Your sidewalk is not made of cement. It's made of concrete which contains cement. Basic concrete is a mixture of rock, sand and cement. In combination with the oxygen in water the three dry elements bond together to make good old fashioned concrete. You'll recognize it as the slab in your yard with the cracks you're perpetually trying to patch.

Putting in a concrete walk, curb or patio takes quite a bit of expertise. If you don't want to undertake such a project yourself, it's important that you know what to look for from a contractor.

Although most concrete surfaces look pretty much alike, when it comes to strength there are differences. Most of us use the number of sacks of cement mixed into each cubic yard as a criterion for purchasing concrete. But this can be a poor measure of what you are buying.

For example: four-and-a-half-sack concrete can be stronger than six-sack mix depending on the size of the rock used. When gravel is used instead of rock, it takes more cement in the mix to make the resultant concrete as strong as the concrete that contains rock.

It is wise to purchase concrete even for a patio by its "compressive strength" rating. For most jobs the concrete used should be one that develops about 2500 pounds per square inch (PSI) of compressive strength. (3000 psi is better yet, 2000 psi not as good). Most concrete mixing companies have their own set of formulas to create different strengths of concrete. The stronger the concrete, the more resistant it will be to cracking by natural forces, such as ground heaving.

But the concrete alone is not the only measure for lasting quality. Reinforcing steel is equally important. Concrete expands and contracts with temperature change, and reinforcing steel holds things together when this happens.

Concrete is brittle and therefore has a tendency to crack under the best of conditions. But when the proper amount of steel reinforcing is used, the cracks tend to be only hair-line in size. Without steel reinforcing, cracks often open wider. Shifting in height also occurs more readily.

Two kinds of reinforcing are popular reinforcing bars and welded wire fabric. Unfortunately, even though it is popular, welded wire fabric is not as strong as reinforcing bars. Regardless of the type of reinforcing that is used, it is extremely important that it remain centered between the top and the bottom of the concrete slab. When concrete is poured, the reinforcing sometimes sinks to the bottom. If this happens it's almost like not having reinforcing at all. So, be sure to check the reinforcing material as the concrete is being poured, and pull it back up into the middle of the slab if necessary.

Another important factor is curing. At installation time (during pouring and finishing), if the water in the concrete evaporates too quickly, cracking can also occur. The slower the concrete dries the less the chance of cracking and the stronger the concrete will be. It is best to pour concrete when the temperature is moderate, but never when it is colder than fifty-five degrees Fahrenheit. Spraying the finished surface of the concrete with curing oil will slow the drying process by reducing the rate that water evaporates out of the concrete and will help to prevent cracks.

If too much water is used to mix the concrete, it will be weakened. It is important not to add a lot of water to the concrete once it arrives at your house from the concrete batch plant.

Finally under certain conditions it may be necessary to install a base of sand, gravel or compacted rock under the concrete before it is poured. This is something that you should discuss with a licensed soils engineer if a lot of concrete will be poured. The base recommended by the soils engineer will depend upon the type of soil on your property. There is no rule of thumb.

In summary, make sure that you purchase concrete by PSI rating rather than by amount of cement used. Don't add much more water to the concrete mix once the truck arrives at your home from the batch plant. Make sure that you pour concrete during the cool part of the day so it won't cure (dry) too quickly. Use reinforcing steel centered between the top and bottom of the slab for a longer-lasting and stronger result. And, check with a soils engineer on big jobs like driveways and patios to determine if sand, gravel or compacted base rock should be used below the concrete for needed additional strength.