subject: Ready Mix Concrete [print this page] Concrete is usually a mixture of paste mainly made of sand and rock. Concrete is the final product when cement, water, fine sand and coarse aggregates (rocks) are mixed together to form a solid mass . Concrete is known for its remarkable characteristic of being flexible (when newly mixed) and durable(when hardened) . Perhaps these characteristics of concrete have made it the most sought after building material. Concrete has been responsible for a lot of build skyscrapers, bridges, sidewalks and superhighways, houses and dams across the globe.
Strong and durable concrete is the outcome of careful proportioning and mixing of the ingredients. If the concrete mixture does not have enough paste to fill up the voids between the aggregates will result in rough, honeycombed surfaces and porous concrete. Excess of cement paste will produce a smooth surface; however, this type of concrete will be more likely to crack and will prove to be uneconomical.
A properly proportioned concrete mixture will possess the desired workability for the fresh concrete and the required durability and strength for the hardened concrete. Typically, a mixture is by volume about 10 to 15 percent cement, 60 to 75 percent aggregates, and 15 to 20 percent water. Entrained air bubbles in many concrete mixtures may also take up another 5 to 8 percent
The character of concrete is determined by the quality of the paste. The strength of the paste, in turn, depends on the ratio of water to cement. The water-cement ratio is the weight of the mixing water divided by the weight of the cement. High-quality concrete is producedby lowering the water-cement ratio as much as possible without sacrificing the workability of fresh concrete. Generally, using less water produces a higher quality concrete provided the concrete is properly placed, consolidated and cured.
Although most drinking water is suitable for use in concrete, aggregates are chosen carefully. Aggregates comprise 60 to 75 percent of the total volume of concrete. The type and size of the aggregate mixture depends on the thickness and purpose of the final concrete product.
Relatively thin building sections call for small coarse aggregate, while aggregates up to six inches in diameter have been used in large dams. A continuous gradation of particle sizes is desirable for efficient use of the paste. In addition, aggregates should be clean and free from any matter that might affect the quality of the concrete.
