed after casting that would contain the mix along the edges during casting, and provide me a nice underside. I would use the recipe on page 15 of the Catalog of Products and Techniquescalled the "Spray Recipe" (Don't let the name fool you, it would be used for casting in this case). I would add the decorative aggregate into a 'face mix' that was cast into the mold, then follow that with a fiber reinforced 'backer/structural mix'. I would make the thickness between 3/4"- 1" thick, depending on how big my aggregate was- 3/4" for aggregate 1/4" or less OR 1" for aggregate larger than 1/4".
The reason I would choose casting upside down, as opposed to right side up, is for a few reasons. I would have more control over the aggregate distribution,. When troweling aggregates are moved around based on the action of the trowel, meaning that they can be forced down and harder to expose during polishing. I would also not want to stand around and have to trowel a piece flat after casting, and if you aren't perfectly flat then polishing becomes an arduous task.
Is this the Right way to go about this project? Not necessarily. Again, context should drive your decisions, and every situation is different. There is no reason this countertop cannot be cast right side up. You could make it 1/2" thick if great cautions were taken because that thickness was absolutely necessary (I wouldn't recommend it though).