Various types of building materials are used during the construction work at different stages according to structural requirements.
Cement concrete of different qualities or mix are used in the building at different stages according to the type of building. Cement concrete is the most important part of constructional work; it needs special attention during the construction. Cement concrete works should be mixed and vibrated mechanically. Cement concrete is a mixture of cement, fine aggregate, coarse aggregate and water.
Classification of concrete mixes:-
The cement concrete mixes should be as per (IS: 456-2000). Concrete is generally classified by the method adopted for determining the proportions of cement, aggregate and water either as design mix concrete or nominal mix concrete. Compressive strength is considered to be the most important property of concrete and has been held as an index of its over all quality. Concrete is therefore designated by the grades according to its characteristic compressive strength, which is defined as the strength of concrete.
I S: 456-2000 has grouped various grades of concrete in to ordinary, standard and high strength concrete. Minimum grade of concrete for plain and reinforced concrete shall be M15 and M20 respectively for various exposure conditions. M refers to the mix and number to the specified characteristic compressive strength of 150 mm cubes of concrete at 28 days. Concrete of grades lower than M15 may be used for plain concrete construction, lean concrete, simple foundations, foundations for masonry walls and other simple or temporary reinforced concrete structures.
Design mix concrete:-
The design mix concrete is that concrete in which the proportions of cement, aggregates and water are determined by proper design using statistical concepts to produce a concrete of required workability and target mean strength. In order to ensure that not more than the specified proportion of test results is likely to fall below the characteristic strength, the concrete mix has to be designed for higher average compressive strength.
Nominal Mix Concrete:-
In the nominal mix concretes the constituents of concrete are measured by volume and the proportions are pre- determine such as 1:8:16, 1:4:8 1;3;6 and 1:2;4 etc. The unit of measurement of cement is a bag of 50 kg having a volume of 0.035 cum. The batch boxes are made with the size 35x25x40cm which corresponds to one bag of cement. Nominal mix concretes 1:5:10, 1:4:8, 1:3:6, 1:2:4 and 1;1.5; 3 roughly correspond to M5, M7.5, M 10,M15 and M20 grades of concrete so far as their compressive strength is concerned. The following ratios are generally applied for site requirement.
1. Cement concrete 1:8:16:-
Where cement concrete 1:8:16 ratio is used it means 1 part of cement 8 parts of fine aggregate/ sand and 16 parts of coarse aggregate. This ratio cement concrete is very low in strength and is recommended for the following type of building and work:-
• Used in foundation of walls of ordinary and single story building and used as base coat under floors and pavement.
2. Cement concrete1:6:12:-
Where cement concrete 1:6:12 ratio is used it means 1 part of cement 6 parts of sand and 12 parts of coarse aggregate. This ratio cement concrete is low in strength and is recommended for the following type of building and works:-
• Used in foundation of two or three story buildings, used in foundation of abetments, piers and retaining walls and used as basecoat under taxi tracks, pavement and cement concrete road.
3. Cement Concrete 1:4:8:-
Where cement concrete 1:4:8 ratio is used it means 1 part cement 4 parts of sand/fine aggregate and 8 parts of coarse aggregate. This ratio cement concrete has medium strength and is recommended for the following type of buildings and works:-
• Used in foundation of multi story buildings, under foundation of RCC columns, stairs, raft, RCC wall and air strips and taxi tracks base coat.
4. Cement Concrete 1:3:6:-
Where cement concrete 1:3:6 ratio is used it means 1 part cement 3 parts of fine aggregate or coarse sand and 6 parts of coarse aggregate. This ratio cement concrete has medium strength and is recommended for the following type of buildings and works:-
• Used in mass concrete, bed plates, concrete blocks, canal lining and chowkats hold fast etc.
5. Cement Concrete 1:2:4:-
Where cement concrete 1:2; 4 ratio is used it means 1 part cement 2 parts of fine aggregate or coarse sand and 4 parts of coarse aggregate. This ratio cement concrete has good strength and is recommended for the following type of buildings and works:-
• Used in footings of columns and raft foundation, used in beams, slabs, columns, stairs and walls of ordinary, single story and temporary buildings and used in retaining walls, pavements, floors and bedplates etc.
Where cement concrete 1:1:5:3 ratio is used it means 1 part cement 1.5 parts of fine aggregate or coarse sand and 3 parts of coarse aggregates. This ratio cement concrete has very good strength and is recommended for the following type of buildings and works:-
• Important RCC structures, piles, arches, impermeable construction against water heads. This ratio of cement concrete is safe against earth quake.