Concrete Volume Batching: Definition and Example
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Concrete Volume Batching
WHAT IS CONCRETE VOLUME BATCHING?
Concrete Volume Batching means measuring different ingredients of concrete (i.e. cement, sand, coarse aggregate and water) before mixing it. When this measurement is done on the basis of volume, we call it Volume Batching. Below are mentioned some of important points to remember before adopting volume batching method in field.
- 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.
HOW CONCRETE VOLUME BATCHING IS DONE IN FIELD?
Before batching concrete ingredients in terms of volume, we need to know two things.
- What is the relative proportion of concrete ingredients in terms of volume?
- What is the water-cement ratio?
After knowing these two things you can proceed to batch concrete ingredients in field.
The table shown below can be used as a reference to find out relative proportion of different ingredients, for different grades of concrete.
|Grade of Concrete||Relative Proportion of Ingredients by Volume||In General We Specify It As|
|Cement||Fine Aggregate||Coarse Aggregate|
The water-cement ratio to be used must be as per the contract.
STEP-1-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.
STEP-2-BATCHING OF AGGREGATE (BY VOLUME)
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|
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 gravel or crushed rock||1.25-2.5||20-40|
STEP-3-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.
Now let us solve an example, so that you become more confident in the field. In this example I have also tried o show you, how correction for bulking is applied and how correction of any surface moisture is applied.
Relative proportion of cement : sand : coarse aggregate is 1:2:4
Water-cement ratio is 0.6
Moisture content in fine aggregate (i.e. sand) is 6% by volume
Moisture content in coarse aggregate is 1.5% by volume
Bulking of fine aggregate is 20%
|Step-1-Calculating Volume of Dry material Required|
|Ratio of materials by volume||=1:2:4|
|Volume of one bag of cement||=35 litres|
|Volume of dry fine aggregate required||=35*2=70 litres|
|Volume of dry coarse aggregate required||=35*4=140 litres|
|Total water required||=0.60*50=30 litres|
|Step-2-Applying Correction for Moisture Present in Aggregate|
|Moisture percent in fine aggregate (F.A.)||=6%|
|Amount of moisture present in F.A.||=70*(6/100)=4.2 litres|
|Moisture percent in coarse aggregate (C.A.)||=1.5%|
|Amount of moisture present in C.A.||=140*(1.5/100)=2.1 litres|
|So extra amount of moisture present in C.A & F.A.||=4.2+2.1=6.3 litres|
|Net water to be added||=Total water-extra amount of water present as moisture content=30-6.3=23.7 litres|
|Step-3-Applying Correction for Bulking of fine aggregate|
|Bulking of fine aggregate||=20% of total volume of F.A.=70*(20/100)=14 litres|
|Moist fine aggregate required||=70+14=84 litres|
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