Accurate measurement of cement, fine aggregate and coarse aggregate is most necessary for producing good concrete.

Cement.

Cement should always be measured by weight and not by volume, because of the fact that the same container filled once with loose cement and secondly with tight cement will differ largely in its contents and this difference may be even up to 40 %. Cement is packed in jute bags weighing 50 kg. But during transportation and handling some quantity of cement is lost through the pores of the bag and as such the weight of cement bags must be checked at site before its use. the deficit in weight must be made up by adding cement in the mixture.


Sand.

Sand should be measured by weight, but alternately may be measured by volume with proper allowance of bulking.


Bulking of Sand

The increase in volume of dry sand due to absorption of moisture which forms a thin film of water around the particles of sand by frictional resistance is called bulking of sand. A moisture content of 2 to 5 percent increases the volume from 15 to 30 percent. The finer the sand, the greater is the bulking. When the moisture content is increased by adding water gradually until the sand is completely saturated, bulking diminishes pratically to nil and the sand occupies the least volume as in the dry state.
The amount to bulking can be readily determined at site by a suitable method as follows:
Sufficient quantity of sand shall be put loosely into container of uniform cross section, levelled off but not pressed down and its depth is measured. The sand is then well mixed and stirred with plenty of water and allowed to settle. The top surface of the inundated sand shall be smoothed and levelled. The new depth of sand is then measured and this is practically equal to that volume which would be occupied by the same weight of sand when dry. Now the amount of loss in the volume of sand is the amount of Bulking.
If D = depth of sand when damp; D1 = depth of sand after settling under water then percentage of bulking = D - D1 / D1   X  100, and this additional percentage of sand determined at the time of mixing shall be added for correct proportioning of a mixture.


Coarse Aggregate

Coarse aggregate may be measured either by volume or by weight. If the aggregate be wet, allowance for surface water should be made while determining the quantity of water to be used in the mix.
In case of ordinary works the measurements are done by volume, but in case of important or specially for controlled concrete works the measurement must be done by weight only.


Water

Water is measured by volume, as so many litres per bag of cement as specified according to slump test.
The strength and workability of concrete depend to a great extent on the amount of water used in mixing. An increase of 10 % of water above the optimum amount increases workability but may reduce the strength by 15 % approximately and an increase 50 % of water above the optimum amount may reduce the strength by 50 %. With an excess of more than 50 %, the concrete becomes inadhesive. Excessive amount of water in concrete mixing, not only produces low strength but also increases about 10 % less than minimal may be insufficient to ensure complete setting of the cement and may produce an unworkable concrete.
A rule for determining the approximate quantity of water in gallons / litres to give a strong concrete of reasonable workability may be given by :
Water requirement of concrete = 28 % by weight of cement + 4 % by total weight of aggregates.
Modern method to determine the quantity of water in a mixture is water cement ratio. The ratio of the volume of mixing of water to the volume of cement is called the water-cement ratio.
In practice the amount of water is actually determined at site, by means of 'slump test' with trial mixtures.

According to the British standard specifications.

Proportion                                  1:2:4     1: 1 1/2 : 3     1:1:2

Water cement ratio (by wt)     0.58           0.51           0.43

Quantity of water may now easily be calculated from water cement ratio as.

Weight of water
-----------------------   =  0.58 for 1:2:4 mix or Ww / Wc  = 29 litres
Weight of cement

(Note : one litres of water weighs one kilogram).