Composition of Concrete

      

Concrete is a mixture of sand, gravel or other aggregates, and Portland cement (not a brand name) mixed with enough water to form a semi-fluid state. The mixture is then placed into a form to harden. In its premixed state, it is known as "cement." Once mixed and hard, the term becomes "concrete," e.g., a "bag of cement;" "a concrete driveway or walkway."

There are many manufacturers of cement, which comes packaged in 1-cubic foot bags that weigh 94 pounds. There are five basic cement types. The type most used in home construction is Type I, Type II is used for massive structures such as bridges and pilings and provides some sulfate resistance. Type III hardens quickly and it generates more heat in the drying process. It most commonly is used on commercial structures such as smoke stacks, in which the forms are moved quickly as the material sets up. It is also a product used for winter construction and rush jobs. Type IV is a low-heat producing cement and it is used for massive structures. Type V offers a high sulfate resistance; it is used in areas with high sulfate content in water and the soil.

AGGREGATES:
The second material used in concrete is called aggregate which is a fancy catchall term for sand and gravel. Aggregate ranges in size from dust-sized pieces of sand up to 2-1/2 inch stones used as larger "fill." Ideally, aggregate combines these sizes to provide the strongest type of concrete; the small particles fill in around the larger particles.

For most residential masonry projects, the aggregates used are sand and gravel or crushed stone. Do not use so-called "sharp sand" that is sometimes used for mortar. "Bank run" sand is best because the rounded, various-sized particles of this type of sand work to an advantage in the concrete. The sand particle size can be as large as 1/4-inch in diameter.

Gravel/stone-
The second aggregate, which is called coarse aggregate, can be either gravel or crushed stone. The stones may be as large as 1-inch. They can be screened for uniform size, or they can be bank run, which may also include some coarse sand along with the gravel sizes.

Usually, the larger the aggregate, the more economical the material will be to buy. The concrete will require less cement and the finished project will suffer less from shrinkage. However, don't use aggregate larger than one quarter of the thickness of the placement. This means that if the concrete will be 4 inches thick, you can use up to 1-inch size gravel or crushed stone. A 6-inch thick slab can include 1-1/2 inch stones.

Another consideration is the practicality of moving a mix containing the larger stones. If you have to shovel the concrete some distance down a form, it is a lot harder to shovel the larger stones than the smaller aggregates.

It is a general rule to place concrete as close to its final location as possible. Excessive movement can separate the water from the cement aggregates. If you must move the concrete more than 10 feet in the form, use 1/2-inch or smaller aggregate. Larger 3/4-inch, stone can be used in a flat slab where you won't have to move the material quite as far. For a large foundation, where you will be dumping directly into the forms and you won't need a smooth surface, you can use larger aggregates. Aggregate size also depends on the size of the spacing in the reinforcing bars or mesh. The stones should be no larger than three-fourths the opening between the bars, or the stones can get caught in the mesh of the bars and prevent the concrete from settling into position.

WATER:
The third ingredient in concrete is water. The water should be free of foreign materials and impurities. A good rule is to use only water that is fit to drink.

You don't want to use water out of a stream or lake that is muddy and dirty. If you do use clean lake water, however, filter it some way so it is fee of any foreign materials such as leaves and grass.

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