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 (Figure 4).
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.
4.Openings for appliances
and drop-in sinks are cut through the top in advance.
The counter is then formed to the inside of the
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 (Figure 5).
|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
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
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 (Figure 6).
Figure 6.These faucet holes are formed by
PVC pipe set into a piece of rigid foam. 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.
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).
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. Stainless-steel pencil rod (right)
provides extra reinforcement at high-stress areas
around the sink opening and along the
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
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
8.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