To ask why concrete should be divided into strength grades, we should first know what is concrete strength grade. Concrete strength grade is artificially divided according to the concrete cube compressive strength value, it is the characteristic strength of concrete. According to the current rules and regulations of relevant standards, cube compressive strength of concrete is made according to standard methods for the standard size of 150 ㎜ cube specimens, consistent with the ISO test method for temperature of 20 2 ℃, humidity is more than 95% of the standard curing room, maintenance to 28 d age, according to the standard test method for compressive strength of concrete cube.

According to the relevant standards, the strength grade of building materials should be expressed by the material name plus its strength standard value. Therefore, the Concrete strength grade is divided into C10, C15, C20, C25, C30, C35, C40, C45, C50, C55, C60, C65, C70, C75, C80 by the symbol C(Abbreviation of English Concrete) and the standard value of the cube compressive strength behind it.

Because the load of the concrete in different parts of the general engineering is not the same, some big, some small, can not completely use a strength grade of concrete. To bear the place with big pressure, be about to use high intensity grade concrete, to bear the place with little pressure, be about to use the concrete of low intensity grade, accordingly, concrete is divided into different intensity grade, in order to get used to the need of different project, choose through design calculation.