Types of concrete
Concrete is produced in a variety of compositions and performance characteristics to meet a wide range of needs. Modern concrete mix designs can be complex; the choice of a concrete mix depends on the need of the project both in terms of strength and appearance and in relation to local legislation and building codes. The design begins by determining the requirements of the concrete; these requirements take into consideration the weather conditions that the concrete will be exposed to in service, the required design strength. The compressive strength of a concrete is determined by taking standard molded, standard-cured cylinder samples. Many factors need to be taken into account, from the cost of the various additives and aggregates, to the trade offs between the "slump" for easy mixing and placement and ultimate performance. A mix is designed using cement and fine aggregates and chemical admixtures; the method of mixing will be specified, as well as conditions that it may be used in. This allows a user of the concrete to be confident.
Various types of concrete have been developed for specialist application and have become known by these names. Concrete mixes can be designed using software programs; such software provides the user an opportunity to select their preferred method of mix design and enter the material data to arrive at proper mix designs. Concrete has been used since ancient times. Regular Roman concrete for example was made from volcanic ash, hydrated lime. Roman concrete was superior to other concrete recipes used by other cultures. Besides volcanic ash for making regular Roman concrete, brick dust can be used. Besides regular Roman concrete, the Romans invented hydraulic concrete, which they made from volcanic ash and clay. Regular concrete is the lay term for concrete, produced by following the mixing instructions that are published on packets of cement using sand or other common material as the aggregate, mixed in improvised containers; the ingredients in any particular mix depends on the nature of the application.
Regular concrete can withstand a pressure from about 10 MPa to 40 MPa, with lighter duty uses such as blinding concrete having a much lower MPa rating than structural concrete. Many types of pre-mixed concrete are available which include powdered cement mixed with an aggregate, needing only water. A batch of concrete can be made by using 1 part Portland cement, 2 parts dry sand, 3 parts dry stone, 1/2 part water; the parts are in terms of weight – not volume. For example, 1-cubic-foot of concrete would be made using 22 lb cement, 10 lb water, 41 lb dry sand, 70 lb dry stone; this would weigh about 143 lb. The sand should be mortar or brick sand and the stone should be washed if possible. Organic materials should be removed from the stone to ensure the highest strength. High-strength concrete has a compressive strength greater than 40 MPa. In the UK, BS EN 206-1 defines High strength concrete as concrete with a compressive strength class higher than C50/60. High-strength concrete is made by lowering the water-cement ratio to 0.35 or lower.
Silica fume is added to prevent the formation of free calcium hydroxide crystals in the cement matrix, which might reduce the strength at the cement-aggregate bond. Low W/C ratios and the use of silica fume make concrete mixes less workable, likely to be a problem in high-strength concrete applications where dense rebar cages are to be used. To compensate for the reduced workability, superplasticizers are added to high-strength mixtures. Aggregate must be selected for high-strength mixes, as weaker aggregates may not be strong enough to resist the loads imposed on the concrete and cause failure to start in the aggregate rather than in the matrix or at a void, as occurs in regular concrete. In some applications of high-strength concrete the design criterion is the elastic modulus rather than the ultimate compressive strength. Stamped concrete is an architectural concrete. After a concrete floor has been laid, floor hardeners are impregnated on the surface and a mold that may be textured to replicate a stone / brick or wood is stamped on to give an attractive textured surface finish.
After sufficient hardening, the surface is cleaned and sealed to provide protection. The wear resistance of stamped concrete is excellent and hence found in applications like parking lots, walkways etc. High-performance concrete is a new term for concrete that conforms to a set of standards above those of the most common applications, but not limited to strength. While all high-strength concrete is high-performance, not all high-performance concrete is high-strength; some examples of such standards used in relation to HPC are: Ultra-high-performance concrete is a new type of concrete, being developed by agencies concerned with infrastructure protection. UHPC is characterized by being a steel fibre-reinforced cement composite material with compressive strengths in excess of 150 MPa, up to and exceeding 250 MPa. UHPC is characterized by its constituent material make-up: fine-grained sand, silica fume, small steel fibers, special blends of high-strength Portland cement. Note that there is no large aggregate.
