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 admixtures (except air-entraining admixtures when necessary). Whenever possible, comparison should be made between these alternatives to determine which is more economical or convenient. Any admixture should be added according to current specifications and under the direction of the crew leader.
Materials, such as hydrated lime and bentonite, are used to improve workability. These materials increase the fines in a concrete mix when an aggregate is tested deficient in fines (that is, lacks sufficient fine material).
The deliberate adding of millions of minute disconnected air bubbles to cement paste, if 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.
The only accepted accelerator for general concrete work is calcium chloride with not more than 2 percent by weight of the cement being used. This accelerator is added as a solution to the mix water and is used to speed up the strength gain. Although the final strength is not affected, the strength gain for the first 7 days is greatly affected. The strength gain for the first 7 days can be as high as 1,000 pounds per square inch (psi) over that of normal concrete mixes.
The accepted use for retarders is to reduce the rate of hydration. This permits the placement and consolidation of concrete before initial set. Agents normally used are fatty acids, sugar, and starches.
Portland cement is packed in cloth or paper sacks, each weighing 94 pounds. A 94-pound sack of cement amounts to about 1 cubic foot by loose volume.
Cement will retain its quality indefinitely if it does not come in contact with moisture. If allowed to absorb appreciable moisture in storage, however, it sets more slowly and strength is reduced. Sacked cement should be stored in warehouses or sheds made as watertight and airtight as possible. All cracks in roofs and walls should be closed, and there should be no openings between walls and roof. The floor should be above ground to protect the cement against dampness. All doors and windows should be kept closed.
Sacks should be stacked against each other to prevent circulation of air between them, but they should not be stacked against outside walls. If stacks are to stand undisturbed for long intervals, they should be covered with tarpaulins.
When shed or warehouse storage cannot be provided, sacks that must be stored in the open should be stacked on raised platforms and covered with waterproof tarps. The tarps should extend beyond the edges of the platform to deflect water away from the platform and the cement.
Cement sacks stacked in storage for long periods sometimes acquire a hardness called warehouse pack. This can usually be loosened by rolling the sack around. However, cement that has lumps or is not free flowing should not be used.
LEARNING OBJECTIVE: Upon completing this section, you should be able to calculate concrete mix designs.
Before proportioning a concrete mix, you need information concerning the job, such as size and shapes of structural members, required strength of the concrete, and exposure conditions. The end use of the concrete and conditions at time of placement are additional factors to consider.
The ingredient proportions for the concrete on a particular job are usually set forth in the speci- fications under "CONCRETE - General Require- merits." See table 6-3 for examples of normalContinue Reading