Concrete countertops are works of art. They are also structural pieces of concrete. Just as you don't have to be a trained artist to create an aesthetically pleasing concrete countertop, you don't have to be an engineer to build one that is structurally sound — but it helps to understand the physical stresses at play.
"Reinforcement is one of the most misunderstood topics in concrete countertops," observes Jeffrey Girard, a civil engineer and president of The Concrete Countertop Institute.
Too little, too much — how much is just right? Unfortunately, there is not a simple rule of thumb when it comes to reinforcement. Each concrete countertop has its own criteria: span, width, thickness, load and the material properties of the concrete itself. What is critical to understand, Girard says, is that "concrete countertops are beams, not slabs on grade." He maintains that if you understand where load, compression and tension occur in concrete countertops you can figure out the appropriate reinforcement design.
Types of reinforcement
It's probably a good idea to point out at the start that there are two kinds of reinforcement: primary and secondary. Primary reinforcement you can't do without. Secondary reinforcements can contribute additional characteristics that may make a better product, but are not absolutely necessary.
Primary reinforcement materials are structural, plain and simple. Rebar, ladder wire and structural carbon fiber grid are all primary reinforcement materials and each has different characteristics and uses.
Conventional rebar is indeed a strong reinforcement material, but for many concrete countertop applications it's just too big. "Conventional 3/8-inch, 1/2-inch or 3/4-inch rebar is way too big for a typical 1 1/2-inch thick concrete countertop," Girard explains. "No material more than 3/16-inch in diameter should be used in a countertop less than 3 inches thick."
Michael Karmody, a founding partner of Stone Soup Concrete, agrees that rebar is only useful in a situation where you have a large volume of concrete. "We typically pour 1 1/2 inches. Rebar takes up too much space and can cause cracks."
The profile of the rebar or steel used may also be important. Tom Ralston, president and CEO of Tom Ralston Concrete, observes that smooth bars don't grab to the concrete as well as ribbed bar.
Ladder wire and wire mesh are used by many concrete countertop contractors, and for good reason: Both materials are strong and small enough to use effectively in countertops. The major differences are the overall shape and gauge. Ladder wire is 9-gauge structural wire that is a narrow parallel set of strands. Wire mesh is large, open mesh, often 10-gauge in diameter, but there are heavier gauges, such as 6-gauge, which is a larger diameter.
Mesh with enough steel in it to provide the required tensile capacity is a very efficient way of reinforcing large, simple slabs (such as rectangles), Girard says. On the other hand, he points out, ladder wire is very efficient for reinforcing complex shapes and around sink holes, where the fixed spacing of welded wire mesh would require extensive cutting and splicing. "Both can work together to make very effective, efficient and economical reinforcing," he says.
Ralston likes flat wire mesh because it is easier to work with. "You cut the shape you need and it can be used throughout a monolithic unit," he says. He has found that it is better to use 2.1 gauge for more substantial reinforcing. "Also, you can address the corners and thinner areas with small, 1/4-inch stainless bar that has been grooved with a grinder for better bonding," he adds.
Structural carbon fiber is relatively new to the concrete countertop trade. John Carson, director of commercial development for TechFab LLC, explains that carbon fiber was originally developed for the precast industry and used to produce CarbonCast precast commercial building systems. But use of carbon fiber has migrated to concrete countertops because it has high tensile strength and is thin enough to fit in tight spots.
Some countertop contractors use other reinforcement materials, such as expanded metal, and report good results. Some experts, however, express concern related to the variable properties of some of these materials, especially when they are used for primary reinforcement.
Fibers are frequently used in concrete countertop mix design, but reinforcing fibers should only be used as secondary reinforcement. Most fibers are synthetic — polypropylene or nylon — and so they are physically unable to provide significant tensile reinforcement. What a matrix of fibers in the mix can do is to help stabilize the wet concrete as it cures. The fibers help by distributing shrinkage stresses, thereby minimizing or eliminating large cracks.
But Ralston cautions that fibers can be problematic if they clump and are not mixed or dispersed well.
Some fibers are structural, such as hooked steel fibers, chopped carbon fibers, polyvinyl alcohol and chopped alkali-resistant glass fibers. While these individual fibers are strong, and they improve the mechanical properties and tensile strength of concrete, Girard cautions they are not a replacement for reinforcing steel.