The current types in
An embankment dam is a large artificial dam. It is created by the placement and compaction of a complex semi-plastic mound of various compositions of soil, clay, or rock, it has a semi-pervious waterproof natural covering for a dense, impervious core. This makes such a dam impervious to seepage erosion; such a dam is composed of fragmented independent material particles. The friction and interaction of particles binds the particles together into a stable mass rather than by the use of a cementing substance. Embankment dams come in two types: the earth-filled dam made of compacted earth, the rock-filled dam. A cross-section of an embankment dam shows a shape like hill. Most have a central section or core composed of an impermeable material to stop water from seeping through the dam; the core can be of concrete, or asphalt concrete. This dam type is a good choice for sites with wide valleys, they can be built on softer soils. For a rock-fill dam, rock-fill is blasted using explosives to break the rock.
Additionally, the rock pieces may need to be crushed into smaller grades to get the right range of size for use in an embankment dam. The building of a dam and the filling of the reservoir behind it places a new weight on the floor and sides of a valley; the stress of the water increases linearly with its depth. Water pushes against the upstream face of the dam, a nonrigid structure that under stress behaves semiplastically, causes greater need for adjustment near the base of the dam than at shallower water levels, thus the stress level of the dam must be calculated in advance of building to ensure that its break level threshold is not exceeded. Overtopping or overflow of an embankment dam beyond its spillway capacity will cause its eventual failure; the erosion of the dam's material by overtopping runoff will remove masses of material whose weight holds the dam in place and against the hydraulic forces acting to move the dam. A small sustained overtopping flow can remove thousands of tons of overburden soil from the mass of the dam within hours.
The removal of this mass unbalances the forces that stabilize the dam against its reservoir as the mass of water still impounded behind the dam presses against the lightened mass of the embankment, made lighter by surface erosion. As the mass of the dam erodes, the force exerted by the reservoir begins to move the entire structure; the embankment, having no elastic strength, would begin to break into separate pieces, allowing the impounded reservoir water to flow between them and removing more material as it passes through. In the final stages of failure the remaining pieces of the embankment would offer no resistance to the flow of the water and continue to fracture into smaller and smaller sections of earth or rock until these would disintegrate into a thick mud soup of earth and water. Therefore, safety requirements for the spillway are high, require it to be capable of containing a maximum flood stage, it is common for its specifications to be written such. A number of embankment dam overtopping protection systems have been developed.
These techniques include the concrete overtopping protection systems, timber cribs, sheet-piles and gabions, reinforced earth, minimum energy loss weirs, embankment overflow stepped spillways and the precast concrete block protection systems. Earth structure Gravity dam List of largest dams in the world Embankment dams Table of contents An introduction to embankment dams 100 Years of Embankment Dam Design and Construction in the U. S. Bureau of Reclamation
Hydroelectricity is electricity produced from hydropower. In 2015, hydropower generated 16.6% of the world's total electricity and 70% of all renewable electricity, was expected to increase about 3.1% each year for the next 25 years. Hydropower is produced in 150 countries, with the Asia-Pacific region generating 33 percent of global hydropower in 2013. China is the largest hydroelectricity producer, with 920 TWh of production in 2013, representing 16.9 percent of domestic electricity use. The cost of hydroelectricity is low, making it a competitive source of renewable electricity; the hydro station consumes no water, unlike gas plants. The average cost of electricity from a hydro station larger than 10 megawatts is 3 to 5 U. S. cents per kilowatt hour. With a dam and reservoir it is a flexible source of electricity since the amount produced by the station can be varied up or down rapidly to adapt to changing energy demands. Once a hydroelectric complex is constructed, the project produces no direct waste, in many cases, has a lower output level of greenhouse gases than fossil fuel powered energy plants.
Hydropower has been used since ancient times to perform other tasks. In the mid-1770s, French engineer Bernard Forest de Bélidor published Architecture Hydraulique which described vertical- and horizontal-axis hydraulic machines. By the late 19th century, the electrical generator was developed and could now be coupled with hydraulics; the growing demand for the Industrial Revolution would drive development as well. In 1878 the world's first hydroelectric power scheme was developed at Cragside in Northumberland, England by William Armstrong, it was used to power a single arc lamp in his art gallery. The old Schoelkopf Power Station No. 1 near Niagara Falls in the U. S. side began to produce electricity in 1881. The first Edison hydroelectric power station, the Vulcan Street Plant, began operating September 30, 1882, in Appleton, with an output of about 12.5 kilowatts. By 1886 there were 45 hydroelectric power stations in the U. S. and Canada. By 1889 there were 200 in the U. S. alone. At the beginning of the 20th century, many small hydroelectric power stations were being constructed by commercial companies in mountains near metropolitan areas.
