In construction, there should be individual strength for each building element.
To maintain the strength of concrete for different elements, various types of concrete grades are required. The strength needed for foundation, beam, slab etc. will be different.
IS 456-2000 has specified the concrete mixes into a number of grades as M10, M15, M20, M25, M30, M35 and M40. In the following construction video tutorial, brief explanation is given on M20 grade of concrete.
In M20, M denotes Mix and 20 refers to the characteristic strength (fck) of that mix i.e. 20mpa. Cement, sand and aggregates are used for mixing in the ratio of 1 : 1.5 : 3. M20 signifies mixture of cement, sand and aggregate which are prepared in such a manner that a cement concrete cube of size 15 cm x 15 cm x 15 cm is formed with characteristic strength (fck) of 20mpa while examining it after being cured for 28 days.
The characteristic strength (fck) signifies the strength under which not over 5% of test results are predictable to fail.
Cement is always calculated with weight. Commonly it is applied in terms of bags. The weight of one bag of cement is 50 kg and it contains a volume of 35 litres (or, 0.035m3). A gauge box is employed for batching of fine and coarse aggregate by volume.
To get more brief information, watch the following video tutorial.