The key requirement for pouring and finishing a concrete slab is speed. On a dry, windy day, it may take only three hours for freshly poured concrete to become too stiff to work. For a 10-by-12 slab, two people will need about an hour for the heavy work of pouring, leveling, and smoothing the concrete, plus up to three hours to finish the surface. Until you have some experience, it’s best to pour concrete in sections no larger than 120 square feet.
A good source of concrete is a ready-mix company specializing in small loads—unless the slab is far from the street. In that case, haul the concrete in a rented trailer and mixer.
When making concrete yourself, test the mixture’s consistency frequently (opposite). Between checks, add water sparingly—1 cup per cubic foot of concrete at a time.
Smooth and textured finishes to concrete (pages 141-142) need no advance preparation, but surfacing with other materials requires forethought. For example, if you intend to apply a pebble-aggregate surface (page 143), you will need a stiffer-than-average concrete mix. Furthermore, you must thoroughly wet the pebbles before pouring the concrete. Order ⅓ cubic yard of gravel for every 100 square feet of slab.
After a slab is finished, it must be cured—kept moist and warm for at least a week to allow for the gradual chemical reactions that give concrete its full structural strength. The most common method of curing is to cover the slab with a polyethylene sheet. Colored slabs and pebble-aggregate surfaces are air-cured, however—left uncovered and sprinkled several times a day with water. Wait until the slab has cured to remove the outside forms.
Mixing the correct amount of water into concrete is crucial: Too little and a smooth finish on the concrete will be difficult to attain, too much and the material will lack strength and durability. The concrete in the near photograph, although appearing somewhat dry, is actually just right; light troweling produces a smooth surface. A dash or two more water produces a mudlike mixture that is too wet for use (far).
Adding the concrete
Support the wire mesh on bricks, and oil the form boards to prevent sticking. Dump enough concrete into the first form to overfill by ½ inch a 3- to 4-foot-wide section between the form boards, packing each load against the preceding one with a shovel. Use the shovel after each load to push the concrete into form corners and against the joint filler. Work a flat spade between the forms and the concrete to force the stones in the mix away from the sides. Then jab the spade vertically into the concrete throughout the section, to eliminate air pockets. If the wire reinforcement sags into the gravel base under the concrete’s weight, hook it with a rake and lift it to the middle of the concrete.
Screeding the concrete
As each 3- to 4-foot section is filled, set a screed—a straight 2-by-4, cut 2 feet wider than the width of the form—on edge across the form boards. With the aid of a helper, lift and lower the screed in a chopping motion to force the aggregate down into the concrete. Then, starting at one end of the filled section, pull the screed across the surface of the concrete, simultaneously sliding it from side to side in a sawlike motion. Tilt the screed toward you as you pull it, so that the bottom of the board acts as a cutting edge. To level any remaining low spots or bumps, pull the screed across the concrete again, tilting it away from you. In areas around obstacles such as steps or window wells, use a short screed, cut to fit the space. Fill and level the rest of the form in successive 3- to 4-foot sections.
Bull-floating the surface
To compact and smooth the concrete, first push the float forward, tilting the front edge of the blade upward. Then draw it back, keeping the blade flat against the surface. Shovel fresh concrete into any remaining depressions. To reach areas beyond arm’s length, bridge the wet concrete with a ladder supported on concrete blocks. Bull-float the surface again.
Edging the concrete
When the concrete is firm enough to hold its shape, run a mason’s trowel between the form boards and the outside edge of the slab to separate the top inch of concrete from the wood (above, right). Push an edger back and forth along the slot (above, left), tilting the leading edge of the tool slightly upward to avoid gouging the concrete. Any deep indentations will be difficult to fill during later finishing steps. Wait for any surface water to evaporate from the slab before applying the finish.
Troweling a smooth finish
Place a pair of knee boards—1- by 2-foot pieces of ⅜-inch plywood with 2-by-2 handles nailed at the ends—on the slab. Kneeling on the boards, smooth the concrete with a hand float, holding it flat and sweeping it in overlapping arcs across the surface. Then sweep a rectangular steel trowel, held flat, across the same area. Similarly float and trowel the rest of the slab, moving the knee boards as necessary. (The concrete will be firm enough to walk on at this stage.) After floating and troweling, go over the slab again with the trowel alone, this time tilting the tool slightly. Work the surface until no concrete collects on the trowel and the blade makes a ringing sound indicating that the concrete is too firm to work any further. Run the edger between the form boards and the edges of the slab (above) to restore edging lines.
Brooming a skidproof surface
Hand-float the concrete and trowel it once (page 141). Instead of the final troweling, draw a damp, stiff-bristled utility brush across the surface. Either score straight lines at right angles to the forms or move the broom in arcs to produce a curved pattern. If the broom picks up small lumps of concrete, hose down the bristles to clean them; give the slab a few more minutes’ drying time before you continue. If you have to press hard to score the concrete, work fast; the concrete will soon be too firm to take a finish.
Creating a flagstone effect
Immediately after bull-floating the concrete (page 140), score the surface with irregularly spaced grooves, ½- to ¾-inch deep, using a convex jointer. Place a ladder across the forms as a bridge to reach inaccessible spots. After surface water has evaporated, hand-float and trowel the surface, then retool the grooves to restore the flagstone pattern to its original clarity. Brush out the grooves with a dry paintbrush to remove any remaining loose bits of concrete.
Preparing the surface
Fill the form with concrete as on page 140, but pack it even with the tops of the boards rather than above them. Level the concrete with a screed notched at each end so that its bottom edge rides ½ inch below the tops of the form boards, then bull-float the surface. Scatter the damp pebbles evenly over the concrete with a shovel. Cover the surface with a single layer of stones, using a ladder bridge, if necessary, to reach inner areas.
Embedding the aggregate
Tap the stones into the concrete with a bull float, forcing them just below the surface. After you have gone over the entire slab with the bull float, press down any stones that are still visible with a hand float, using a ladder bridge, if needed, to reach the interior of the slab. Then run the hand float across the surface as on page 141, covering the stones with a thin, smooth layer of concrete.
Exposing the aggregate
After surface water has evaporated and the concrete is firm enough to resist indentation, brush the surface lightly with a stiff nylon broom to expose the tops of the stones. While a helper sprays the slab with water, brush it again, uncovering between a quarter and a half of the stones’ circumference. If you dislodge any stones, stop brushing and wait until the concrete is a bit firmer before continuing. If the concrete is difficult to wash off, work quickly to expose the aggregate before the surface becomes too stiff. After exposing the stones, continue to spray the surface until there is no noticeable cement film left on the aggregate. Scrub individual spots missed in the general wash with a scrub brush and a pail of water. Two to four hours after exposing the aggregate, wash and lightly brush the surface again to remove any cloudy residue from the stones.