A previous column about concrete discussed grading, forms and form oil, and preparing the soil.
This column describes other important steps that contribute to a successful concrete pour:
Concrete alone isn’t much stronger than plain old dirt. Pour a slab of concrete over a bed of adobe soil and the adobe will soon crack the concrete to bits unless the concrete has gobs of steel bracing.
Steel reinforcing bars, or “rebars,” are used to substantially enhance the strength of concrete. The bars are placed perpendicular to each other creating a kind of “tic-tac-toe” grid that adds strength to concrete. The amount of strength depends on the diameter of the rods and the distance between them.
The most common configuration uses half-inch rebars placed 2 feet on center in both directions. Each intersection of the rebars is tied together with heavy “tie wire.” The steel bars are lifted to and held within the center of the proposed slab with small square concrete spacers called “dobies.”
Each intersection of the rebar mat is supported by a doby acting as a spacer between the rebar mat and the ground. A steel mat laid on the ground beneath the concrete provides no support. To properly strengthen the slab, the rebar mat must be as close to the center of the slab as possible.
Keep in mind that adding steel is an inexpensive way of substantially strengthening concrete.
There are many different kinds of concrete, and the many combinations of rock (or gravel), sand and cement can result in various concrete strengths.
Concrete should not be bought based on the “number of sacks of cement” it contains. Five-sack concrete mix (five sacks of cement per cubic yard) is not always better than four-sack mix. More cement in a given mix does not necessarily mean more strength.
Buy concrete based on its “compressive strength,” that is, how much pressure it will take to crush it. The higher the compressive strength, the stronger the concrete. Concrete rated 2,500 PSI is about standard; 3,000 PSI is better, recommended and worth the extra cost.
If you have ever seen a commercial concrete pour you might have noticed someone watering the area with a garden hose. This is done to saturate the soil so it will not draw water from the concrete before the water and the cement in the concrete mix have time to properly harden, or “cure.” This is why it is so important not to pour concrete on an extremely hot day. If it cures too rapidly, the concrete will be weak and the lasting quality will diminish.
Wetting the ground helps reduce the escape of water through the bottom of the concrete. But water also can escape from the top as evaporation. This is prevented by applying a light layer of oil, called “curing compound,” to the top of the concrete.
The oil holds the moisture in the concrete from the top in the same way plastic wrap prevents food from drying out. The clear oil will eventually evaporate, but it remains long enough to slow down and stabilize the curing process.
For more home improvement tips and information from James and Morris Carey, visit their Web site at www.onthehouse.com or call 800-737-2474 Saturdays from 6 to 10 a.m. The Careys are also on KRKO (1380-AM) from 6 to 8 a.m. every Saturday.
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