Everyone knows that concrete is composed of four basic ingredients: portland cement, large aggregate, small aggregate, and water. When a contractor orders concrete, he typically specifies the number of bags of cement that he wants per cubic yard with the understanding that he will obtain the desired strength and properties. The amount of gravel (large aggregate) and sand (small agggregate) are often at the discretion of the supplier. Water content, the other key ingredient, typically is a function of the slump.
Beyond the basics, admixtures often are added to the mix to alter the basic mechanical properties or to affect changes to the desired characteristics of the mix. These include retarders, accelerators, water reducers, and a host of other admixtures. A percentage of the portland cement also can be replaced with byproducts, such as slag or fly ash, resulting in what is called a blended cement. With all of the possible cmbinations and permutations, it can be confusing. Adding to mix variability are different cement types, cement from different manufacturers, and of course, the aggregates comprising 60% to 75% of the mix, which vary in strength, porosity, and relative size.
There are five basic parameters for which concrete is mixed: workability, consistency, density, strength, and durability. The parameters that typically are involved in design for a residential wall are: workability, the property that determines the capacity to be placed and consolidated without harmful segregation; strength, the capacity of concrete to resist compression and tension at 28 days; and, consistency, the relative mobility of the concrete that is measured in slump. Shrinkage and economy are additional considerations that are important in residential mix designs.
Fortunately, the design process has been streamlined. Most readymix companies can tell you what properties are found in their most common mixes. You simply tell them your conditions and desired results and they will commend a mix.
Concrete mixtures typically are proportioned or designed through the cooperation of a ready-mix and cement supplier and a testing lab specializing in concrete. Mix design is based on knowledge of the materials, experience, and testing. If job conditions are different from lab conditions, the trials should extend to the field to replicate job conditions. The end result is a mix design in which the quantity of each component is specified per cubic yard of concrete. Cement is specified by type and manufacturer. Fine aggregate (dry weight) as well as the source, coarse aggregate including maximum size and source, and water, are specified in pounds. Air entrainment and chemical admixtures, if used, will be specified in their appropriate units. It is important to note that components such as fine aggregate may have different weights if they are stored outside (which is typical) where they may hold moisture. This simple fact can change the amount of water and fine aggregate in the mix.
After testing and verification, the laboratory typically will report on a slump range, unit weight, air content, strength obtained at a stipulated time of curing, and the temperature range in which the tests were performed.
Fortunately, there are volumes of research compiled to shorten your work. An example includes the work done by the Concrete Foundations Association on design mixes for cold-weather applications. The association and its consultants designed 44 mixes, tested more than 800 refrigerated and frozen cylinders, and produced more than a dozen full-sized walls in frozen field conditions to develop mix designs that would give multiple options for wall contractors in cold climates. The results from this research alone have reshaped the thinking of how mix design impacts cold-weather performance along with validating the importance of maturity testing and prediction as the best method for determining concrete strength and performance.
For more information on mix design research for residential applications, visit www.cfawalls.org
- Ed Sauter is the executive director of the Tilt-Up Concrete Association. He can be reached at 319-895-6911 or email@example.com