What is Concrete?

Concrete, as you know is an artificial stone made by mixing cement and sand with gravel, broken stone, or other aggregate. These materials must be mixed with sufficient water to cause the cement to set and bind the entire mass.

There are various types of concrete used during construction operations, such as precast, prestressed, and many others. Not matter how the concrete is expected to be used, the makeup is basically the same the world over. That is, concrete is a synthetic construction material, made by mixing cement, fine aggregate (usually sand), coarse aggregate (usually gravel or crushed stone), and water together in proper proportions. The product is not concrete unless all four of these ingredients are present. A mixture of cement, sand and water, without the coarse aggregate, s not concrete but mortar or grout. Many people mistakenly call a concrete wall or floor a cement wall or floor. There is no such thing as cement wall or floor.

The fine and coarse aggregates in a concrete mix are called inert ingredients; the cement and water are the active ingredients. The inert ingredients and the cement are thoroughly mixed together first. As soon as the water is added, a chemical reaction between the water and the cement begins, and it is this reacting (which is called Hydration) that causes the concrete to harden. Remember that the hardening process is caused by hydration of the cement by the water not by drying out of the mix. Instead of being dried out. The concrete must be kept as moist as possible during the initial hydration process. Drying out would cause a drop in water content below the amount required for satisfactory hydration of the cement. In fact, concrete hill harden just s well underwater as it will in the air.

In mixing concrete, the most important proportion to remember is that between cement and water. As long as a mix is workable, the amount of aggregate may be varied considerably. However, for a given strength of concrete, the ratio between cement and water is fixed. The relationship between strength of the concrete and the relative quantities of water and cement is expressed more definitely by the concrete experts:

For given materials and conditions of handling, the strength of the concrete is determined primarily by the ratio of the volume of the mixing water to the volume of cement as long as the mixture is plastic and workable.

In other words , if 6 gallons of water are used for each sack of cement in a mixture, the strength of the concrete at a certain age is already determined. The only extra provisions are that the mixture be plastic and workable and that the aggregates be strong, clean, and made up sound particles. More water will mean less strength and les water greater strength.

Following this principle, modern practice is to state the amount of mixing water foe each sack of cement to produce “pastes” of different strengths. Common combinations are 5 gallon paste 6-galon paste, and 7-gallon paste, to be selected according to the type of work to be done.