landscape project Las Vegas

This article possibly contains original research. Please improve it by verifying the claims made and adding inline citations. Statements consisting only of original research should be removed. (July 2016) (Learn how and when to remove this message)

A lawn (/lɔːn/) is an area of soil-covered land planted with grasses and other durable plants such as clover which are maintained at a short height with a lawn mower (or sometimes grazing animals) and used for aesthetic and recreational purposes—it is also commonly referred to as part of a garden. Lawns are usually composed only of grass species, subject to weed and pest control, maintained in a green color (e.g., by watering), and are regularly mowed to ensure an acceptable length.^[1] Lawns are used around houses, apartments, commercial buildings and offices. Many city parks also have large lawn areas. In recreational contexts, the specialised names turf, parade, pitch, field or green may be used, depending on the sport and the continent.

The term "lawn", referring to a managed grass space, dates to at least the 16th century. With suburban expansion, the lawn has become culturally ingrained in some areas of the world as part of the desired household aesthetic.^[2] However, awareness of the negative environmental impact of this ideal is growing.^[3] In some jurisdictions where there are water shortages, local government authorities are encouraging alternatives to lawns to reduce water use. Researchers in the United States have noted that suburban lawns are "biological deserts" that are contributing to a "continental-scale ecological homogenization."^[4] Lawn maintenance practices also cause biodiversity loss in surrounding areas.^[5][6] Some forms of lawn, such as tapestry lawns, are designed partly for biodiversity and pollinator support.

Lawn is a cognate of Welsh llan ( Cornish and Breton *lann* which is derived from the Common Brittonic word landa (Old French: lande) that originally meant heath, barren land, or clearing.^[7][8]

Areas of grass grazed regularly by rabbits, horses or sheep over a long period often form a very low, tight sward similar to a modern lawn. This was the original meaning of the word "lawn", and the term can still be found in place names. Some forest areas where extensive grazing is practiced still have these seminatural lawns. For example, in the New Forest, England, such grazed areas are common, and are known as lawns, for example Balmer Lawn.^{[citation needed]}

Lawns may have originated as grassed enclosures within early medieval settlements used for communal grazing of livestock, as distinct from fields reserved for agriculture.^{[citation needed]} Low, mown-meadow areas may also have been valued because they allowed those inside an enclosed fence or castle to view those approaching. The early lawns were not always distinguishable from pasture fields. The damp climate of maritime Western Europe in the north made lawns possible to grow and manage. They were not a part of gardens in most other regions and cultures of the world until contemporary influence.^[9]

In 1100s Britain, low-growing area of grasses and meadow flowers were grazed or scythed to keep them short, and used for sport.^[10] Lawn bowling, which began in the 12th or 13th century, required short turf.^[10]

Establishing grass using sod instead of seed was first documented in a Japanese text of 1159.^[10]

Lawns became popular with the aristocracy in northern Europe from the Middle Ages onward. In the fourteen hundreds, open expanses of low grasses appear in paintings of public and private areas; by the fifteen hundreds, such areas were found in the gardens of the wealthy across northern and central Europe. Public meadow areas, kept short by sheep, were used for new sports such as cricket, soccer, and golf.^[10] The word "laune" is first attested in 1540 from the Old French lande "heath, moor, barren land; clearing".^[11] It initially described a natural opening in a woodland.^[10] In the sixteen hundreds, "lawn" came to mean a grassy stretch of untilled land, and by mid-century, there were publications on seeding and transplanting sod. In the seventeen hundreds, "lawn" came to mean specifically a mown stretch of meadow.^[10]

Lawns similar to those of today first appeared in France and England in the 1700s when André Le Nôtre designed the gardens of the Palace of Versailles that included a small area of grass called the tapis vert, or "green carpet", which became a common feature of French gardens. Large, mown open spaces became popular in Europe and North America.^[10] The lawn was influenced by later seventeen-hundreds trends replicating the romantic aestheticism of grassy pastoralism from Italian landscape paintings.^[12]

Before the invention of mowing machines in 1830, lawns were managed very differently. They were an element of wealthy estates and manor houses, and in some places were maintained by labor-intensive scything and shearing (for hay or silage). They were also pasture land maintained through grazing by sheep or other livestock.^{[citation needed]}

It was not until the 17th and 18th century that the garden and the lawn became a place created first as walkways and social areas. They were made up of meadow plants, such as camomile, a particular favourite (see camomile lawn). In the early 17th century, the Jacobean epoch of gardening began; during this period, the closely cut "English" lawn was born. By the end of this period, the English lawn was a symbol of status of the aristocracy and gentry.^{[citation needed]}

In the early 18th century, landscape gardening for the aristocracy entered a golden age, under the direction of William Kent and Lancelot "Capability" Brown. They refined the English landscape garden style with the design of natural, or "romantic", estate settings for wealthy Englishmen.^[13] Brown, remembered as "England's greatest gardener", designed over 170 parks, many of which still endure. His influence was so great that the contributions to the English garden made by his predecessors Charles Bridgeman and William Kent are often overlooked.^[14]

His work still endures at Croome Court (where he also designed the house), Blenheim Palace, Warwick Castle, Harewood House, Bowood House, Milton Abbey (and nearby Milton Abbas village), in traces at Kew Gardens and many other locations.^[15] His style of smooth undulating lawns which ran seamlessly to the house and meadow, clumps, belts and scattering of trees and his serpentine lakes formed by invisibly damming small rivers, were a new style within the English landscape, a "gardenless" form of landscape gardening, which swept away almost all the remnants of previous formally patterned styles. His landscapes were fundamentally different from what they replaced, the well-known formal gardens of England which were criticised by Alexander Pope and others from the 1710s.^[16]

The open "English style" of parkland first spread across Britain and Ireland, and then across Europe, such as the garden à la française being replaced by the French landscape garden. By this time, the word "lawn" in England had semantically shifted to describe a piece of a garden covered with grass and closely mown.^[17]

Wealthy families in America during the late 18th century also began mimicking English landscaping styles. British settlers in North America imported an affinity for landscapes in the style of the English lawn. However, early in the colonization of the continent, environments with thick, low-growing, grass-dominated vegetation were rare in the eastern part of the continent, enough so that settlers were warned that it would be difficult to find land suitable for grazing cattle.^[18] In 1780, the Shaker community began the first industrial production of high-quality grass seed in North America, and a number of seed companies and nurseries were founded in Philadelphia. The increased availability of these grasses meant they were in plentiful supply for parks and residential areas, not just livestock.^[17]

Thomas Jefferson has long been given credit for being the first person to attempt an English-style lawn at his estate, Monticello, in 1806, but many others had tried to emulate English landscaping before he did. Over time, an increasing number towns in New England began to emphasize grass spaces. Many scholars link this development to the romantic and transcendentalist movements of the 19th century. These green commons were also heavily associated with the success of the Revolutionary War and often became the homes of patriotic war memorials after the Civil War ended in 1865.^[17]

Before the mechanical lawn mower, the upkeep of lawns was possible only for the extremely wealthy estates and manor houses of the aristocracy. Labor-intensive methods of scything and shearing the grass were required to maintain the lawn in its correct state, and most of the land in England was required for more functional, agricultural purposes.^{[citation needed]}

This all changed with the invention of the lawn mower by Edwin Beard Budding in 1830. Budding had the idea for a lawn mower after seeing a machine in a local cloth mill which used a cutting cylinder (or bladed reel) mounted on a bench to trim the irregular nap from the surface of woolen cloth and give a smooth finish.^[19] Budding realised that a similar device could be used to cut grass if the mechanism was mounted in a wheeled frame to make the blades rotate close to the lawn's surface. His mower design was to be used primarily to cut the lawn on sports grounds and extensive gardens, as a superior alternative to the scythe, and he was granted a British patent on 31 August 1830.^[20]

Budding went into partnership with a local engineer, John Ferrabee, who paid the costs of development and acquired rights to manufacture and sell lawn mowers and to license other manufacturers. Together they made mowers in a factory at Thrupp near Stroud.^[21] Among the other companies manufacturing under license the most successful was Ransomes, Sims & Jefferies of Ipswich which began mower production as early as 1832.^[22]

However, his model had two crucial drawbacks. It was immensely heavy (it was made of cast iron) and difficult to manoeuvre in the garden, and did not cut the grass very well. The blade would often spin above the grass uselessly.^[22] It took ten more years and further innovations, including the advent of the Bessemer process for the production of the much lighter alloy steel and advances in motorization such as the drive chain, for the lawn mower to become a practical proposition. Middle-class families across the country, in imitation of aristocratic landscape gardens, began to grow finely trimmed lawns in their back gardens.^{[citation needed]}

In the 1850s, Thomas Green of Leeds introduced a revolutionary mower design called the Silens Messor (meaning silent cutter), which used a chain to transmit power from the rear roller to the cutting cylinder. The machine was much lighter and quieter than the gear driven machines that preceded them, and won first prize at the first lawn mower trial at the London Horticultural Gardens.^[22] Thus began a great expansion in the lawn mower production in the 1860s. James Sumner of Lancashire patented the first steam-powered lawn mower in 1893.^[23] Around 1900, Ransomes' Automaton, available in chain- or gear-driven models, dominated the British market. In 1902, Ransomes produced the first commercially available mower powered by an internal combustion gasoline engine. JP Engineering of Leicester, founded after World War I, invented the first riding mowers.^{[citation needed]}

This went hand-in-hand with a booming consumer market for lawns from the 1860s onward. With the increasing popularity of sports in the mid-Victorian period, the lawn mower was used to craft modern-style sporting ovals, playing fields, pitches and grass courts for the nascent sports of football, lawn bowls, lawn tennis and others.^[24] The rise of Suburbanisation in the interwar period was heavily influenced by the garden city movement of Ebenezer Howard and the creation of the first garden suburbs at the turn of the 20th century.^[25] The garden suburb, developed through the efforts of social reformer Henrietta Barnett and her husband, exemplified the incorporation of the well manicured lawn into suburban life.^[26] Suburbs dramatically increased in size. Harrow Weald went from just 1,500 to over 10,000 while Pinner jumped from 3,00 to over 20,000. During the 1930s, over 4 million new suburban houses were built and the 'suburban revolution' had made England the most heavily suburbanized country in the world by a considerable margin.^[27]

Lawns began to proliferate in America from the 1870s onwards. As more plants were introduced from Europe, lawns became smaller as they were filled with flower beds, perennials, sculptures, and water features.^[28] Eventually the wealthy began to move away from the cities into new suburban communities. In 1856, an architectural book was published to accompany the development of the new suburbia that placed importance on the availability of a grassy space for children to play on and a space to grow fruits and vegetables that further imbued the lawn with cultural importance.^[17] Lawns began making more appearances in development plans, magazine articles, and catalogs.^[29] The lawn became less associated with being a status symbol, instead giving way to a landscape aesthetic. Improvements in the lawn mower and water supply enabled the spread of lawn culture from the Northeast to the South, where the grass grew more poorly.^[17] This in combination with setback rules, which required all homes to have a 30-foot gap between the structure and the sidewalk meant that the lawn had found a specific place in suburbia.^[28] In 1901, the United States Congress allotted $17,000 to the study of the best grasses for lawns, creating the spark for lawn care to become an industry.^[30]

After World War II, a surplus of synthetic nitrogen in the United States led to chemical firms such as DuPont seeking to expand the market for fertilizers.^[31] The suburban lawn offered an opportunity to market fertilizers, previously only used by farmers, to homeowners. In 1955, DuPont released Uramite, a slow-release nitrogen fertilizer specifically marketed for lawns. The trend continued throughout the 1960s, with chemical firms such as DuPont and Monsanto utilizing television advertising and other forms of advertisement to market pesticides, fertilizers, and herbicides.^[32] The environmental impacts of this widespread chemical use were noticed as early as the 1960s, but suburban lawns as a source of pollution were largely ignored.^[33]

Due to the harmful effects of excessive pesticide use, fertilizer use, climate change and pollution, a movement developed in the late 20th century to require organic lawn management. By the first decade of the 21st century, American homeowners were using ten times more pesticides per acre than farmers, poisoning an estimated 60 to 70 million birds yearly.^[34] Lawn mowers are a significant contributor to pollution released into Earth's atmosphere, with a riding lawn mower producing the same amount of pollution in one hour of use as 34 cars.^[34]

In recent years,^[when?] some municipalities have banned synthetic pesticides and fertilizers and required organic land care techniques be used.^[35] There are many locations with organic lawns that require organic landscaping.^{[citation needed]}

Lawn seating

A Memorial Day concert on the west lawn of the U.S. Capitol Building

Prior to European colonization, the grasses on the East Coast of North America were mostly broom straw, wild rye, and marsh grass. As Europeans moved into the region, it was noted by colonists in New England, more than others, that the grasses of the New World were inferior to those of England and that their livestock seemed to receive less nutrition from it. In fact, once livestock brought overseas from Europe spread throughout the colonies, much of the native grasses of New England disappeared, and an inventory list from the 17th century noted supplies of clover and grass seed from England. New colonists were even urged by their country and companies to bring grass seed with them to North America. By the late 17th century, a new market in imported grass seed had begun in New England.^[17]

Much of the new grasses brought by Europeans spread quickly and effectively, often ahead of the colonists. One such species, Bermuda grass (Cynodon dactylon), became the most important pasture grass for the southern colonies.^{[citation needed]}

Kentucky bluegrass (Poa pratensis) is a grass native to Europe or the Middle East. It was likely carried to Midwestern United States in the early 1600s by French missionaries and spread via the waterways to the region around Kentucky. However, it may also have spread across the Appalachian Mountains after an introduction on the east coast.^{[citation needed]}

Farmers at first continued to harvest meadows and marshes composed of indigenous grasses until they became overgrazed. These areas quickly fell to erosion and were overrun with less favorable plant life. Soon, farmers began to purposefully plant new species of grass in these areas, hoping to improve the quality and quantity of hay to provide for their livestock as native species had a lower nutritive value. While Middle Eastern and Europeans species of grass did extremely well on the East Coast of North America, it was a number of grasses from the Mediterranean that dominated the Western seaboard. As cultivated grasses became valued for their nutritional benefits to livestock, farmers relied less and less on natural meadows in the more colonized areas of the country. Eventually even the grasses of the Great Plains were overrun with European species that were more durable to the grazing patterns of imported livestock.^[17]

A pivotal factor in the spread of the lawn in America was the passage of legislation in 1938 of the 40-hour work week. Until then, Americans had typically worked half days on Saturdays, leaving little time to focus on their lawns. With this legislation and the housing boom following the Second World War, managed grass spaces became more commonplace.^[28] The creation in the early 20th century of country clubs and golf courses completed the rise of lawn culture.^[17]

According to study based on satellite observations by Cristina Milesi, NASA Earth System Science, its estimates: "More surface area in the United States is devoted to lawns than to individual irrigated crops such as corn or wheat.... area, covering about 128,000 square kilometers in all."^[36]

