The concrete mix ratio refers to the proportional relationship between the constituent materials in the concrete.  First of all, we need to know what substances are contained in concrete. The four materials of water, cement, sand, and gravel are the most basic components of concrete. Secondly, we need to choose according to the purpose of the concrete. conventional grade-c20-c25-c30-concrete mix ratio, the following design methods are carried out.

Basic requirements for designing concrete mix ratios:

1. Meet the strength grade of concrete design.
2. Concrete and workability to meet construction requirements.
3. To meet the durability requirements of concrete use.
4. Meet the above conditions to save cement and reduce concrete costs. Concrete is divided into several strength grades according to strength, and the strength grade of concrete is strongly divided by compression. The strength of concrete is divided into twelve grades such as C7.5, C10, C15, C20, C25, C30, C35, C40, C45, C50, C55 and C60.

Concrete mix ratios are expressed in two ways: one is expressed in materials used per cubic meter of concrete (Water: Cement: Sand: Gravel), and the other is expressed as a ratio of various materials. for example, C:S:G = 1:2.3:4.2, and W/C = 0.6.

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

## Concrete strength and its standard value symbol

The strength of building materials is expressed by “f”. The concrete cube has a compressive strength of “fcu”. Among them, “cu” means cube. The standard value of the cubic compressive strength is expressed by “fcu,k”, where “k” means the standard value.

## Concrete preparation strength calculation

fcu, 0 ≥fcu, k +1.645σ

σ—the standard deviation of concrete strength (N/mm²).  σ = 5.00 (N / mm²);

fcu,σ  —— concrete preparation strength (N / mm²);

fcu,k ——standard value of compressive strength of concrete cube (N/mm²);

## strength

C15
C20
C25
C30
C35
C40
C45
C50
C55
C60
C65
C70
C75
C80
fcu,k
10.0
13.4
16.7
20.1
23.4
26.8
29.6
32.4
35.5
38.5
41.5
44.5
47.4
50.2

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