How to buy, mix and pour concrete

Concrete is a mixture of portland cement, sand, coarse aggregate (gravel), and water. Use it within 11/2 hours after it has been mixed. Arrange to have someone help you with your project.

Caution: Wet concrete is caustic. Wear goggles, waterproof gloves, long pants and sleeves, and high rubber boots. Promptly wash splatters off your skin. Rinse clothing when you finish working. Use lanolin cream to relieve mild irritation. See a doctor if discomfort persists.

Planning a project
Make your measurements on the ground or in a scale drawing, then calculate your needs in cubic yards: multiply the dimensions of your area and divide by 27 (there are 27 cubic feet in 1 cubic yard); add 10 to 15 percent for a safe margin. If your project is over 3 cubic yards, call a supplier of ready-mixed concrete; he sells by the cubic yard.

Buying and mixing
When you need a small amount-for setting fence posts, for example-buy dry mixed concrete in 90-pound bags (2/3 cubic feet) from a lumberyard or home center. Pour part of the dry mix into a large metal bucket or a contractor’s wheelbarrow (rent one from a tool rental store). Add a little water and stir with a shovel or hoe. If still too dry, add a little more water. It’s right for pouring when it will hold a mound shape, the smoothed surface of which retains a trowel slash mark.

To mix concrete for a job requiring 1 to 3 cubic yards, rent a portable mixer. Don’t exceed its capacity, which is just over half its total volume. The concrete-mix ratio for a driveway or walk is 1 part cement, 2 parts sand, 4 parts aggregate, and 1/2 part water. Place the mixer at the work site to avoid wheelbarrowing. Hose the drum after each batch; spray other tools often. Pouring, finishing, and curing Pour only on a level gravel bed or level and slightly damp ground. Pour on a dry day, with the temperature between 50°F and 70°F

For a large job have ready-mixed concrete delivered and dumped directly into the formwork if possible. In regions that have frost, be sure to order airentrained concrete.

When pouring from a mixer or barrow, deposit each load against the previous, beginning in a corner. In a slab, spread the concrete with a shovel or hoe. In a wall formwork, ram the mixture up and down with a square end shovel to remove air bubbles and rock pockets; give particular attention to areas near the forms.

To level the surface, place a strikeoff board across the side forms. Move it back and forth to shift the excess concrete into the voids. Use a bull float to level any gaps and embed the aggregate below the concrete surface.

After the concrete has set enough to hold its shape, run an edger back and forth between the side forms and the concrete. Next, cut control joints in the slab to allow for expansion (one every 4 feet for a walkway; one every 10 feet for a driveway). Next, use a wooden or metal float to level lumps and voids, then smooth the surface several times with a trowel. For a nonskid finish, sweep with a damp, stiff broom. Care forms from the “green” concrete one day after the pour.

Curing should take 5 days in 70°F weather, 7 when cooler. During this period, keep concrete moist by sprinkling, or cover it with a plastic sheet.