Lawn monoculture was a reflection of more than an interest in offsetting depreciation, it propagated the homogeneity of the suburb itself. Although lawns had been a recognizable feature in English residences since the 19th century, a revolution in industrialization and monoculture of the lawn since the Second World War fundamentally changed the ecology of the lawn. Money and ideas flowed back from Europe after the U.S. entered WWI, changing the way Americans interacted with themselves and nature, and the industrialization of war hastened the industrialization of pest control.^[37] Intensive suburbanization both concentrated and expanded the spread of lawn maintenance which meant increased inputs in not only petrochemicals, fertilizers, and pesticides, but also natural resources like water.^[2][17][28]

Lawns became a means of performing class values for the urban middle class, in which the condition of the lawn becomes representative of moral character and social reliability. The social values associated with lawns are promoted and upheld by social pressure, laws, and chemical producers. Social pressure comes from neighbors or homeowner associations who think that the unkempt lawns of neighbors may affect their own property values or create eyesores. Pressures to maintain a lawn are also legal; there are often local or state laws against letting weeds get too tall or letting a lawn space be especially unkempt, punishable by fees or litigation. Chemical producers unwilling to lose business propagate the ideal of a lawn, making it seem unattainable without chemical aid.^[12]

Front lawns became standardized in the 1930s when, over time, specific aspects such as grass type and maintenance methods became popular. The lawn-care industry boomed, but the Great Depression of the 1930s and in the period prior to World War II made it difficult to maintain the cultural standards that had become heavily associated with the lawn due to grass seed shortages in Europe, America's main supplier. Still, seed distributors such as Scotts Miracle-Gro Company in the United States encouraged families to continue to maintain their lawns, promoting it as a stress-relieving hobby. During the war itself, homeowners were asked to maintain the appearances of the home front, likely as a show of strength, morale, and solidarity. After World War II, the lawn aesthetic once again became a standard feature of North America, bouncing back from its minor decline in the decades before with a vengeance, particularly as a result of the housing and population boom post-war.^[17]

The VA loan in the United States let American ex-servicemen buy homes without providing a down payment, while the Federal Housing Administration offered lender inducements that aided the reduction of down payments for the average American from 30% to as little as 10%. These developments made owning your own home cheaper than renting, further enabling the spread of suburbia and its lawns.^[28]

Levittown, New York, was the beginning of the industrial suburb in the 20th century, and by proxy the industrial lawn. Between 1947 and 1951, Abraham Levitt and his sons built more than seventeen thousand homes, each with its own lawn. Abraham Levitt wrote "No single feature of a suburban residential community contributes as much to the charm and beauty of the individual home and the locality as well-kept lawns". Landscaping was one of the most important factors in Levittown's success – and no feature was more prominent than the lawn. The Levitts understood that landscaping could add to the appeal of their developments and claimed that, "increase in values are most often found in neighborhoods where lawns show as green carpets" and that, over the years, "lawns trees and shrubs become more valuable both aesthetically and monetarily".^[38] During 1948, the first spring that Levittown had enjoyed, Levitt and Sons fertilized and reseeded all of the lawns free of charge.^[28]

The economic recession that began in 2008 has resulted in many communities worldwide to dig up their lawns and plant fruit and vegetable gardens.^{[citation needed]} This has the potential to greatly change cultural values attached to the lawn, as they are increasingly viewed as environmentally and economically unviable in the modern context.^[39]

The appearance of the lawn in Australia followed closely after its establishment in North America and parts of Europe. Lawn was established on the so-called "nature strip" (a uniquely Australian term) by the 1920s and was common throughout the developing suburbs of Australia. By the 1950s, the Australian-designed Victa lawn mower was being used by the many people who had turned pastures into lawn and was also being exported to dozens of countries.^[40] Prior to the 1970s, all brush and native species were stripped from a development site and replaced with lawns that utilized imported plant species. Since the 1970s there has been an interest in using indigenous species for lawns, especially considering their lower water requirements.^[41] Lawns are also established in garden areas as well as used for the surface of sporting fields.^{[citation needed]}

Over time, with consideration to the frequency of droughts in Australia, the movement towards "naturalism", or the use of indigenous plant species in yards, was beneficial. These grasses were more drought resistant than their European counterparts, and many who wished to keep their lawns switched to these alternatives or allowed their green carpets to revert to the indigenous scrub in an effort to reduce the strain on water supplies.^[39] However, lawns remain a popular surface and their practical and aesthetically pleasing appearance reduces the use of water-impervious surfaces such as concrete. The growing use of rainwater storage tanks has improved the ability to maintain them.^{[citation needed]}

Following recent droughts,^[when?] Australia has seen a change to predominately warm-season turfgrasses, particularly in the southern states like New South Wales and Victoria which are predominately temperate climates within urban regions. The more drought tolerant grasses have been chosen by councils and homeowners for the choice of using less water compared to cool-season turfgrasses like fescue and ryegrass. Mild dormancy seems to be of little concern when high-profile areas can be oversown for short periods or nowadays, turf colourants (fake green) are very popular.^{[citation needed]}

Lawns are a common feature of private gardens, public landscapes and parks in many parts of the world. They are created for aesthetic pleasure, as well as for sports or other outdoor recreational use. Lawns are useful as a playing surface both because they mitigate erosion and dust generated by intensive foot traffic and because they provide a cushion for players in sports such as rugby, football, soccer, cricket, baseball, golf, tennis, field hockey, and lawn bocce.^{[citation needed]}

Lawns and the resulting lawn clipping waste can be used as an ingredient in making compost and is also viewed as fodder, used in the production of lawn clipping silage which is fed to livestock^[42][43] as a sustainable feed source.

Lawns need not be, and have not always been, made up of grasses alone. There exist, for instance, moss lawns, clover lawns, thyme lawns, and tapestry lawns (made from diverse forbs). Sedges, low herbs and wildflowers, and other ground covers that can be walked upon are also used.^{[citation needed]}

Thousands of varieties of grasses and grasslike plants are used for lawns, each adapted to specific conditions of precipitation and irrigation, seasonal temperatures, and sun/shade tolerances. Plant hybridizers and botanists are constantly creating and finding improved varieties of the basic species and new ones, often more economical and environmentally sustainable by needing less water, fertilizer, pest and disease treatments, and maintenance. The three basic categories are cool season grasses, warm season grasses, and grass alternatives.^{[citation needed]}

Many different species of grass are currently used, depending on the intended use and the climate. Coarse grasses are used where active sports are played, and finer grasses are used for ornamental lawns for their visual effects. Some grasses are adapted to oceanic climates with cooler summers, and others to tropical and continental climates with hotter summers. Often, a mixture of grass or low plant types is used to form a stronger lawn when one type does better in the warmer seasons and the other in the colder ones. This mixing is taken further by a form of grass breeding which produces what are known as cultivars. A cultivar is a cross-breed of two different varieties of grass and aims to combine certain traits taken from each individual breed. This creates a new strain which can be very specialised, suited to a particular environment, such as low water, low light or low nutrient.^{[citation needed]}

Cool season grasses start growth at 5 °C (41 °F), and grow at their fastest rate when temperatures are between 10 °C (50 °F) and 25 °C (77 °F), in climates that have relatively mild/cool summers, with two periods of rapid growth in the spring and autumn.^[44] They retain their color well in extreme cold and typically grow very dense, carpetlike lawns with relatively little thatch.^{[citation needed]}

• Bluegrass (Poa spp.)
• Bentgrass (Agrostis spp.)
• Ryegrasses (Lolium spp.)
• Fescues (Festuca spp.)
• Feather reed grass (Calamagrostis spp.)
• Tufted hair grass (Deschampsia spp.)

Warm season grasses only start growth at temperatures above 10 °C (50 °F), and grow fastest when temperatures are between 25 °C (77 °F) and 35 °C (95 °F), with one long growth period over the spring and summer (Huxley 1992). They often go dormant in cooler months, turning shades of tan or brown. Many warm season grasses are quite drought tolerant, and can handle very high summer temperatures, although temperatures below −15 °C (5 °F) can kill most southern ecotype warm season grasses. The northern varieties, such as buffalograss and blue grama, are hardy to 45 °C (113 °F).

• Zoysiagrass (Zoysia spp.)
• Bermudagrass (Cynodon spp.)
• St. Augustine grass (Stenotaphrum secundatum)
• Bahiagrass (Paspalum spp.)
• Centipedegrass (Eremochloa ophiuroides)
• Carpet grass (Axonopus spp.)
• Buffalograss (Bouteloua dactyloides)
• Grama grass (Bouteloua spp.)
• Kikuyu grass (Pennisetum clandestinum)

Grass seed mixes have been developed to include only grass seed species that grow will in low sunlight conditions. These seed mixes are designed to deal with light shade caused by trees that can create patchiness, or slightly heavier shade that prevents the full growth of grass. Most lawns will experience shade in some shape or form due to surrounding fences, furniture, trees or hedges and these grass seed species' are especially useful in the Northern Hemisphere and Northwestern Europe.^[45]

• Festuca rubra subsp. commutata (Chewings Fescue)
• Poa pratensis (Smooth Stalked Meadow Grass)
• Festuca ovina (Sheeps Fescue)
• Festuca trachyphylla (hard fescue)
• Festuca rubra (Strong Creeping Red Fescue)

Carex species and cultivars are well represented in the horticulture industry as 'sedge' alternatives for 'grass' in mowed lawns and garden meadows. Both low-growing and spreading ornamental cultivars and native species are used in for sustainable landscaping as low-maintenance and drought-tolerant grass replacements for lawns and garden meadows. Wildland habitat restoration projects and natural landscaping and gardens also use them for 'user-friendly' areas. The J. Paul Getty Museum has used Carex pansa (meadow sedge) and Carex praegracilis (dune sedge) expansively in the Sculpture Gardens in Los Angeles.^[46]

Some lower sedges used are:

• Carex caryophyllea (cultivar 'The Beatles')
• C. divulsa (Berkeley sedge)^[46]
• C. glauca (blue sedge) (syn. C. flacca)
• C. pansa (meadow sedge)^[46]
• C. praegracilis (dune sedge)^[46]
• C. subfusca (mountain sedge)^[46]
• C. tumulicola (foothill sedge) (cultivar 'Santa Cruz Mnts. selection')^[46]
• C. uncifolia (ruby sedge)

Moss lawns do well in shaded areas under trees, and require only about 1% of the water of a traditional grass lawn once established.^[48][47][49] Clover lawns do especially well in damp, alkaline soils. Yarrow lawns are drought resistant, can be mowed to form a soft, comfortable turf; common yarrow is native throughout Europe, North America, and parts of Asia, and spreads vegetatively to cover the ground.^[50][51][52] Camomile lawns and thyme lawns are fragrant (and native to Europe an North Africa). Soleirolia soleirolii favours shaded, damp spaces (and is often used in tsubo-niwas); it is native to the European side of the Mediterranean, and can be invasive elsewhere.^[53]

Other low ground covers suitable for lawns include Corsican mint (native to three mediterranean islands, invasive), Ophiopogon planiscapus (native to Japan),^[53] Lippia^[54] and lawnleaf,^[55] (native to Central America and southern North America),^[55][54] purple flowering Mazus (native to East Asia), grey Dymondia (native to South Africa), creeping sedums (various species native to various continents),^[54] Cotula species (ditto),^[55] and creeping jenny (native to Europe).^[54]

The examples and perspective in this section may not represent a worldwide view of the subject. You may improve this section, discuss the issue on the talk page, or create a new section, as appropriate. (October 2024) (Learn how and when to remove this message)

Some plants native to Eastern North America that can be used as alternatives to grass lawns or incorporated into lawns are:^{[56][57][58][59]}

• Common yarrow
• Virginia springbeauty^[60]
• Wild strawberry
• Dwarf cinquefoil
• Moss phlox
• Creeping phlox
• Sensitive fern
• Canadian wild ginger
• Cinnamon fern
• Lyreleaf sage^[61]
• Allegheny pachysandra
• Woodland stonecrop
• Green-and-gold
• Beetleweed
• Blue-eyed grass
• Common blue violet
• Dwarf crested iris
• Wild pink
• Purple wood sorrel
• Spotted cranesbill

Alternatives to lawns include meadows, drought-tolerant xeriscape gardens, natural landscapes, native plant habitat gardens, paved Spanish courtyard and patio gardens, butterfly gardens, rain gardens, and kitchen gardens. Trees and shrubs in close proximity to lawns provide habitat for birds in traditional, cottage and wildlife gardens.^{[62][citation needed]}

Seasonal lawn establishment and care varies depending on the climate zone and type of lawn grown.^{[citation needed]}

Early autumn, spring, and early summer are the primary seasons to seed, lay sod (turf), plant 'liners', or 'sprig' new lawns, when the soil is warmer and air cooler. Seeding is the least expensive, but may take longer for the lawn to be established. Aerating just before planting/seeding may promote deeper root growth and thicker turf.^[63]

Sodding (American English), or turfing (British English), provides an almost instant lawn, and can be undertaken in most temperate climates in any season, but is more expensive and more vulnerable to drought until established. Hydroseeding is a quick, less expensive method of planting large, sloped or hillside landscapes. Some grasses and sedges are available and planted from 'liner' and 4-inch (100 mm) containers, from 'flats', 'plugs' or 'sprigs', and are planted apart to grow together.^{[citation needed]}

Lawn growth, 20-hour time lapse

Various organic and inorganic or synthetic fertilizers are available, with instant or time-release applications. Pesticides, which includes biological and chemical herbicides, insecticides and fungicides, treating diseases like gray leaf spot, are available. Consideration for their effects on the lawn and garden ecosystem and via runoff and dispersion on the surrounding environment, inform laws constraining their use. For example, the Canadian province of Quebec and over 130 municipalities prohibit the use of synthetic lawn pesticides.^[64] The Ontario provincial government promised in September 2007 to also implement a province-wide ban on the cosmetic use of lawn pesticides, for protecting the public. Medical and environmental groups supported such a ban.^[65]

On 22 April 2008, the Provincial Government of Ontario announced that it would pass legislation that would prohibit, province-wide, the cosmetic use and sale of lawn and garden pesticides.^[66] The Ontario legislation would also echo Massachusetts law requiring pesticide manufacturers to reduce the toxins they use in production.^[67] Experts^[who?] advise that a healthy lawn contains at least some "weeds" and insects, discouraging indiscriminate use of potentially harmful chemicals.^[34]

Sustainable gardening uses organic horticulture methods, such as organic fertilizers, biological pest control, beneficial insects, and companion planting, among other methods, to sustain an attractive lawn in a safe garden. An example of an organic herbicide is corn gluten meal, which releases an 'organic dipeptide' into the soil to inhibit root formation of germinating weed seeds. An example of an organic alternative to insecticide use is applying beneficial nematodes to combat soil-dwelling grubs, such as the larvae of chafer beetles. The Integrated Pest Management approach is a coordinated low impact approach.^[68]

Maintaining a rough lawn requires only occasional cutting with a suitable machine, or grazing by animals. Maintaining a smooth and closely cut lawn, be it for aesthetic or practical reasons or because social pressure from neighbors and local municipal ordinances requires it,^[69] necessitates more organized and regular treatments. Usually once a week is adequate for maintaining a lawn in most climates. However, in the hot and rainy seasons of regions contained in hardiness zones greater than 8, lawns may need to be maintained up to two times a week.^[70]

