Mix design is a process of determining the right quality materials and their relative proportions to prepare concrete of desired properties like workability, strength, setting time and durability.

While following a mix design is advised to optimise the material consumption, it is not possible at site to always come up with Mix design. Nominal mix concrete is prepared by approximate proportioning of cement, sand and aggregate to obtain target compressive strength.

Mix ratio of concrete defines the ratio of cement sand and aggregate by volume in that order. So a mix ratio of 1:2:4 represents Cement: Sand: Aggregate – 1:2:4 (by Volume)


Material requirement for producing 1 Cum of Nominal Concrete Mix
The following are the materials required to produce 1 Cum of Concrete of a given Nominal Mix Proportion.


Method-1: 
 
DLBD method to determine material requirement for Nominal Concrete Mix (M20 – 1:2:4)

The DLBD (Dry Loose Bulk Densities) method is an accurate method to calculate cement, sand and aggregate for a given nominal mix concrete. This gives accurate results as it takes into account the Dry Loose Bulk Densities of materials like Sand and Aggregate which varies based on the local source of the material.

For calculation, We consider a nominal concrete mix proportion of 1:2:4 (~M20).


Step-1: 

 Calculated Volume of materials required.:-

01 bag (50 kg) of cement = 35 litres or 0.035 cubic meter (cum)

Since we know the ratio of cement to sand (1:2) and cement to aggregate (1:4)

Volume of Sand required would be = 0.035 x 2 = 0.07 cubic meter (cum)

Volume of Aggregate required would be = 0.035 x 4 = 0.14 cubic meter (cum)

Step-2: 

 Convert Volume requirement to weights:-

To convert Sand volume into weight we assume,  we need the dry loose bulk density (DLBD). This density for practical purposes has to be determined at site for arriving at the exact quantities. We can also assume the following dry loose bulk densities for calculation.

DLBD of Sand = 1600 kgs/cum

DLBD of Aggregate = 1450 Kgs/Cum

So, Sand required = 0.07 x 1600 = 112 kgs

and Aggregate required = 0.14 x 1450 =203 kgs

Considering water/cement (W/C) ratio of 0.55

We can also arrive at the Water required = 50 x 0.55 = 27.5 kg

So, One bag of cement (50 Kgs) has to be mixed with 112 kgs of Sand, 203 Kgs of aggregate and 27.5 kgs of water to produce M20 grade concrete.

Step-3: 

 Calculate Material requirement for producing 1 cum Concrete

From the above calculation, we have already got the weights of individual ingredients in concrete.

So, the weight of concrete produced with 1 Bag of cement (50 Kgs) =50 kg + 140 kg + 203 kg + 27.5 kg = 420.5 kg say 420 kgs

Considering concrete density = 2400 kg/cum,

One bag of cement and other ingredients can produce = 420/2400 = 0.175 Cum of concrete (1:2:4)

01 bag cement yield = 0.175 cum concrete with a proportion of 1:2:4

01 cum of concrete will require

Cement required = 1/0.175 = 5.72 Bags

Sand required = 140/0.175 = 800 Kgs

Aggregate required = 203/0.175 = 1160 kgs


Method-2:
 
Empirical method to determine material requirement for Nominal Concrete Mix:-

Although empirical method is easy to use in determining the materials requirement for Nominal Concrete mix, it sometimes doesnt give accurate results as it doesn’t take into factor the local variations in the materials.
Let’s design M20 grade concrete. Ratio for M20 concrete is 1 : 2 : 4

Step-1: 

 Calculate the Volumes of material required in 1 Cum concrete

The dry volume of concrete mixture is always greater than the wet volume. The ratio of dry volume to the wet volume of concrete is 1.54.

So 1.54 Cum of dry materials (cement, sand and aggregate) is required to produce 1 Cum of concrete

Volume of Cement required = 1/(1+2+4) X 1.54 = 1/7 X 1.54 = 0.22 Cum

Volume of Sand required = 2/7 X 1.54 = 0.44 Cum or 15.53 cft

Volume of Aggregate required = 4/7 X 1.54 = 0.88 Cum or 31.05 cft

Note: 1 cubic meter = 35.29 cubic feet.

Step-2: 

 Calculate the weights of materials required in 1 Cum concrete:-

Density of Cement (loose) = 1440 kgs/cum

So weight of cement required = 1440 X 0.22 = 316.2 Kgs or 6.32 bags

Density of Sand = 1600 Kgs/cum

Weight of Sand required  = 1600 X 0.44 = 704 kgs

Density of Aggregate = 1450 kgs/cum


Weight of aggregate required = 1450 X 0.88 = 1,276 Kgs