**Concrete strength grade selection**

**Concrete strength grade selection**

**Concrete strength grade**

In accordance with the national standard GB 50010-2002 “Concrete Structure Design Code”, the strength grade of concrete should be determined by the standard value of cubic compressive strength. Cube compressive strength standard value refers to the standard method of production and maintenance of the side length of 150mm cube specimen, measured at 28d age with a standard test method with 95% guarantee of compressive strength, to fcu, k said.

Ordinary concrete is divided into fourteen strength classes: C15, C20, C25, C30, C35, C40, C45, C50, C55, C60, C65, C70, C75 and C80. concrete strength class is an important basis for concrete structure design, construction quality control and project acceptance. Different construction projects and building parts need to use different strength grades of concrete, generally have a certain range of choice.

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**Concrete strength grade selection range**

Different construction projects, different parts often use different strength grades of concrete, in the current level of concrete engineering in China, the general range of choice is as follows.

① C10 ~ C15 – for the mat, foundation, flooring and the structure of the force is not large.

②C20 ~ C25 – for beams, slabs, columns, stairs, roofing and other ordinary reinforced concrete structures.

③C25～C30 – for large-span structures, structures requiring high durability, prefabricated elements, etc.

④C40～C45 – for prestressed reinforced concrete members, crane beams and special structures, etc., for 25～30 storeys.

⑤C50～C60 – for high-rise buildings of 30 to 60 stories or more.

⑥C60～C80 – for high-rise buildings with high performance concrete.

⑦C80 ~ C120 – using ultra-high strength concrete in high-rise buildings.

In the future, the use of concrete up to C130 or more may be promoted. Concrete is divided into several strength levels according to strength, the strength level of concrete is divided by the standard value of cubic compressive strength fcu,k. The standard value of cubic compressive strength is a value in the overall distribution of cubic compressive strength, the strength is not more than 5% of the worthy percentage, that is, there is a 95% guarantee rate.

**Concrete ratio**

Concrete ratio refers to the proportional relationship between the constituent materials (cement, water, sand, stone) in concrete. There are two ways to express: one is to 1 cubic meter of concrete in the amount of various materials, such as cement 300 kg, water 180 kg, sand 690 kg, stone 1260 kg; another is to use a unit mass of cement and the ratio of the amount of various materials and the water-cement ratio of concrete to express, for example, the former example can be written as: C:S:G = 1:2.3:4.2, W / C = 0.6.

①. Commonly used grade

C20

Water: 175kg Cement: 343kg Sand: 621kg Stone: 1261kg

Matching ratio is: 0.51:1:1.81:3.68

C25

Water: 175kg Cement: 398kg Sand: 566kg Stone: 1261kg

Mixing ratio: 0.44:1:1.42:3.17

C30

Water: 175kg Cement: 461kg Sand: 512kg Stone: 1252kg

Mixing ratio: 0.38:1:1.11:2.72

②. Normal concrete ratio reference:

C20 concrete ratio

Conditions: slump 35–50mm; sand type: coarse sand, formulation strength: 28.2MPa; stone: river stone; maximum particle size: 31.5mm; cement strength grade 32.5, actual strength 35.0MPa .

C20 concrete ratio (weight ratio): cement: sand: gravel: water = 1: 1.83: 4.09: 0.50

In each cubic meter of concrete, cement content: 326Kg; sand content: 598Kg; crushed stone: 1332Kg

C25 concrete ratio

Conditions: slump 35–50mm; sand type: coarse sand, formulated strength: 28.2MPa; stone: river stone (pebble); maximum particle size: 31.5mm; cement strength grade 32.5, actual strength 35.0MPa .

C25 concrete mix ratio (weight ratio): cement: sand: gravel: water = 1:1.48:3.63:0.44

In each cubic meter of concrete, the cement content: 370Kg; sand content: 549Kg; crushed stone: 1344Kg

In the actual construction process, the matching ratio of mortar and concrete will vary according to the construction conditions and material conditions, and should be adjusted on site according to the actual situation. Therefore, the above ratio is only a reference value. But the magnitude of change will not be too big.

