Decorative Concrete Expert

Figure 7. it's important to properly reinforce concrete counters. Here, the form carpenter cuts 6x6 wire mesh around the opening for a drop-in sink. The wire on the back wall will reinforce the integral backsplash. Stainlesssteel pencil rod (right) provides extra reinforcement at high-stress areas around the sink opening and along the nosing.

Reinforcement
Concrete counters should contain some kind of reinforcement like rebar, wire mesh, or fiber mesh in the mix. We normally use 6x6 flat wire mesh. You can also use number three rebar, but only in thicker slabs. There's not enough coverage for rebar in a 2-inch counter. We try to add extra reinforcement at stress points such as overhangs, inside corners, and areas where the counter goes from wide to narrow (Figure 7).

Reinforcement should run through the entire counter and into the integral backsplash. We place the wire mesh close to the center of the slab and hold it in about 1/2 inch from edges. We use tie wire and drywall screws to secure the mesh to the plywood. That prevents it from shifting and poking through at the edges.

Regular reinforcing materials work fine, but if you're concerned about corrosion, you can use galvanized or even stainless-steel materials. Theoretically, water could pass through a crack, rust the reinforcing materials, and cause even more cracking. I have seen this happen to pool copings but never to concrete counters.

The Right Mix
The concrete should contain Portland cement, sand, potable water, and well-graded angular aggregate. We use as little water as possible for a good, stiff mix with only 3 or 4 inches of slump (Figure 8).

We prefer a regular six-sack mix; each yard contains six 94-pound bags of cement. It cures to 3,000 or 4,000 psi, which is plenty for a countertop. Increasing the proportion of cement boosts strength but may increase cracking. One way around this is to use an admixture that prevents cracks.

A good ready-mix company can save the trouble of mixing the material, adding color, and cleaning up the mess, but most concrete companies can produce only a limited number of colors. Plus, the working time is short if the material spends a long time in transit.

It's more work to mix our own concrete, but it allows us to use any pigment or additives we want. Batches are limited by the size of the mixer, so we typically have to make multiple batches. The only way they're going to match is if they contain the exact same proportions of ingredients. We use a 5-gallon bucket and a portable scale to weigh materials. To reduce errors, we always try to make each batch the same size. We use a 1/3-yard mixer, SO 11/4 yards could be mixed in four batches (three at 1/3 yard and one at 1/4 yard). But problems are less likely if we mix it in five equal 1/4-yard batches.

A third option is to buy premixed, bagged concrete. I've done training seminars for Buddy Rhodes and have

had good luck using the Buddy Rhodes Bag Mix. (Buddy Rhodes Studio, San Francisco, Calif.; 877/706-5303, www.buddyrhodes.com). It contains sand, white or gray cement, perlite aggregate, and a clay additive to enhance trowelability.

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