Concrete Formulas

      

HOW MUCH CONCRETE MATERIAL DO YOU NEED?:
There are several techniques to prepare concrete for a project:

1. You can buy the dry ingredients separately and mix them.
2. You can buy a dry complete mix to which you simply add water and stir.
3. You can buy concrete from an already-mixed company. This is delivered to the project site and is ready to go.

In the first technique, you will need a mixing machine or a box in which to mix the different dry ingredients. This is hot and heavy work, although it doesn't take much skill.

In the second technique, you also will need mixing facilities. This technique is expensive because the yield from a typical 80 pound bag is 2/3 square foot. This bagged material is excellent for small projects such as anchoring a basketball goal or clothesline post or used as a pothole or broken curb patch.

The third technique is ideal for larger jobs. You build the necessary forms, call the company, and a driver arrives at an appointed time and dumps the concrete load where you want it. It is up to you, of course, to shovel, level, float, and finish it. Be aware that most already-mixed plants won't deliver less than 1/2 yard and some won't deliver less than one full yard of concrete.

CONCRETE FORMULAS:
Cement, sand, coarse aggregate and water must be present in the correct proportions to create a durable, long-lasting job. Incorrect proportions result in a project that will crack, fake, or chip. There should be enough large aggregates to make an economical mix, yet enough small aggregates to f ill the spaces around the larger ones-and enough cement to hold all materials together. In addition, there should be the right proportion of water to provide proper hydration. Either too much or too little water can cause big trouble.

Adding water-
Adding the correct amount of water in a concrete mix is a tough problem. It is compounded by the fact that sand contains varying amounts of water. Sand falls into several wet categories: damp, wet, very wet. To determine which, squeeze a handful of sand into a ball. If it holds its shape, yet leaves no noticeable amount of moisture on your hand, it is considered wet sand.

Damp sand will fall apart after being squeezed. Very wet sand holds its shape and leaves moisture in your hand. In fact, water may run out of your hand. Construction sand can usually be considered "wet."

Proportions also vary according to the size of the coarse aggregates in your mix, and the addition (or exclusion) of air-entraining agents.

Air-entrained concrete-
Air-entrained concrete is necessary in areas in which concrete must withstand freezing and thawing temperature shifts and deicing treatments. Air-entrainment is the process of introducing millions of microscopic air bubbles into the concrete. The air bubbles permit enough space for the absorbed water to expand when the water freezes, so the concrete does not crack or break. Air-entrained concrete is also easier to place and finish than regular concrete, because the tiny air bubbles act as lubricants while the concrete is still plastic.

Air-entrained Portland cement in specially marked bags can be purchased at many building supply stores. However, if it is not available in your area, you can add an air-entraining admixture. This is available from a hard materials dealer-one who handles cement, stone, rock, gravel, and so on.

Whether you buy already bagged air-entrained cement or add the agent, hand mixing is ineffective for entraining air. A mixing machine must be used.

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