Grenoble, France held the International Exhibition of Hydropower and Tourism with over one million visitors. By 1920 as 40% of the power produced in the United States was hydroelectric, the Federal Power Act was enacted into law; the Act created the Federal Power Commission to regulate hydroelectric power stations on federal land and water. As the power stations became larger, their associated dams developed additional purposes to include flood control and navigation. Federal funding became necessary for large-scale development and federally owned corporations, such as the Tennessee Valley Authority and the Bonneville Power Administration were created. Additionally, the Bureau of Reclamation which had begun a series of western U. S. irrigation projects in the early 20th century was now constructing large hydroelectric projects such as the 1928 Hoover Dam. The U. S. Army Corps of Engineers was involved in hydroelectric development, completing the Bonneville Dam in 1937 and being recognized by the Flood Control Act of 1936 as the premier federal flood control agency.
Hydroelectric power stations continued to become larger throughout the 20th century. Hydropower was referred to as white coal for its plenty. Hoover Dam's initial 1,345 MW power station was the world's largest hydroelectric power station in 1936; the Itaipu Dam opened in 1984 in South America as the largest, producing 14,000 MW but was surpassed in 2008 by the Three Gorges Dam in China at 22,500 MW. Hydroelectricity would supply some countries, including Norway, Democratic Republic of the Congo and Brazil, with over 85% of their electricity; the United States has over 2,000 hydroelectric power stations that supply 6.4% of its total electrical production output, 49% of its renewable electricity. The technical potential for hydropower development around the world is much greater than the actual production: the percent of potential hydropower capacity that has not been developed is 71% in Europe, 75% in North America, 79% in South America, 95% in Africa, 95% in the Middle East, 82% in Asia-Pacific.
The political realities of new reservoirs in western countries, economic limitations in the third world and the lack of a transmission system in undeveloped areas result in the possibility of developing 25% of the remaining technically exploitable potential before 2050, with the bulk of that being in the Asia-Pacific area. Some countries have developed their hydropower potential and have little room for growth: Switzerland produces 88% of its potential and Mexico 80%. Most hydroelectric power comes from the potential energy of dammed water driving a water turbine and generator; the power extracted from the water depends on the volume and on the difference in height between the source and the water's outflow. This height difference is called the head. A large pipe delivers water from the reservoir to the turbine; this method produces electricity to supply high peak demands by moving water between reservoirs at different elevations. At times of low electrical demand, the excess generation capacity is used to pump water into the higher reservoir.
When the demand becomes greater, water is released back into the lower reservoir through a turbine. Pumped-storage schemes provide the most commercially important means of large-scale grid energy storage and improve the daily capacity factor of the generation system. Pumped storag
Columbia, South Carolina
Columbia is the capital and second largest city of the U. S. state of South Carolina, with a population estimate of 134,309 as of 2016. The city serves as the county seat of Richland County, a portion of the city extends into neighboring Lexington County, it is the center of the Columbia metropolitan statistical area, which had a population of 767,598 as of the 2010 United States Census, growing to 817,488 by July 1, 2016, according to 2015 U. S. Census estimates; the name Columbia is a poetic term used for the United States, originating from the name of Christopher Columbus. The city is located 13 miles northwest of the geographic center of South Carolina, is the primary city of the Midlands region of the state, it lies at the confluence of the Saluda River and the Broad River, which merge at Columbia to form the Congaree River. Columbia is home to the University of South Carolina, the state's flagship university and the largest in the state, is the site of Fort Jackson, the largest United States Army installation for Basic Combat Training.
Columbia is located 20 miles west of the site of McEntire Joint National Guard Base, operated by the U. S. Air Force and is used as a training base for the 169th Fighter Wing of The South Carolina Air National Guard. Columbia is the location of the South Carolina State House, the center of government for the state. In 1860, the city was the location of the South Carolina Secession Convention, which marked the departure of the first state from the Union in the events leading up to the Civil War. At the time of European encounter, the inhabitants of the area that became Columbia were a people called the Congaree. In May 1540, a Spanish expedition led by Hernando de Soto traversed what is now Columbia while moving northward; the expedition produced the earliest written historical records of the area, part of the regional Cofitachequi chiefdom. From the creation of Columbia by the South Carolina General Assembly in 1786, the site of Columbia was important to the overall development of the state; the Congarees, a frontier fort on the west bank of the Congaree River, was the head of navigation in the Santee River system.