Low-maintenance alternatives to traditional turfgrass lawns reduce the need for frequent mowing, watering and chemical inputs.^[71]

The prevalence of the lawns in films such as Pleasantville (1998) and Edward Scissorhands (1990) alludes to the importance of the lawn as a social mechanism that gives great importance to visual representation of the American suburb as well as its practised culture. It is implied that a neighbor whose lawn is not in pristine condition is morally corrupt, emphasizing the role a well-kept lawn plays in neighborly and community relationships. In both of these films, green space surrounding a house in the suburbs becomes an indicator of moral integrity as well as of social and gender norms – lawn care has long been associated^{[by whom?]} with men. These lawns also reinforce class and societal norms by subtly excluding those who may not have been able to afford a house with a lawn.^[72]

The lawn as a reflection of someone's character and the neighborhood at large is not restricted to films; the same theme appears in The Great Gatsby (1925), by American novelist F. Scott Fitzgerald.^[73] Character Nick Carraway rents the house next to Gatsby's and fails to maintain his lawn according to West Egg standards. The rift between the two lawns troubles Gatsby to the point that he dispatches his gardener to mow Carraway's grass and thereby establish uniformity.^{[citation needed]}

Most lawn-care equipment over the decades has been advertised to men, and companies have long associated good lawn-care with good citizenship in their marketing campaigns. The appearance of a healthy lawn was meant^{[by whom?]} to imply the health of the man taking care of it; controlled weeds and strict boundaries became a practical application of the desire to control nature, as well as an expression of control over personal lives once working full-time became central to suburban success. Women were encultured over time to view the lawn as part of the household, as an essential furnishing, and to encourage their husbands to maintain a lawn for the family and community reputation.^[17]

During World War II (1939–1945), women became the focus of lawn-care companies in the absence of their husbands and sons. These companies promoted lawn care as a necessary means by which women could help support their male family-members and American patriotism as a whole. The image of the lawn changed from focusing on technology and manhood to emphasizing aesthetic pleasure and the health benefits derived from its maintenance; advertisers at lawn care companies assumed that women would not respond positively to images of efficiency and power. The language of these marketing campaigns still intended to imbue the female population with notions of family, motherhood, and the duties of a wife; it has been argued^{[by whom?]} that this was done so that it would be easier for men returning from war to resume the roles which their wives had taken over in their absence. This was especially apparent in the 1950s and 1960s, when lawn-care rhetoric emphasized the lawn as a husband's responsibility and as a pleasurable hobby when he retired.^[17]

There are differences in the particulars of lawn maintenance and appearance, such as the length of the grass, species (and therefore its color), and mowing.^[41][74]

On average, greater amounts of chemical fertilizer, herbicide and pesticide are used to maintain a given area of lawn than on an equivalent area of cultivated farmland.^[75][28] The use of these products causes environmental pollution, disturbance in the lawn ecosystem, and health risks to humans and wildlife.^[76]

In response to environmental concerns, organic landscaping and organic lawn management systems have been developed and are mandated in some municipalities and properties. In the United Kingdom, the environmental group Plantlife has encouraged gardeners to refrain from mowing in the month of May to encourage plant diversity and provide nectar for insects.^[77]

Other concerns, criticisms, and ordinances regarding lawns arise from wider environmental consequences:

• Lawns can reduce biodiversity, especially when the lawn covers a large area.^[78] Traditional lawns often replace plant species that feed pollinators, requiring bees and butterflies to cross "wastelands" to reach food and host plants.^[79] Lawns promote homogenization and are normally cleared of unwanted plant and animal species, typically with synthetic pesticides, which can also kill unintended target species. They may be composed of introduced species not native to the area, particularly in the United States. This can produce a habitat that supports a reduced number of wildlife species.^[80]
• Lawn maintenance commonly involves use of fertilizers and synthetic pesticides, which can cause great harm. Some are carcinogens and endocrine disruptors. They may permanently linger in the environment and negatively affect the health of potentially all nearby organisms. The United States Environmental Protection Agency estimated in 2012 that nearly 32,000,000 kilograms (71,000,000 lb) of active pesticide ingredients are used on suburban lawns each year in the United States.^[81] There are indications of an emerging regulatory response to this issue. For example, Sweden, Denmark, Norway, Kuwait, and Belize have placed restrictions on the use of the herbicide 2,4-D.
• It has been estimated that nearly 64,000,000 litres (14,000,000 imp gal; 17,000,000 US gal) of gasoline are spilled each summer while re-fueling garden and lawn-care equipment in the United States: approximately 50% more than that spilled during the Exxon Valdez incident.^[28]
• The use of pesticides and fertilizers, requiring fossil fuels for manufacturing, distribution, and application, has been shown to contribute to global warming. (Sustainable organic techniques have been shown to help reduce global warming.)^[82] A hectare of lawn in Nashville, Tennessee, produces greenhouse gases equivalent to 697 to 2,443 kg of carbon dioxide a year. The higher figure is equivalent to a flight more than halfway around the world. Lawn mowing is one element of lawn culture that causes a great amount of emissions (which can be mitigated by replacing lawn mowers with grazing livestock). ^[83]

Maintaining a green lawn sometimes requires large amounts of water. While natural rainfall is usually sufficient to maintain a lawn's health in the temperate British Isles- the birthplace of the concept of the lawn- in times of drought hosepipe bans may be implemented by the water suppliers.^[84] Conversely, exportation of the lawn ideal to more arid regions (e.g. U.S. Southwest and Australia) strains water supply systems when water supplies are already scarce. This necessitates upgrades to larger, more environmentally invasive equipment to deal with increased demand due to lawn watering. Grass typically goes dormant during periods of cold or heat outside of its preferred temperature ranges; dormancy reduces the grasses' water demand. Most grasses typically recover quite well from a drought, but many property owners become concerned about the brown appearance and increase watering during the summer months. Water in Australia observed 1995 data that up to 90% of the water used in Canberra during summer drought periods was used for watering lawns.^[85]

In the United States, 50 to 70% of residential water is used for landscaping, with most used to water lawns.^[81] A 2005 NASA study estimated conservatively 128,000 square kilometres (49,000 sq mi; 32,000,000 acres) of irrigated lawn in the US, three times the area of irrigated corn.^[86] That translates to about 200 US gallons (760 L; 170 imp gal) of drinking-quality fresh water per person per day is required to keep up United States' lawn surface area.^{[citation needed]}

In 2022, the state of Nevada pass a bill that not only banned the installation of new lawns in the state, but also mandated the removal of any lawn deemed "nonfunctional." This was in response to a years-long drought in the state. ^[87]

An increased concern from the general public over pesticide and fertilizer use and their associated health risks, combined with the implementation of the legislation, such as the US Food Quality Protection Act, has resulted in the reduced presence of synthetic chemicals, namely pesticides, in urban landscapes such as lawns in the late 20th century.^[88] Many of these concerns over the safety and environmental impact of some of the synthetic fertilizers and pesticides has led to their ban by the United States Environmental Protection Agency and many local governments.^[76] The use of pesticides and other chemicals to care for lawns has also led to the death of nearly 7 million birds each year, a topic that was central to the novel Silent Spring by the conservationist Rachel Carson.^[28]

The use of lawn chemicals made its first appearance in the 18th century through the introduction of "English garden" fads. These types of lawns put precise hedging, clean cut grass, and extravagant plants on display. Following the initial introduction of lawn chemicals, they have still been continually used throughout North America. Because many of the turf-grass species in North America are not native to our ecosystems, they require extensive maintenance. According to the United States Geological Survey, 99% of the urban water samples that were tested contained one or more types of pesticides. In addition to water contamination, chemicals are making their way into houses which can lead to chronic exposure. Currently, standards for pesticide management practices have been put in place through the Food Quality Protection Act.^[12]

In the United States, lawn heights are generally maintained by gasoline-powered lawn mowers, which contribute to urban smog during the summer months.^[89] The EPA found, in some urban areas, up to 5% of smog was due to small gasoline engines made before 1997, such as are typically used on lawn mowers. Since 1997, the EPA has mandated emissions controls on newer engines in an effort to reduce smog.^[90]

A 2010 study seemed to show lawn care inputs were balanced by the carbon sequestration benefits of lawns, and they may not be contributors to anthropogenic global warming.^[91][92] Lawns with high maintenance (mowing, irrigation, and leaf blowing) and high fertilization rates have a net emission of carbon dioxide and nitrous oxide that have large global warming potential.^[93] Lawns that are fertilized, irrigated, and mowed weekly have a lower species diversity.^[94]

Replacing turf grass with low-maintenance groundcovers or employing a variety of low-maintenance perennials, trees and shrubs^[80] can be a good alternative to traditional lawn spaces, especially in hard-to-grow or hard-to-mow areas, as it can reduce maintenance requirements, associated pollution and offers higher aesthetic and wildlife value.^[95][71] Growing a mixed variety of flowering plants instead of turfgrass is sometimes referred to as meadowscaping.^[96]

Lawns take up space that could otherwise be used more productively, such as for urban agriculture or home gardening. This is the case in many cities and suburbs in the United States, where open or unused spaces are "not generally a result of a positive decision to leave room for some use, but rather is an expression of a pastoral aesthetic norm that prizes spacious lawns and the zoning restrictions and neighborhood covenants that give these norms the force of law."^[97]

In urban and suburban spaces, growing food in front yards and parking strips can not only provide fresh produce but also be a source of neighborhood pride.^[98] While converting lawn space into strictly utilitarian farms is not common, incorporating edible plants into front yards with sustainable and aesthetically pleasing design is of growing interest in the United States.^[99]

• Bacterial lawn
• Moss lawn
• Tapestry lawn
• Organic lawn management
• Gardening
• List of organic gardening and farming topics

• Bormann, F. Herbert, et al. (1993) Redesigning the American Lawn.
• Hessayon, D. G. (1997). The Lawn Expert. Expert. ISBN 978-0-903505-48-2.
• Huxley, A., ed. (1992). New RHS Dictionary of Gardening. Lawns: Ch. 3: pp. 26–33. Macmillan. ISBN 0-333-47494-5.
• Jenkins, V. S. (1994). The Lawn: A History of an American Obsession. Smithsonian Books. ISBN 1-56098-406-6.
• Steinberg, T. (2006). American Green, The Obsessive Quest for the Perfect Lawn. W.W. Norton & Co. ISBN 0-393-06084-5.
• Wasowski, Sally and Andy (2004). Requiem for a Lawnmower.

Wikimedia Commons has media related to Lawns.

Wikisource has the text of the 1920 Encyclopedia Americana article Lawns.

• "Planting and care of Lawns" from the UNT Govt. Documents Dept.
• Integrated Pest Management Program: website & search-engine
• How to look after your Lawn
• Lawn Care University at Michigan State University
• "EPA Management of Polluted Runoff: Nonpoint Source Pollution" (includes mismanagement of lawns problems.)

Artificial water basin for swimming

For other uses, see Swimming pool (disambiguation).

 

A swimming pool, swimming bath, wading pool, paddling pool, or simply pool, is a structure designed to hold water to enable swimming and associated activities. Pools can be built into the ground (in-ground pools) or built above ground (as a freestanding construction or as part of a building or other larger structure), and may be found as a feature aboard ships. In-ground pools are most commonly constructed from materials such as concrete, natural stone, metal, plastic, composite or fiberglass, and may follow a standardized size, the largest of which is the Olympic-size swimming pool, or be of a custom shape.

Many health clubs, fitness centers, and private clubs have pools for their members, often used for exercise. In much of the world, local governments provide publicly-run pools for their citizens. Many of these are outdoors; indoor pools are typically part of a leisure centre. Many hotels have a pool for the use of their guests. Pools as a feature in hotels are more common in tourist areas or near convention centers. Many universities and other institutional communities provide pools for their members., often as part of an institution-specific athletic or recreational complex. Apartment complexes and residential subdivisions may provide a pool for the use of their residents. Private residences, particularly in areas with warm climates, may have their own pools.

Educational facilities such as high schools and universities often have pools for physical education classes, recreational activities, leisure, and competitive athletics such as swimming teams. Hot tubs and spas are small heated pools used for relaxation or hydrotherapy. Specialised pools are also used for diving, water sports, and physical therapy, as well as for training of lifeguards and astronauts. Swimming pools most commonly use chlorinated water, or salt water, and may be heated or unheated.

History

[edit]

The "Great Bath" at the site of Mohenjo-Daro in modern-day Pakistan was most likely the first swimming pool, dug during the 3rd millennium BC. This pool is 12 by 7 metres (39 by 23 feet), is lined with bricks, and was covered with a tar-based sealant.^[1]

Ancient Greeks and Romans built artificial pools for athletic training in the palaestras, for nautical games and for military exercises. Roman emperors had private swimming pools in which fish were also kept, hence one of the Latin words for a pool was piscina. The first heated swimming pool was built by Gaius Maecenas in his gardens on the Esquiline Hill of Rome, likely sometime between 38 and 8 BC.^[2] Gaius Maecenas was a wealthy imperial advisor to Augustus and considered one of the first patrons of arts.^[3]

Ancient Sinhalese built a pair of pools called "Kuttam Pokuna" in the kingdom of Anuradhapura, Sri Lanka, in the 6th century AD. They were decorated with flights of steps, punkalas or pots of abundance, and scroll design.^[4][5]

Swimming pools became popular in Britain in the mid-19th century. As early as 1837, six indoor pools with diving boards existed in London, England.^[6] The Maidstone Swimming Club in Maidstone, Kent is believed to be the oldest surviving swimming club in Britain. It was formed in 1844, in response to concerns over drownings in the River Medway, especially since would-be rescuers would often drown because they themselves could not swim to safety. The club used to swim in the River Medway, and would hold races, diving competitions and water polo matches. The South East Gazette July 1844 reported an aquatic breakfast party: coffee and biscuits were served on a floating raft in the river. The coffee was kept hot over a fire; club members had to tread water and drink coffee at the same time. The last swimmers managed to overturn the raft, to the amusement of 150 spectators.^[7]

The Amateur Swimming Association was founded in 1869 in England,^{[citation needed]} and the Oxford Swimming Club in 1909.^[8] The presence of indoor baths in the cobbled area of Merton Street might have persuaded the less hardy of the aquatic brigade to join. So, bathers gradually became swimmers, and bathing pools became swimming pools.^{[citation needed]} In 1939, Oxford created its first major public indoor pool at Temple Cowley.

