used. This occurs because of the nature of these methods. It does not necessarily imply that one method is better than another. Each method begins by assuming certain needs or requirements and then proceeds to determine the other variables. Since the methods begin differently and use different procedures, the final proportions vary slightly. This is to be expected, and it further points out the necessity of trial mixes in determining the final mix proportions.
Construction crews in the field convert the designed trial mix proportions into field mix proportions suitable for the mixing equipment available. It must be remembered, however, that the trial mix method was designed under controlled conditions based on certain assumptions that may not exist in the field. For this reason, it often becomes necessary for the field crews to adjust the mix for moisture and entrained air.
Admixtures include all materials other than portland cement, water, and aggregates that are added to concrete, mortar, or grout immediately before or during mixing. Admixtures are sometimes used in concrete mixtures to improve certain qualities, such as workability, strength, durability, watertightness, and wear resistance. They may also be added to reduce segregation, reduce heat of hydration, entrain air, and accelerate or retard setting and hardening. The same results can often be obtained by changing the mix proportions or by selecting other suitable materials without resorting to the use of admixtures (except air-entraining admixtures when necessary). Whenever possible, comparison should be made between these alternatives to determine which is more economical and/or convenient. Any admixture to be in concrete should be added according to current specifications and under the direction of the engineer in charge.
The most commonly used admixture in concrete mixtures is an air-entraining agent of the type discussed in the previous section on "Mix Adjustments" for entrained air. In general, air-entraining agents are derivatives of natural wood resins, animal or vegetable fats or oils, alkali salts of sulfated or sulfonated organic compounds, and water-soluble soaps. Most air-entraining agents are in liquid form for use in the mix water. The instructions for the use of the various agents to produce a specified air content are provided by the manufacturer.
Automatic dispensers, made available by some manufacturers, permit more accurate control of the quantities of air-entraining agents used in the mix. The main reason for using intentionally entrained air is to improve the resistance of the concrete to freezing and thawing exposure. However, there are other important beneficial effects in both freshly mixed and hardened concrete, which include workability, resistance to deicers, sulfate resistance, strength, abrasion resistance, and watertightness.
The slump test measures the consistency of concrete. Do not use it to compare mixes having wholly different proportions or containing different sizes of aggregates. When different batches are tested, changes in slump indicate changes in materials, mix
Table 3-13. - Recommended Slumps for Various Types of Construction