A ferry was established by the colonial government in 1754 to connect the fort with the growing settlements on the higher ground on the east bank. Like many other significant early settlements in colonial America, Columbia is on the fall line from the Piedmont region; the fall line is the spot where a river becomes unnavigable when sailing upstream and where water flowing downstream can power a mill. State Senator John Lewis Gervais of the town of Ninety Six introduced a bill, approved by the legislature on March 22, 1786, to create a new state capital. There was considerable argument over the name for the new city. According to published accounts, Senator Gervais said he hoped that "in this town we should find refuge under the wings of COLUMBIA", for, the name which he wished it to be called. One legislator insisted on the name "Washington", but "Columbia" won by a vote of 11–7 in the state senate; the site was chosen as the new state capital in 1786, due to its central location in the state.
The State Legislature first met there in 1790. After remaining under the direct government of the legislature for the first two decades of its existence, Columbia was incorporated as a village in 1805 and as a city in 1854. Columbia received a large stimulus to development when it was connected in a direct water route to Charleston by the Santee Canal; this canal connected the Cooper rivers in a 22-mile-long section. It was first chartered in 1786 and completed in 1800, making it one of the earliest canals in the United States. With increased railroad traffic, it ceased operation around 1850; the commissioners designed a town of 400 blocks in a 2-mile square along the river. The blocks were sold to speculators and prospective residents. Buyers had to build a house at least 30 feet long and 18 feet wide within three years or face an annual 5% penalty; the perimeter streets and two through streets were 150 feet wide. The remaining squares were divided by thoroughfares 100 feet wide; the commissioners comprised the local government until 1797 when a Commission of Streets and Markets was created by the General Assembly.
Three main issues occupied most of their time: public drunkenness and poor sanitation. As one of the first planned cities in the United States, Columbia began to grow rapidly, its population was nearing 1,000 shortly after the start of the 19th century. In 1801, South Carolina College was founded in Columbia; the original building survives. The city was chosen as the site of the institution in part to unite the citizens of the Upcountry and the Lowcountry and to discourage the youth from migrating to England for their higher education. At the time, South Carolina sent more young men to England; the leaders of South Carolina wished to monitor the development of the school. Columbia received its first charter as a town in 1805. An intendant and six wardens would govern the town. John Taylor, the first elected intendant served in both houses of the General Assembly, both houses of Congress, as governor. By 1816, there were a population of more than one thousand. Columbia became chartered with an elected mayor and six aldermen.
Two years Columbia had a police force consisting of a full-time chief and nine patrolmen. The city continued to grow at a rapid
Recreation is an activity of leisure, leisure being discretionary time. The "need to do something for recreation" is an essential element of human psychology. Recreational activities are done for enjoyment, amusement, or pleasure and are considered to be "fun"; the term recreation appears to have been used in English first in the late 14th century, first in the sense of "refreshment or curing of a sick person", derived turn from Latin. Humans spend their time in activities of daily living, sleep, social duties, leisure, the latter time being free from prior commitments to physiologic or social needs, a prerequisite of recreation. Leisure has increased with increased longevity and, for many, with decreased hours spent for physical and economic survival, yet others argue that time pressure has increased for modern people, as they are committed to too many tasks. Other factors that account for an increased role of recreation are affluence, population trends, increased commercialization of recreational offerings.
While one perception is that leisure is just "spare time", time not consumed by the necessities of living, another holds that leisure is a force that allows individuals to consider and reflect on the values and realities that are missed in the activities of daily life, thus being an essential element of personal development and civilization. This direction of thought has been extended to the view that leisure is the purpose of work, a reward in itself, "leisure life" reflects the values and character of a nation. Leisure is considered a human right under the Universal Declaration of Human Rights. Recreation is difficult to separate from the general concept of play, the term for children's recreational activity. Children may playfully imitate activities, it has been proposed that play or recreational activities are outlets of or expression of excess energy, channeling it into acceptable activities that fulfill individual as well as societal needs, without need for compulsion, providing satisfaction and pleasure for the participant.
A traditional view holds that work is supported by recreation, recreation being useful to "recharge the battery" so that work performance is improved. Work, an activity performed out of economic necessity and useful for society and organized within the economic framework, however can be pleasurable and may be self-imposed thus blurring the distinction to recreation. Many activities may be work for one person and recreation for another, or, at an individual level, over time recreational activity may become work, vice versa. Thus, for a musician, playing an instrument may be at one time a profession, at another a recreation, it may be difficult to separate education from recreation as in the case of recreational mathematics. Recreation is an essential part of human life and finds many different forms which are shaped by individual interests but by the surrounding social construction. Recreational activities can be communal or solitary, active or passive, outdoors or indoors, healthy or harmful, useful for society or detrimental.
