Since the implementation of the General Code for Concrete Structures GB55008-2021, whether the foundation cushion should also comply with the provision that "the concrete strength grade of plain concrete structural components shall not be lower than C20" has been controversial. Recently, the Qingdao Municipal Bureau of Housing and Urban-Rural Development's "Technical Q&A List for the Review of Construction Drawing Design of Construction Projects (2023 Edition)" gave an official answer to this question.
Screenshot of "Qingdao Construction Engineering Construction Drawing Design Review Technical Q&A List (2023 Edition)".
The official reply once again clarified the provisions in the general regulations, which means that the foundation cushion is not a structure, and the standard of the minimum strength of structural concrete C20 does not apply to the cushion.
So, how to determine the minimum strength of foundation cushion concrete?
Since the foundation cushion belongs to the foundation division, it should refer to the current "Code for Design of Building Foundation Foundation" GB50007-2011, and the relevant provisions stipulate that the strength grade of cushion concrete should not be lower than C10.
Screenshot of Code for Design of Building Foundations GB 50007-2011
However, because there is no C10 grade for commercial concrete, for a long time, the foundation cushion has been constructed with C15 concrete, and the minimum strength requirements for cushion concrete have been increased from C10 to C15 in many local standards.
To sum up, the minimum strength of foundation cushion concrete is summarized in one sentence: national standard C10, actual C15, no need for C20.