The modern Olympic Games started in 1896 and included swimming races, after which the popularity of swimming pools began to spread. In the US, the Racquet Club of Philadelphia clubhouse (1907) boasts one of the world's first modern above-ground swimming pools. The first swimming pool to go to sea on an ocean liner was installed on the White Star Line's Adriatic in 1906.^[9] The oldest known public swimming pool in the U.S., Underwood Pool, is located in Belmont, Massachusetts.^[10]

Interest in competitive swimming grew following World War I. Standards improved and training became essential. Home swimming pools became popular in the United States after World War II and the publicity given to swimming sports by Hollywood films such as Esther Williams' Million Dollar Mermaid made a home pool a desirable status symbol. More than 50 years later, the home or residential swimming pool is a common sight. Some small nations enjoy a thriving swimming pool industry (e.g., New Zealand pop. 4,116,900 – holds the record in pools per capita with 65,000 home swimming pools and 125,000 spa pools).^[12]

A two-storey, white concrete swimming pool building composed of horizontal cubic volumes built in 1959 at the Royal Roads Military College is on the Canadian Register of Historic Places.^[13]

According to the Guinness World Records, the largest swimming pool in the world is San Alfonso del Mar Seawater pool in Algarrobo, Chile. It is 1,013 m (3,323 ft) long and has an area of 8 ha (20 acres). At its deepest, it is 3.5 m (11 ft) deep.^[14] It was completed in December 2006.^[15]

The largest indoor wave pool in the world is at DreamWorks Water Park within the American Dream shopping and entertainment complex at the Meadowlands Sports Complex in East Rutherford, New Jersey, United States, and the largest indoor pool in North America is at the Neutral Buoyancy Lab in the Sonny Carter Training Facility at NASA JSC in Houston.^[16][17]

In 2021, Deep Dive Dubai, located in Dubai, UAE, was certified by the Guinness Book of World Records as the world's deepest swimming pool reaching 60 metres (200 ft).^[18][19] The Y-40 swimming pool at the Hotel Terme Millepini in Padua, Italy, previously held the record, 42.15 m (138.3 ft), from 2014 until 2021.^[20]

The Fleishhacker Pool in San Francisco was the largest heated outdoor swimming pool in the United States. Opened on 23 April 1925, it measured 1,000 by 150 ft (300 by 50 m) and was so large that the lifeguards required kayaks for patrol. It was closed in 1971 due to low patronage.^[21]

In Europe, the largest swimming pool opened in 1934 in ElblÄ…g (Poland), providing a water area of 33,500 square metres (361,000 sq ft).^[22]

One of the largest swimming pools ever built was reputedly created in Moscow after the Palace of Soviets remained uncompleted. The foundations of the palace were converted into the Moskva Pool open-air swimming pool after the process of de-Stalinisation.^[23] However, after the fall of communism, Christ the Saviour Cathedral was re-built on the site between 1995 and 2000; the cathedral had originally been located there.^{[citation needed]}

The highest swimming pool is believed to be in Yangbajain (Tibet, China). This resort is located at 4,200 m (13,800 ft) AMSL and has two indoor swimming pools and one outdoor swimming pool, all filled with water from hot springs.^[24]

Length: Most pools in the world are measured in metres, but in the United States pools are often measured in feet and yards. In the UK most pools are calibrated in metres, but older pools measured in yards still exist. In the US, pools tend to either be 25 yards (SCY-short course yards), 25 metres (SCM-short course metres) or 50 metres (LCM - long course meters). US high schools and the NCAA conduct short course (25 yards) competition. There are also many pools 33+1⁄3 m long, so that 3 lengths = 100 m. This pool dimension is commonly used to accommodate water polo.^{[citation needed]}

USA Swimming (USA-S) swims in both metric and non-metric pools. However, the international standard is metres, and world records are only recognized when swum in 50 m pools (or 25 m for short course) but 25-yard pools are very common in the US. In general, the shorter the pool, the faster the time for the same distance, since the swimmer gains speed from pushing off the wall after each turn at the end of the pool.

Width: The width of the pool depends on the number of swimming lanes and the width of each individual lane. In an Olympic swimming pool each lane is 2.5 meters wide^[25] and contains 10 lanes, thus making the pool 25 meters wide.

Depth: The depth of a swimming pool depends on the purpose of the pool, and whether it is open to the public or strictly for private use. If it is a private casual, relaxing pool, it may go from 1.0 to 2.0 m (3.3 to 6.6 ft) deep. If it is a public pool designed for diving, it may slope from 3.0 to 5.5 m (10 to 18 ft) in the deep end. A children's play pool may be from 0.3 to 1.2 m (1 to 4 ft) deep. Most public pools have differing depths to accommodate different swimmer requirements. In many jurisdictions, it is a requirement to show the water depth with clearly marked depths affixed to the pool walls,^[26][27] although this may not be the case for private pools in some jurisdictions.^[28]

Pools can be either indoors or outdoors. They can be of any size and shape, and inground or above ground. Most pools are permanent fixtures, while others are temporary, collapsible structures.

Private pools are usually smaller than public pools, on average 3.7 m × 7.3 m (12 ft × 24 ft) to 6.1 m × 12.2 m (20 ft × 40 ft) whereas public pools usually start at 20 m (66 ft).^{[citation needed]} Home pools can be permanently built-in, or be assembled above ground and disassembled after summer. Privately owned outdoor pools in backyards or gardens started to proliferate in the 1950s in regions with warm summer climates, particularly in the United States with desegregation.^[29] A plunge pool is a smaller, permanently installed swimming pool, with a maximum size of approximately 3 m × 6 m (10 ft × 20 ft).^[30][31][32]

Construction methods for private pools vary greatly. The main types of in-ground pools are gunite shotcrete, concrete, vinyl-lined, and one-piece fiberglass shells.

Many countries now have strict pool fencing requirements for private swimming pools, which require pool areas to be isolated so that unauthorized children younger than six years cannot enter. Many countries require a similar level of protection for the children residing in or visiting the house, although many pool owners prefer the visual aspect of the pool in close proximity to their living areas, and will not provide this level of protection. There is no consensus between states or countries on the requirements to fence private swimming pools, and in many places they are not required at all, particularly in rural settings.^[33]

Inexpensive temporary polyvinyl chloride pools can be bought in supermarkets and taken down after summer. They are used mostly outdoors in yards, are typically shallow, and often their sides are inflated with air to stay rigid. When finished, the water and air can be let out and this type of pool can be folded up for convenient storage. They are regarded in the swimming pool industry as "splasher" pools intended for cooling off and amusing toddlers and children, not for swimming, hence the alternate name of "kiddie" pools.^{[citation needed]}

Toys are available for children and other people to play with in pool water. They are often blown up with air so they are soft but still reasonably rugged, and can float in water.

• 
  A black Labrador Retriever bathing in a kiddie pool
• 
  Children playing in an inflatable pool

Public pools are often part of a larger leisure center or recreational complex. These centres often have more than one pool, such as an indoor heated pool, an outdoor (chlorinated, saltwater or ozonated) pool which may be heated or unheated, a shallower children's pool, and a paddling pool for toddlers and infants. There may also be a sauna and one or more hot tubs or spa pools ("jacuzzis").

Many upscale hotels and holiday resorts have a swimming pool for use by their guests. If a pool is in a separate building, the building may be called a natatorium. The building may sometimes also have facilities for related activities, such as a diving tank. Larger pools sometimes have a diving board affixed at one edge above the water.

Many public swimming pools are rectangles 25 m or 50 m long, but they can be any size and shape. There are also elaborate pools with artificial waterfalls, fountains, splash pads, wave machines, varying depths of water, bridges, and island bars.

Some swimming facilities have lockers for clothing and other belongings. The lockers can require a coin to be inserted in a slot, either as deposit or payment. There are usually showers – sometimes mandatory – before and/or after swimming. There are often also lifeguards to ensure the safety of users.

Wading or paddling pools are shallow bodies of water intended for use by small children, usually in parks. Concrete wading pools come in many shapes, traditionally rectangle, square or circle. Some are filled and drained daily due to lack of a filter system. Staff chlorinate the water to ensure health and safety standards.^{[citation needed]}

See: #Dimensions (above) and Swimming (sport)#Competition pools

The Fédération Internationale de la Natation (FINA, International Swimming Federation) sets standards for competition pools: 25 or 50 m (82 or 164 ft) long and at least 1.35 m (4.4 ft) deep. Competition pools are generally indoors and heated to enable their use all year round, and to more easily comply with the regulations regarding temperature, lighting, and automatic officiating equipment.

An Olympic-size swimming pool (first used at the 1924 Olympics) is a pool that meets FINA's additional standards for the Olympic Games and for world championship events. It must be 50 by 25 m (164 by 82 ft) wide, divided into eight lanes of 2.5 m (8.2 ft) each, plus two areas of 2.5 m (8.2 ft) at each side of the pool. Depth must be at least 2 m (6.6 ft).^[34]

The water must be kept at 25–28 °C (77–82 °F) and the lighting level at greater than 1500 lux. There are also regulations for color of lane rope, positioning of backstroke flags (5 metres from each wall), and so on.^[34] Pools claimed to be "Olympic pools" do not always meet these regulations, as FINA cannot police use of the term. Touchpads are mounted on both walls for long course meets and each end for short course.

A pool may be referred to as fast or slow, depending on its physical layout.^[35] Some design considerations allow the reduction of swimming resistance making the pool faster: namely, proper pool depth, elimination of currents, increased lane width, energy absorbing racing lane lines and gutters, and the use of other innovative hydraulic, acoustic and illumination designs.

In the last two decades, a new style of pool has gained popularity. These consist of a small vessel (usually about 2.5 × 5 m) in which the swimmer swims in place, either against the push of an artificially generated water current or against the pull of restraining devices. These pools have several names, such as swim spas, swimming machines, or swim systems. They are all examples of different modes of resistance swimming.

This section does not cite any sources. Please help improve this section by adding citations to reliable sources. Unsourced material may be challenged and removed. (January 2018) (Learn how and when to remove this message)

Hot tubs and spa pools are common heated pools used for relaxation and sometimes for therapy. Commercial spas are common in the swimming pool area or sauna area of a health club or fitness center, in men's clubs, women's clubs, motels and exclusive five-star hotel suites. Spa clubs may have very large pools, some segmented into increasing temperatures. In Japan, men's clubs with many spas of different size and temperature are common.

Commercial spas are generally made of concrete, with a mosaic tiled interior. More recently^[when?] with the innovation of the pre-form composite method where mosaic tiles are bonded to the shell this enables commercial spas to be completely factory manufactured to specification and delivered in one piece. Hot tubs are typically made somewhat like a wine barrel with straight sides, from wood such as Californian redwood held in place by metal hoops. Immersion of the head is not recommended in spas or hot tubs due to a potential risk of underwater entrapment from the pump suction forces. However, commercial installations in many countries must comply with various safety standards which reduce this risk considerably.

Home spas are a worldwide retail item in western countries since the 1980s, and are sold in dedicated spa stores, pool shops, department stores, the Internet, and catalog sales books. They are almost always made from heat-extruded acrylic sheet Perspex, often colored in marble look-alike patterns. They rarely exceed 6 m² (65 sq ft) and are typically 1 m (3 ft 3 in) deep, restricted by the availability of the raw sheet sizes (typically manufactured in Japan). There is often a mid-depth seating or lounging system, and contoured lounger style reclining seats are common.

Upmarket spas often include a drinks tray, lights, LCD flat-screen TV sets and other features that make the pool a recreation center. Due to their family-oriented nature, home spas are normally operated from 36 to 39 °C (97 to 102 °F). Many pools are incorporated in a redwood or simulated wood surround, and are termed "portable" as they may be placed on a patio rather than sunken into a permanent location. Some portable spas are shallow and narrow enough to fit sideways through a standard door and be used inside a room. Low power electric immersion heaters are common with home spas.

Whirlpool tubs first became popular in the U.S. during the 1960s and 1970s. A spa is also called a "jacuzzi" there, as the word became a generic after-plumbing component manufacturer; Jacuzzi introduced the "spa whirlpool" in 1968. Air bubbles may be introduced into the nozzles via an air-bleed venturi pump that combines cooler air with the incoming heated water to cool the pool if the temperature rises uncomfortably high. Some spas have a constant stream of bubbles fed via the seating area of the pool, or a footwell area. This is more common as a temperature control device where the heated water comes from a natural (uncontrolled heat) geothermal source, rather than artificially heated.

Water temperature is usually very warm to hot – 38 to 42 °C (100 to 108 °F) – so bathers usually stay in for only 20 to 30 minutes. Bromine or mineral sanitizers are often recommended as sanitizers for spas because chlorine dissipates at a high temperature, thereby heightening its strong chemical smell. Ozone is an effective bactericide and is commonly included in the circulation system with cartridge filtration, but not with sand media filtration due to clogging problems with turbid body fats.

In the early 20th century, especially in Australia, ocean pools were built, typically on headlands by enclosing part of the rock shelf, with water circulated through the pools by flooding from tidal tanks or by regular flooding over the side of the pools at high tide. This continued a pre-European tradition of bathing in rockpools with many of the current sites being expanded from sites used by Aboriginal Australians or early European settlers. Bathing in these pools provided security against both rough surf and sea life. There were often separate pools for women and men, or the pool was open to the sexes at different times with a break for bathers to climb in without fear of observation by the other sex.^[36] These were the forerunners of modern "Olympic" pools. A variation was the later development of sea- or harbour-side pools that circulated sea water using pumps. A pool of this type was the training ground for Australian Olympian Dawn Fraser.

There are currently about 100 ocean baths in New South Wales, which can range from small pools roughly 25 metres long and "Olympic Sized" (50m) to the very large, such as the 50 × 100 m baths in Newcastle. While most are free, a number charge fees, such as the Bondi Icebergs Club pool at Bondi Beach. Despite the development of chlorinated and heated pools, ocean baths remain a popular form of recreation in New South Wales.

A semi-natural ocean pool exists on the central coast of New South Wales; it is called The Bogey Hole.

The Marina Bay Sands SkyPark Infinity Pool in Singapore, viewed from the poolside (left) and near the edge (right)

An infinity pool (also named negative edge or vanishing edge pool) is a swimming pool which produces a visual effect of water extending to the horizon, vanishing, or extending to "infinity". Often, the water appears to fall into an ocean, lake, bay, or other similar body of water. The illusion is most effective whenever there is a significant change in elevation, though having a natural body of water on the horizon is not a limiting factor.^{[citation needed]}

Natural pools were developed in central and western Europe in the early and mid-1980s by designers and landscape architects with environmental concerns. They have recently been growing in popularity as an alternative to traditional swimming pools.^[37][38] Natural pools are constructed bodies of water in which no chemicals or devices that disinfect or sterilize water are used, and all the cleaning of the pool is achieved purely with the motion of the water through biological filters and plants rooted hydroponically in the system. In essence, natural pools seek to recreate swimming holes and swimmable lakes, the environment where people feel safe swimming in a non-polluted, healthy, and ecologically balanced body of water.

Water in natural pools has many desirable characteristics. For example, red eyes, dried-out skin and hair, and bleached swimsuits associated with overly chlorinated water are naturally absent in natural pools.^{[citation needed]} Natural pools, by requiring a water garden to be a part of the system, offer different aesthetic options and can support amphibious wildlife such as snails, frogs, and salamanders, and even small fish if desired.

A zero-entry swimming pool, also called a beach entry swimming pool, has an edge or entry that gradually slopes from the deck into the water, becoming deeper with each step, in the manner of a natural beach. As there are no stairs or ladders to navigate, this type of entry assists older people, young children and people with accessibility problems (e.g., people with a physical disability) where gradual entry is useful.

