PRODUCTION: HOW CONCRETE IS MADE
Three components make up concrete: cement, aggregate, and water. Cement in concrete acts as a binding agent. Mixed with water, the resulting paste coats and fills the spaces between the aggregate of gravel, rocks, or sand. The term hydraulic cement refers to the chemical change that takes place when cement is mixed with water. The hardening or curing process results in a hard, dense material. However, pure cement is fairly useless except as a binding agent due to its excessive shrinking and cracking. Portland Cement is the most common type of cement used in mixing concrete today. A mixture of clay and limestone is burned in a kiln until it fuses into a lump called clinker, which is ground into a fine powder. There are several types of Portland Cement, each of which imparts particular desired qualities based on anticipated weather, need for extra strength, need to counteract excessive heat buildup, or where resistance to sulfates is necessary. Pozzolans, including fly ash, silica fume, and slag from steel production, have the qualities of Roman volcanic ash. They can be added to the cement mix in powder form. Pozzolans improve the performance of concrete, all while incorporating recycled products. Metakaolin may be added to impart an aesthetically desirable white color.
Water is mixed with cement to create a paste-like substance. Water used to make concrete must be potable (drinkable), and impurities such as chlorides, sulfates, alkalis, and solids must be limited. Impurities can affect the setting time or strength of the concrete. They can also cause staining, corrosion of reinforcement, volume instability, and reduced durability. There is an art and science to getting the ratio of cement to water just right. The cement must be fluid enough to pour and allow the concrete to be spread. However, too much water will cause excessive shrinkage and cracking. Less water results in stronger
Decorating Gallery 5.2 Slump Test. How a slump test works: (1) Place a sample of wet concrete in a cone; (2) Tamp concrete in a prescribed manner; (3) Lift the cone; and (4) Measure the “slump” (vertical settling). concrete, but the mixture must have enough water to allow it to coat each piece of aggregate with cement paste and to fill the voids among the pieces of aggregate. If the mix is too stiff, it may not be workable and the concrete may be porous and rough. A slump test is used to test mixed concrete for moisture content; the mix is tamped into a cone and then turned onto a flat surface. The amount of vertical settling is measured to determine if an acceptable standard is met.
Aggregate refers to sand, gravel, or crushed stone. The cement/water/paste binds the aggregate to form a solid mass. Aggregates are categorized as fine (sand-sized up to 14-inch in diameter) to coarse (usually 4 to 1/ inches in diameter. Aggregate up to 6 inches in diameter may be used for large concrete projects such as dams. Aggregate must be clean and free from contaminants to avoid chemical reactions that can affect the strength of the concrete. The shape of the aggregate affects the concrete as well. Round aggregates require less water and air and result in better consolidation. Aggregates can be exposed, adding color, texture, and pattern to the surface. Lightweight concrete may have a shale or slate aggregate. Perlite, a spherical volcanic glass with insulative properties can be used to improve the insulation value of concrete.
A typical concrete mixture may be made up of 10 to 15 percent cement, 60 to 75 percent aggregate, and 15 to 20 percent water. The performance of concrete is enhanced by the addition of admixtures that are chosen for a specific desired outcome. The term admixture refers to something added to a mix. In the case of concrete, admixtures improve or change the performance of the final product. From 5 to 8 percent of the concrete mix may be composed of admixtures. A common admixture is entrained air, an intentional use of air pockets for improved performance during freeze/thaw cycles. Some chemical admixtures are used to accelerate the curing process, whereas other admixtures are retarders that slow the process in order to allow more time to place and work the mix. Plasticizers and super-plasticizers are admixtures that reduce the need for water, which can result in increased strength. Pigments may be added as coloring agents for aesthetic purposes.
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As concrete is poured, it must be compacted to ensure that there are no unwanted air pockets, that the form is completely filled, and that there are no honeycombing voids that occur when cement does not completely fill the spaces between the particles of aggregate. When an exterior slab is poured, a flat board or an aluminum tool called a screed is used to cut off excess concrete, smoothing and leveling the surface. A wood or metal bull float is used to further smooth the surface, leaving it relatively even but slightly rough. At this point, water will appear on the surface, giving the concrete a glossy appearance. No further finishing is done until the sheen evaporates. A broom finish leaves a slip-resistant surface on exterior surfaces. On an interior, a hard trowel finish provides a smooth surface.
Hydration is the chemical process that occurs as concrete hardens and cures, becoming progressively stiffer, harder, and stronger. Curing begins when the bleed, or moisture sheen, has disappeared from the surface and the exposed surface begins to harden. The stiffening, hardening, and strengthening development is progressive. In about 28 days, concrete has reached most of its strength, but it continues to gain strength for years. Concrete should not cure too rapidly, so measures are taken to slow the process by keeping it moist, such as sprinkling with water fog, covering with a moisture-retaining fabric, or sealing with plastic or special sprays.
The mass of plain concrete makes it extremely strong in compression, but it cannot withstand wind, earthquakes, vibrations, or bending or stretching (tension) stresses. Steel is strong in tension, and the hybrid that results from combining the two is referred to as reinforced concrete or Ferroconcrete (ferro from “ferrous,” meaning iron), strong in both tension and compression. When concrete is poured into the molds or formwork that will determine the final shape, steel is placed where the greatest stress is anticipated. The steel must also be located where it will be adequately covered by concrete for protection from corrosion and fire. Steel reinforcement may be in the form of rods or bars with ribs for better bonding or in the form of welded wire fabric, a grid of steel wires. Vertical and horizontal sections of concrete are tied together by steel. Steel reinforcement also helps to minimize shrinking and cracking and helps control thermal expansion and contraction.
When concrete is site formed, or poured in place, the size and shape of the building or object can be unique. Most concrete is delivered to the site in a concrete truck with a revolving drum. This “ready mix” concrete may be mixed at a plant, mixed while in transit in the revolving drum of the concrete truck, and/or remixed on site. Formwork holds the concrete in place until it is cured. Forms for concrete walls may be prefabricated and reusable. Wood formwork can be designed to leave an imprint such as board patterns of wood on the concrete. Square or rectangular columns also use wood forms, but round columns are usually formed with a compressed resin-impregnated paper tube that is disposable. Forms are held in place by a framework of vertical wood studs and horizontal reinforcement boards known as walers. Bracing and shoring may be necessary. Spreaders and form ties keep the forms separated as they are filled with concrete. Formwork must be treated with a parting compound, such as oil, wax, or plastic, for easy removal. When concrete is to remain exposed, the designer can carefully place the form ties to create a pattern or design on the finished concrete surface. When concrete cures, the forms are removed. The resulting surface may be left exposed or used as a substrate for other materials. As an alternate to using shoring, floor slabs can be cast on the ground and lifted into place using a crane. Similarly, wall slabs can be cast on the ground and tilted into place.