Why is the mix design sheet important in operations?

According to the ABNT (Brazilian Association of Technical Standards) standard NBR 7212 of 04/2021, Ready-mixed concrete - Preparation, supply and control, the mix design sheet serves to record the materials that make up the formulation (recipe or mix design) of a concrete on the date it was created. The technical manager prepares the recipe and formalizes it through a document called a MIX DESIGN SHEET, after analyzing each component (cement, gravel, sand, water, admixtures and other additions) separately.

On large projects, throughout the execution period, the characteristics of the concrete components may change, and these changes must be reported to the client according to the prior agreement. Anyone who hires a concreting services company has the right to request the composition of the initial mix design and to be notified when there are changes to the quantities of the materials used in the concrete.

The concreting services company must present this information through the mix design sheet, which must contain at least:

• date the mix design sheet was created;
• mix design identification code;
• concrete specifications;
• materials used and their respective absolute specific masses;
• input suppliers;
• mass quantity of each concrete component;
• expected air content;
• specific mass of the concrete in its fresh state;
• signature of the technical manager.

What is a concrete mix design?

Concrete is the product of mixing cement, water, stone and sand. When the cement is hydrated by water, it forms a strong paste that adheres to the aggregate fragments (stone and sand), resulting in a monolithic block.

The proportion between all the materials that make up the concrete is known as dosing or mix design. Concrete with special characteristics can be obtained by adding admixtures, polystyrene, pigments, fibers or other types of additions to the mix. Each material to be used in the dosing must be analyzed beforehand in a laboratory (in accordance with ABNT standards) in order to verify quality and obtain the data needed to develop the mix design (specific mass, particle size, etc.).

When defining the mix design, care must be taken with the quality and quantity of the water used, as it is responsible for triggering the chemical reaction that transforms the cement into a binding paste. If the quantity is too little, the reaction will not be complete; if it is higher than ideal, strength will decrease due to the pores that form when this excess evaporates.

The importance of dosing

Dosing is one of the most important stages in concrete production, so much so that it appears in the title of standard NBR 7212 (Ready-mixed concrete). This standard establishes the basic requirements for the production of ready-mixed concrete and defines procedures for the various stages of the process, including the storage of the concrete's constituent materials, dosing, mixing, transport, concrete receiving, quality control, inspection and acceptance, among others.

Considering dosing as the proportioning by mass or volume of the materials to obtain the concrete, a dosing study must be carried out to achieve the best result. This depends on the analysis of each material used in the concrete, followed by the production of concrete in a laboratory, with different proportions of each of these materials, until reaching the proportioning with the best cost-benefit ratio, thus defining the final composition of the concrete (dosing / mix design / recipe).

Dosing can also be understood as the quantification of the concrete's component materials at the time of production, that is, placing the correct proportions of each material inside the mixer truck.

The weighing systems for the concrete's component materials in production should preferably be automated. Automation must perform operations with a degree of precision appropriate to the standard, keep weighing records to ensure load traceability, and record the differences between the requested quantity and the amount actually dosed for each component material of the mix design. In addition, it must alert to unwanted deviations that occur during dosing, something that is almost impossible to achieve manually.

For this, you can count on Kmix, a platform created by Kartrak to make everything easier and more precise.