Indoor pools are located inside a building with a roof and are insulated by at least three walls. Built for year-round swimming or training, they are found in all climate types. Since the buildings around indoor pools are insulated, heat escapes much less, making it less expensive to heat indoor pools than outdoor pools (all of whose heat escapes).^[39]

Architecturally, an indoor pool may look like the rest of the building, but extra heating and ventilation and other engineering solutions are required to ensure comfortable humidity levels. In addition to drainage and automatic pool covers, there are a number of ways to remove the humidity present in the air in any wet indoor environment. Efficient dehumidification in the indoor pool environment prevents structural damage, lowers energy costs for cooling or heating, and improves the indoor climate to provide a comfortable swimming environment.^{[citation needed]}

Some colleges, universities, and high schools have buildings that use the term "natatorium" in their names, especially when the building houses more than just a swimming pool, for example a diving well or facilities for water polo. The word natatorium was borrowed from Late Latin "place for swimming" into English in New England in 1890.^[40][41]

This type of swimming pool is suspended high above the ground. A prominent example is the Sky Pool in London's Embassy Gardens, the world's first floating pool.^[42]

Swimming pools are also used for events such as synchronized swimming, water polo, canoe polo and underwater sports such as underwater hockey, underwater rugby, finswimming and sport diving as well as for teaching diving, lifesaving and scuba diving techniques. They have also been used for specialist tasks such as teaching water-ditching survival techniques for aircraft and submarine crews and astronaut training. Round-cornered, irregular swimming pools, such as the Nude Bowl, were drained of water and used for vertical skateboarding.

Levels of bacteria and viruses in swimming pool water must be kept low to prevent the spread of diseases and pathogens. Bacteria, algae and insect larvae can breed in the pool if water is not properly sanitized. Pumps, mechanical sand filters, and disinfectants are often used to sanitise the water.

Chemical disinfectants, such as chlorine (usually as a hypochlorite salt, such as calcium hypochlorite) and bromine, are commonly used to kill pathogens. If not properly maintained, chemical sanitation can produce high levels of disinfection byproducts. Sanitized swimming pool water can theoretically appear green if a certain amount of iron salts or copper chloride are present in the water.^[43]

Acesulfame potassium has been used to estimate how much urine is discharged by swimmers into a pool.^[44] A Canadian study estimated that swimmers had released 75 litres of urine into a large pool that had about 830,000 litres of water and was a third of the size of an olympic pool. Hot tubs were found to have higher readings of the marker. While urine itself is relatively harmless, its degradation products may lead to asthma.^[44]

Swimming pool heating costs can be significantly reduced by using a pool cover. Use of a pool cover also can help reduce the amount of chemicals (chlorine, etc.) required by the pool. Outdoor pools gain heat from the sun, absorbing 75–85% of the solar energy striking the pool surface. Though a cover decreases the total amount of solar heat absorbed by the pool, the cover eliminates heat loss due to evaporation and reduces heat loss at night through its insulating properties. Most swimming pool heat loss is through evaporation.^[45]

In areas which reach freezing temperature, it is important to close a pool properly. This varies greatly between in-ground and above-ground pools. By taking steps to properly secure the pool, it lessens the likelihood that the superstructure will be damaged or compromised by freezing water.^[46]

In preparation for freezing temperatures, an in-ground swimming pool's pipes must be emptied. An above-ground pool should also be closed, so that ice does not drag down the pool wall, collapsing its structure. The plumbing is sealed with air, typically with rubber plugs, to prevent cracking from freezing water. The pool is typically covered to prevent leaves and other debris from falling in. The cover is attached to the pool typically using a stretch cord, similar to a bungee cord, and hooks fitted into the pool surround. The skimmer is closed off or a floating device is placed into it to prevent it from completely freezing and cracking.

Floating objects such as life rings or basketballs can be placed in the pool to avoid its freezing under the cover. Sand or DE filters must be backwashed, with the main drain plug removed and all water drained out. Drain plugs on the pool filter are removed after the filter has been cleaned. The pool pump motor is taken under cover. Winter chemicals are added to keep the pool clean. The innovation of a composite construction of fiberglass, with an epoxy coating and porcelain ceramic tiles has led to the pre-form, composite-type with significant advantages over older methods; however, it also has increased sensitivity to metal staining.^{[citation needed]}

In climates where there is no risk of freezing, closing down the pool for winter is not so important. Typically, the thermal cover is removed and stored. Winter sunlight can create an algae mess when a cover that has been left on all winter is removed. The pool is correctly pH-balanced and super-chlorinated. One part algaecide for every 50,000 parts of pool water should be added, and topped up each month. The pool should be filtered for one to two hours daily to keep the automated chlorination system active.^{[citation needed]}

Pools pose a risk of drowning, which may be significant for swimmers who are inexperienced, suffer from seizures, or are susceptible to a heart or respiratory condition. Lifeguards are employed at most pools to execute water rescues and administer first aid as needed in order to reduce this risk.

Diving in shallow areas of a pool may also lead to significant head and neck injuries; diving, especially head-first diving, should be done in the deepest point of the pool, minimally 2.4 m (7 ft 10 in), but desirably 3.7 m (12 ft), deeper if the distance between the water and the board is great.

Pools present a risk of death due to drowning particularly in young children. In regions where residential pools are common, drowning is a major cause of childhood death. As a precaution, many jurisdictions require that residential pools be enclosed with fencing to restrict unauthorized access. Many products exist, such as removable baby fences. The evidence for floating alarms and window/door alarms to reduce the risk of drowning is poor.^[47] Some pools are equipped with computer-aided drowning prevention or other forms of electronic safety and security systems.

Suspended ceilings in indoor swimming pools are safety-relevant components. The selection of materials under tension should be done with care. Especially the selection of unsuitable stainless steels can cause problems with stress corrosion cracking.^[48]

In public swimming pools, dress code may be stricter than on public beaches, and in indoor pools stricter than outdoor pools. For example, in countries where women can be topless on the beach, this is often not allowed in a swimming pool, and a swimsuit must be worn. For men, wearing ordinary shorts and a T-shirt to go in the water at a beach may be considered acceptable, but pools usually require real swimsuits or other dedicated water wear as swimming with regular clothes can potentially weigh a swimmer down should they need to be rescued. In France and some other European countries, board shorts are usually not allowed for "hygienic" reasons. In Nordic countries, in particular Iceland, rules about clothing and hygiene are especially strict.^[49] When diving from a high board, swimsuits are sometimes worn doubled up (one brief inside another) in case the outer suit tears on impact with the water.

• Media related to Swimming pools at Wikimedia Commons

This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. Find sources: "Concrete" – news · newspapers · books · scholar · JSTOR (July 2022) (Learn how and when to remove this message)

Concrete is a composite material composed of aggregate bonded together with a fluid cement that cures to a solid over time. Concrete is the second-most-used substance in the world after water,^[1] and is the most widely used building material.^[2] Concrete is the most manufactured material on Earth.^[3]

When aggregate is mixed with dry Portland cement and water, the mixture forms a fluid slurry that can be poured and molded into shape. The cement reacts with the water through a process called hydration^[4] that hardens it over several hours to form a solid matrix that binds the materials together into a durable stone-like material that has many uses.^[5] This time allows concrete to not only be cast in forms, but also to have a variety of tooled processes performed. The hydration process is exothermic, which means that ambient temperature plays a significant role in how long it takes concrete to set. Often, additives (such as pozzolans or superplasticizers) are included in the mixture to improve the physical properties of the wet mix, delay or accelerate the curing time, or otherwise modify the finished material. Most structural concrete is poured with reinforcing materials (such as steel rebar) embedded to provide tensile strength, yielding reinforced concrete.

Before the invention of Portland cement in the early 1800s, lime-based cement binders, such as lime putty, were often used. The overwhelming majority of concretes are produced using Portland cement, but sometimes with other hydraulic cements, such as calcium aluminate cement.^[6][7] Many other non-cementitious types of concrete exist with other methods of binding aggregate together, including asphalt concrete with a bitumen binder, which is frequently used for road surfaces, and polymer concretes that use polymers as a binder.

Concrete is distinct from mortar.^[8] Whereas concrete is itself a building material, and contains both coarse (large) and fine (small) aggregate particles, mortar contains only fine aggregates and is mainly used as a bonding agent to hold bricks, tiles and other masonry units together.^[9] Grout is another material associated with concrete and cement. It also does not contain coarse aggregates and is usually either pourable or thixotropic, and is used to fill gaps between masonry components or coarse aggregate which has already been put in place. Some methods of concrete manufacture and repair involve pumping grout into the gaps to make up a solid mass in situ.

The word concrete comes from the Latin word "concretus" (meaning compact or condensed),^[10] the perfect passive participle of "concrescere", from "con-" (together) and "crescere" (to grow).

Concrete floors were found in the royal palace of Tiryns, Greece, which dates roughly to 1400 to 1200 BC.^[11][12] Lime mortars were used in Greece, such as in Crete and Cyprus, in 800 BC. The Assyrian Jerwan Aqueduct (688 BC) made use of waterproof concrete.^[13] Concrete was used for construction in many ancient structures.^[14]

Mayan concrete at the ruins of Uxmal (AD 850–925) is referenced in Incidents of Travel in the Yucatán by John L. Stephens. "The roof is flat and had been covered with cement". "The floors were cement, in some places hard, but, by long exposure, broken, and now crumbling under the feet." "But throughout the wall was solid, and consisting of large stones imbedded in mortar, almost as hard as rock."

Small-scale production of concrete-like materials was pioneered by the Nabatean traders who occupied and controlled a series of oases and developed a small empire in the regions of southern Syria and northern Jordan from the 4th century BC. They discovered the advantages of hydraulic lime, with some self-cementing properties, by 700 BC. They built kilns to supply mortar for the construction of rubble masonry houses, concrete floors, and underground waterproof cisterns. They kept the cisterns secret as these enabled the Nabataeans to thrive in the desert.^[15] Some of these structures survive to this day.^[15]

In the Ancient Egyptian and later Roman eras, builders discovered that adding volcanic ash to lime allowed the mix to set underwater. They discovered the pozzolanic reaction.^[16]

The Romans used concrete extensively from 300 BC to AD 476.^[18] During the Roman Empire, Roman concrete (or opus caementicium) was made from quicklime, pozzolana and an aggregate of pumice.^[19] Its widespread use in many Roman structures, a key event in the history of architecture termed the Roman architectural revolution, freed Roman construction from the restrictions of stone and brick materials. It enabled revolutionary new designs in terms of both structural complexity and dimension.^[20] The Colosseum in Rome was built largely of concrete, and the Pantheon has the world's largest unreinforced concrete dome.^[21]

Concrete, as the Romans knew it, was a new and revolutionary material. Laid in the shape of arches, vaults and domes, it quickly hardened into a rigid mass, free from many of the internal thrusts and strains that troubled the builders of similar structures in stone or brick.^[22]

Modern tests show that opus caementicium had a similar compressive strength to modern Portland-cement concrete (c. 200 kg/cm² [20 MPa; 2,800 psi]).^[23] However, due to the absence of reinforcement, its tensile strength was far lower than modern reinforced concrete, and its mode of application also differed:^[24]

Modern structural concrete differs from Roman concrete in two important details. First, its mix consistency is fluid and homogeneous, allowing it to be poured into forms rather than requiring hand-layering together with the placement of aggregate, which, in Roman practice, often consisted of rubble. Second, integral reinforcing steel gives modern concrete assemblies great strength in tension, whereas Roman concrete could depend only upon the strength of the concrete bonding to resist tension.^[25]

The long-term durability of Roman concrete structures has been found to be due to its use of pyroclastic (volcanic) rock and ash, whereby the crystallization of strätlingite (a complex calcium aluminosilicate hydrate)^[26] and the coalescence of this and similar calcium–aluminium-silicate–hydrate cementing binders helped give the concrete a greater degree of fracture resistance even in seismically active environments.^[27] Roman concrete is significantly more resistant to erosion by seawater than modern concrete; it used pyroclastic materials which react with seawater to form Al-tobermorite crystals over time.^[28][29] The use of hot mixing and the presence of lime clasts have been proposed to give the concrete a self-healing ability, where cracks that form become filled with calcite that prevents the crack from spreading.^[30][31]

The widespread use of concrete in many Roman structures ensured that many survive to the present day. The Baths of Caracalla in Rome are just one example. Many Roman aqueducts and bridges, such as the magnificent Pont du Gard in southern France, have masonry cladding on a concrete core, as does the dome of the Pantheon.

After the Roman Empire, the use of burned lime and pozzolana was greatly reduced. Low kiln temperatures in the burning of lime, lack of pozzolana, and poor mixing all contributed to a decline in the quality of concrete and mortar. From the 11th century, the increased use of stone in church and castle construction led to an increased demand for mortar. Quality began to improve in the 12th century through better grinding and sieving. Medieval lime mortars and concretes were non-hydraulic and were used for binding masonry, "hearting" (binding rubble masonry cores) and foundations. Bartholomaeus Anglicus in his De proprietatibus rerum (1240) describes the making of mortar. In an English translation from 1397, it reads "lyme ... is a stone brent; by medlynge thereof with sonde and water sement is made". From the 14th century, the quality of mortar was again excellent, but only from the 17th century was pozzolana commonly added.^[32]

The Canal du Midi was built using concrete in 1670.^[33]

Perhaps the greatest step forward in the modern use of concrete was Smeaton's Tower, built by British engineer John Smeaton in Devon, England, between 1756 and 1759. This third Eddystone Lighthouse pioneered the use of hydraulic lime in concrete, using pebbles and powdered brick as aggregate.^[34]

A method for producing Portland cement was developed in England and patented by Joseph Aspdin in 1824.^[35] Aspdin chose the name for its similarity to Portland stone, which was quarried on the Isle of Portland in Dorset, England. His son William continued developments into the 1840s, earning him recognition for the development of "modern" Portland cement.^[36]

Reinforced concrete was invented in 1849 by Joseph Monier.^[37] and the first reinforced concrete house was built by François Coignet^[38] in 1853. The first concrete reinforced bridge was designed and built by Joseph Monier in 1875.^[39]

Prestressed concrete and post-tensioned concrete were pioneered by Eugène Freyssinet, a French structural and civil engineer. Concrete components or structures are compressed by tendon cables during, or after, their fabrication in order to strengthen them against tensile forces developing when put in service. Freyssinet patented the technique on 2 October 1928.^[40]

Concrete is an artificial composite material, comprising a matrix of cementitious binder (typically Portland cement paste or asphalt) and a dispersed phase or "filler" of aggregate (typically a rocky material, loose stones, and sand). The binder "glues" the filler together to form a synthetic conglomerate.^[41] Many types of concrete are available, determined by the formulations of binders and the types of aggregate used to suit the application of the engineered material. These variables determine strength and density, as well as chemical and thermal resistance of the finished product.

Construction aggregates consist of large chunks of material in a concrete mix, generally a coarse gravel or crushed rocks such as limestone, or granite, along with finer materials such as sand.

