maximum strength and durability will still not be attained unless the sand and coarse aggregate you use consist of well-graded, clean, hard, and durable particles free of undesirable substances (figure 6-1).

WATERTIGHTNESS OF CONCRETE

The ideal concrete mix is one with just enough water required for complete hydration of the cement. However, this results in a mix too stiff to pour in forms. A mix fluid enough to be poured in forms always contains a certain amount of water over and above that which will combine with the cement. This water eventually evaporates, leaving voids, or pores, in the concrete. Penetration of the concrete by water is still impossible if these voids are not inter- connected. They may be interconnected, however, as a result of slight sinking of solid particles in the mix during the hardening period. As these particles sink, they leave water-tilled channels that become voids when the water evaporates. The larger and more numerous these voids are, the more the watertightness of the concrete is impaired. The size and number of the voids vary directly with the amount of water used in excess of the amount required to hydrate the cement. To keep the concrete as watertight as possible, you must not use more water than the minimum amount required to attain the necessary degree of workability.

GENERAL REQUIREMENTS FOR GOOD CONCRETE

The first requirement for good concrete is to use a cement type suitable for the work at hand and have a satisfactory supply of sand, coarse aggregate, and water. Everything else being equal, the mix with the best graded, strongest, best shaped, and cleanest aggregate makes the strongest and most durable concrete.

Second, the amount of cement, sand, coarse aggregate, and water required for each batch must be carefully weighed or measured according to project specifications.

Third, even the best designed, best graded, and highest quality mix does not make good concrete if it is not workable enough to fill the form spaces thoroughly. On the other hand, too much fluidity also results in defects. Also, improper handling during the overall concrete making process, from the initial aggregate handling to the final placement of the mix, causes segregation of aggregate particles by sizes, resulting in nonuniform, poor-quality concrete.

Finally, the best designed, best graded, highest quality, and best placed mix does not produce good concrete if it is not proper] y cured, that is, properly protected against loss of moisture during the earlier stages of setting.

CONCRETE INGREDIENTS

LEARNING OBJECTIVE: Upon completing this section, you should be able to identify the ingredients essential for good concrete.

The essential ingredients of concrete are cement, aggregate, and water. A mixture of only cement and water is called cement paste. In large quantities, however, cement paste is prohibitively expensive for most construction purposes.

PORTLAND CEMENT

Most cement used today is portland cement. This is a carefully proportioned and specially processed combination of lime, silica, iron oxide, and alumina. It is usually manufactured from limestone mixed with shale, clay, or marl. Properly proportioned raw materials are pulverized and fed into kilns where they are heated to a temperature of 2,700F and maintained at that temperature for a specific time. The heat produces chemical changes in the mixture and transforms it into clinker - a hard mass of fused clay and limestone. The clinker is then ground to a fineness that will pass through a sieve containing 40,000 openings per square inch.

Types of Cement

There are five types of Portland cement covered under "Standard Specifications for Portland Cement." These specifications are governed by the American Society for Testing and Material (ASTM) types. Separate specifications, such as those required for air-entraining portland cements, are found under a separate ASTM. The type of construction, chemical composition of the soil, economy, and requirements for use of the finished concrete are factors that influence the selection of the kind of cement to be used.

TYPE I. - Type I cement is a general-purpose cement for concrete that does not require any of the special properties of the other types. In general, type I cement is intended for concrete that is not subjected