Typical Concrete Mix Design

Design Basics

• When designing a concrete mix, most construction workers use the American Concrete Institute's report entitled "Proportions for Normal, Heavyweight, and Mass Concrete." This report gives the proportions of materials to use for a variety of circumstances and uses. For example, the report details different mixes that take into account considerations, such as weather, durability, strength and appearance. Contractors must also consider the placability of the concrete -- the ability of the concrete to be poured and to resist separating -- and slump, which is a measurement of the consistency of the concrete and its ability to remain in place.

Air

• Air is an important component of concrete. Each cubic foot of concrete contains millions of tiny pockets of trapped air. These air pockets expand and contract with changes in temperature, relieving pressure on the concrete. The air pockets also provide space for trapped water to expand so the concrete does not crack in freezing weather. The air content in concrete is altered by using "air-entrained" Portland cement or other agents that help trap air as the concrete is being mixed. The amount of trapped air in a concrete mix is generally between 4 and 7 percent of the total volume, but this varies, depending on temperature and what the concrete will be used for.

Designing Concrete

• When contractors design a concrete mix, they generally begin by choosing the size of aggregate needed. Larger aggregates prevent shrinkage and curling better than smaller-sized aggregates. Often, contractors will use a mixture of coarse and fine aggregate. The contractor then estimates the air and water content by using the guidelines in the American Concrete Institute's tables. These tables will also allow the contractor to select the water to cement ratio for each particular project. For example, a water-cement ratio below 0.50 will improve concrete durability and strength. This ratio will determine the amount of water and cement to add to the aggregate. The amount of water can then be adjusted, depending on whether the aggregate is wet or dry. Before pouring the cement, you should always mix up a trial batch to make sure you have the correct proportions for your particular job.

Different Mixes

• A typical concrete mix uses between 60 and 80 percent aggregates, 7 to 15 percent cement, 14 to 18 percent water and 2 to 8 percent air. In hot weather, the concrete may tend to slump more. To prevent this, you can use a higher proportion of cement to water. In cold weather, you may want to add an accelerator, such as calcium chloride, to help the concrete set faster. For high performance concrete, such as that used in driveways, you need to use enough air so that if water penetrates into the concrete, it does not crack the concrete. Some contractors will use a color hardener to give the concrete a surface color and increase its hardness.