About Laying Concrete

Laying concrete sidewalks, driveways and patios that are both durable and attractive requires careful preparation of the surface, selection of the right materials, and use of correct concrete placement techniques for the job and the weather. Skimping on materials or taking shortcuts in the processor could result in cracked concrete that is structurally unsound.

Features

The durability of concrete constructions of all sorts is dependent upon a concrete mix with proper ratios of aggregate, Portland cement and water. For the strongest possible concrete, as little water should be used as possible. In cold climates an air-entrained mix containing enough cement to cure to 3000 to 4000 psi strength will make for the most durable sidewalk, driveway or patio. A lower-strength concrete can be used in temperate climates.

Potential

Thorough surface preparation ensures sufficient drainage around and down from the finished concrete. Prior to laying concrete, the surface should be graded so that it is level and evenly compacted. Also, the surface should be graded at an angle so that water will run off the concrete slab away from existing building foundations. The surface is then covered with a layer of loose gravel which allows water to pass under the concrete slab. The gravel is covered with a waterproof membrane so that the concrete will not absorb moisture from the soil.

Size

To contain the liquid concrete while it cures, a wood form is constructed. 2 inch by 6 inch boards are commonly used for forms, stacked edge to edge if the slab is thicker than 6 inches. The form is staked on the outside, every 3 to 4 feet, as reinforcement against the weight of the concrete. If the slab is large, reinforcing metal rods or wire mesh are laid over the prepared surface; these will become part of the concrete slab. Very large slabs will need a form divided into sections of workable size to be poured and finished one by one.

Function

Pouring, leveling and finishing are the final steps in laying concrete. These must be completed efficiently since concrete begins to cure as soon as it is mixed. As the concrete is poured, it is tamped to force out air pockets that could compromise strength of the slab. A large straight edge is used to level the concrete. As curing begins, the concrete surface is finished by troweling, either by hand or with a troweling machine. Joints or scores for stress cracks are also cut during finishing.

Warning

Extreme weather can prevent the slow, even curing that produces strong concrete. Freezing during curing robs concrete of at least 50% of it's ultimate strength. Excessive evaporation in hot weather inhibits curing of the concrete surface, causing a crumbly texture. In temperatures below 40 degrees Fahrenheit, curing concrete must be protected from freezing and extreme variations between surface and interior temperature. In temperatures above 90 degrees Fahrenheit, concrete should be kept wet during curing.

Resources

• Concrete: Scientific Principles
• How to Build a Straight Form for Concrete
• How to Lay a Concrete Sidewalk