Rebar is a major component of structural concrete construction, but for decorative concrete flooring and countertops, it is not widely used. There are several reasons why. First of all, decorative concrete is made up of relatively thin slabs and putting rebar in them can cause cracking. If positioned close to the slab surface, rebar will rust due to moisture absorption, and rust corroded rebar can cause concrete to crack, particularly thinner concrete and concrete not made with gravel. Finally, rebar can also cause ghosting in thin slabs, unsightly white lines that are visible on the slab surface.
Fortunately there are various alternatives for reinforcement of decorative concrete, defined as any poured concrete with a slab thickness of less than one and a half inches. Threaded rod, welded wire mesh, ladder wire, chicken wire, hardware cloth and expanded wire lath all have their uses here. All of them are made of steel, which, though it is prone to rusting, has the more important property of a thermal expansion coefficient which matches that of concrete. In other words, aluminum, copper and bronze expand and contract due to temperature faster or slower than concrete does, which causes cracking, unlike steel.
Expanded Metal Lath
This is a sturdy form of metal mesh first developed as a backing for stucco. It works well on flat surfaces and is stronger than chicken wire or hardware cloth, which are more suited to reinforcing more complex non-flat shapes like statuary or ornaments. Metal mesh reinforces slabs by binding the concrete together so it will expand and contract as one unit. For avoiding cracking in L-shaped countertops it is second to none.
Your concrete mix should be composed of small grained aggregate, the size of sand, rather than gravel; this allows the aggregate the flow through the openings in the expanded metal lath, and create an effective bond for the reinforcement. You should also allow a margin of one inch with all slab edges for the mesh so that it does not protrude through the slab when cured.
The metal mesh can be cut with tin snips, but be sure to wear protective gloves, as the edges of the cut mesh are sharp. Mesh is placed into the concrete when half the concrete mix has been poured into the slab or mold, as it is intended to be in the middle of the concrete.
You can find expanded metal lath in large home centers, or stores that sell masonry supplies. It is available in 27 X 96 inch sheets, or from some suppliers by the foot. Get the galvanized type if it is available, it will resist corrosion better.
Ladder wire is composed of pairs of 9 gauge wire rods connected by welded crosspieces in a ladder configuration. It is used by masons as reinforcement in concrete block walls, but decorative concrete craftsmen have made good use of it as a substitute for rebar, since it is made of thinner steel rods. The crosspieces help the ladder wire stiffen concrete even better than simple wire lengths, and their galvanized finish allows for better adhesion and rust resistance.
Ladder wire is available from masonry suppliers. Trim it to size with either bolt cutters or a hacksaw to fit your slab, leaving a margin at the edges. The ladder wires should be set in the concrete both lengthwise and crosswise in order to create rigidity across both directions.
Carbon Fiber Mesh
This is a more recent type of mesh reinforcement that is manufactured from epoxy and carbon fibers. It looks like metal mesh except it is greyish black in color. The main advantage is its strength- pound for pound it is 8 to 10 times stronger than steel, but it handles almost like cloth and can be cut with a pair of scissors. Additionally, it is not prone to rusting, so it can be set near the concrete surface; this makes it ideal for use in thinner slabs of concrete such as tiles.
Loose fibers can be added to the concrete mix for reinforcement also for protection against fine surface cracks and shrinkage cracks. Nylon, polypropylene and glass fiber can be purchased in bulk in bags from concrete supply outlets. They work to strengthen concrete by straddling areas which could otherwise be prone to cracking.
Polypropylene fibers are the most commonly used fiber. Follow the manufacturer’s instructions for mixing into concrete; generally you will use one or two bags per cubic yard of concrete. For small decorative projects, typically you will need only a handful of fiber.
Photo by Brian Birke, Creative Commons Attribution License