Concrete batching means measuring the various components of concrete (ie cement, sand, coarse aggregate and water) before mixing them. When this measurement is done on the basis of volume, we call it volume batching, Some important points to remember before adopting volume batching method in the field are mentioned below.
- Volume batching is not a good method for measuring solid materials.
- This does not apply in the case of reinforced concrete structures.
- This method of concrete batching may not be economical.
- It can only be used for insignificant concrete or small concrete works.
Before batching solid material in terms of quantity, we need to know two things.
- What is the relative ratio of the solid components by volume?
- What is the water-cement ratio?
Knowing these two things you can proceed to batch the solid material in the field.
The table below can be used as a reference to find out the relative proportions of the different ingredients for different grades of concrete.
grade of concrete | Relative ratio of ingredients to quantity | In general we specify it as: | ||
cement | fine aggregate | coarse aggregate | ||
M10 | 1 | 3 | 6 | 1:3:6 |
M15 | 1 | 2 | 4 | 1:2:4 |
M20 | 1 | 1.5 | 3 | 1:1.5:3 |
M25 | 1 | 1 | 2 | 1:1:2 |
The water-cement ratio to be used should be as per the contract.
Table of Contents
Step-1-Batching of Cement
Cement is always measured by weight. Mostly it is used as a bag. A bag of cement weighs 50 kg and has a volume of 35 liters (or, 0.035 m.)^{3}) Cement should not be mixed on a volume basis as its weight per unit volume varies according to the way the container is filled.
Step-2-Batch of Sets (According to Quantity)
A gauge box is used (as shown in the figure) for bulk batching of fine and coarse aggregates by volume. The box should not be too shallow. It should be completely filled with aggregates. The top of the material should be hit level with a straight edge. The ratio of the content of concrete decides the capacity of the box. Normally the capacity of the gauge box is equal to the volume of a bag (ie 35 litres) of cement. The convenient sizes of gauge boxes to suit different ratios are given in the table below.
Capacity in liters | cm. Inside Dimensions of Gauge Box in | ||
Length | Width | 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 |
pay attention: While adopting volume batching, the moisture present in the aggregate and sand agglomeration should be taken into account.
The following table shows the estimated surface moisture carried by the aggregate as per IS-456-2000.
position of the set | Approximate amount of surface moisture | |
Percentage by Mass | liter per square meter^{3} | |
very wet sand | 7.5 | 120 |
medium wet sand | 5.0 | 80 |
moist sand | 2.5 | 40 |
moist gravel or crushed rock | 1.25-2.5 | 20-40 |
Step-3-Water Batching
Water is measured in either kilos or liters which can be convenient. In this case, the two units are equal, since the density of water is one kilo per liter. The amount of water required is calculated by multiplying the water-cement ratio with the weight of the cement.
Let us now solve an example, so that you become more confident in the field. In this example I have also tried to show you how to apply correction for bulking and how to apply moisture correction to any surface.
data given
- relative ratio of cement : sand : coarse aggregate Is 1:2:4
- Water-cement ratio 0.6 . Is
- Moisture content in fine aggregates (ie sand) is 6% by volume
- Moisture content in coarse aggregate is 1.5% by volume
- The bulk of the fine aggregate is 20%
Solution
Step-1-Calculation of Amount of Dry Ingredients Required | |
ratio of ingredients by volume | =1:2:4 |
volume of a bag of cement | =35 liters |
Quantity of dry fine aggregate required | =35*2=70 liters |
Quantity of dry coarse aggregate required | =35*4=140 liters |
water-cement ratio | =0.60 |
total water requirement | =0.60*50=30 liters |
Step 2 – Applying correction for moisture in the aggregate | |
Moisture percentage (FA) in fine aggregate | ,6% |
FA. moisture content in | ,70*(6/100)=4.2 liters |
Moisture Percentage (CA) in Coarse Aggregates | = 1.5% |
CA . moisture content in | =140*(1.5/100)=2.1 liters |
Hence the excess amount of moisture present in Ca and FA | =4.2+2.1=6.3 liters |
pure water to be added | =Total water- additional amount of water present as moisture content=30-6.3
=23.7 liters |
Step-3 – Applying corrections to the bulk of the fine aggregate | |
bulk of fine aggregate | = 20% of total quantity of FA=70*(20/100)
=14 liters |
moist fine aggregate required | =70+14=84 liters |
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