BATCHING OF CONCRETE

Here is all about Batching of concrete which a engineer should be knowing for a better workmanship.
Batching of concrete means measuring different ingredients of concrete (i.e. cement, sand, coarse aggregate and water) before mixing it. Batching can be done by
· Volume Batching
· weight Batching




When this measurement is done on the basis of volume, we call it Volume Batching
VOLUME BATCHING :
BATCHING OF CEMENT
Cement is always measured by weight. Mostly it is used in terms of bags. One bag of cement weighs 50 kg and has a volume of 35 litres (or, 0.035m3). Cement should not be batched by volume because its weight per unit volume varies according to the way the container is filled.

 BATCHING OF AGGREGATE (BY VOLUME)


            Gauge Box or farma

 A gauge box is used (as shown in the picture) for batching of fine and coarse aggregate by volume. The box should not be too shallow. It should be completely filled with aggregates. The top of the materials should be struck off level with a straight edge. The ratio of materials of concrete decides the capacity of the box. Generally capacity of a gauge box is equal to the volume of one bag of cement (i.e. 35 litres). Convenient sizes of gauge boxes to suit different proportions are given in the table below.

Capacity In Litres Inside Dimensions of gauge box in cm
Length Breadth Depth
25 25 25 40
30 25 25 48
35 27 27 48
40 29 29 48
45 30 30 50
50 31 31 52
Note: The moisture present in aggregates and bulking of sand must be accounted for while volume batching is adopted.
The following table gives the approximate surface moisture carried by aggregates as per IS-456-2000.

Condition of Aggregates Approximate Quantity of Surface Mositure
Percent by Mass Litre per m3
Very wet sand 7.5 120
Moderately wet sand 5.0 80
Moist sand 2.5 40
Moist gravel or crushed rock 1.25-2.5 20-40

BATCHING OF WATER
Water is measured either in kg or litres as may be convenient. In this case, the two units are same, as the density of water is one kg per litre. The quantity of water required is calculated by multiplying water-cement ratio with weight of cement.
Points to remember
 Volume batching is not a good method for measuring concrete materials.
 It is not applicable in case of reinforced concrete structure.
 This method of concrete batching may not be economical.
 It can be only used for unimportant concrete or for small concrete works

WEIGHT BATCHING :
When this measurement is done on the basis of weight, we call it weight Batching
· The concrete ingredients are batched / proportioned based on weight.
· Batching by mass is achieved by using weigh batching machine / plant.
· Materials batched by mass are exact in measurement taking cognisance of the differences in the material properties.
· Water for mixing is weighed out precisely as a function of the weight of cement used for each mix proportion. Hence uniform water-cement ratio can be maintained, which plays an important role in achieving strength of concrete.
· Each batch is weighed which makes it easy to detect overruns or incorrect dosages.
· Concrete is of uniform and consistent quality.