Where you need to place reinforcement depends on your countertop. In Girard's view, because countertops are beams, they should all have the reinforcement placed in the bottom. "There should be between 1/4-inch and no more than 3/8-inch of concrete between the bottom of the countertop and the reinforcement. And there should be at least 1/2-inch of concrete between the reinforcement and any edge," he explains.
Ralston has a different take. He believes that the plastic shrinkage cracking begins at the top of the slab, thus the reinforcing should be closer to the top to hold together any cracking right from the onset.
What if your countertop is also acting as a cantilever? You will definitely need reinforcement at the top. Girard recommends placing the top reinforcement between 1/4-inch and 3/8-inch from the finished top. The recommendation remains the same even if you will be grinding the top. "Usually exposed aggregate countertops are ground down a maximum of 1/8-inch to expose the aggregate, so the reinforcing would start out 3/8-inch from the top and end up 1/4-inch from the top after grinding is complete," Girard explains.
Because it is relatively flat and strong, Karmody likes to use wire mesh in both bottom and top placements.
When he knows where a cantilevered section will go, Buddy Rhodes, of Buddy Rhodes Studio, uses a two-layer approach to beef up the countertop. "The two-layer approach is to put steel reinforcing (ladder wire) near the bottom of the slab to counteract the flexural forces at the bottom of the countertop and carbon fiber grid at the top to counteract the flexural forces produced when the cantilever section has weight on it. All this is done in the same pour." He explains that for a right-side-up mold, you pour about a third of the thickness of the slab and place the ladder wire, then pour another third and place the grid, then top off the last third and strike off, then trowel smooth. He says, conversely, an upside-down mold would have the grid first and the steel reinforcing in the top (bottom) of the slab.
Ralston, who casts-in-place most of the time, uses another technique: "If we have large cantilevers extending beyond the countertop cabinet more than 10 inches, we use small pieces of L bar to connect the countertop and then fasten wire and rebar to that." If the countertop extends beyond 12 inches, Ralston sometimes recommends the use of corbels.
One could argue that cast-in-place countertops are not beams because there is plywood supporting them. However, if you think about it, even in construction, plywood isn't a structural component; it is flexible. Also, one could point out that cast-in-place countertops are not moved about like precast countertops. Girard emphasizes that the same engineering principles apply to cast-in-place, but he concedes reinforcement may not be as critical — shrinkage control and mix design become the dominant issues.
Dave Pettigrew, owner of Diamond D Co., casts countertops in place; he stresses the importance of good mix design, including the use of angular rock instead of pea gravel. "Angular rock interlocks and provides strength," he says. Still, Pettigrew uses a variety of reinforcement materials in his countertops.
How does reinforcement affect the final appearance? Well, it shouldn't.
Some contractors are worried about rust forming if the reinforcement isn't galvanized or epoxy coated. Karmody says he actually doesn't mind a little rust because it helps the concrete and steel create a better bond.
What about ghosting? This phenomenon is frequently attributed to having the reinforcement too close to the surface. Actually, reports Girard, ghosting (or shadowing) occurs when the reinforcement is pushed down into the concrete or the concrete is poured through the reinforcement. "This pushes the aggregate (or larger sand grains) aside and forms a trench that fills in with the finer particles of the cement paste. The result is that the physical composition of the concrete above the reinforcing is different than the rest of the concrete." Because those lines of concrete are different in composition, they will cure differently and take stain differently.
And what about surface fuzz if you use fiber reinforcement? Different contractors have different solutions to this pesky problem.
Pettigrew sands off any fiber fuzz and applies two clear finish coats, or he'll use a microtopping over a concrete blank that contains the fiber reinforcement.
Karmody says if he grinds the surface of the countertop he'll sometimes pour a veneer layer on top.
Rhodes points out, "We use fibers in the back of the slab so they don't poke out of the finished surface and give you a hairy countertop. These hairs can be sanded or burned off, but it's better to avoid the peach fuzz."
When it comes to concrete countertops and reinforcement, Carson observes, "There's skill in creating concrete countertops and you have to understand some of the design implications. There's a wide array of technologies that people use… but reinforcement is important."
Also important, says Ralston, are experience and analysis of the project.
In the end, it all boils down to the right mix of reinforcement: not too little, not too much.
(Reprinted, with the permission of The Concrete Countertop Institute, from the textbook for Precast Concrete Countertops 101.)