The mix ratio for concrete is typically expressed in terms of the proportions of cement, sand, coarse aggregates and water, sometimes with additives like reducers or admixtures. The mix ratio varies depending on the strength and durability requirements of the concrete, as well as factors like the size and shape of aggregates used.
British Standard (BS) provides specifications for various concrete grades. Common grades include C12/15, C16/20, C20/25, C25/30, C30/35, C35, C40, etc. The numbers represent the compressive strength of the concrete in megapascals (MPa) at 28 days.
Concrete grade C12/15 means a characteristic compressive strength of 12 MPa when tested with a cylinder of 15 cm diameter and 30 cm long and 15 MPa when tested with a cube of 15 cm × 15 cm × 15 cm after 28 days.
Concrete grade C16/20 means a characteristic compressive strength of 16 MPa when tested with a cylinder of 15 cm diameter and 30 cm long and 20 MPa when tested with a cube of 15 cm × 15 cm × 15 cm after 28 days.
Concrete grade C20/25 means a characteristic compressive strength of 20 MPa when tested with a cylinder of 15 cm diameter and 30 cm long and 25 MPa when tested with a cube of 15 cm × 15 cm × 15 cm after 28 days.
Concrete grade C25/30 means a characteristic compressive strength of 25 MPa when tested with a cylinder of 15 cm diameter and 30 cm long and 30 MPa when tested with a cube of 15 cm × 15 cm × 15 cm after 28 days.
Concrete grade C30/35 means a characteristic compressive strength of 30 MPa when tested with a cylinder of 15 cm diameter and 30 cm long and 35 MPa when tested with a cube of 15 cm × 15 cm × 15 cm after 28 days.
Here’s are the mix ratio for C12/15, C16/20, C20/25, C25/30 & C30/35 concrete:
- A typical mix ratio for C12/15 concrete could be 1:2:4 by volume, which means 1 part cement mix to 2 parts sand and 4 parts coarse aggregates mixed with 0.6 parts water. That is, 1 bucket of Portland cement mixed with 2 buckets of sand and 4 buckets of stone.
- A typical mix ratio for C16/20 concrete could be 1:1.5:3 by volume, which means 1 part cement mix to 1.5 parts sand and 3 parts coarse aggregates mixed with 0.55 parts water. That is, 1 bucket of Portland cement mixed with 1.5 buckets of sand and 3 buckets of stone.
- A typical mix ratio for C20/25 concrete could be 1:1:2 by volume, which means 1 part cement mix to 1 parts sand and 2 parts coarse aggregates mixed with 0.5 parts water. That is, 1 bucket of Portland cement mixed with 1 bucket of sand and 2 buckets of stone.
- A typical mix ratio for C25/30 concrete could be 1:0.75:1.5 by volume, which means 1 part cement mix to 0.75 parts sand and 1.5 parts coarse aggregates mixed with 0.45 parts water. That is, 1 bucket of Portland cement mixed with 0.75 buckets of sand and 1.5 buckets of stone.
- A typical mix ratio for C30/35 concrete could be 1:0.5:1 by volume, which means 1 part cement mix to 0.5 parts sand and 1 parts coarse aggregates mixed with 0.4 parts water. That is, 1 bucket of Portland cement mixed with 1/2 buckets of sand and 1 bucket of stone.
To make 1 m3 of C16/20 concrete at 1:1.5:3 volumetric ratio, you would need to mix 8 x 50kg bags of cement with 706 kg of sand, 1,302 kg of stone and approximately 200-220 litres of water.
To make 1 m3 of C12/15 concrete at 1:2:4 volumetric ratio, you would need to mix 7 x 50kg bags of cement with 704 kg of sand, 1,276 kg of stone and approximately 175-190 litres of water.
To make 1 m3 of C20/25 concrete at 1:1:2 volumetric ratio, you would need to mix 11 x 50kg bags of cement with 624 kg of sand, 1,194 kg of stone and approximately 220-240 litres of water.
To make 1 m3 of C25/30 concrete at 1:0.75:1.5 volumetric ratio, you would need to mix 14 x 50kg bags of cement with 0.36 m3 of sand, 0.71 m3 of stone and approximately 310 litres of water.
Here’s are the table for C12/15, C16/20, C20/25, C25/30 & C30/35 concrete:
| Concrete Grade | Concrete mix ratio |
| C12/15 | 1:2:4 |
| C16/20 | 1:1.5:3 |
| C20/25 | 1:1:2 |
| C25/30 | 1:0.75:1.5 |
| C30/35 | 1:0.5:1 |
Conclusions:
In general, a typical mix ratio for C12/15 concrete could be 1:2:4 by volume, a typical mix ratio for C16/20 concrete could be 1:1.5:3 by volume, a typical mix ratio for C20/25 concrete could be 1:1:2 by volume, and a typical mix ratio for C25/30 concrete could be 1:0.75:1.5 by volume.