The number of bags of cement required for one cubic metre (1 m3) of concrete depends on the mix ratio being used for the concrete and factors such as entrained air, wastage, and shrinkage. The following processes apply assuming we are using a mix ratio of 1: 2: 4.
Step 1: Specify the density of the cement, fine aggregate, and coarse aggregate
Density of cement = 1440 kg/m3
Density of fine aggregate = 1600 kg/m3
Density of coarse aggregate = 1650 kg/m3
Step 2: Calculate the sum of the concrete mix ratio
Sum of ratio = 1 + 2 + 4 = 7
Considering 2% of entrained air
2% of 1 m3 = (2/100) x 1 = 0.02 m3
Considering 10% of waste
10% of 1 m3 = (10/100) x 1 = 0.1 m3
Considering 33% of shrinkage
35% of 1 m3 = (35/100) x 1 = 0.35 m3
Step 3: Calculate the volume of materials
Volume of cement = (1/7) x 1.47 = 0.21 m3
Volume of fine aggregate = (2/7) x 1.47 = 0.42 m3
Volume of coarse aggregate = (4/7) x 1.47 = 0.84 m3
Step 4: Determine the mass of quantities from the volume (Mass = density x volume)
Mass of cement = 0.21 x 1440 = 302.4 kg
Mass of fine aggregate = 0.42 x 1600 = 672 kg
Mass of coarse aggregate = 0.84 x 1650 = 1386 kg
Step 5: Determine the number of bags of cement, tonnage of fine aggregate and coarse aggregate
In 1 m3 of concrete, we have 302.4 kg of cement = 302.4/50 = 6.05 bags, say 6.1 bags
In 1 m3 of concrete, we have 672 kg of fine aggregate = 672/1000 = 0.672 ton, say 0.7 ton
In 1 m3 of concrete, we have 1386 kg of coarse aggregate = 1386/1000 = 1.386 ton, say 1.4 ton
We can directly calculate the tonnage of fine aggregate and coarse aggregate from the volume of the two constituents using the relation: 1 m3 = 1.344 ton (the value may be affected by the density of the material).
Fine aggregate = 0.42 m3 = 0.42 x 1.344 = 0.565 ton
Coarse aggregate = 0.84 m3 = 0.84 x 1.344 = 1.13 ton
Observation: the values are lower than the ones specified through weight due to the factor of the densities of the materials, hence, it is ideal to specify through weight.
Step 6: Specify the number of trips
Fine and coarse aggregates are usually brought to the site using trucks. Each full truckload is called a trip. To determine the number of trips required for a 1 m3 of concrete depends on the tonnage of the truck, say 7-ton truck, 30-ton truck, etc. Simply divide the calculated tonnage for the material by the tonnage of the truck to be used. For instance, the calculated tonnage for fine aggregate was 0.7 tons. Assuming we intend to bring the material to the site using a 7-ton truck, the number of trips would be 0.7/7 = 0.1 trip which would not fill the truck, though.
In reality, large volumes of concrete are required for work. If 20 m3 of concrete is required, the tonnage of fine aggregate required = 20 x 0.7 = 14 tons. The number of trips required with a 7-ton truck would be 14/7 = 2 trips.
To calculate the number of bags of cement for any volume of concrete and mix ratio, visit: CONCRETE CALCULATOR