Cement paste, most commonly made of Portland cement, is the most prevalent kind of concrete binder. For cementitious binders, water is mixed with the dry cement powder and aggregate, which produces a semi-liquid slurry (paste) that can be shaped, typically by pouring it into a form. The concrete solidifies and hardens through a chemical process called hydration. The water reacts with the cement, which bonds the other components together, creating a robust, stone-like material. Other cementitious materials, such as fly ash and slag cement, are sometimes added—either pre-blended with the cement or directly as a concrete component—and become a part of the binder for the aggregate.^[42] Fly ash and slag can enhance some properties of concrete such as fresh properties and durability.^[42] Alternatively, other materials can also be used as a concrete binder: the most prevalent substitute is asphalt, which is used as the binder in asphalt concrete.

Admixtures are added to modify the cure rate or properties of the material. Mineral admixtures use recycled materials as concrete ingredients. Conspicuous materials include fly ash, a by-product of coal-fired power plants; ground granulated blast furnace slag, a by-product of steelmaking; and silica fume, a by-product of industrial electric arc furnaces.

Structures employing Portland cement concrete usually include steel reinforcement because this type of concrete can be formulated with high compressive strength, but always has lower tensile strength. Therefore, it is usually reinforced with materials that are strong in tension, typically steel rebar.

The mix design depends on the type of structure being built, how the concrete is mixed and delivered, and how it is placed to form the structure.

Portland cement is the most common type of cement in general usage. It is a basic ingredient of concrete, mortar, and many plasters.^[43] It consists of a mixture of calcium silicates (alite, belite), aluminates and ferrites—compounds, which will react with water. Portland cement and similar materials are made by heating limestone (a source of calcium) with clay or shale (a source of silicon, aluminium and iron) and grinding this product (called clinker) with a source of sulfate (most commonly gypsum).

Cement kilns are extremely large, complex, and inherently dusty industrial installations. Of the various ingredients used to produce a given quantity of concrete, the cement is the most energetically expensive. Even complex and efficient kilns require 3.3 to 3.6 gigajoules of energy to produce a ton of clinker and then grind it into cement. Many kilns can be fueled with difficult-to-dispose-of wastes, the most common being used tires. The extremely high temperatures and long periods of time at those temperatures allows cement kilns to efficiently and completely burn even difficult-to-use fuels.^[44] The five major compounds of calcium silicates and aluminates comprising Portland cement range from 5 to 50% in weight.

Combining water with a cementitious material forms a cement paste by the process of hydration. The cement paste glues the aggregate together, fills voids within it, and makes it flow more freely.^[45]

As stated by Abrams' law, a lower water-to-cement ratio yields a stronger, more durable concrete, whereas more water gives a freer-flowing concrete with a higher slump.^[46] The hydration of cement involves many concurrent reactions. The process involves polymerization, the interlinking of the silicates and aluminate components as well as their bonding to sand and gravel particles to form a solid mass.^[47] One illustrative conversion is the hydration of tricalcium silicate:

Cement chemist notation:    C₃S + H → C-S-H + CH + heat

Standard notation:               Ca₃SiO₅ + H₂O → CaO・SiO₂・H₂O (gel) + Ca(OH)₂ + heat

Balanced:                             2 Ca₃SiO₅ + 7 H₂O → 3 CaO・2 SiO₂・4 H₂O (gel) + 3 Ca(OH)₂ + heat

                                             (approximately as the exact ratios of CaO, SiO₂ and H₂O in C-S-H can vary)^[47]

The hydration (curing) of cement is irreversible.^[48]

Fine and coarse aggregates make up the bulk of a concrete mixture. Sand, natural gravel, and crushed stone are used mainly for this purpose. Recycled aggregates (from construction, demolition, and excavation waste) are increasingly used as partial replacements for natural aggregates, while a number of manufactured aggregates, including air-cooled blast furnace slag and bottom ash are also permitted.

The size distribution of the aggregate determines how much binder is required. Aggregate with a very even size distribution has the biggest gaps whereas adding aggregate with smaller particles tends to fill these gaps. The binder must fill the gaps between the aggregate as well as paste the surfaces of the aggregate together, and is typically the most expensive component. Thus, variation in sizes of the aggregate reduces the cost of concrete.^[49] The aggregate is nearly always stronger than the binder, so its use does not negatively affect the strength of the concrete.

Redistribution of aggregates after compaction often creates non-homogeneity due to the influence of vibration. This can lead to strength gradients.^[50]

Decorative stones such as quartzite, small river stones or crushed glass are sometimes added to the surface of concrete for a decorative "exposed aggregate" finish, popular among landscape designers.

Admixtures are materials in the form of powder or fluids that are added to the concrete to give it certain characteristics not obtainable with plain concrete mixes. Admixtures are defined as additions "made as the concrete mix is being prepared".^[51] The most common admixtures are retarders and accelerators. In normal use, admixture dosages are less than 5% by mass of cement and are added to the concrete at the time of batching/mixing.^[52] (See § Production below.) The common types of admixtures^[53] are as follows:

• Accelerators speed up the hydration (hardening) of the concrete. Typical materials used are calcium chloride, calcium nitrate and sodium nitrate. However, use of chlorides may cause corrosion in steel reinforcing and is prohibited in some countries, so that nitrates may be favored, even though they are less effective than the chloride salt. Accelerating admixtures are especially useful for modifying the properties of concrete in cold weather.
• Air entraining agents add and entrain tiny air bubbles in the concrete, which reduces damage during freeze-thaw cycles, increasing durability. However, entrained air entails a tradeoff with strength, as each 1% of air may decrease compressive strength by 5%.^[54] If too much air becomes trapped in the concrete as a result of the mixing process, defoamers can be used to encourage the air bubble to agglomerate, rise to the surface of the wet concrete and then disperse.
• Bonding agents are used to create a bond between old and new concrete (typically a type of polymer) with wide temperature tolerance and corrosion resistance.
• Corrosion inhibitors are used to minimize the corrosion of steel and steel bars in concrete.
• Crystalline admixtures are typically added during batching of the concrete to lower permeability. The reaction takes place when exposed to water and un-hydrated cement particles to form insoluble needle-shaped crystals, which fill capillary pores and micro-cracks in the concrete to block pathways for water and waterborne contaminates. Concrete with crystalline admixture can expect to self-seal as constant exposure to water will continuously initiate crystallization to ensure permanent waterproof protection.
• Pigments can be used to change the color of concrete, for aesthetics.
• Plasticizers increase the workability of plastic, or "fresh", concrete, allowing it to be placed more easily, with less consolidating effort. A typical plasticizer is lignosulfonate. Plasticizers can be used to reduce the water content of a concrete while maintaining workability and are sometimes called water-reducers due to this use. Such treatment improves its strength and durability characteristics.
• Superplasticizers (also called high-range water-reducers) are a class of plasticizers that have fewer deleterious effects and can be used to increase workability more than is practical with traditional plasticizers. Superplasticizers are used to increase compressive strength. It increases the workability of the concrete and lowers the need for water content by 15–30%.
• Pumping aids improve pumpability, thicken the paste and reduce separation and bleeding.
• Retarders slow the hydration of concrete and are used in large or difficult pours where partial setting is undesirable before completion of the pour. Typical retarders include sugar, sodium gluconate, citric acid, and tartaric acid.^[55]

Inorganic materials that have pozzolanic or latent hydraulic properties, these very fine-grained materials are added to the concrete mix to improve the properties of concrete (mineral admixtures),^[52] or as a replacement for Portland cement (blended cements).^[59] Products which incorporate limestone, fly ash, blast furnace slag, and other useful materials with pozzolanic properties into the mix, are being tested and used. These developments are ever growing in relevance to minimize the impacts caused by cement use, notorious for being one of the largest producers (at about 5 to 10%) of global greenhouse gas emissions.^[60] The use of alternative materials also is capable of lowering costs, improving concrete properties, and recycling wastes, the latest being relevant for circular economy aspects of the construction industry, whose demand is ever growing with greater impacts on raw material extraction, waste generation and landfill practices.

• Fly ash: A by-product of coal-fired electric generating plants, it is used to partially replace Portland cement (by up to 60% by mass). The properties of fly ash depend on the type of coal burnt. In general, siliceous fly ash is pozzolanic, while calcareous fly ash has latent hydraulic properties.^[61]
• Ground granulated blast furnace slag (GGBFS or GGBS): A by-product of steel production is used to partially replace Portland cement (by up to 80% by mass). It has latent hydraulic properties.^[62]
• Silica fume: A by-product of the production of silicon and ferrosilicon alloys. Silica fume is similar to fly ash, but has a particle size 100 times smaller. This results in a higher surface-to-volume ratio and a much faster pozzolanic reaction. Silica fume is used to increase strength and durability of concrete, but generally requires the use of superplasticizers for workability.^[63]
• High reactivity metakaolin (HRM): Metakaolin produces concrete with strength and durability similar to concrete made with silica fume. While silica fume is usually dark gray or black in color, high-reactivity metakaolin is usually bright white in color, making it the preferred choice for architectural concrete where appearance is important.
• Carbon nanofibers can be added to concrete to enhance compressive strength and gain a higher Young's modulus, and also to improve the electrical properties required for strain monitoring, damage evaluation and self-health monitoring of concrete. Carbon fiber has many advantages in terms of mechanical and electrical properties (e.g., higher strength) and self-monitoring behavior due to the high tensile strength and high electrical conductivity.^[64]
• Carbon products have been added to make concrete electrically conductive, for deicing purposes.^[65]
• New research from Japan's University of Kitakyushu shows that a washed and dried recycled mix of used diapers can be an environmental solution to producing less landfill and using less sand in concrete production. A model home was built in Indonesia to test the strength and durability of the new diaper-cement composite.^[66]

Concrete production is the process of mixing together the various ingredients—water, aggregate, cement, and any additives—to produce concrete. Concrete production is time-sensitive. Once the ingredients are mixed, workers must put the concrete in place before it hardens. In modern usage, most concrete production takes place in a large type of industrial facility called a concrete plant, or often a batch plant. The usual method of placement is casting in formwork, which holds the mix in shape until it has set enough to hold its shape unaided.

Concrete plants come in two main types, ready-mix plants and central mix plants. A ready-mix plant blends all of the solid ingredients, while a central mix does the same but adds water. A central-mix plant offers more precise control of the concrete quality. Central mix plants must be close to the work site where the concrete will be used, since hydration begins at the plant.

A concrete plant consists of large hoppers for storage of various ingredients like cement, storage for bulk ingredients like aggregate and water, mechanisms for the addition of various additives and amendments, machinery to accurately weigh, move, and mix some or all of those ingredients, and facilities to dispense the mixed concrete, often to a concrete mixer truck.

Modern concrete is usually prepared as a viscous fluid, so that it may be poured into forms. The forms are containers that define the desired shape. Concrete formwork can be prepared in several ways, such as slip forming and steel plate construction. Alternatively, concrete can be mixed into dryer, non-fluid forms and used in factory settings to manufacture precast concrete products.

Interruption in pouring the concrete can cause the initially placed material to begin to set before the next batch is added on top. This creates a horizontal plane of weakness called a cold joint between the two batches.^[67] Once the mix is where it should be, the curing process must be controlled to ensure that the concrete attains the desired attributes. During concrete preparation, various technical details may affect the quality and nature of the product.

Design mix ratios are decided by an engineer after analyzing the properties of the specific ingredients being used. Instead of using a 'nominal mix' of 1 part cement, 2 parts sand, and 4 parts aggregate, a civil engineer will custom-design a concrete mix to exactly meet the requirements of the site and conditions, setting material ratios and often designing an admixture package to fine-tune the properties or increase the performance envelope of the mix. Design-mix concrete can have very broad specifications that cannot be met with more basic nominal mixes, but the involvement of the engineer often increases the cost of the concrete mix.

Concrete mixes are primarily divided into nominal mix, standard mix and design mix.

Nominal mix ratios are given in volume of $\displaystyle \textCement : Sand : Aggregate$. Nominal mixes are a simple, fast way of getting a basic idea of the properties of the finished concrete without having to perform testing in advance.

Various governing bodies (such as British Standards) define nominal mix ratios into a number of grades, usually ranging from lower compressive strength to higher compressive strength. The grades usually indicate the 28-day cure strength.^[68]

Thorough mixing is essential to produce uniform, high-quality concrete.

Separate paste mixing has shown that the mixing of cement and water into a paste before combining these materials with aggregates can increase the compressive strength of the resulting concrete.^[69] The paste is generally mixed in a high-speed, shear-type mixer at a w/c (water to cement ratio) of 0.30 to 0.45 by mass. The cement paste premix may include admixtures such as accelerators or retarders, superplasticizers, pigments, or silica fume. The premixed paste is then blended with aggregates and any remaining batch water and final mixing is completed in conventional concrete mixing equipment.^[70]

Resonant acoustic mixing has also been found effective in producing ultra-high performance cementitious materials, as it produces a dense matrix with low porosity.^[71]

Workability is the ability of a fresh (plastic) concrete mix to fill the form/mold properly with the desired work (pouring, pumping, spreading, tamping, vibration) and without reducing the concrete's quality. Workability depends on water content, aggregate (shape and size distribution), cementitious content and age (level of hydration) and can be modified by adding chemical admixtures, like superplasticizer. Raising the water content or adding chemical admixtures increases concrete workability. Excessive water leads to increased bleeding or segregation of aggregates (when the cement and aggregates start to separate), with the resulting concrete having reduced quality. Changes in gradation can also affect workability of the concrete, although a wide range of gradation can be used for various applications.^[72][73] An undesirable gradation can mean using a large aggregate that is too large for the size of the formwork, or which has too few smaller aggregate grades to serve to fill the gaps between the larger grades, or using too little or too much sand for the same reason, or using too little water, or too much cement, or even using jagged crushed stone instead of smoother round aggregate such as pebbles. Any combination of these factors and others may result in a mix which is too harsh, i.e., which does not flow or spread out smoothly, is difficult to get into the formwork, and which is difficult to surface finish.^[74]

Workability can be measured by the concrete slump test, a simple measure of the plasticity of a fresh batch of concrete following the ASTM C 143 or EN 12350-2 test standards. Slump is normally measured by filling an "Abrams cone" with a sample from a fresh batch of concrete. The cone is placed with the wide end down onto a level, non-absorptive surface. It is then filled in three layers of equal volume, with each layer being tamped with a steel rod to consolidate the layer. When the cone is carefully lifted off, the enclosed material slumps a certain amount, owing to gravity. A relatively dry sample slumps very little, having a slump value of one or two inches (25 or 50 mm) out of one foot (300 mm). A relatively wet concrete sample may slump as much as eight inches. Workability can also be measured by the flow table test.

Slump can be increased by addition of chemical admixtures such as plasticizer or superplasticizer without changing the water-cement ratio.^[75] Some other admixtures, especially air-entraining admixture, can increase the slump of a mix.

High-flow concrete, like self-consolidating concrete, is tested by other flow-measuring methods. One of these methods includes placing the cone on the narrow end and observing how the mix flows through the cone while it is gradually lifted.

After mixing, concrete is a fluid and can be pumped to the location where needed.

Concrete must be kept moist during curing in order to achieve optimal strength and durability.^[76] During curing hydration occurs, allowing calcium-silicate hydrate (C-S-H) to form. Over 90% of a mix's final strength is typically reached within four weeks, with the remaining 10% achieved over years or even decades.^[77] The conversion of calcium hydroxide in the concrete into calcium carbonate from absorption of CO₂ over several decades further strengthens the concrete and makes it more resistant to damage. This carbonation reaction, however, lowers the pH of the cement pore solution and can corrode the reinforcement bars.

