Asked how to get good quality residential concrete, Dave Shepard, director of sustainable development for the Portland Cement Association (PCA), Skokie, Ill, says, "Contract with a well-educated concrete contractor. When they have a good attitude about learning and caring for their work, everything falls into place."
Well-defined standards and guidelines exist for every residential concrete use. They are published by the American Concrete Institute (ACI), the International Residential Code (IRC), and the Concrete Foundation Association (CFA). Compromised work results when standards aren't followed by jobsite personnel or when codes aren't enforced.
Concrete Mix Basics
Concrete is made from blends of coarse aggregate (crushed rock or gravel), fine aggregate (sand), portland cement, admixtures, water, and increasingly, the addition of pozzolans-fly ash or ground granulated blast-furnace slag (referred to as "slag"). You may have heard that aggregates are "gap-graded" or "well-graded" in a mix. Residential concrete mixes tend to be less complicated so most of them are gap-graded, with aggregates that pass through one sieve size. Well-graded mixes have several coarse aggregate sizes (sometimes fine aggregates also) included. The goal is to minimize void spaces, use less cementitous material, and less water. Higher strength, greater durability, and less shrinkage result.
There are many good reasons to include pozzolans in concrete mixes. A current one is to make concrete "greener." Manufacturing portland cement is energy intensive-one ton of portland cement releases one ton of carbon dioxide. Fly ash and slag, being waste products from other industries, require only a small amount of energy for use in concrete mixes. By replacing up to 20% of the portland cement specified in a mix with pozzolans, concrete becomes greener because less carbon dioxide goes into the atmosphere. Fly ash is more popular material because it's more available throughout the country. Both products improve the strength and durability of concrete. The downside is that strength gains are slower (compressive strength usually is specified to be checked at 56 days instead of 28 days) and they retard initial setting times.
Water is the most troublesome for concrete. Most of it is "water of convenience" to make placement easier. The water content is measured as slump, water-cement ratio (w/c), and "total water." On the jobsite, testing companies measure slump by filling an approved cylindrical cone with fresh concrete. When the cone is removed, the amount that the concrete sags is measured in inches. This becomes the "slump" of the mix. A slump of 4 to 5 inches is considered to be in the proper range. The w/c ratio is the weight comparison of water to cementitious material. For residential work it should be 0.40 to 0.55 (exterior flatwork should-n't exceed 0.45). For well-graded mixes water is more accurately expressed as "total water." Good concrete has between 29 to 33 gallons of water per cubic yard.
A wide range of admixtures can be added to concrete when it is mixed. They enhance the performance characteristics of concrete by making placement easier and countering harsh weather affects during placement. The most common residential admixtures include calcium chloride to accelerate setting time, and air entrainment to provide freeze/thaw protection. In nonfreeze/thaw locations small amounts help to control bleeding. Sometimes water reducing admixtures are included to reduce the water content of a mix by at least 5%. Occasionally, superplasticizers are used to raise the "placing slump" without adding water, significantly increasing concrete strength in the process.
The problem with water is that some contractors add too much. As mentioned earlier, the best slump is between 4 and 5 inches. But job-site addition increases can be as high as 9 inches, making concrete flow long distances in foundation forms or on floor placements.When you add too much water to concrete strength decreases, shrinkage increases (resulting in more cracking), durability decreases as concrete becomes more porous and permeable, and air entrainment ratios change.
Footings are sized according to soil conditions. Some soils tolerate higher loads per square foot than others. The compressive strength requirement is usually 2000 to 3000 psi. Forms usually aren't very tight so concrete is placed at low slump.