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Concrete

Concrete

Background

Concrete is a hardened building material created by combining a chemically inert mineral aggregate (usually sand, gravel, or crushed stone), a binder (natural or synthetic cement), chemical additives, and water. Although people commonly use the word "cement" as a synonym for concrete, the terms in fact denote different substances: cement, which encompasses a wide variety of fine-ground powders that harden when mixed with water, represents only one of several components in modern concrete. As concrete dries, it acquires a stone-like consistency that renders it ideal for constructing roads, bridges, water supply and sewage systems, factories, airports, railroads, waterways, mass transit systems, and other structures that comprise a substantial portion of the U.S. wealth. According to the National Institute of Standards and Technology (NIST), building such facilities is in itself one of the nation's largest industries and represents about 10 percent of the gross national product. Over $4 billion worth of hydraulic cement, a variety which hardens under water, is produced annually in the United States for use in $20 billion worth of concrete construction. The value of all cement-based structures in the United States is in the trillions of dollarsroughly commensurate with the anticipated cost of repairing those structures over the next twenty years.

The words cement and concrete are both of Latin origin, reflecting the likelihood that the ancient Romans were the first to use the substances. Many examples of Roman concrete construction remain in the countries that encircle the Mediterranean, where Roman builders had access to numerous natural cement deposits. Natural cement consists mainly of lime, derived from limestone and often combined with volcanic ash. It formed the basis of most civil engineering until the eighteenth century, when the first synthetic cements were developed.

The earliest manmade cement, called hydraulic lime, was developed in 1756, when an English engineer named John Smeaton needed a strong material to rebuild the Eddystone lighthouse off the coast of Devon. Although the Romans had used hydraulic cement, the formula was lost from the collapse of their empire in the fifth century A.D. until Smeaton reinvented it. During the early nineteenth century several other Englishmen contributed to the refinement of synthetic cement, most notably Joseph Aspdin and Isaac Charles Johnson. In 1824 Aspdin took out a patent on a synthetic blend of limestone and clay which he called Portland cement because it resembled limestone quarried on the English Isle of Portland. However, Aspdin's product was not as strong as that produced in 1850 by Johnson, whose formula served as the basis of the Portland cement that is still widely used today. Concrete made with Portland cement is considered superior to that made with natural cement because it is stronger, more durable, and of more consistent quality. According to the American Society of Testing of Materials (ASTM), Portland cement is made by mixing calcareous (consisting mostly of calcium carbonate) material such as limestone with silica-, alumina-, and iron oxide-containing materials. These substances are then burned until they fuse together, and the resulting admixture, or clinker, is ground to form Portland cement.

Although Portland cement quickly displaced natural cement in Europe, concrete technology in the United States lagged considerably behind. In America, natural cement rock was first discovered during the early 1800s, when it was used to build the Erie Canal. The construction of such inland waterways led to the establishment of a number of American companies producing natural cement. However, because of Portland cement's greater strength, many construction engineers preferred to order it from Europe, despite the additional time and expense involved. Thomas Edison was very interested in Portland cement and even cast phonograph cabinets of the material. When United States industry figured out how to make Portland cement during the early 1870s, the production of natural cement in America began to decline.

After the refinement of Portland cement, the next major innovation in concrete technology occurred during the late nineteenth century, when reinforced concrete was invented. While concrete easily resists compression, it does not tolerate tension well, and this weakness meant that it could not be used to build structureslike bridges or buildings with archesthat would be subject to bending action. French and English engineers first rectified this deficiency during the 1850s by embedding steel bars in those portions of a concrete structure subject to tensile stress. Although the concrete itself is not strengthened, structures built of reinforced concrete can better withstand bending, and the technique was used internationally by the early twentieth century.

Another form of strengthened concrete, prestressed concrete, was issued a U.S. patent in 1888. However, it was not widely used until World War II, when several large docks and bridges that utilized it were constructed. Rather than reinforcing a highly stressed portion of a concrete structure with steel, engineers could now compress a section of concrete before they subjected it to stress, thereby increasing its ability to withstand tension.

Today, different types of concrete are categorized according to their method of installation. Ready- or pre-mixed concrete is batched and mixed at a central plant before it is delivered to a site. Because this type of concrete is sometimes transported in an agitator truck, it is also known as transit-mixed concrete. Shrink-mixed concrete is partially mixed at the central plant, and its mixing is then completed en route to the site.

