Aaron Sanders, age 15, of Shreveport, La., for his question:

HAS CONCRETE BEEN USED FOR A LONG TIME?

Concrete is an artificial engineering material made from a mixture of portland cement, water, fine and coarse aggregates and a small amount of air. The durability of concrete is evidenced by the fact that concrete columns built by the ancient Egyptians more than 3,600 years ago are still standing.

Many of the great works of Roman architecture were constructed of concrete nearly 2,000 years ago.

So, you can see, concrete has definitely been around for a long time.

Concrete is the most widely used construction material in the world, as a matter of fact. In the United States almost twice as much concrete is used as all other building materials combined.

Concrete is the only major building material that can be delivered to the job site in a pliable state. This unique quality makes concrete most desirable as a building material because it can be molded to virtually any form or shape and provides for continuity of structural action.

Concrete provides a wide latitude in surface textures and colors and can be adapted to many uses, including construction of streets and highways, dams, bridges, large buildings, airport runways, irrigation structures, breakwaters, piers and docks, sidewalks, silos and farm buildings, homes and even barges and ships.

Under normal conditions, concrete grows stronger as it grows older. The chemical reactions between cement and water that cause the paste to harden and bind the aggregates together requires time. The reactions take place very rapidly at first and then more slowly over a long period of time.

In the presence of moisture, concrete continues to gain strength for years.                             

Concrete mixtures are usually specified in terms of the dry volume ratios of cement, sand, and coarse aggregates used. A 1:2:3 mixture, for instance, consists of one part by volume of cement, two parts of sand and three parts of coarse aggregate.

Depending on the applications, the proportions of the ingredients in the concrete can be altered to produce specific changes in its properties, particularly strength and durability. The ratios can vary from 1:2:3 to 1:2:4 and 1:3:5. The amount of water that has to be added to the cement mixture is about 1 to 1 1/2 times the volume of the cement.

For most small jobs and minor repairs, concrete can be mixed by hand, but machine mixing ensures more uniform batches and therefore, superior performance. For most home repairs and improvements, such as with walks, driveways, patios and garden pools, the recommended portions are a 1:2:3 mix.

The aggregates should be clean and free of vegetable matter. All sand should also be dirt free.