The workability of concrete is governed by the amount of aggregate in the mix. Where reduction of aggregate (or increase in cement) is impractical, workability is increased by adding a plasticizer. Air-entraining agents, when used, are plasticizers. Other substances include calcium chloride, lime, fly ash, and other pozzolans. Calcium chloride is also an accelerator. Lime increases the cementing properties of cement, as do pozzolans combined with lime. Fly ash is inexpensive compared to cement and is used as a partial replacement (up to as much as 50 percent) of the cement. It changes both the plastic and the hardened properties of concrete. Fly ash improves workability and reduces segregation, bleeding, and the heat of hydration. The concrete will not be as watertight as a cement-only concrete, nor will it have as much initial strength. Additional tests may have to be made to determine when to remove the forms. Its final strength, however, will be as great as a cement-only concrete.

Dense concrete is required in some types of construction, such as in prestressed structures. This density is achieved when cement particles are separated evenly throughout the mix or at least prevented from attaching to each other (flocculating). A detergent admixture will disperse the particles individually and will create a more uniform paste. These admixtures also reduce the formation of a cement gel that expands at the early stages of hydration and pushes the particles apart, thus increasing the volume. Prevention of this expansion results in a denser paste.

Watertightness can be controlled to a great extent by lowering the water-cement ratio. This may not always be practical, and sometimes even with a low water-cement ratio, capillaries still form through the concrete. Densifying or using an accelerator like calcium chloride improves the watertightness.

The greatest improvement in watertightness and resistance to deterioration under freezing and thawing is obtained by incorporating 4 to 6 percent, by volume, of entrained air into the mix. Workability of fresh concrete is enhanced by such entrained air. Air-entrained cement contains the necessary admixture.

Soaps, butylstearate, some of the fine pozzolans, and several proprietary compounds are available for use as air-entraining admixtures with ordinary cements. These agents minimize the formation of capillaries and plug the tiny holes with a water-repellant or sealing material. They provide small, uniformly spaced, discrete air voids that prevent the buildup of damaging pressures from the expansion of freezing water into ice.

Concrete does not develop its full strength until the chemical process of curing (or hydration) is complete. Curing takes place over an extended periodthe most critical portion of which is from the day of placement through the 10th day. The extent and rate of curing depends upon the presence of moisture and the correct temperature within the mix.

The ideal temperatures for concrete work are between 55F and 70F. Above this, rapid evaporation of moisture creates a problem. At lower temperatures, the curing or setting is delayed. Temperatures below 32F stop the hydration process. Since the chemical action gives off some heat, some method must be used to keep the heat within the structure during times of low temperatures. Cold weather construction may even require heating the ingredients, or mix, and covering the emplaced concrete or providing a heated enclosure. In hot weather, extra care is required to prevent a high temperature rise and too rapid drying of the fresh concrete. Moistening the aggregate with cool water will lower the generated temperature. The water is kept cool as possible by the application of reflective white or aluminum paint to water supply lines and storage tanks. On massive construction, such as dams and heavy retaining walls, the mixing water is often cooled artificially or ice is substituted for part of the water. This ice must be melted by the time the concrete is fully mixed and ready to leave the mixer. Cement replace-ment materials (such as pozzolans of diatomaceous earth, pumicites, or fly ash) may be used to depress concrete temperature by reduction of the heat of hydration in a structure; however, pozzolans vary widely and may have adverse effects on strength, air content, and durability, if used in excessive amounts.

Figure 13-20.Increase of concrete compressive strenth with curing age.

figure 13-20. Note the long-time gain in strength that occurs when proper temperature and moisture conditions are maintained.

Several tests, such as slump, air content, and weight determination, are necessary to determine the quality of freshly mixed concrete. In addition, strengths tests are needed to determine whether a hardened concrete satisfies specified strength requirements. This section briefly discusses those tests.

As you know, the measure of the workability or consistency of a concrete mix is its slump. With too little slump, the mixture may be too difficult to work into the forms and around the reinforcing steel. On the other hand, with too much slump, the concrete ingredients may segregate and excessive bleeding or migration of water to the top surface of the freshly placed concrete may occur. Excess bleeding increases the water-cement ratio near the top surface of the concrete and results in a weak top layer with poor durability.

To determine whether a freshly mixed concrete satisfies the specified requirements for slump, you must perform a slump test. By now, you should be thoroughly

familiar with the procedures of slump testing. If not, you should review the discussion of slump testing that is in the EA3 TRAMAN.