First, concrete classification

 

Concrete can be divided into apparent density according to apparent density: heavy concrete, ordinary concrete and light concrete. It can also be divided according to the intensity level. For example, we often hear C20 and C30 as the strength grade, C60 below is called ordinary concrete, C60~C100 is high-strength concrete, and C100 or above is called super-high-strength concrete.

 

The fourteen strength grades commonly used in general engineering are C15, C20, C25, C30, C35, C40, C45, C50, C55, C60, C65, C70, C75 and C80. The strength level is determined by the standard value of the cubic compressive strength. Concrete strength grade is also an important basis for concrete structure design, construction quality control and engineering acceptance. Different engineering structures and different parts of the building should require the use of concrete of different strength grades.

 

 

1. Selection range of concrete strength grade

 

At present, in China's engineering construction, the general selection of concrete is as follows: 1C10 ~ C15 - for cushions, foundations, floors and structures with little force; 2C20 ~ C25 - for beams, slabs, columns, stairs, Ordinary reinforced concrete structure such as roof truss; 3C25～C30——for large-span structures, structures requiring high durability, prefabricated components, etc.; 4C40～C45—for prestressed reinforced concrete members, crane beams and special structures, etc. 25 to 30 layers; 5C50 to C60 - for high-rise buildings of 30 to 60 stories; 6C60 to C80 - for high-rise buildings, using high-performance concrete;

 

The strength grade of concrete is graded according to the standard value of cubic compressive strength (in MPa) fcu. The standard value of the compressive strength of the cube is a value in the overall distribution of the cubic compressive strength, and the strength is less than 5% of the percentage of the value, that is, a 95% guarantee rate. Concrete with a strength class of C30 means 30 MPa ≤ fcu < 35 MPa. Generally, the main concrete used in rural self-built houses is C15~C30.

 

 

Second, the concrete mix ratio

 

The concrete mix ratio refers to the proportional relationship between the various constituent materials (cement, water, sand, stone) in concrete. There are two ways to express: one is the amount of various materials in 1 cubic meter of concrete, such as 300 kg of cement, 180 kg of water, 690 kg of sand, and 1,260 kg of stone; the other is the use of cement and various materials per unit mass. The ratio of the amount of use and the water-cement ratio of concrete are shown, for example, the previous example can be written as: C: S: G = 1: 2.3: 4.2, W / C = 0.6.

 

 

Third, the actual construction of the concrete label selection

 

The influence of the concrete marking on the column and the shear wall (axial pressure ratio control): increasing the number can significantly reduce the size of the column wall and increase the actual utilization rate of the building. The influence of the concrete marking on the beam: under normal circumstances, it has almost no influence on the bearing capacity of the beam, so the influence on the section and reinforcement of the beam is small, and high marking is not suitable.