Grades and types of concrete
The strength of the concrete is termed by grade. The grade reflects the compressive strength which is the characteristic strength of concrete after 28 days. It is depicted by a letter M followed by a number. For instance, if the grade of the concrete is said to be M20, it means that the concrete mix (M) has a compressive strength of 20 N/mm2. Various grades of concrete are used for different applications in home constructions.
Different types of concrete including ordinary, standard and high strength concrete include use of various grades. Ordinary concrete includes grades of M10, M15 and M20. The standard ones include M25, M30, M35, M40, M45, M50, and M55. The high strength concrete includes M60 and M80 grades. Generally M10 and M15 grades of concrete are used for leveling course, and bedding for footings. Standard concrete and concrete of grade M20 is applicable for Reinforced Cement Concrete (RCC) works for slabs, beams, columns and so on. The high strength concrete has its applications for high rise buildings.
The proportion of the materials in the concrete mix also varies for different applications. For instance, a proportion of 1:2:4 (1 part of cement, 2 parts of sand, and 4 parts of aggregates) is used for footings of pillars and raft foundations. It is even used for RCC works in lintel shelves and other small works. The quantity of water required for this proportion per bag of cement (50 kg) is about 30 liters. However, for RCC work in beams, pillars, walls and others, a proportion of 1:1.5:3 is used which requires about 25 liters of water per bag of cement.
Quality of concrete
The stress applied on the concrete footings, pillars, and slabs affects the strength of the building. Therefore you should not compromise on the quality of concrete or materials used for these key elements. The quality of concrete depends on the quality of cement, sand, aggregates and water used in the mixture.
Good quality cement is greenish gray in color without any lumps or dust. You should feel a cool finish when a pinch of cement is rubbed between your fingers. Coming to sand, it should be free of grass, leaves, and other foreign organic matter. Too much clay and silt content of the sand also affects its quality.
The other component of concrete is aggregates. Clay or dirt coating on the aggregates prevents the adhesion of cement to aggregates. This in turn slows down the setting and hardening process, reducing the strength of concrete. Therefore ensure that quality aggregates are used. Good quality aggregates are dense, inert, hard and durable. They are capable of maintaining well bonding with the cement paste.
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