A significant section of recreational activities are designated as hobbies which are activities done for pleasure on a regular basis. A list of typical activities could be endless including most human activities, a few examples being reading, playing or listening to music, watching movies or TV, fine dining, sports and travel; some recreational activities - such as gambling, recreational drug use, or delinquent activities - may violate societal norms and laws. Public space such as parks and beaches are essential venues for many recreational activities. Tourism has recognized that many visitors are attracted by recreational offerings. In support of recreational activities government has taken an important role in their creation and organization, whole industries have developed merchandise or services. Recreation-related business is an important factor in the economy. S. economy and generates 6.5 million jobs. A recreation center is a place for recreational activities administered by a municipal government agency.
Swimming, weightlifting and kids' play areas are common. Many recreational activities are organized by public institutions, voluntary group-work agencies, private groups supported by membership fees, commercial enterprises. Examples of each of these are the National Park Service, the YMCA, the Kiwanis, Walt Disney World. Recreation has many health benefits, accordingly, Therapeutic Recreation has been developed to take advantage of this effect; the National Council for Therapeutic Recreation Certification is the nationally recognized credentialing organization for the profession of Therapeutic Recreation. Professionals in the field of Therapeutic Recreation who are certified by the NCTRC are called "Certified Therapeutic Recreation Specialists"; the job title "Recreation Therapist" is identified in the U. S. Dept of Labor's Occupation Outlook; such therapy is applied in rehabilitation, psychiatric facilities for youth and adults, in the care of the elderly, the disabled, or people with chronic diseases.
Recreational physical activity is important to reduce obesity, the risk of osteoporosis and of cancer, most in men that of colon and prostate, in women that of the breast. Extreme adventure recreation carries its own ha
Lake Murray (South Carolina)
Lake Murray is a reservoir in the U. S. state of South Carolina. It is 50,000 acres in size, has 500 miles of shoreline, it was impounded in the late 1920s to provide hydroelectric power to the state of South Carolina. Lake Murray is fed by the Saluda River, which flows from upstate South Carolina near the North Carolina state line; the Saluda Dam was an engineering feat at the time of its construction. The dam, using the native red clay soil and bedrock, was the largest earthen dam in the world when it was completed in 1930. Lake Murray itself is named after William S. Murray; the Saluda Dam is 1.5 miles long and 220 feet high. Lake Murray is 41 miles long, 14 miles wide at its widest point. At the time when the lake was finished, it was the world's largest man-made reservoir. In addition to serving as a source of hydroelectric power for the region, the lake has become a recreational attraction, with fishing and boating being popular activities. Dreher Island State Recreation Area, located in the Western part of the lake, provides multiple activities—all focused on the lake.
The Saluda River was named after the Saluda Indian tribe. For reasons unclear, the Saluda tribe migrated to Pennsylvania beginning in the early 18th century and were replaced by Cherokee from the north; the lower Saluda River valley was settled in the early 1750s by German and Swiss emigrants. The region had two major settlements: the Saxe-Gotha township. In 1755, the Cherokee signed a peace treaty with the British and the Cherokee withdrew from the area, leaving much of the land for open settlement; the Dutch Fork was the most densely settled, becoming home to 483 settler families by 1760. It has been estimated that by the year 1765 there were about 8,000 Dutch-Germans and German-Swiss and an additional 1000 Moravians of German origin who had come to the province of South Carolina. A total of 9000 Germans was the number or 8.4% of population in 1765. Because of this common nationality and language, the Dutch Fork community remained cohesive and somewhat isolated through the years. Today, the surnames of area reflect this: Sligh, Cannon, Lindler, Corley, Sease, Bowers, Kinard, Summer, Dreher, Dominic, Epting, Huffstetler, McCartha, etc.
Many of these family groups live on land, under the original land grant from the King of England still today. During the American Revolution, the Dutch Fork area was patriot, unlike the surrounding regions that held large groups of English settlers; the only major engagement of the Revolution, fought in the vicinity occurred in the nearby town of Ninety-Six, located up the Saluda River. It was the first land battle south of New England in the war; the Saluda River was a strategic boundary, since there was no bridge on the river at that time, the ferries near the Dutch Fork area were vital to the movement of troops and material westward toward the frontier. The most important of these ferries were Kimpson's Ferry. During the war, Hessian mercenaries came to South Carolina to fight for the British. Many of them had been pressed into the service and brought to the Colonies against their will, therefore many deserted the army and found shelter in Dutch-German settlements such as the Dutch Fork. Today, many locals know of specific ancestors that were brought to fight the young United States and became citizens.
