The concrete mix ratio refers to the proportion of the components in the concrete. The concrete mix ratio is usually represented by the quality of various materials per cubic meter of concrete, or amount proportion of various materials.

**The basic requirements for the design of concrete mix ratio are:**

1, meet the strength grade of concrete design.

2, meet the concrete peaceability.

3, meet the durability of the use of concrete.

4, meet the above conditions to save cement and reduce the cost of concrete.

**The strength of concrete is divided into twelve grades, such as C7.5, C10, C15, C20, C25, C30, C35, C40, C45, C50, C55, C60 and so on.**

The concrete mix ratio refers to the proportion of the components in concrete (cement, water, sand, and stone). There are two methods: one is to use a variety of materials amount per 1 cubic meters, such as cement 300 kg, water 180 kg, sand 690 kg, gravel 1260 kg; the other is that the water cement ratio and cement ratio and concrete content of different materials per unit mass of the cases, such as precedent can be written as: C:S:G=1:2.3:4.2, W/C=0.6.

**Common grade:**

**C20**

Water: 175KG

cement: 343kg

sand: 621kg

stone: 1261kg

The mix ratio is: 0.51:1:1.81:3.68

**C25**

Water: 175KG

cement: 398kg

sand: 566kg

stone: 1261kg

The mix ratio is: 0.44:1:1.42:3.17

**C30**

Water: 175KG

cement: 461kg

sand: 512kg

stone: 1252kg

The mix ratio is: 0.38:1:1.11:2.72

**Calculation of concrete compounding strength:**

**fcu, 0≥fcu，k+1.645σ**

In which:

σ——Standard deviation of concrete strength（N/mm2) σ=5.00（N/mm2）;

fcu, 0——Compounding strength of concrete（N/mm2）;

fcu, k——Standard value of compressive strength of concrete cubes（N/mm2）, fcu，k=20（N/mm2）;

After calculation: fcu,0=20+1.645×5.00=28.23（N/mm2）