Raw Materials

Structural concrete normally contains one part cement to two parts fine mineral aggregate to four parts coarse mineral aggregate, though these proportions are often varied to achieve the strength and flexibility required in a particular setting. In addition, concrete contains a wide range of chemicals that imbue it with the characteristics desired for specific applications. Portland cement, the kind most often used in concrete, is made from a combination of a calcareous material (usually limestone) and of silica and alumina found as clay or shale. In lesser amounts, it can also contain iron oxide and magnesia. Aggregates, which comprise 75 percent of concrete by volume, improve the formation and flow of cement paste and enhance the structural performance of concrete. Fine grade comprises particles up to. 20 of an inch (five millimeters) in size, while coarse grade includes particles from. 20 to. 79 of an inch (20 millimeters). For massive construction, aggregate particle size can exceed 1.50 inches (38 millimeters).

Aggregates can also be classified according to the type of rock they consist of: basalt, flint, and granite, among others. Another type of aggregate is pozzolana, a siliceous and aluminous material often derived from volcanic ash. Reacting chemically with limestone and moisture, it forms the calcium silicate hydrates that are the basis of cement. Pozzolana is commonly added to Portland cement paste to enhance its densification. One type of volcanic mineral, an aluminum silicate, has been combined with siliceous minerals to form a composite that reduces weight and improves bonding between concrete and steel surfaces. Its applications have included precast concrete shapes and asphalt/concrete pavement for highways. Fly ash, a coal-burning power plant byproduct that contains an aluminosilicate and small amounts of lime, is also being tested as a possible pozzolanic material for cement. Combining fly ash with lime (CaO) in a hydrothermal process (one that uses hot water under pressure) also produces cement.

A wide range of chemicals are added to cement to act as plasticizers, superplasticizers, accelerators, dispersants, and water-reducing agents. Called admixtures, these additives can be used to increase the workability of a cement mixture still in the nonset state, the strength of cement after application, and the material's water tightness. Further, they can decrease the amount of water necessary to obtain workability and the amount of cement needed to create strong concrete. Accelerators, which reduce setting time, include calcium chloride or aluminum sulfate and other acidic materials. Plasticizing or superplasticizing agents increase the fluidity of the fresh cement mix with the same water/cement ratio, thereby improving the workability of the mix as well as its ease of placement. Typical plasticizers include polycarboxylic acid materials; superplasticizers are sulphanated melamine formaldehyde or sulphanated naphthalene formaldehyde condensates. Setretarders, another type of admixture, are used to delay the setting of concrete. These include soluble zinc salts, soluble borates, and carbohydrate-based materials. Gas forming admixtures, powdered zinc or aluminum in combination with calcium hydroxide or hydrogen peroxide, are used to form aerated concrete by generating hydrogen or oxygen bubbles that become entrapped in the cement mix.

Cement is considered a brittle material; in other words, it fractures easily. Thus, many additives have been developed to increase the tensile strength of concrete. One way is to combine polymeric materials such as polyvinyl alcohol, polyacrylamide, or hydroxypropyl methyl cellulose with the cement, producing what is sometimes known as macro-defect-free cement. Another method entails adding fibers made of stainless steel, glass, or carbon. These fibers can be short, in a strand, sheet, non-woven fabric or woven fabric form. Typically, such fiber represents only about one percent of the volume of fiber-reinforced concrete.

The Manufacturing
Process

The manufacture of concrete is fairly simple. First, the cement (usually Portland cement) is prepared. Next, the other ingredientsaggregates (such as sand or gravel), admixtures (chemical additives), any necessary fibers, and waterare mixed together with the cement to form concrete. The concrete is then shipped to the work site and placed, compacted, and cured.

