A concrete mix design is the recipe used to make portland cement concrete. Mix designs usually are based upon volumetric measurements, but concrete is usually mixed (at least in a commercial setting) according to weight of materials. Therefore, a mix design most commonly gives directions for making the mix based upon weight.

A very simple method for mixing concrete is the "1:2:3" method. This type of concrete would use one part of cement, two parts of fine aggregate (sand), and three parts of coarse aggregate (gravel). To this mixture is added enough water to bring the concrete to the desired consistency. This will make a good concrete, but it is rather innefficient, since it tends to use more cement than is necessary.

As shown in the above example, a mix design will usually give direction as to the quantity of cement, coarse aggregate, fine aggregate, and water in a specified amount of concrete (usually a cubic yard). A mix design will also give directions as to amounts of admixtures such as air entraining agents, water reducers, or corrosion inhibiting agents. Below is described the Absolute Volume Method of concrete mix design:

The first step in creating a concrete mix design is to determine the desired compressive strength of the finished concrete. This is expressed in terms of the 28-day compressive strength; the strength the concrete will achieve in 28 days. This is usually based upon statistical data of similar mixes, or there are accepted formulas that will account for statistical variations in concrete strengths if data is not available.

After determining the design strength, a water-cement ratio is selected. The common source for this is Figure 7-1 of the Portland Cement Association's Design and Control of Concrete Mixtures, the standard manual for designing concrete. Simultaneously with this, Table 7-6 is consulted, which tabulates various water contents per cubic yard for varying levels of slump, coarse aggregate size, and air entrainment. So we now have the amount of water in the mix, and its ratio to the cement. The amount of water is divided by the water-cement ratio, yielding the weight of cement needed for the mix.

Water and cement will make the "paste" that holds the aggregates together. The aggregate selection begins by consulting Table 7-5, which relates maximum aggregate size of the coarse aggregate, and fineness modulus of the fine aggregate to show the volume of coarse aggregate per unit of volume of concrete. Common values for this will range from about 0.5 to 0.7.

After the volume of coarse aggregate has been determined, the weight of each material (water, cement, and coarse aggregate) per cubic yard of concrete is converted to volume through the use of specific gravity. The total of these is then subtracted from one cubic yard, and the difference made up using fine aggregate.

This is a general outline of the process for designing a concrete mix. There are other concerns, such as air entrainment levels, sulfate resistance, aggregate sizing, and mix water characteristics, which must also be considered. The information here comes from the Portland Cement Association's publication Design and Control of Concrete Mixtures. It covers mix design, as well as concerns for placement and usage of concrete.

The Portland Cement Association (PCA), can be contacted at:
Portland Cement Association
5420 Old Orchard Road
Skokie, IL 60077-1083

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