Concrete Mix Design Basics

The standard for designing a concrete mix is ACI 211.1, Proportions for Normal, Heavyweight, and Mass Concrete. Like most ACI committee reports, this document is excellent, albeit highly technical. If you want more detail, written in a more user-friendly way, get a copy of the Portland Cement Association's Design and Control of Concrete Mixtures. That's the bible for concrete mix design.

ACI 211.1 notes that the "selection of concrete proportions involves a balance between economy and requirements for placeability, strength, durability, density, and appearance." Finding the right balance to achieve all these requirements comes mainly from experience.


Placeability is an important attribute of a good mix design.
Atlanta Brick & Concrete, Atlanta, GA.

Placeability is simply the traits that the wet or plastic concrete has that allow it to be placed and finished. Placeability includes a mix that doesn't segregate but that can be consolidated. The ability to be pumped is another consideration. The classic way to measure placeability is slump, although two different mixes can have the same slump and behave very differently depending on the aggregates, the air content, and the admixtures.

Strength is nearly always specified for a concrete mix. The International Residential Code, for example, specifies that interior slabs have a minimum compressive strength of 2500 psi. In structural concrete applications, strength is indeed critical, since that's what the designer is counting on to hold up the building. For flatwork, though, strength is seldom the controlling factor and higher strength concrete mixes, that produce very stiff concrete, can result in more shrinkage, which shows up as curling and cracking.

Long term durability is at least as important as placeability and strength, but it is sometimes sacrificed if there are compromises to be made. Durability is achieved by getting a low-permeability, low-shrinkage concrete that has the proper amount and distribution of entrained air. Durable concrete must resist freeze-thaw action and prevent chloride from penetrating to the reinforcing steel to contribute to corrosion.


To retain its appearance, exterior concrete must be designed to handle freeze-thaw action.
Becker Architectural Concrete, South St. Paul, MN.

Appearance issues are obviously more important for decorative concrete than other applications. Nothing sours a client more that a beautiful floor that is cracked or spalled or a decorative wall that is honeycombed. With slabs, shrinkage is the number one concern and lower strength concrete (with less cement paste) will often shrink and curl less. And since with decorative concrete, surface hardeners will provide a high strength, more wear resistant, less permeable, surface layer, the overall concrete strength can be less.

Now let's talk about the choices you will (or should) make in concrete materials.

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