Pouring Concrete

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Small, onsite mixers will make about half a large wheelbarrow load of concrete at a time, taking 15 or 20 minutes per batch.

Site-mixing lets you pace the job yourself, without the hassle of directing a full size cement truck into your back yard. On the other hand, you still need to have sand, cement and crushed stone trucked in, and then you have to shovel it into the mixer. As far as dollars and cents go, mixing it yourself might save you 10 percent to 20 percent over ready-mix.

Ready-mix, truck-delivered concrete offers two advantages: convenience and superior formulation. The disadvantage is the size of check you'll write when the project is all done. Since the cost of ready-mix varies, sometimes wildly, shop around, especially if you stumble onto a supplier who demands a home delivery surcharge. Not all do. Where I live, the cost of concrete dropped from more than $110 per cubic yard in the late 1980s to about $85 per cubic yard after a second concrete company opened shop in the area. (Competition's great, as long as it's not in your business.)

Besides deciding on how much readymix to order (the desk jockey at the plant can help you calculate that), you have two other options to consider, both of which you should ask for by name. The first is the addition of reinforcing fibers into the concrete mix. These are synthetic strands added at the plant to increase the tensile strength of the hardened product. Fiber costs a bit more, but it's worth it, especially for outdoor pads. Another thing to ask for is air entrainment. This is the incorporation of a controlled amount of tiny air bubbles into the concrete while it's mixed. This boosts the ability of the hardened pad to resist frost-induced flaking, a common cause of concrete failure.

Site Preparation

If you've ever visited a tropical area, perhaps you've noticed how good the concrete looks. The reason is the lack of subzero temperatures and all the havoc they wreak on concrete in colder parts of the world.

When you're preparing a site for a concrete pad in areas that receive winter weather, you're fighting against the tendency for soil to heave up and down as it freezes and thaws, which cracks concrete. You could completely solve this problem by preparing a foundation for your pad that would extend down into the soil below the frost line, but this isn't practical for a pad of any size. You have to attack the problem by combining several strategies.

The first is site preparation. The reason soil heaves when it freezes is because it contains water. Dry soil won't heave at all, no matter how cold it gets. That's why in frost-prone regions you should start any pad-pouring project by replacing the soil directly under the area with a compacted layer of something coarse, like crushed stone. This promotes drainage that reduces the heaving hazard. How deep should you go? Six to 12 inches is the general advice. This won't get you under the frost line, but it will remove the most active soil. To prevent settling after the concrete has been poured, any coarse fill you add to a pad area must be compacted mechanically using a gas-powered compactor. Otherwise, the settling would cause cracking, too.

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