For work requiring more than one cubic yard of material, concrete is usually ordered from a ready-mix supplier for delivery to the job site. The supplier will need to know the minimum compressive strength, the maximum aggregate size, and any special requirements such as air entrainment for added freeze-thaw durability. The supplier will then select a mix design that is appropriate for your needs. If you are mixing small batches of concrete on site, you will need to understand the basic principles of concrete mix design yourself. The proportion of dry ingredients and the ratio of water to cement are the two most important factors.
Cement and aggregates provide strength, durability, and volume stability in concrete, but too much or too little of one in relation to the other reduces quality.
■ Lean or oversanded mixes with low cement content and high aggregate proportions are harsh and have poor workability.
■ Fat or undersanded mixes with high cement content and low aggregate proportions are sticky and expensive.
Within the range of normal concrete strengths, compressive strength is inversely related to water content. That is, the more water you use, the lower the concrete strength. But increasing water content increases fluidity and workability. Since water is required for workability, and since workability is required for high-quality concrete, the low water requirements for strength and high water requirements for workability must be balanced. The ratio of water to cement is the weight of water divided by the weight of cement. Water-cement ratio affects the consistency of a concrete mix. The consistency, in turn, affects how easily the concrete can be poured, moved around in the forms, compacted, and finished. Up to a point, a mix with more water is easier to work with than one that has less water and is therefore stiffer. Too much water, though, will cause the ingredients to separate during the pouring, placing, and handling and will destroy the integrity of the concrete. Too much water also lowers strength, increases the porosity and water permeability of the cured concrete, and makes it more prone to shrinkage cracking. The trick is to use enough water to make the fresh concrete workable, but not so much that it creates weak or porous structures.
Air content for ready-mix concrete should generally be 3 to 6-1/2%, depending on the maximum aggregate size (see Figure below). Concrete that is batched on site can be made with either an air-entrained cement or an air-entraining admixture. Using an air-entrained cement will yield an air content within the proper range. When using a separate airentraining agent, carefully follow the manufacturer’s instructions to determine the correct amount to add to the mix. For job-site mixing, air-entrained cement is usually easier to work with.
It is easier to measure concrete consistency or slump than to calculate water-cement ratio. The concrete mix consistency produced by adding various amounts of water is measured by slump tests in which fresh concrete is poured into a special mold called a slump cone. You can buy one from a building supply yard. Place the concrete into the cone in three layers. Tamp each layer with a metal rod to assure that it is completely consolidated and does not contain air pockets. When the cone is full, scrape off any excess concrete, leaving a level top. Then remove the cone and measure the amount of slump or settlement with a rod and ruler (Figure 2-11). The wetter the mix, the higher the slump measurement, and the drier the mix, the lower the slump measurement. The slump recommended to assure proper water-cement ratio for residential concrete is 3 to 5 inches. Slump tests can also be used to ensure consistent mixes from batch to batch.
As a general guideline for ordering ready-mix concrete, Figure 2-12 shows recommended mix requirements for various exposure conditions. The weathering regions indicated on the map are intended only as a guide. Particularly in mountainous regions, local conditions can change within a very short distance and may be more or less severe than indicated by the region classification. Severe exposures are those in which deicing salts are used because of significant snowfall combined with extended periods in which natural thawing does not occur. If you are in doubt about which classification applies, always use the more severe exposure. Actual concrete ingredient proportions can be measured either by volume or by weight.