Preparing Portland cement

  • 1 The limestone, silica, and alumina that make up Portland cement are dry ground into a very fine powder, mixed together in predetermined proportions, preheated, and calcined (heated to a high temperature that will burn off impurities without fusing the ingredients). Next the material is burned in a large rotary kiln at 2,550 degrees Fahrenheit (1,400 degrees Celsius). At this temperature, the material partially fuses into a substance known as clinker. A modern kiln can produce as much as 6,200 tons of clinker a day.
  • 2 The clinker is then cooled and ground to a fine powder in a tube or ball mill. A ball mill is a rotating drum filled with steel balls of different sizes (depending on the desired fineness of the cement) that crush and grind the clinker. Gypsum is added during the grinding process. The final composition consists of several compounds: tricalcium silicate, dicalcium silicate, tricalcium aluminate, and tetracalcium aluminoferrite.

Mixing

  • 3 The cement is then mixed with the other ingredients: aggregates (sand, gravel, or crushed stone), admixtures, fibers, and water. Aggregates are pre-blended or added at the ready-mix concrete plant under normal operating conditions. The mixing operation uses rotation or stirring to coat the surface of the aggregate with cement paste and to blend the other ingredients uniformly. A variety of batch or continuous mixers are used.
  • 4 Fibers, if desired, can be added by a variety of methods including direct spraying, premixing, impregnating, or hand laying-up. Silica fume is often used as a dispersing or densifying agent.

Transport to work site

  • 5 Once the concrete mixture is ready, it is transported to the work site. There are many methods of transporting concrete, including wheelbarrows, buckets, belt conveyors, special trucks, and pumping. Pumping transports large quantities of concrete over large distances through pipelines using a system consisting of a hopper, a pump, and the pipes. Pumps come in several typesthe horizontal piston pump with semi-rotary valves and small portable pumps called squeeze pumps. A vacuum provides a continuous flow of concrete, with two rotating rollers squeezing a flexible pipe to move the concrete into the delivery pipe.

Placing and compacting

  • 6 Once at the site, the concrete must be placed and compacted. These two operations are performed almost simultaneously. Placing must be done so that segregation of the various ingredients is avoided and full compactionwith all air bubbles eliminatedcan be achieved. Whether chutes or buggies are used, position is important in achieving these goals. The rates of placing and of compaction should be equal; the latter is usually accomplished using internal or external vibrators. An internal vibrator uses a poker housing a motor-driven shaft. When the poker is inserted into the concrete, controlled vibration occurs to compact the concrete. External vibrators are used for precast or thin in situ sections having a shape or thickness unsuitable for internal vibrators. These type of vibrators are rigidly clamped to the formwork, which rests on an elastic support. Both the form and the concrete are vibrated. Vibrating tables are also used, where a table produces vertical vibration by using two shafts rotating in opposite directions.

Curing

  • 7 Once it is placed and compacted, the concrete must cured before it is finished to make sure that it doesn't dry too quickly. Concrete's strength is influenced by its moisture level during the hardening process: as the cement solidifies, the concrete shrinks. If site constraints prevent the concrete from contracting, tensile stresses will develop, weakening the concrete. To minimize this problem, concrete must be kept damp during the several days it requires to set and harden.

Quality Control

Concrete manufacturers expect their raw material suppliers to supply a consistent, uniform product. At the cement production factory, the proportions of the various raw materials that go into cement must be checked to achieve a consistent kiln feed, and samples of the mix are frequently examined using X-ray fluorescence analysis.

The strength of concrete is probably the most important property that must be tested to comply with specifications. To achieve the desired strength, workers must carefully control the manufacturing process, which they normally do by using statistical process control. The American Standard of Testing Materials and other organizations have developed a variety of methods for testing strength. Quality control charts are widely used by the suppliers of ready-mixed concrete and by the engineer on site to continually assess the strength of concrete. Other properties important for compliance include cement content, water/cement ratio, and workability, and standard test methods have been developed for these as well.

The Future

Though the United States led the world in improving cement technology from the 1930s to the 1960s, Europe and Japan have since moved ahead with new products, research, and development. In an effort to restore American leadership, The National Science Foundation has established a Center for Science and Technology of Advanced Cement-Based Materials at Northwestern University. The ACBM center will develop the science necessary to create new cement-based materials with improved properties. These will be used in new construction as well as in restoration and repair of highways, bridges, power plants, and waste-disposal systems.

The deterioration of the U.S. infrastructure has shifted the highway industry's emphasis from building new roads and bridges to maintaining and replacing existing structures. Because better techniques and materials are needed to reduce costs, the Strategic Highway Research Program (SHRP), a 5-year $150 million research program, was established in 1987. The targeted areas were asphalt, pavement performance, concrete structures, and highway operations.

