Dare Concrete - Outer Banks, North Carolina

Countertops - Poured-In-Place Steps

The right mix for a countertop is essential. We use Type K Portland Cement (this type is used in bridges to allow flexibility, less shrinkage and cracking). A mix must be rich and relatively dry. Shrinkage compensating admixtures are also used to control cracks. In this picture the ready mix concrete is being dumped into forms built over a set of frameless kitchen cabinets. When finished with an acid-based stain (inserted photo), the counter looks like burnished leather. Your steel (rebar) must be tied tight on top of a layer of plastic sheeting (2 mil poly)

Steel sheet reinforces a wide overhang. We bolt a piece of _-in. steel to a cabinet top to support a 10-in concrete overhang. Bolt heads are inside the cabinets. Concrete covers the edge of the steel. By pulling the edge of the form 1 in. away from the steel, we ensure that concrete will hide the rough edge. Braces are screwed to cabinet tops. To support the forms for the 2 1/4-in. overhang, we use deck screws to attach 1x bracing to the inside of the cabinets.

Although well-made cabinets should not have any trouble supporting the weight of concrete counters, adequate bracing is crucial until concrete sets up.
Add vertical supports. Beneath each brace, we wedge a 1x post. The bracing will prevent the forms at the overhang from flexing as the concrete is poured and troweled.

For a wide overhang, brackets help. Simple brackets made from 1x material provide additional support to a wide cantilevered edge. Screw holes don’t matter, because the cabinet side will be covered with paneling.

A gauge block sets counter thickness. We use a 2-in. wide block to locate the form edge. Because the screed will ride on the top of the form, accuracy is important.

For added strength and crack resistance, fiber-reinforced concrete for the counter will be poured over _-in. steel rebar that has been wired into a 6-in. grid. We prefer use wiremesh as a substitute for reinforcement of the concrete mix. Wire it up. Intersecting pieces of rebar should be wired together so that the grid doesn’t shift as concrete is poured around it. A bag tier makes short work of this otherwise tedious process.

The sink goes there. Built-up layers of flakeboard cut to the sink manufacture’s paper pattern will keep concrete out of the sink opening. Duct tape makes a good moisture barrier.

Steel runs the full length. A single piece of _-in. rebar strengthens the concrete backsplashes. Wood blocks keep the steel from sinking to the bottom of the form when the concrete is poured, and braces prevent the form from bowing.


Two Tips for Smooth Edges - Vegetable oil too far gone for cooking is perfect as a form release agent. We spread the oil with a cheap brush to keep the concrete from sticking to the wood. Two coats may be needed. Vibrate out the bubbles. A palm sander (without the sandpaper) settles the concrete and reduces the honey-combing that could appear in the finished edge.

Pack the concrete into the forms.	A steel edging tool makes a clean radius at the outside edge.
Remove the braces, and screed the top.

The mag float flattens the surface.

A steel trowel polishes the surface. A smooth surface takes time and patience. Keep tools flat to avoid marking the surface. Count on troweling more than once.

The final finish – Acid etches the cured top. After curing for 14 days, the counter is washed with a solution of water and muriatic acid, which will help the acid-based stain to bite into the surface. Wear gloves; safety goggles are a recommended idea as well.

An acid-based stain makes concrete achieve a variegated look; unique, there aren’t two alike. After the muriatic acid wash has dried, two coats of stain are applied 24 hours apart. Your concrete countertop just turned into burnished leather.