Mixing concrete is a specific and deliberate process with many variables! One of the most important considerations that often becomes an oversight is the “Shear”.

Shear in concrete mixing refers to the mechanical force that causes layers or particles within the concrete mix to slide past each other. This force is crucial because it affects how well the ingredients (cement, water, aggregates, and any additives) chemically combine into a uniform, workable mix.

More specifically:

• Shear forces occur during mixing when the mixer blades or paddles move through the materials.
• These forces help break up clumps, distribute the cement evenly, and coat the aggregates thoroughly with the cement paste.
• Proper shear ensures homogeneous mixing, which is key to the concrete’s strength, workability, and durability.
• If the shear is too low, the mix may be lumpy or segregated.
• If it’s too high, it can break down the aggregates or cause excessive air entrainment, which might weaken the final concrete.

In short, shear is the internal sliding force applied to the concrete materials during mixing that helps achieve a uniform and workable concrete mix.

How do you ensure a good shear and a consistent mix in a volumetric concrete mixer?

There are factors within the type of mix design being poured, the materials used, cement type, and admixes. The most important part of a good shear, however, comes down to the auger.

• Maintenance is key: Always ensure your auger wear blades are in good repair, the sealing rubbers are not worn, and the mechanical components are working as intended.
• Auger speed: The faster the auger is spinning, the less mixing time is typically applied to your concrete. Finding the delicate balance that works best for your mix design is very important to a good shear.
• Auger Pitch/Angle: Adjusting the mix auger angle allows for greater or lower shear. Bay-Lynx recommends to typically use a 35-45 degree angle, depending on the application.

The final factor to consider is the knowledge and experience of the operator. Making adjustments on the fly is easy, but knowing exactly what concrete should look like and how it is affected by changes in temperature, water content, and mix designs takes skill. Always ensure your operators know what to look for and what the best way to approach changes in shear are.