Hydration and hardening of concrete during the first three days is critical. Abnormally fast drying and shrinkage due to factors such as evaporation from wind during placement may lead to increased tensile stresses at a time when it has not yet gained sufficient strength, resulting in greater shrinkage cracking. The early strength of the concrete can be increased if it is kept damp during the curing process. Minimizing stress prior to curing minimizes cracking. High-early-strength concrete is designed to hydrate faster, often by increased use of cement that increases shrinkage and cracking. The strength of concrete changes (increases) for up to three years. It depends on cross-section dimension of elements and conditions of structure exploitation.^[50] Addition of short-cut polymer fibers can improve (reduce) shrinkage-induced stresses during curing and increase early and ultimate compression strength.^[78]

Properly curing concrete leads to increased strength and lower permeability and avoids cracking where the surface dries out prematurely. Care must also be taken to avoid freezing or overheating due to the exothermic setting of cement. Improper curing can cause spalling, reduced strength, poor abrasion resistance and cracking.

During the curing period, concrete is ideally maintained at controlled temperature and humidity. To ensure full hydration during curing, concrete slabs are often sprayed with "curing compounds" that create a water-retaining film over the concrete. Typical films are made of wax or related hydrophobic compounds. After the concrete is sufficiently cured, the film is allowed to abrade from the concrete through normal use.^[79]

Traditional conditions for curing involve spraying or ponding the concrete surface with water. The adjacent picture shows one of many ways to achieve this, ponding—submerging setting concrete in water and wrapping in plastic to prevent dehydration. Additional common curing methods include wet burlap and plastic sheeting covering the fresh concrete.

For higher-strength applications, accelerated curing techniques may be applied to the concrete. A common technique involves heating the poured concrete with steam, which serves to both keep it damp and raise the temperature so that the hydration process proceeds more quickly and more thoroughly.

Asphalt concrete (commonly called asphalt,^[80] blacktop, or pavement in North America, and tarmac, bitumen macadam, or rolled asphalt in the United Kingdom and Ireland) is a composite material commonly used to surface roads, parking lots, airports, as well as the core of embankment dams.^[81] Asphalt mixtures have been used in pavement construction since the beginning of the twentieth century.^[82] It consists of mineral aggregate bound together with asphalt, laid in layers, and compacted. The process was refined and enhanced by Belgian inventor and U.S. immigrant Edward De Smedt.^[83]

The terms asphalt (or asphaltic) concrete, bituminous asphalt concrete, and bituminous mixture are typically used only in engineering and construction documents, which define concrete as any composite material composed of mineral aggregate adhered with a binder. The abbreviation, AC, is sometimes used for asphalt concrete but can also denote asphalt content or asphalt cement, referring to the liquid asphalt portion of the composite material.

Graphene enhanced concretes are standard designs of concrete mixes, except that during the cement-mixing or production process, a small amount of chemically engineered graphene (typically < 0.5% by weight) is added.^[84][85] These enhanced graphene concretes are designed around the concrete application.

Bacteria such as Bacillus pasteurii, Bacillus pseudofirmus, Bacillus cohnii, Sporosarcina pasteuri, and Arthrobacter crystallopoietes increase the compression strength of concrete through their biomass. However some forms of bacteria can also be concrete-destroying.^[86] Bacillus sp. CT-5. can reduce corrosion of reinforcement in reinforced concrete by up to four times. Sporosarcina pasteurii reduces water and chloride permeability. B. pasteurii increases resistance to acid.^[87] Bacillus pasteurii and B. sphaericuscan induce calcium carbonate precipitation in the surface of cracks, adding compression strength.^[88]

Nanoconcrete (also spelled "nano concrete"' or "nano-concrete") is a class of materials that contains Portland cement particles that are no greater than 100 μm^[89] and particles of silica no greater than 500 μm, which fill voids that would otherwise occur in normal concrete, thereby substantially increasing the material's strength.^[90] It is widely used in foot and highway bridges where high flexural and compressive strength are indicated.^[88]

Pervious concrete is a mix of specially graded coarse aggregate, cement, water, and little-to-no fine aggregates. This concrete is also known as "no-fines" or porous concrete. Mixing the ingredients in a carefully controlled process creates a paste that coats and bonds the aggregate particles. The hardened concrete contains interconnected air voids totaling approximately 15 to 25 percent. Water runs through the voids in the pavement to the soil underneath. Air entrainment admixtures are often used in freeze-thaw climates to minimize the possibility of frost damage. Pervious concrete also permits rainwater to filter through roads and parking lots, to recharge aquifers, instead of contributing to runoff and flooding.^[91]

Polymer concretes are mixtures of aggregate and any of various polymers and may be reinforced. The cement is costlier than lime-based cements, but polymer concretes nevertheless have advantages; they have significant tensile strength even without reinforcement, and they are largely impervious to water. Polymer concretes are frequently used for the repair and construction of other applications, such as drains.

Plant fibers and particles can be used in a concrete mix or as a reinforcement.^[92][93][94] These materials can increase ductility but the lignocellulosic particles hydrolyze during concrete curing as a result of alkaline environment and elevated temperatures^[95][96][97] Such process, that is difficult to measure,^[98] can affect the properties of the resulting concrete.

Sulfur concrete is a special concrete that uses sulfur as a binder and does not require cement or water.

Volcanic concrete substitutes volcanic rock for the limestone that is burned to form clinker. It consumes a similar amount of energy, but does not directly emit carbon as a byproduct.^[99] Volcanic rock/ash are used as supplementary cementitious materials in concrete to improve the resistance to sulfate, chloride and alkali silica reaction due to pore refinement.^[100] Also, they are generally cost effective in comparison to other aggregates,^[101] good for semi and light weight concretes,^[101] and good for thermal and acoustic insulation.^[101]

Pyroclastic materials, such as pumice, scoria, and ashes are formed from cooling magma during explosive volcanic eruptions. They are used as supplementary cementitious materials (SCM) or as aggregates for cements and concretes.^[102] They have been extensively used since ancient times to produce materials for building applications. For example, pumice and other volcanic glasses were added as a natural pozzolanic material for mortars and plasters during the construction of the Villa San Marco in the Roman period (89 BC – 79 AD), which remain one of the best-preserved otium villae of the Bay of Naples in Italy.^[103]

Waste light is a form of polymer modified concrete. The specific polymer admixture allows the replacement of all the traditional aggregates (gravel, sand, stone) by any mixture of solid waste materials in the grain size of 3–10 mm to form a low-compressive-strength (3–20 N/mm²) product^[104] for road and building construction. One cubic meter of waste light concrete contains 1.1–1.3 m³ of shredded waste and no other aggregates.

This section does not cite any sources. Please help improve this section by adding citations to reliable sources. Unsourced material may be challenged and removed. (October 2024) (Learn how and when to remove this message)

Recycled aggregate concretes are standard concrete mixes with the addition or substitution of natural aggregates with recycled aggregates sourced from construction and demolition wastes, disused pre-cast concretes or masonry. In most cases, recycled aggregate concrete results in higher water absorption levels by capillary action and permeation, which are the prominent determiners of the strength and durability of the resulting concrete. The increase in water absorption levels is mainly caused by the porous adhered mortar that exists in the recycled aggregates. Accordingly, recycled concrete aggregates that have been washed to reduce the quantity of mortar adhered to aggregates show lower water absorption levels compared to untreated recycled aggregates.

The quality of the recycled aggregate concrete is determined by several factors, including the size, the number of replacement cycles, and the moisture levels of the recycled aggregates. When the recycled concrete aggregates are crushed into coarser fractures, the mixed concrete shows better permeability levels, resulting in an overall increase in strength. In contrast, recycled masonry aggregates provide better qualities when crushed in finer fractures. With each generation of recycled concrete, the resulting compressive strength decreases.

Concrete has relatively high compressive strength, but much lower tensile strength.^[105] Therefore, it is usually reinforced with materials that are strong in tension (often steel). The elasticity of concrete is relatively constant at low stress levels but starts decreasing at higher stress levels as matrix cracking develops. Concrete has a very low coefficient of thermal expansion and shrinks as it matures. All concrete structures crack to some extent, due to shrinkage and tension. Concrete that is subjected to long-duration forces is prone to creep.

Tests can be performed to ensure that the properties of concrete correspond to specifications for the application.

The ingredients affect the strengths of the material. Concrete strength values are usually specified as the lower-bound compressive strength of either a cylindrical or cubic specimen as determined by standard test procedures.

The strengths of concrete is dictated by its function. Very low-strength—14 MPa (2,000 psi) or less—concrete may be used when the concrete must be lightweight.^[106] Lightweight concrete is often achieved by adding air, foams, or lightweight aggregates, with the side effect that the strength is reduced. For most routine uses, 20 to 32 MPa (2,900 to 4,600 psi) concrete is often used. 40 MPa (5,800 psi) concrete is readily commercially available as a more durable, although more expensive, option. Higher-strength concrete is often used for larger civil projects.^[107] Strengths above 40 MPa (5,800 psi) are often used for specific building elements. For example, the lower floor columns of high-rise concrete buildings may use concrete of 80 MPa (11,600 psi) or more, to keep the size of the columns small. Bridges may use long beams of high-strength concrete to lower the number of spans required.^[108][109] Occasionally, other structural needs may require high-strength concrete. If a structure must be very rigid, concrete of very high strength may be specified, even much stronger than is required to bear the service loads. Strengths as high as 130 MPa (18,900 psi) have been used commercially for these reasons.^[108]

The cement produced for making concrete accounts for about 8% of worldwide CO₂ emissions per year (compared to, e.g., global aviation at 1.9%).^[110][111] The two largest sources of CO₂ are produced by the cement manufacturing process, arising from (1) the decarbonation reaction of limestone in the cement kiln (T ≈ 950 °C), and (2) from the combustion of fossil fuel to reach the sintering temperature (T ≈ 1450 °C) of cement clinker in the kiln. The energy required for extracting, crushing, and mixing the raw materials (construction aggregates used in the concrete production, and also limestone and clay feeding the cement kiln) is lower. Energy requirement for transportation of ready-mix concrete is also lower because it is produced nearby the construction site from local resources, typically manufactured within 100 kilometers of the job site.^[112] The overall embodied energy of concrete at roughly 1 to 1.5 megajoules per kilogram is therefore lower than for many structural and construction materials.^[113]

Once in place, concrete offers a great energy efficiency over the lifetime of a building.^[114] Concrete walls leak air far less than those made of wood frames.^[115] Air leakage accounts for a large percentage of energy loss from a home. The thermal mass properties of concrete increase the efficiency of both residential and commercial buildings. By storing and releasing the energy needed for heating or cooling, concrete's thermal mass delivers year-round benefits by reducing temperature swings inside and minimizing heating and cooling costs.^[116] While insulation reduces energy loss through the building envelope, thermal mass uses walls to store and release energy. Modern concrete wall systems use both external insulation and thermal mass to create an energy-efficient building. Insulating concrete forms (ICFs) are hollow blocks or panels made of either insulating foam or rastra that are stacked to form the shape of the walls of a building and then filled with reinforced concrete to create the structure.

Concrete buildings are more resistant to fire than those constructed using steel frames, since concrete has lower heat conductivity than steel and can thus last longer under the same fire conditions. Concrete is sometimes used as a fire protection for steel frames, for the same effect as above. Concrete as a fire shield, for example Fondu fyre, can also be used in extreme environments like a missile launch pad.

Options for non-combustible construction include floors, ceilings and roofs made of cast-in-place and hollow-core precast concrete. For walls, concrete masonry technology and Insulating Concrete Forms (ICFs) are additional options. ICFs are hollow blocks or panels made of fireproof insulating foam that are stacked to form the shape of the walls of a building and then filled with reinforced concrete to create the structure.

Concrete also provides good resistance against externally applied forces such as high winds, hurricanes, and tornadoes owing to its lateral stiffness, which results in minimal horizontal movement. However, this stiffness can work against certain types of concrete structures, particularly where a relatively higher flexing structure is required to resist more extreme forces.

As discussed above, concrete is very strong in compression, but weak in tension. Larger earthquakes can generate very large shear loads on structures. These shear loads subject the structure to both tensile and compressional loads. Concrete structures without reinforcement, like other unreinforced masonry structures, can fail during severe earthquake shaking. Unreinforced masonry structures constitute one of the largest earthquake risks globally.^[117] These risks can be reduced through seismic retrofitting of at-risk buildings, (e.g. school buildings in Istanbul, Turkey).^[118]

Concrete is one of the most durable building materials. It provides superior fire resistance compared with wooden construction and gains strength over time. Structures made of concrete can have a long service life.^[119] Concrete is used more than any other artificial material in the world.^[120] As of 2006, about 7.5 billion cubic meters of concrete are made each year, more than one cubic meter for every person on Earth.^[121]

The use of reinforcement, in the form of iron was introduced in the 1850s by French industrialist François Coignet, and it was not until the 1880s that German civil engineer G. A. Wayss used steel as reinforcement. Concrete is a relatively brittle material that is strong under compression but less in tension. Plain, unreinforced concrete is unsuitable for many structures as it is relatively poor at withstanding stresses induced by vibrations, wind loading, and so on. Hence, to increase its overall strength, steel rods, wires, mesh or cables can be embedded in concrete before it is set. This reinforcement, often known as rebar, resists tensile forces.^[123]

Reinforced concrete (RC) is a versatile composite and one of the most widely used materials in modern construction. It is made up of different constituent materials with very different properties that complement each other. In the case of reinforced concrete, the component materials are almost always concrete and steel. These two materials form a strong bond together and are able to resist a variety of applied forces, effectively acting as a single structural element.^[124]

Reinforced concrete can be precast or cast-in-place (in situ) concrete, and is used in a wide range of applications such as; slab, wall, beam, column, foundation, and frame construction. Reinforcement is generally placed in areas of the concrete that are likely to be subject to tension, such as the lower portion of beams. Usually, there is a minimum of 50 mm cover, both above and below the steel reinforcement, to resist spalling and corrosion which can lead to structural instability.^[123] Other types of non-steel reinforcement, such as Fibre-reinforced concretes are used for specialized applications, predominately as a means of controlling cracking.^[124]

Precast concrete is concrete which is cast in one place for use elsewhere and is a mobile material. The largest part of precast production is carried out in the works of specialist suppliers, although in some instances, due to economic and geographical factors, scale of product or difficulty of access, the elements are cast on or adjacent to the construction site.^[125] Precasting offers considerable advantages because it is carried out in a controlled environment, protected from the elements, but the downside of this is the contribution to greenhouse gas emission from transportation to the construction site.^[124]

Advantages to be achieved by employing precast concrete:^[125]

• Preferred dimension schemes exist, with elements of tried and tested designs available from a catalogue.
• Major savings in time result from manufacture of structural elements apart from the series of events which determine overall duration of the construction, known by planning engineers as the 'critical path'.
• Availability of Laboratory facilities capable of the required control tests, many being certified for specific testing in accordance with National Standards.
• Equipment with capability suited to specific types of production such as stressing beds with appropriate capacity, moulds and machinery dedicated to particular products.
• High-quality finishes achieved direct from the mould eliminate the need for interior decoration and ensure low maintenance costs.