**Cement mortar ratio**

Matching ratio of M5 cement mortar.

Conditions: construction level, general; sand, medium sand; sand moisture content: 2.5%; actual strength of cement: 32.5 MPa

M5 mix ratio (weight ratio): cement: medium sand = 1:5.23. In each cubic meter of brick masonry, the required M5 cement mortar is 0.238 m3, of which 67.59 Kg of cement; 354 Kg of medium sand (0.26 m3)

The ratio of M7.5 cement mortar.

Conditions: construction level, general; sand, medium sand; sand moisture content: 2.5%; actual strength of cement: 32.5 MPa

M7.5 ratio (weight ratio) cement: sand = 1: 4.82. Each cubic meter of brick masonry requires M7.5 cement mortar is 0.251 m3, of which 77.31 Kg of cement; 373 Kg of sand (0.27 m3)

**The construction of concrete grade selection**

**The construction of concrete grade selection**

**The influence of concrete grade on column, shear wall, beam and floor slab**

The effect of concrete grade on columns and shear walls (axial pressure ratio control): raising the grade can significantly reduce the size of columns and walls and increase the actual usage rate of the building.

Effect of concrete grade on beams: Under normal circumstances, it has almost no effect on the bearing capacity of beams, so it has little effect on the cross-section and reinforcement of beams, and it is not advisable to use high grades.

Effect of concrete grade on floor slabs: Under normal circumstances, it has almost no effect on the bearing capacity of the slab, but it may increase the structural reinforcement rate of the slab and increase the potential risk of cracking of the slab, so a low grade should be used as far as possible.

**The choice of concrete grade in the actual project is recommended**

Ordinary structural beam concrete grade is generally C25, C20. C30 can be used for the concrete grade of beams and slabs under greater stress, such as the basement floor and roof slabs, roof garden slabs, etc.; shear walls, columns concrete grade is controlled by the axial pressure ratio, so that it is as close as possible to the upper limit of the axial pressure ratio, and at the same time, the majority of vertical members for structural reinforcement.

General beams and columns with a grade, columns and walls will have to look at the calculation. Multi-story best not to use c40 and above, because c40 can not be mixed on site. Generally speaking, the residential beams and slabs c25 on the line. If the large public buildings are all c30, basement roof and sidewalls are c30. The above are common practices.

For the general multi-storey structure, if the building is not particularly tough on the vertical component cross-sectional size, we do not particularly want to use high-grade concrete, generally use C25-C40, commonly used C30-C40. concrete grade, although to a certain extent to reduce the amount of reinforcement, but the efficiency of this reduction compared to the cost of concrete and construction quality, the level of requirements of the increase in cost. The economic significance of the increase in cost brought about seems to be small. Therefore, unless there are special circumstances, neither the A, nor the designer, nor the builder is willing to use high-grade concrete.

From the mechanical point of view, for multi-story structure, the axial pressure ratio of column is not very high, so it is not very meaningful to increase fc to improve the bearing capacity of members; as beam is a bending member, the economic benefit of increasing fc is not very obvious under the premise that the height of beam can be guaranteed; in general, increasing the strength grade is not pleasing to the eye.

However, with the increase of structural layers, for high-rise and super high-rise structures, usually the axial pressure ratio of vertical members – columns and walls at the ground floor will be higher, at this time, the increase of fc will reduce the axial pressure ratio of vertical members, reduce the cross-sectional size of members, reduce the reinforcement and other benefits. members at the ground floor. For horizontal members – beams, the same reason as above, usually up to C40. for beams to improve the fc will have a little contribution to the bearing capacity, but the load may be deformation, crack control, and coupled with the ductility of high-strength concrete members is also poor, need to add restraint reinforcement to improve the ductility, so high-strength concrete The benefits will be greatly reduced or even outweighed by the benefits.