The Center for Building Technology at NIST is also conducting research to improve concrete performance. The projects include several that are developing new methods of field testing concrete. Other projects involve computer modeling of properties and models for predicting service life. In addition, several expert systems have been developed for designing concrete mixtures and for diagnosing causes of concrete deterioration.

Another cement industry trend is the concentration of manufacturing in a smaller number of larger-capacity production systems. This has been achieved either by replacing several older production lines with a single, high-capacity line or by upgrading and modernizing an existing line for a higher production yield. Automation will continue to play an important role in achieving these increased yields. The use of waste byproducts as raw materials will continue as well.

Where To Learn More

Books

American Concrete Institute. Cement and Concrete Terminology. 1967.

Mindess, S. Advances in Cementitious Materials. The American Ceramic Society, 1991. Vol. 16: Ceramic Transactions.

Neville, A. M. and J. J. Brooks. Concrete Technology. John Wiley & Sons, Inc., 1987.

Skalny, Jan P. Materials Science of Concrete I. The American Ceramic Society, 1989.

Skalny, J. and S. Mindess. Materials Science of Concrete II. The American Ceramic Society, 1991.

Periodicals

Holterhoff, A. "Implementing SPC in the Manufacture of Calcium Aluminate Cements." Ceramic Bulletin, 1991.

Jiang, W. and D. Roy. "Hydrothermal Processing of New Fly Ash Cement." Ceramic Bulletin, 1992.

Sheppard, L. "Cement Renovations Improve Concrete Durability." Ceramic Bulletin, 1991.

L. S. Millberg

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concrete

concrete. Building material made by mixing fragments of hard material (aggregate—usually broken stone) with mortar (fine aggregate—usually sand, water, and a binding-agent—now usually Portland cement). Historically, concrete was made with lime, sand, and water, with brick-dust, crushed volcanic rock, and other materials added. A type of concrete was used in Roman construction called opus caementicium, consisting of undressed stones bedded in a mix of lime and pozzolan, which dried out quickly, so had to be laid in courses. By C1 ad the drying-out process could be retarded, thanks to the evolution of slow-drying mixes, and this facilitated the evolution of huge vaulted structures covering vast spaces. The Romans used types of concrete made of lime, with tufa (porous, light, volcanic rock found around Rome) and other aggregates for these vaults, often in association with brick or stone reinforcement, and this created an architecture where the inner volumes were more important, perhaps, than the exteriors. Early examples of Roman architecture covered by concrete vaults are the Domus Aurea (Golden House) by Severus, and the enormous Pantheon in Rome, with its coffered dome

Types of concrete were in use for Byzantine structures but fell from favour until revived in C18, notably in France and England. Concrete was used by Smirke in the structure of the British Museum, and concrete laid over hollow-brick vaulting was used by Henry Roberts for fire-proof construction in work-ing-class housing during the 1850s. The discovery by Joseph Aspdin (1779–1855) of Portland cement made from lime and clay facilitated the development of immensely strong concrete structures as well as the evolution of a scientifically based theory. Strong in compression, concrete is weak in tension, so the weakness has to be eliminated if concrete is to be used in members subjected to tension, such as beams. Reinforcement with metal was experimented with in the early C19, and Loudon (1832) recorded concrete floors reinforced with interlacing iron bars. Other pioneers include Coignet, Monier, and Louis-Joseph Vicat (1786–1861—who produced cements that set under water, and classified them as ‘hydraulic’), William E. Ward (1821–1906—who built a concrete house at Chester, NY, in 1873), and Thaddeus Hyatt (1816–1901). The last two published theoretical works in the USA in the 1870s, but the theoretical basis for reinforced concrete evolved from the early work of William Boutland Wilkinson (1819–1902—who patented a reinforced-concrete floor system in 1854), and Joseph-Louis Lambot (1814–87— who exhibited his system of wire-mesh reinforced concrete at the 1855 Exposition Universelle, Paris). Monier licensed his patents in Germany in 1885 through Gustav Adolf Wayss (1850–1917), who in turn commissioned Matthias Koenen (1849–1924) to research the theory of reinforced concrete, but a major advance came when Hennebique developed concrete reinforced with steel (1892). In the USA advances were made by Ernest L. Ransome (1884–1911) and Albert Kahn, leading to standardization and the mass-production of building components.

