Admixtures
Admixtures include all materials added to a mix other than Portland cement, water, and aggregates. 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 the heat of hydration, entrain air, and accelerate or retard setting and hardening.
We should note that the same results can often be obtained by changing the mix proportions or by selecting other suitable materials without resorting to the use of dmixtures, except air-entraining admixtures when necessary. Whenever possible, compare these alternatives to determine which is more economical or convenient. Seabees rely on local suppliers for appropriate basic concrete mixes. Add any admixture according to current specifications determined by the Engineering Aid and upon approval of the Quality Control department.
Workability Agents
Materials such as hydrated lime and bentonite improve workability. These materials increase the fines in a concrete mix when an aggregate is tested deficient in fines, i.e., lacking sufficient fine material.
Air-Entraining Agents
The deliberate adding of millions of minute disconnected air bubbles to cement paste, if those bubbles are evenly diffused, changes the basic concrete mix and increases durability, workability, and strength. The acceptable amount of entrained air in a concrete mix, by volume, is 3 to 7 percent. Air-entraining agents, used with types I, II, or III cement, are derivatives of natural wood resins, animal or vegetable fats, oils, alkali salts of sulfated organic compounds, and water soluble soaps. Most air-entraining agents are in liquid form for use in the mixing water.
Accelerating admixtures
Accelerating admixtures speed up the setting and hardening of concrete. They are especially useful in cold weather because concrete hardens slowly at temperatures below about 50°F. In the past, the most common of these admixtures was calcium chloride. However, calcium chloride in concrete increases the potential for corrosion of reinforcing steel and some other metals. When required by the specifications, nonchloride accelerators are available.
Retarders
The accepted use for retarders is to reduce the rate of hydration. They are often used in warm weather to keep the concrete from setting before it can be placed and finished. Agents normally used are fatty acids, sugar, and starches.