INTRODUCTION
1.1 WHAT IS CONCRETE
Concrete is an artificial engineering material made from a mixture of Portland cement, water, fine and course aggregates, and a small amount of air. It is the most widely used construction material in the world.
Concrete is the only major building material that can be delivered to the job site in a plastic state. this unique quality makes concrete desirable as a building material because it can be molded to virtually any form or shape. Concrete provides a wide latitude in surface textures and colours and can be used to construct a wide variety of structures, such as highways, and streets, bridges, dams, barge buildings, airport runways, irrigation structures, breakwaters, piers and docks, sidewalks, soles and farm buildings, homes and even barges and ships.
Other desirable qualities of concrete as a building material are its strength, economy, and durability. Depending on the mixture of material used, concrete will support, in compression, 700 or more kg/sq cm (10,000 or more 1b/sq in). the ensile strength of concrete is much lower, but by using properly designed still reinforcing, structural members can be made that are as strong in tension as they are in compression. The durability of concrete is evidenced by the fact that concrete columns built by the Egyptians more than 3000 years ago are still standing.
There are however, many different types of concrete, the names of some are distinguished by the types, sizes and densities of aggregates e.g. eight weight, normal weight or heavy weight. Concrete are similar in composition to mortar, which are used to bond unit masonry. Mortars however, are normally made with sand as a hole aggregates.
Whereas, concrete contain much larger aggregates and this usually have greater strength. As a result, concrete have a much wider range of structural application, including pavements, footings, pipes, unit majoring, walls, dams and tanks. Because ordinary concrete is much weaker in tension than in compression, it is usually prestressed or reinforced with a much stronger material, such as steel, to resort tension.
There are various methods employed for carting ordering concrete. For very small projects, sacks of prepared mixes may be purchased and mixed on the site with water, usually a drem-type, portable, mechanical mixer.
For large projects, mix ingredient are weighed separately and deposited in a stationary batch mixer or a continuous mixer. Concrete mixed or agitated in a truck is called ready mixed concrete. In general, concrete is placed and consolidation is forms by hand tamping or pudding around reinforcing steel or by spreading at or near vertical surface. Another technique vibration or mechanical pudding, which is the most satisfactory one for achieving proper consolidation.
CONSTITUENT OF CONCRETE
The two major components of concretes are cement parts and inert materials. The cement parts consists of Portland cement, water, and some air either in the form of naturally entrapped air voids or minute, intentionally entrained air bubbles. The inert materials are usually composed of fire aggregate, which is a material such as sand, and course aggregate, which is a material such as gravel, crushed stone, or slag. In general, fire aggregate particular are smaller than 6.4mm (.25mm) in size, and course aggregates a particles are large than 6.4mm (.025mm). Depending on the thickness of structure to be built, the size is used, when Portland cement is mixed with water, the components of the cement react to form a cementing medium. In properly mixed concrete, each particles of sand and course aggregates is completely surrounded and coated by this paste, and all spaces between the particular are filled with it. As the cement part sets and hardens, it binds the aggregates into a solid mass.
Under normal conditions, concrete grows stronger as it grows older. The chemical reactions between cement and water that cause the parts to harden and bind the aggregates together require time. The reactions take place very rapidly at first and then slowly over a long period of time.