Finishing Concrete

Finishing concrete is a specialized skill that you can’t learn quickly. If the project is large, have someone on hand who knows finishing. If it’s a small project and appearance isn’t important, however, it’s a good time to learn a new skill. Timing is critical. If you start too soon, you’ll weaken the surface. Start too late and the concrete will be unworkable. The waiting period depends on the weather and type of concrete. Start when the water sheen is gone from the surface and the concrete will carry foot pressure without sinking more than 1/4 inch.

Tools: Steel trowel, edger, jointer, plywood for kneelers, circular saw with masonry blade, wood float, broom.

Hiring a Professional

■ If your project is larger than 250 square feet, you may need to hire a professional even if you or a friend have finishing experience. Choose someone who is reliable; if your finisher doesn’t show up, you’re sunk.

■ Check the Yellow Pages or go to job sites and ask for a finisher. Even if finishers work regularly for a company, many will work for others after-hours. Tell them how many square feet the job is and if there are steps or other complications. They may charge by the area or the hour.

1. Edge the corners. Edging creates rounded edges that prevent damage from chipping and add an attractive finish. Edging also compacts and hardens the concrete along the form. Hold the edger against the form and flat on the concrete surface. Tilt the front edge up slightly while you push it forward. Raise the rear slightly when drawing the edger backward. Use short back-and-forth strokes to shape the edge and to work larger pieces of gravel deeper into the concrete. Complete the edging by holding the tool level and using long, smooth strokes. Repeat the edging process after each finishing task.

2. Make control joints. Jointing results in a series of grooves that prevent concrete slabs from cracking randomly. To be effective, a control joint should be one-fourth as deep as the thickness of the slab. Create control joints by finishing them into the wet concrete with a jointer or by sawing grooves in the surface after the concrete has hardened. Work the jointer as you would an edger, using a straight 2x4 as a guide. To cut concrete with a circular saw, use a masonry cutting blade. Cut the concrete as soon as it is hard, but not set—usually a few hours after Step 5.

3. Float with a hand tool. Floating pushes aggregate deeper into the concrete, smooths the surface, and draws a wet mixture of sand and cement to the surface, making further finishing possible. A wood float produces a coarse-textured surface; a magnesium float makes a smoother surface. If water begins to surface when you begin floating, stop floating and wait a while before trying again. Hold the float nearly flat and sweep it in wide arcs to fill low spots and flatten lumps. Smooth the marks left by edging and jointing.

4. Finish with a steel trowel. To achieve a smooth, dense surface, switch to a steel trowel after floating. Hold the trowel blade nearly flat against the surface, with the leading edge raised slightly. Overlap each pass by one-half of the tool’s length so you end up troweling all the surface twice in the first operation. For an even smoother surface, trowel the surface a second or even third time. On the final troweling, the trowel should make a ringing sound as you move it over the concrete.

5. Make a broom finish. For a nice-looking, slip-resistant surface on steps, walks, and drives, pull a damp broom across the surface of the just-troweled concrete. The stiffer the bristle, the coarser will be the texture. Only pull the broom—do not push. Have a brick or a piece of 2x4 handy on which you can knock the broom now and then to keep it clear of concrete buildup.

Adding Custom Finishes

Most custom finishes end up with a rough surface. But you will still need someone with good concrete finishing skills to bring the concrete to the correct level and smoothness to take the custom finish. One exception is the brick pattern. In that case, most do-it-yourselfers can start with a tamped sand or gravel bed, mix and pour small amounts of concrete at a time, and proceed from one form to the next.

Tools: Garden hose, trowels, brushes, joint strike.

Finishing Options

- For a broom finish, trowel the concrete. Then use a straw broom to make swirling patterns on the surface of the concrete. Use either wide arcs or squiggly strokes.

- To create patterns, you can rent concrete embossing stampers in many patterns. They work best if your concrete contains gravel no larger than 1/4 inch. To move the job along, rent at least two stampers. When the surface has begun to set, but is still soft, press the stamper in place firmly by stepping on it. Work quickly, alternating stampers.

1. To make an aggregate surface, sprinkle stones on the surface. Pour the concrete and screed it to about 1/2 inch below the top of the forms. As soon as the water evaporates from the surface (watch carefully), sprinkle stones in a uniform layer over the concrete.

2. Push the stones in. With a wooden float, carefully press the stones into the concrete until the surface is smooth, even, and as free of stroke marks as a floated slab. If the slab is a large one, apply a curing retarder to give you extra working time.

3. Expose the stones. When the concrete becomes firm (after about an hour), gently brush away excess concrete with a stiff broom. If the stones come out, stop working and wait for the concrete to harden further. Spray a fine mist on the surface. Continue sweeping until runoff water is clear and the stones are exposed.

Make a travertine finish. This finish should be used only in areas not subject to freezing (see box, right). Pour, screed, and float the concrete. As soon as the water evaporates from the surface, use a brush to spatter on pigmented mortar the consistency of thick paint. After the mortar has stiffened slightly, use a steel trowel to smooth the high spots to make the distinctive travertine pattern.

Rock Salt Finish - This technique produces pits in the concrete surface, giving it a weathered appearance. Like the travertine finish shown at left, a rock salt finish is not recommended for areas subject to freezing. Water sits in the pits and freezes, cracking the surface of the concrete. After finishing the concrete, sprinkle some rock salt—not too much or you’ll weaken the slab—over the surface. Use a trowel to embed the salt in the concrete. After the concrete hardens, wash and brush away the remaining salt.

Carve a flagstone pattern. To make your surface look like flagstones or geometric shapes, score the concrete soon after you finish it with a bull float or darby and after water has evaporated from the surface. A joint strike works well for this technique. Go over the marks each time you do other finishing operations.

1. To make a brick pattern, pour concrete into a form. This method is ideal for small jobs. These brick-shaped forms are available at home centers. Set the form on top of the gravel or sand bed and shovel concrete into it. Make sure each section is filled to the top of the form but no higher.

2. Remove form and finish with a trowel. After water has evaporated from the concrete surface water, wiggle the form out carefully so clean-looking lines remain. Flick away crumbs and carefully give the surface a trowel finish. Once the concrete is cured, fill the gaps with mortar.

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