SECTION I. BASIC CONSIDERATION
1-3. Concrete is a mixture of aggregate, and often controlled amounts of entrained air, held together by a hardened paste made from cement and water. Although there are other kinds of cement, the word cement in common usage refers to portland cement. A chemical reaction between the portland cement and water--not drying of the mixture--causes concrete to harden to a stone-like condition. This reaction is called hydration. Hydration gives off heat, known as the heat of hydration. Because hydration--not air drying--hardens concrete, freshly placed concrete submerged underwater will harden. When correctly proportioned, concrete is at first a plastic mass molded into nearly any size or shape. Upon hydration of the cement by the water, concrete becomes stone-like in strength, durability, and hardness.
1-4. Portland cement is the most commonly used modern hydraulic cement. In this case, the word hydraulic is the cement's characteristic of holding aggregate together by using water or other low-viscosity fluids. Portland cement is a carefully proportioned and specially processed chemical combination of lime, silica, iron oxide, and alumina. It was named after the Isle of Portland in the English Channel.
1-5. Unless tests or experience indicates that a particular water source is satisfactory, water should be free from acids, alkalis, oils, and organic impurities. The basic ratio of cement to water determines the concrete's strength; generally, the less water in the mix, the stronger, more durable and watertight the concrete. The concrete should be workable but not too stiff to use. Too much water dilutes the cement paste (binder), resulting in weak and porous concrete. Concrete quality varies widely, depending on the characteristics of its ingredients and the proportions of the mix.
1-6. Inert filler materials (usually sand and stone or gravel) make up between 60 and 80 percent of the volume of normal concrete. In air-entrained concrete, the air content ranges up to about 8 percent of the volume. Aggregate is often washed when impurities are found that can retard cement hydration or deteriorate the concrete's quality. 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 aggregates passing the number 4 sieve (16 openings per square inch), it is called mortar, stucco, or cement plaster. When cement is mixed with coarse aggregate (CA) of more than 1/4 inch plus fine aggregate (FA) and water, the product is concrete. The aggregate's physical and chemical properties also affect concrete properties; aggregate size, shape, and grade influence the amount of water required. For example, limestone aggregate requires more water than similar size marble aggregate. Aggregate surface texture influences the bond between the aggregate and the cement paste. In properly mixed concrete, the paste 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 paste's 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's quality.
1-7. These substances are added to the concrete mixture to accelerate or retard the initial set, improve workability, reduce water requirements, increase strength, or otherwise alter concrete properties. They usually cause a chemical reaction within the concrete. Admixtures are normally classified into accelerators, retardants, air-entraining agents, workability agents, damp-proofing and permeability-reducing agents, pozzolans, color pigments, and miscellaneous materials. Many admixtures fall into more than one classification.
CONCRETE AS A BUILDING MATERIAL
1-8. Concrete has a great variety of applications; it not only meets structural demands but 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.
1-9. Concrete and cement are among the most important construction materials. Concrete is fireproof, watertight, economical, and easy to make. It offers surface continuity (absence of joints) and solidity; it will bond with other materials. Concrete is usually locally available worldwide.
1-10. Certain limitations of concrete cause cracking and other structural weaknesses that detract from the appearance, serviceability, and useful life of concrete structures. Listed below are some principal limitations and disadvantages of concrete.
Low Tensile Strength
1-11. Concrete members subject to tensile stress must be reinforced with steel bars or mesh to prevent cracking and failure.
1-12. During setting and hardening, the heat of hydration raises the concrete temperature and then it gradually cools. These temperature changes can cause severe thermal strains and early cracking. In addition, hardened concrete expands and contracts with changes in temperature (at roughly the same rate as steel); therefore, expansion and contraction joints must be provided in many types of concrete structures to prevent failures.
Drying Shrinkage and Moisture Movements
1-13. Concrete shrinks as it dries and as it hardens. It expands and contracts with wetting and drying. These movements require that control joints be provided at intervals to avoid unsightly cracks. To prevent drying shrinkage in newly placed concrete, its surface is kept moist continuously during the curing process. Moisture is applied when the concrete is hard enough so as not to damage the concrete's surface.
1-14. Concrete deforms gradually (creeps) under load. This deformation does not recover completely when the load is removed.
1-15. Even the best quality concrete is not entirely impervious to moisture. Concrete normally contains soluble compounds that are leached out in varying amounts by water, unless properly constructed joints allow water to enter the mass. Impermeability is particularly important in reinforced concrete--the concrete must prevent water from reaching the steel reinforcement.
1-16. The unit of measure for concrete is cubic foot (cu ft). Thus, a standard sack of portland cement weighs 94 pounds and equals 1 loose cubic foot. FAs and CAs are measured by loose volume, whereas water is measured by the gallon. Concrete is usually referred to by cubic yards (cu yd)
|David L. Heiserman, Editor||
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Revised: June 06, 2015