Compressive strength and grade of concrete in structures

The paper summarizes the experience of the Department of Buildings and Facilities Structures Research in determining the compressive strength and grade of concrete in the structures, which characterize one of the main requirements for ensuring the structures mechanical strength and stability. Their unambiguous interpretation at the stages of concrete composition selection, products manufacture and structures operation leads to a conflict of interests and corruption risks between a concrete producer, a builder and an investor. Two approaches to the concrete strength and grade assessment are considered: the first one reasons from the economic interests of a concrete producer (the possibility of cement saving at a stable well-organized production facility), the second one takes into account the consumer's point of view (design indicators ensuring). The first approach is based on the coefficient of the tested control concrete cubes strength variation declared by the concrete mixture manufacturer. The calculation of the relationship between the average and characteristic compressive resistance of concrete at various coefficients of concrete compressive strength variation and grades showed that this coefficient can significantly change the assessment results. The second approach is based on the use of reference core samples cut directly from the structure, which are tested and interpreted according to established international experience. The difference in strength assessments is shown for the cases with the use of samples tests results selection compared to groups in which the smallest values are removed. Both approaches are analyzed based on the experience of determining the concrete compressive strength grade using the core samples from the entire floor slab in the existing structure and from some its areas; the results were far from straightforward and not consistent with the project. It is necessary to clearly define the areas of application of norms and standards that, firstly, serve the technology and the production market of concrete and concrete and reinforced concrete products, and secondly, ensure obtaining the actual characteristics of existing products, structures and facilities and their conformity to the project. It is advisable, especially for the structures of the higher levels of responsibility and in some controversial matters, to verify the results obtained with the first approach application to the concrete mix test specimens by testing core samples cut from the structures.