Due to cement's exothermic chemical reaction while setting up, large concrete structures such as dams, navigation locks, large mat foundations, and large breakwaters generate excessive heat during hydration and associated expansion. To mitigate these effects, post-cooling^[126] is commonly applied during construction. An early example at Hoover Dam used a network of pipes between vertical concrete placements to circulate cooling water during the curing process to avoid damaging overheating. Similar systems are still used; depending on volume of the pour, the concrete mix used, and ambient air temperature, the cooling process may last for many months after the concrete is placed. Various methods also are used to pre-cool the concrete mix in mass concrete structures.^[126]

Another approach to mass concrete structures that minimizes cement's thermal by-product is the use of roller-compacted concrete, which uses a dry mix which has a much lower cooling requirement than conventional wet placement. It is deposited in thick layers as a semi-dry material then roller compacted into a dense, strong mass.

Raw concrete surfaces tend to be porous and have a relatively uninteresting appearance. Many finishes can be applied to improve the appearance and preserve the surface against staining, water penetration, and freezing.

Examples of improved appearance include stamped concrete where the wet concrete has a pattern impressed on the surface, to give a paved, cobbled or brick-like effect, and may be accompanied with coloration. Another popular effect for flooring and table tops is polished concrete where the concrete is polished optically flat with diamond abrasives and sealed with polymers or other sealants.

Other finishes can be achieved with chiseling, or more conventional techniques such as painting or covering it with other materials.

The proper treatment of the surface of concrete, and therefore its characteristics, is an important stage in the construction and renovation of architectural structures.^[127]

Prestressed concrete is a form of reinforced concrete that builds in compressive stresses during construction to oppose tensile stresses experienced in use. This can greatly reduce the weight of beams or slabs, by better distributing the stresses in the structure to make optimal use of the reinforcement. For example, a horizontal beam tends to sag. Prestressed reinforcement along the bottom of the beam counteracts this. In pre-tensioned concrete, the prestressing is achieved by using steel or polymer tendons or bars that are subjected to a tensile force prior to casting, or for post-tensioned concrete, after casting.

There are two different systems being used:^[124]

• Pretensioned concrete is almost always precast, and contains steel wires (tendons) that are held in tension while the concrete is placed and sets around them.
• Post-tensioned concrete has ducts through it. After the concrete has gained strength, tendons are pulled through the ducts and stressed. The ducts are then filled with grout. Bridges built in this way have experienced considerable corrosion of the tendons, so external post-tensioning may now be used in which the tendons run along the outer surface of the concrete.

More than 55,000 miles (89,000 km) of highways in the United States are paved with this material. Reinforced concrete, prestressed concrete and precast concrete are the most widely used types of concrete functional extensions in modern days. For more information see Brutalist architecture.

Once mixed, concrete is typically transported to the place where it is intended to become a structural item. Various methods of transportation and placement are used depending on the distances involve, quantity needed, and other details of application. Large amounts are often transported by truck, poured free under gravity or through a tremie, or pumped through a pipe. Smaller amounts may be carried in a skip (a metal container which can be tilted or opened to release the contents, usually transported by crane or hoist), or wheelbarrow, or carried in toggle bags for manual placement underwater.

Extreme weather conditions (extreme heat or cold; windy conditions, and humidity variations) can significantly alter the quality of concrete. Many precautions are observed in cold weather placement.^[128] Low temperatures significantly slow the chemical reactions involved in hydration of cement, thus affecting the strength development. Preventing freezing is the most important precaution, as formation of ice crystals can cause damage to the crystalline structure of the hydrated cement paste. If the surface of the concrete pour is insulated from the outside temperatures, the heat of hydration will prevent freezing.

The American Concrete Institute (ACI) definition of cold weather placement, ACI 306,^[129] is:

• A period when for more than three successive days the average daily air temperature drops below 40 °F (~ 4.5 °C), and
• Temperature stays below 50 °F (10 °C) for more than one-half of any 24-hour period.

In Canada, where temperatures tend to be much lower during the cold season, the following criteria are used by CSA A23.1:

• When the air temperature is ≤ 5 °C, and
• When there is a probability that the temperature may fall below 5 °C within 24 hours of placing the concrete.

The minimum strength before exposing concrete to extreme cold is 500 psi (3.4 MPa). CSA A 23.1 specified a compressive strength of 7.0 MPa to be considered safe for exposure to freezing.

Concrete may be placed and cured underwater. Care must be taken in the placement method to prevent washing out the cement. Underwater placement methods include the tremie, pumping, skip placement, manual placement using toggle bags, and bagwork.^[130]

A tremie is a vertical, or near-vertical, pipe with a hopper at the top used to pour concrete underwater in a way that avoids washout of cement from the mix due to turbulent water contact with the concrete while it is flowing. This produces a more reliable strength of the product. The toggle bag method is generally used for placing small quantities and for repairs. Wet concrete is loaded into a reusable canvas bag and squeezed out at the required place by the diver. Care must be taken to avoid washout of the cement and fines.

Underwater bagwork is the manual placement by divers of woven cloth bags containing dry mix, followed by piercing the bags with steel rebar pins to tie the bags together after every two or three layers, and create a path for hydration to induce curing, which can typically take about 6 to 12 hours for initial hardening and full hardening by the next day. Bagwork concrete will generally reach full strength within 28 days. Each bag must be pierced by at least one, and preferably up to four pins. Bagwork is a simple and convenient method of underwater concrete placement which does not require pumps, plant, or formwork, and which can minimise environmental effects from dispersing cement in the water. Prefilled bags are available, which are sealed to prevent premature hydration if stored in suitable dry conditions. The bags may be biodegradable.^[131]

Grouted aggregate is an alternative method of forming a concrete mass underwater, where the forms are filled with coarse aggregate and the voids then completely filled from the bottom by displacing the water with pumped grout.^[130]

Concrete roads are more fuel efficient to drive on,^[132] more reflective and last significantly longer than other paving surfaces, yet have a much smaller market share than other paving solutions. Modern-paving methods and design practices have changed the economics of concrete paving, so that a well-designed and placed concrete pavement will be less expensive on initial costs and significantly less expensive over the life cycle. Another major benefit is that pervious concrete can be used, which eliminates the need to place storm drains near the road, and reducing the need for slightly sloped roadway to help rainwater to run off. No longer requiring discarding rainwater through use of drains also means that less electricity is needed (more pumping is otherwise needed in the water-distribution system), and no rainwater gets polluted as it no longer mixes with polluted water. Rather, it is immediately absorbed by the ground.^{[citation needed]}

Cement molded into a forest of tubular structures can be 5.6 times more resistant to cracking/failure than standard concrete. The approach mimics mammalian cortical bone that features elliptical, hollow osteons suspended in an organic matrix, connected by relatively weak "cement lines". Cement lines provide a preferable in-plane crack path. This design fails via a "stepwise toughening mechanism". Cracks are contained within the tube, reducing spreading, by dissipating energy at each tube/step.^[133]

This section may be unbalanced towards certain viewpoints. Please improve the article or discuss the issue on the talk page. (January 2024)

The manufacture and use of concrete produce a wide range of environmental, economic and social impacts.

Grinding of concrete can produce hazardous dust. Exposure to cement dust can lead to issues such as silicosis, kidney disease, skin irritation and similar effects. The U.S. National Institute for Occupational Safety and Health in the United States recommends attaching local exhaust ventilation shrouds to electric concrete grinders to control the spread of this dust. In addition, the Occupational Safety and Health Administration (OSHA) has placed more stringent regulations on companies whose workers regularly come into contact with silica dust. An updated silica rule, which OSHA put into effect 23 September 2017 for construction companies, restricted the amount of breathable crystalline silica workers could legally come into contact with to 50 micro grams per cubic meter of air per 8-hour workday. That same rule went into effect 23 June 2018 for general industry, hydraulic fracturing and maritime. That deadline was extended to 23 June 2021 for engineering controls in the hydraulic fracturing industry. Companies which fail to meet the tightened safety regulations can face financial charges and extensive penalties. The presence of some substances in concrete, including useful and unwanted additives, can cause health concerns due to toxicity and radioactivity. Fresh concrete (before curing is complete) is highly alkaline and must be handled with proper protective equipment.

A major component of concrete is cement, a fine powder used mainly to bind sand and coarser aggregates together in concrete. Although a variety of cement types exist, the most common is "Portland cement", which is produced by mixing clinker with smaller quantities of other additives such as gypsum and ground limestone. The production of clinker, the main constituent of cement, is responsible for the bulk of the sector's greenhouse gas emissions, including both energy intensity and process emissions.^[134]

The cement industry is one of the three primary producers of carbon dioxide, a major greenhouse gas – the other two being energy production and transportation industries. On average, every tonne of cement produced releases one tonne of CO₂ into the atmosphere. Pioneer cement manufacturers have claimed to reach lower carbon intensities, with 590 kg of CO₂eq per tonne of cement produced.^[135] The emissions are due to combustion and calcination processes,^[136] which roughly account for 40% and 60% of the greenhouse gases, respectively. Considering that cement is only a fraction of the constituents of concrete, it is estimated that a tonne of concrete is responsible for emitting about 100–200 kg of CO₂.^[137][138] Every year more than 10 billion tonnes of concrete are used worldwide.^[138] In the coming years, large quantities of concrete will continue to be used, and the mitigation of CO₂ emissions from the sector will be even more critical.

Concrete is used to create hard surfaces that contribute to surface runoff, which can cause heavy soil erosion, water pollution, and flooding, but conversely can be used to divert, dam, and control flooding. Concrete dust released by building demolition and natural disasters can be a major source of dangerous air pollution. Concrete is a contributor to the urban heat island effect, though less so than asphalt.

Reducing the cement clinker content might have positive effects on the environmental life-cycle assessment of concrete. Some research work on reducing the cement clinker content in concrete has already been carried out. However, there exist different research strategies. Often replacement of some clinker for large amounts of slag or fly ash was investigated based on conventional concrete technology. This could lead to a waste of scarce raw materials such as slag and fly ash. The aim of other research activities is the efficient use of cement and reactive materials like slag and fly ash in concrete based on a modified mix design approach.^[139]

The embodied carbon of a precast concrete facade can be reduced by 50% when using the presented fiber reinforced high performance concrete in place of typical reinforced concrete cladding.^[140] Studies have been conducted about commercialization of low-carbon concretes. Life cycle assessment (LCA) of low-carbon concrete was investigated according to the ground granulated blast-furnace slag (GGBS) and fly ash (FA) replacement ratios. Global warming potential (GWP) of GGBS decreased by 1.1 kg CO₂ eq/m³, while FA decreased by 17.3 kg CO₂ eq/m³ when the mineral admixture replacement ratio was increased by 10%. This study also compared the compressive strength properties of binary blended low-carbon concrete according to the replacement ratios, and the applicable range of mixing proportions was derived.^[141]

High-performance building materials will be particularly important for enhancing resilience, including for flood defenses and critical-infrastructure protection.^[142] Risks to infrastructure and cities posed by extreme weather events are especially serious for those places exposed to flood and hurricane damage, but also where residents need protection from extreme summer temperatures. Traditional concrete can come under strain when exposed to humidity and higher concentrations of atmospheric CO₂. While concrete is likely to remain important in applications where the environment is challenging, novel, smarter and more adaptable materials are also needed.^[138][143]

There have been concerns about the recycling of painted concrete due to possible lead content. Studies have indicated that recycled concrete exhibits lower strength and durability compared to concrete produced using natural aggregates.^{[148][149][150][151]} This deficiency can be addressed by incorporating supplementary materials such as fly ash into the mixture.^[152]

The world record for the largest concrete pour in a single project is the Three Gorges Dam in Hubei Province, China by the Three Gorges Corporation. The amount of concrete used in the construction of the dam is estimated at 16 million cubic meters over 17 years. The previous record was 12.3 million cubic meters held by Itaipu hydropower station in Brazil.^{[153][154][155]}

The world record for concrete pumping was set on 7 August 2009 during the construction of the Parbati Hydroelectric Project, near the village of Suind, Himachal Pradesh, India, when the concrete mix was pumped through a vertical height of 715 m (2,346 ft).^[156][157]

The Polavaram dam works in Andhra Pradesh on 6 January 2019 entered the Guinness World Records by pouring 32,100 cubic metres of concrete in 24 hours.^[158] The world record for the largest continuously poured concrete raft was achieved in August 2007 in Abu Dhabi by contracting firm Al Habtoor-CCC Joint Venture and the concrete supplier is Unibeton Ready Mix.^[159][160] The pour (a part of the foundation for the Abu Dhabi's Landmark Tower) was 16,000 cubic meters of concrete poured within a two-day period.^[161] The previous record, 13,200 cubic meters poured in 54 hours despite a severe tropical storm requiring the site to be covered with tarpaulins to allow work to continue, was achieved in 1992 by joint Japanese and South Korean consortiums Hazama Corporation and the Samsung C&T Corporation for the construction of the Petronas Towers in Kuala Lumpur, Malaysia.^[162]

The world record for largest continuously poured concrete floor was completed 8 November 1997, in Louisville, Kentucky by design-build firm EXXCEL Project Management. The monolithic placement consisted of 225,000 square feet (20,900 m²) of concrete placed in 30 hours, finished to a flatness tolerance of F_F 54.60 and a levelness tolerance of F_L 43.83. This surpassed the previous record by 50% in total volume and 7.5% in total area.^[163][164]

The record for the largest continuously placed underwater concrete pour was completed 18 October 2010, in New Orleans, Louisiana by contractor C. J. Mahan Construction Company, LLC of Grove City, Ohio. The placement consisted of 10,251 cubic yards of concrete placed in 58.5 hours using two concrete pumps and two dedicated concrete batch plants. Upon curing, this placement allows the 50,180-square-foot (4,662 m²) cofferdam to be dewatered approximately 26 feet (7.9 m) below sea level to allow the construction of the Inner Harbor Navigation Canal Sill & Monolith Project to be completed in the dry.^[165]

Concrete is used as an artistic medium.^{[citation needed]} Its appearance is also imitated in other media: for example Congolese artist Sardoine Mia creates canvases that look like concrete surfaces.^[166]

• Concrete leveling – Process to level concrete by levelling its underlying foundation
• Concrete mixer – Device that combines cement, aggregate, and water to form concrete
• Concrete masonry unit – Standard-sized block used in constructionPages displaying short descriptions of redirect targets
• Concrete plant – Equipment that combines various ingredients to form concrete
• Eurocode 2: Design of concrete structures
• Heavy metals – Loosely defined subset of elements that exhibit metallic properties
• Hempcrete – Biocomposite material used for construction and insulation
• Particulates – Microscopic solid or liquid matter suspended in the Earth's atmosphere
• Schmidt hammer – Type of measuring instrument
• Syncrete – Synthetic form of concrete
• Thermal integrity profiling – Method used to test concrete