Download PDF version (446.4k) Log In or Register to view the full article as a PDF document.

Cast-In-Place Concrete Counters, continued

Fixtures and appliances. Every opening through the counter is formed before the concrete is poured. It's not hard to form holes for faucets, but we need to know in advance what size they are and exactly where they go. Cooktops and drop-in sinks are simple to form because the edge of the opening is covered by a flange. Undermount sinks are trickier because the edge is visible and has to be finished. Undermount sinks are installed flush to the plywood top before we get to the job.

0704ra-09

Openings for appliances and drop-in sinks are cut through the top in advance.

0704ra-08

The counter is then formed to the inside of the cutout.

Undermount sink form. Foam insulation is good for blocking out undermount sinks because it's easy to cut and can be removed by breaking it into pieces. We put the paper template that comes with the sink on the foam and transfer the shape by poking a series of holes through the line. We cut the foam with a jigsaw, then smooth the edges with sandpaper. We wrap the edge with red plastic tape to fill voids and prevent concrete from sticking, then install the foam over the sink with cleats.

0704ra-10

This foil-faced foam is blocking out the opening for an undermount sink. The foam and most of the wood forms pictured here will be removed as soon as the concrete is hard enough to finish. The undermount sink is already in place beneath the Kraft paper.

Faucets. We form faucet holes with short pieces of PVC pipe. The pipe can be removed when the slab begins to set. Most faucet stems are long enough to go through a couple of inches of material. But if the combined thickness of counter and plywood is too great, the plumber will not be able to install the nut that holds the faucet in. To avoid this, we cut an oversized hole through the plywood under the faucet and cover the hole with a piece of foam.

The foam keeps the concrete out of the cabinet, but it's soft enough that we can poke holes in it and use it to hold the PVC. We continually check the pipes for plumb because it's easy to knock them out of position. When the slab is cured, the plumber can easily dig out the foam from below and seat the fastening nuts against the bottom of the slab.

0704ra-11

These faucet holes are formed by PVC pipe set into a piece of rigid foam.

0704ra-12

The foam covers an opening in the plywood and makes that part of the counter thinner so the plumber has plenty of thread to install fixtures.

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.

0704ra-03

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.

0704ra-13

Stainless-steel pencil rod (right) provides extra reinforcement at high-stress areas around the sink opening and along the nosing.

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. 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.

0704ra-04

To maximize strength and minimize shrinkage, use concrete with a very low water-to-cement ratio. The mix should be stiff and difficult to spread.

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 1 1/4 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.