The process of proper and accurate measurement of materials for making concrete is known as batching. It is the first process of making quality concrete, hence due importance should be given.
Generally, there are two methods of batching namely,
- Volume batching and
- Weigh batching
a) Volume batching
The process of measuring materials using their occupied volume is called volume batching.
A gauge box, generally called Farma is used to measure the volume of the materials. They can be made of timber or steel plates.
The volume of the gauge box is made equal to the volume of one bag of cement i.e.,35 litres (1.25 cubic feet) or its multiple. (Read FAQ to know about the volume of cement bag)
The standard dimensions of the farma or gauge box shall be 1 ft x 1 ft x 1.25 ft.
a) Batching of cement
Cement is not usually batched in farma. The volume of one bag of cement is taken as 35 litres. As the volume of one gauge box and one cement bag are the same, we take the cement bag and pour it directly into the mixer. Generally, for each batch mix, one bag of cement is used.
b) Batching of aggregates
The fine aggregates and the coarse aggregates are measured using the gauge box according to the mix proportion. (shown in example below)
c) Batching of water
Water may be measured either in litre or kg (as both are the same for water because its density is one kg/litre). The quantity of water required will depend upon the water-cement(w/c) ratio.
If the w/c ratio is 0.45, then the quantity of water required per bag of cement is 0.45 x 50 kg = 22.5 litres or 22.5 kg.
The batch volume of materials for various mixes
For example, let’s calculate the batch volume for concrete of grade M20.
The mix proportion of M20 is 1: 1.5: 3.
For each batch, one bag of cement is used which is 35 litres. Hence, if we calculate the volume of aggregate with respect to the cement content then,
- The volume of fine aggregate = 1.5*35 = 52.5 litres
- The volume of coarse aggregate = 3*35 = 105 litres
The batch volume of materials for various nominal mixes is shown below.
|Grade||Proportion||Cement (Kg)||Fine aggregate (litres)||Coarse aggregate (litres)|
|M10||1 : 3 : 6||50||105||210|
|M15||1 : 2 : 4||50||70||140|
|M20||1 : 1.5 : 3||50||52.5||105|
|M25||1 : 1 : 2||50||35||70|
Is volume batching, a proper method?
Volume batching is not a good method for proportioning materials because of the following reasons,
- The Volume of moist sand in a loose condition weighs much less than the same volume of dry compacted sand. So there is a high chance of the mix becoming disproportionate. That’s why it prefers not to measure granular material in its volume.
- The packaging of granular materials in the farma for each batch mix is unpredictable. For example, some people may pack sand tightly in farma while others may pack loosely while measuring.
- Various other factors like bulking of sand are not considered which may lead to the change in proportion.
Because of the above reasons, it is necessary to do weigh batching for important concrete works. However, for unimportant works, concrete ingredients may be weighed by volume.
b) Weigh batching
Weight batching is the correct method of measuring materials that are used for making concrete. For important concreting works, weigh batching should be adopted.
There are various types of weigh batchers available. We can select one of the batcher depending upon the nature of the job.
- For smaller works, a spring-loaded weighing arrangement with the dial can be used.
- For larger works and in large batching plants, automatic weighing equipment is used.
When weigh batching is adopted, the measurement of water must be done accurately. Manual addition of water by buckets will not be accepted as there may be a chance of spilling. Hence, the water should be stored in tanks fitted with valves.
Sometimes, water meters are fitted in the exit valve of the tank from which the exact quantity of water can be let into the mixer.
“Weigh batching is best compared to volume batching.”
Is it possible to batch the ingredients of concrete accurately?
No matter how much care we gave, there may be the slightest error in batching. That’s why Indian Standards 4925: 2004, gives the following tolerances for individual batching.
|Cement and other cementitious materials||±1|
Frequently Asked Questions (FAQ)
1) How to calculate the volume of one cement bag?
To calculate it, we need to know the density of cement and the weight of one cement bag. The density of cement is usually around 1440 kg per cubic meter. The weight of one cement bag is 50kg.
Note that 1 cubic meter equals to 1000 litres.
2) How Mass and volume are the same for water?
The density of water is 1000 kg/m3 or 1 kg/litre.
Let’s find out the mass of water of 10 litres,
Mass = Volume x Density = 10 lit x 1 kg/ lit = 10kg.
Hope you get all the information you need to know about batching of concrete. If you have any queries or if you found this article helpful, let us know in the comment section.