Portland Cement. This is the most commonly used of modern hydraulic cements. In this case, the word
hydraulic means that the cement's characteristic of holding aggregate together is caused by water or other low-
viscosity fluids. Portland cement is a carefully proportioned and specially processed chemical combination of
lime, silica, iron oxide, and alumina.
Mixing Water. Unless tests or experience indicates that a particular water source is satisfactory, mixing water
should be free from acids, alkalies, oils, and organic purities. The basic ratio of water to cement determines the
strength of concrete. The less water in the mix, as long as it is workable and not too stiff, the stronger, more
durable, and watertight the concrete. Too much water dilutes cement paste (binder), resulting in weak and porous
concrete. Concrete quality varies widely, depending on the characteristics of its ingredients and the proportion of
the mix.
Aggregates. Inert filler material (usually sand and stone or gravel) make up between 60 to 80 percent of the
volume of normal concrete. Aggregate is often washed when impurities or excess fines that can retard cement
hydration or otherwise deteriorate concrete quality are found. All aggregate is screened to ensure proper size
gradation because concrete differs from other cement-water-aggregate mixtures in the size of its aggregate. For
example, when cement is mixed with water and an aggregate passing the No 4 sieve, it is called mortar, stucco, or
cement plaster. When cement is mixed with coarse aggregate of more than 1/4-inch, plus fine aggregate and
water, the product is concrete. The physical and chemical properties of the aggregate also affect concrete
properties. Aggregate size, shape, and grade influence the amount of water required. Aggregate surface texture
influences the bond between the aggregate and the cement paste. In properly mixed concrete, the paste
completely surrounds each aggregate particle and fills all spaces between the particles. The elastic properties of
the aggregate influence the elastic properties of the concrete and the pastes resistance to shrinkage. Reactions
between the cement paste and the aggregate can either improve or harm the bond between the two and,
consequently, the concrete quality.
Air. All concrete contains some air. If air is chemically induced into the mix, it is called entrained air.
Entrained air adds beneficial qualities to the concrete such as increased freeze-thaw capabilities, durability, and
watertightness. Entrained air can range from 1.5 to 7.5 percent. Air that is added to the mix as a result of the
mixing process is called entrapped air. Entrapped air adds nothing to the mix; however, this air can range from
0.5 to 3 percent.
Admixtures. When mixing concrete, these substances are added to accelerate or retard the initial set, improve
workability, reduce mixing water requirements, increase strength, or otherwise alter concrete properties. They
usually cause a chemical reaction within the concrete. Admixtures are normally classified into accelerators,
retarders, air-entraining agents, water reducers, and pozzolans. Many admixtures fall into more than one
classification.
Concrete as a Building Material
Concrete has a great variety of applications because it not only meets structural demands but also lends itself
readily to architectural treatment. In buildings, concrete is used for footings, foundations, columns, beams,
girders, wall slabs, and roof units--in short, all important building elements. Other important concrete
applications are in road pavements, airport runways, bridges, dams, irrigation canals, water-diversion structures,
sewage-treatment plants, and water-distribution pipelines. A great deal of concrete is used in manufacturing
masonry units, such as concrete blocks and concrete bricks.
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