Sunday - May 20, 2012

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  • Why Concrete?
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Construction Basics

Alatini Saulala and his staff will demonstrate Tongan pride in their daily work.

Subgrade

The subgrade should be compacted and have an even thickness. A standard driveway is 4" thick you want 4" thick continuously, not a 3" to 4" varying thickness.

Many western states have expansive soils. In these conditions, from 2" to 8" of crushed rock should be used as subgrade material depending on the soils level of expansiveness. If you have doubts about the soil characteristics in your area, consult a soils engineer.

The Correct Concrete Mix

A 4000-psi, .50 water to cement ratio is best for driveway construction. This provides better wearability and a "denser" concrete than the typical 2500-psi mix. Concrete is permeable and "wicks" moisture from beneath the slab. With the moisture come salts from the soil which can leave efflorescence on the surface. The .50 w/c mix provides a geometric reduction in this "wicking" action.

Excess water should not be added at the project site, as this will dilute the water to cement ratio.

In cold climates, air entrainment should be added to the concrete at the batch plant. This entrained air allows any moisture which does enter the concrete to expand in the microscopic air pockets during a freeze/thaw cycle instead of putting internal pressure on the concrete.

Joint Placement

Joints should be at least the concrete thickness so a 1" deep joint should be used in a 4" thick driveway. Joints should also be spaced 2-3 times in feet the thickness of the concrete: so a 4" thick driveway should have joints no farther than 8 12.

If joints are spaced too far apart, cracks will often occur where the joints should have been.

Proper Drainage

For best drainage, the concrete should slope at least 1" per running foot away from the home. If proper drainage is prevented do to the area of concrete being locked between two structures, a drain may need to be installed which will collect the water at a low point in the concrete and feed it down the drain.

Reinforcement

Reinforcement can be with either wire mesh, or steel bars placed in a grid pattern. In either case blocks should be used to keep the reinforcement in the center of the concrete. Note: that reinforcement does not eliminate cracks it simply holds them together.

Finishing Techniques

After concrete is bull-floated, it should be left alone until all the bleed water on top of the concrete has evaporated. Starting the finishing operation too soon can trap surface water and create a weak surface.

Of course, there are many other important steps in building a concrete driveway. The items listed here, though, are the most common causes of problems when not done properly.