There are two main reasons for this problem. One is that the raw materials of concrete have changed, and the other is that the concrete engineering is complex and changeable, and the technical indicators of concrete requirements have changed.
The cement and other cementitious materials used in the concrete production process will fluctuate more or less in different batches even if they are of the same strength grade and the same manufacturer. The cement strength or mineral admixture activity index is higher or lower than the test mix ratio. The values are all normal. If the original mix ratio is used, it may cause intensity fluctuations. How to overcome the fluctuation of concrete raw materials to ensure the quality of concrete and truly achieve the design requirement of ≥95% concrete strength guarantee rate is a problem that the laboratory technicians face and need to solve.
In the concrete production process, new raw materials or concrete with special requirements are often rushed for production, and the concrete mix design and test time are insufficient. It is still necessary to verify the workability and short-life strength of concrete before production. For the later strength, such as 28d or longer-age strength, impermeability, frost resistance, electric flux and other durability indicators, it can be verified during production. When designing production mix ratio parameters, sufficient surplus should be ensured to ensure a sufficient qualification rate, and then analysis and optimization should be carried out based on production and test data.
When new materials or concrete with special requirements are rushed for production, due to the lack of test data support, the design of concrete mix ratio needs to rely on past practice and test experience. When using new materials for production, concrete technicians can initially determine the mixing ratio of new materials for production based on the existing raw materials. For concrete with special requirements, concrete technicians can infer (or calculate in accordance with specifications) that after adding special performance requirements based on the test results of ordinary concrete mix ratio, the change trend of concrete mix ratio parameters will adjust the existing production mix ratio for production, and test, Verify the technical indicators of the produced concrete.