The Volume Ratio for Mixing Concrete

Ranges

• Making a concrete mix doesn't follow one set ratio; the amount of each ingredient varies depending on the type of application. Usually concrete mixes are between 7 percent to 15 percent cement, 14 percent to 21 percent water and 60 percent to 80 percent aggregate, by volume. The ratio helps determine the properties of the final product, so engineers might vary it depending on what properties are most important for a particular job.

Calculations

• You can calculate a volume ratio in a couple of ways. Suppose you start out with a weight ratio, e.g., 1:2:2. In this case, you would divide each of the numbers in the weight ratio by the density of that material to get the volume ratio. Water's density is 61.998 lbs. per cubic foot and the density of the other ingredients varies depending on the product.

Percentages

• Another way to calculate the volume ratio is based on the percentages. If you know the product should be 10 percent cement, 15 percent water and 75 percent aggregate, for example, your starting ratio is 10:15:75. You can now simplify this ratio by dividing each number by the first number in the ratio or by a common denominator. Dividing by the number five in this case, for example, would give you 2:3:15.

Considerations

• The water/cement ratio is the most important part. A high water-cement ratio is more workable and flows more readily, so it's easier to pump and pour. A low water/cement ratio, on the other hand, makes the product stronger. In some applications, workability might be the more important factor, whereas in others strength is critical. Engineers usually prefer to proportion by weight because proportioning by volume is less accurate, although you can do this also if you prefer.