Baudot's church of St-Jean de Montmartre, Paris (1894–1902), employed steel reinforcement in its brick-and-concrete construction, and Maillart evolved designs for reinforced-concrete buildings from 1905, developing the theme of unified pier and vault known as mushroom slabs. Max Berg constructed the huge Jahrhunderthalle (Century Hall) in Breslau (now Wrocław) of reinforced concrete in 1910–13, and Auguste Perret began using reinforced concrete almost from the beginning of his career with the Rue Franklin flats, Paris (1903–4). The Royal Liver Building, Liverpool (1908–10), by W. Aubrey Thomas (1859–1934), is an early British example of reinforced-concrete construction on the Hennebique principle, while the same architect's Tower Buildings, near by (1908), expresses the frame more clearly, and is clad in faïence. Reinforced concrete enabled very large cantilevers to be constructed, but its major advantages were that it was capable of withstanding great compressive and tensile loads (as steel can), but with the important advantage of a high degree of fire-resistance. The evolution of complex reinforced-concrete structures was pioneered by Freyssinet with his bridges and parabolic vaults. In later times, Candela and Nervi further developed reinforced-concrete structures (see béton).

Bibliography

A. Allen (1988, 1992);
Bennett (2001, 2002);
P. Collins (1959);
N. Davey (1961);
J. Faber & and Alsop (1976);
B. Fröhlich (2002);
Kind-Barkäuskas et al. (2002);
S. Macdonald (ed.) (2003);
W. McKay (1957);
Mainstone (1975);
Mallinson (1986);
Newby (ed.) (2001);
Stanley (1979);
Jane Turner (1996)

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"concrete." A Dictionary of Architecture and Landscape Architecture. . Encyclopedia.com. 19 Oct. 2017 <http://www.encyclopedia.com>.

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concrete

concrete, structural masonry material made by mixing broken stone or gravel with sand, cement, and water and allowing the mixture to harden into a solid mass. The cement is the chemically active element, or matrix; the sand and stone are the inert elements, or aggregate. Concrete is adaptable to widely varied structural needs, is available practically anywhere, is fire resistant, and can be used by semiskilled workers.

The use of artificial masonry similar to modern concrete dates from a remote period but did not become a standard technique of construction until the Romans adopted it (after the 2d cent. BC) for roads, immense buildings, and engineering works. The concrete of the Romans, formed by combining pozzuolana (a volcanic earth) with lime, broken stones, bricks, and tuff, was easily produced and had great durability (the Pantheon of Rome and the Baths of Caracalla were built with it). Enormous spaces could be roofed without lateral thrusts by vaults cast in the rigid homogeneous material.

Scientifically proportioned concrete formed with cement is an invention of modern times; the name did not appear until c.1830. Modern portland cement has revolutionized the production and potentialities of concrete and has superseded the natural cements, to which it is vastly superior. The component materials of concrete are mixed in varying proportions, according to the strength required and the function to be fulfilled; the proportions were first worked out by Duff Abrams in 1918. The ideal mixture is that which solidifies with the minimum of voids, the mortar and small particles of aggregate filling all interstices. A typical proportioning is 1:2:5, i.e., one part of cement, two parts of sand, and five parts of broken stone or gravel, with the proper amount of water for a pouring consistency. A simple test called a "slump test" is used to confirm the proportions and consistency of the mixture, and it is then poured into wood or steel molds, called forms. Concrete usually takes about five days to cure, or reach acceptable hardness, but a technique called steam saturation can shorten that curing time to less than 18 hours. A wide variety of additives allow the concrete to harden faster or slower, resist scaling, have increased strength, or adopt the final shape more easily.

