Yield of Concrete
There are various factors which affect the yield of concrete for a given proportion of concrete ingredients, such as
- Water-cement ratio
- Size of aggregate
- Method of compaction etc.
Yield of concrete simply means the actual volume of concrete produced when a given proportion of concrete ingredients are mixed in a certain manner.
There exist two methods to calculate yield of concrete, i.e.
- By rule of thumb
- By absolute volume of ingredients
1. By Rule of Thumb
The rule of thumb may be used to estimate the approximate yield of concrete from a given concrete mix, when batched by volume.
If the proportion of concrete is a:b:c i.e., if a parts of cement, b parts of sand and c parts of coarse aggregate are mixed by volume, the resulting concrete will have a volume of (2/3)*(a+b+c).
2. By Absolute Volume of Ingredients
Yield of concrete can be calculated more accurately by considering the absolute volumes of various ingredients used in concrete (cement, sand, coarse aggregate & water) with volume of entrapped air. When concrete is well compacted, the volume of entrapped air is less than 1% and therefore, it can be neglected.
Let w, a, b and c be absolute volumes of water, cement, fine aggregate and coarse aggregate respectively.
If we consider final volume of concrete produced will be 1 then it means,
w + a + b + c = 1
The value of absolute volume can be obtained by the relation:
Absolute volume = weight of the materials / (apparent specific gravity * unit weight of water)
Estimate the yield of concrete per bag of cement for a concrete mix of proportion (1:2:4).
By Rule of Thumb
A bag of cement containing 50 kg of cement is considered to have a volume of 0.035 m3.
Therefore a = 0.035 m3
Then volume of sand becomes, b = 2*0.035 m3
And volume of coarse aggregate becomes, c = 4*0.035 m3
So yield of concrete of proportion (1:2:4)
= (2/3)*(0.035 + 2*0.035 + 4*0.035)
= 0.163 m3
By Absolute Volume Method
Before calculating the absolute volume of different ingredients of concrete following assumptions are made.
- Specific gravity of cement is taken as 3.15
- Specific gravity of sand is taken as 2.65 and unit weight is taken as 16 kN/m3.
- Specific gravity of coarse aggregate is taken as 2.80 and unit weight is taken as 15 kN/m3.
- Water cement ratio is 0.6
- Unit weight of water is 10 kN/m3
Using the formula of absolute volume calculate, the volume of each components of concrete
|Absolute volume of cement (a)
= 0.02 m3
|Absolute volume of sand (b)
||= 2 * 0.035 *16 /(2.65*10)
= 0.04 m3
|Absolute volume of coarse aggregate (c)
= 0.07 m3
|Absolute volume of water (w)
= 0.03 m3
|Total volume of concrete produced i.e.
Yield of Concrete
= 0.16 m3
Notes to Remember
- If ingredients are already tested for specific gravity and unit weights, then that values can be used to find out more accurate results.
- In this example calculation water cement ratio is taken as 0.6. if you are using any other water cement ratio then replace 0.60 with that value while calculating absolute volume of water.
- Never use 0.035 if each bag of cement do not weigh 50kg.