Concrete used without strengthening is termed mass, or plain, concrete and has the structural properties of stone—great strength under compressive forces and almost none under tensile ones. F. Joseph Monier, a French inventor, found that the tensile weakness could be overcome if steel rods were embedded in a concrete member. The new composite material was called reinforced concrete, or ferroconcrete. It was patented in 1857, and a private house in Port Chester, N.Y., first demonstrated (1857) its use in the United States. It is now rivaled in popularity as a structural material only by steel. Concrete reinforced with polypropylene fibers instead of steel yields equivalent strength with a fraction of the thickness. Reinforced concrete was improved by the development of prestressed concrete—that is, concrete containing cables that are placed under tension opposite to the expected compression load before or after the concrete hardens. Another improvement, thin-shell construction, takes advantage of the inherent structural strength of certain geometric shapes, such as hemispherical and elliptical domes; in thin-shell construction great distances are spanned with very little material. The perfecting of reinforced concrete has profoundly influenced structural building techniques and architectural forms.

See A. A. Raafat, Reinforced Concrete in Architecture (1958); J. J. Waddell, Concrete Construction Handbook (1968); D. F. Orchard, Concrete Technology (1976).

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concrete

con·crete • adj. / känˈkrēt; ˈkänˌkrēt; kənˈkrēt/ existing in a material or physical form; real or solid; not abstract: concrete objects like stones. ∎  specific; definite: I haven't got any concrete proof. ∎  (of a noun) denoting a material object as opposed to an abstract quality, state, or action. • n. / ˈkänˌkrēt; känˈkrēt/ a heavy, rough building material made from a mixture of broken stone or gravel, sand, cement, and water, that can be spread or poured into molds and that forms a stonelike mass on hardening: slabs of concrete | [as adj.] the concrete sidewalk. • v. / ˈkänˌkrēt; känˈkrēt/ [tr.] (often be concreted) 1. cover (an area) with concrete. ∎  [tr.] fix in position with concrete. 2. archaic form (something) into a mass; solidify: the juices of the plants are concreted upon the surface. ∎  make real or concrete instead of abstract. PHRASES: be set in concrete (of a policy or idea) be fixed and unalterable. DERIVATIVES: con·crete·ly adv. con·crete·ness n. ORIGIN: late Middle English (in the sense ‘formed by cohesion, solidified’): from French concret or Latin concretus, past participle of concrescere ‘grow together.’ Early use was also as a grammatical term designating a quality belonging to a substance (usually expressed by an adjective such as white in white paper) as opposed to the quality itself (expressed by an abstract noun such as whiteness); later concrete came to be used to refer to nouns embodying attributes (e.g., fool, hero), as opposed to the attributes themselves (e.g., foolishness, heroism), and this is the basis of the modern use as the opposite of ‘abstract.’ The noun sense ‘building material’ dates from the mid 19th cent.

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concrete

concrete Hard, strong building material made by mixing Portland cement, sand, gravel, and water. It is an important building material. Embedded steel rods can reinforce concrete. Pre-stressed concrete contains piano wires instead of steel. Its modern use dates from the early 19th century, although the Romans made extensive use of concrete.

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concrete

concrete †united, composite; opp. to abstract XIV; sb. concreted mass XVII; composition of sand or gravel and cement XIX. — F. concret or L. concrētus, pp. of concrēscere grow together, f. CON- + crēscere grow.
So concretion XVI.

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concrete

concrete Building material composed of cement, aggregate, and water in varying proportions according to use; when mixed together the material hardens to a rock-like consistency.

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concrete

concreteaccrete, beat, beet, bittersweet, bleat, cheat, cleat, clubfeet, compete, compleat, complete, conceit, Crete, deceit, delete, deplete, discreet, discrete, eat, effete, élite, entreat, escheat, estreat, excrete, feat, feet, fleet, gîte, greet, heat, leat, leet, Magritte, maltreat, marguerite, meat, meet, mesquite, mete, mistreat, neat, outcompete, peat, Pete, petite, pleat, receipt, replete, seat, secrete, sheet, skeet, sleet, splay-feet, street, suite, sweet, teat, treat, tweet, wheat •backbeat • heartbeat • deadbeat •breakbeat • offbeat • browbeat •downbeat • drumbeat • upbeat •sugar beet • Blackfeet • flatfeet •forefeet • exegete • polychaete •lorikeet • parakeet •athlete, biathlete, decathlete, heptathlete, pentathlete, triathlete •kick-pleat • paraclete • obsolete •gamete • crabmeat • sweetmeat •mincemeat • forcemeat • backstreet •concrete • window seat

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