After the war ended, things in the Dutch Fork returned to peaceful normalcy until the American Civil War. When South Carolina became the first state to secede from the Union, numerous volunteer regiments were created from people in these settlements. By 1928, about 5,000 people were living in the Saluda River valley; the community included 3 churches, 6 schools, 193 graveyards. There had been interest in water power generation on the Saluda River for more than a hundred years; as the demand for electricity in the developing Southern United States increased, it became apparent that harnessing the flow of large rivers such as the Saluda would be needed. In 1904, Lexington Water Power Company was incorporated by G. A. Guignard of Columbia, South Carolina; the company acquired the flowage rights on the Saluda River from Dreher Shoals to 20 miles upstream. Two dams were considered to be built, one at Dreher Shoals, 10 miles west of Columbia, the other at Bear Creek, five miles upstream. However, in 1907 the company sold the lands necessary for construction of the lower dam at Dreher Shoals to James W. Jackson, of Augusta, Georgia and W. T. Van Brunt of New York.
Between 1908 and 1911, ownership of the Dreher Shoals property shifted several times but it was purchased by the Richland Public Service Company, a subsidiary of Columbia Railway, Gas & Electric Company. Since 1916, a man named Thomas Clay Williams had been proposing the development of hydroelectric power on the Saluda and Cooper rivers in South Carolina, but his propositions did not generate much serious interest. T. C. Williams was not an engineer, his belief that massive power could be generated from the swamps and coastal plains of the state did not meet much enthusiasm with the leading engineers of South Carolina, it was not until the plans were brought before an engineer from New York, William Spencer Murray, that an engineer realized Williams's dream and its potential. William S. Murray was an engineer with much experience in generation. In 1920, Congress
Steel is an alloy of iron and carbon, sometimes other elements. Because of its high tensile strength and low cost, it is a major component used in buildings, tools, automobiles, machines and weapons. Iron is the base metal of steel. Iron is able to take on two crystalline forms, body centered cubic and face centered cubic, depending on its temperature. In the body-centered cubic arrangement, there is an iron atom in the center and eight atoms at the vertices of each cubic unit cell, it is the interaction of the allotropes of iron with the alloying elements carbon, that gives steel and cast iron their range of unique properties. In pure iron, the crystal structure has little resistance to the iron atoms slipping past one another, so pure iron is quite ductile, or soft and formed. In steel, small amounts of carbon, other elements, inclusions within the iron act as hardening agents that prevent the movement of dislocations that are common in the crystal lattices of iron atoms; the carbon in typical steel alloys may contribute up to 2.14% of its weight.
Varying the amount of carbon and many other alloying elements, as well as controlling their chemical and physical makeup in the final steel, slows the movement of those dislocations that make pure iron ductile, thus controls and enhances its qualities. These qualities include such things as the hardness, quenching behavior, need for annealing, tempering behavior, yield strength, tensile strength of the resulting steel; the increase in steel's strength compared to pure iron is possible only by reducing iron's ductility. Steel was produced in bloomery furnaces for thousands of years, but its large-scale, industrial use began only after more efficient production methods were devised in the 17th century, with the production of blister steel and crucible steel. With the invention of the Bessemer process in the mid-19th century, a new era of mass-produced steel began; this was followed by the Siemens–Martin process and the Gilchrist–Thomas process that refined the quality of steel. With their introductions, mild steel replaced wrought iron.
Further refinements in the process, such as basic oxygen steelmaking replaced earlier methods by further lowering the cost of production and increasing the quality of the final product. Today, steel is one of the most common manmade materials in the world, with more than 1.6 billion tons produced annually. Modern steel is identified by various grades defined by assorted standards organizations; the noun steel originates from the Proto-Germanic adjective stahliją or stakhlijan, related to stahlaz or stahliją. The carbon content of steel is between 0.002% and 2.14% by weight for plain iron–carbon alloys. These values vary depending on alloying elements such as manganese, nickel, so on. Steel is an iron-carbon alloy that does not undergo eutectic reaction. In contrast, cast iron does undergo eutectic reaction. Too little carbon content leaves iron quite soft and weak. Carbon contents higher than those of steel make a brittle alloy called pig iron. While iron alloyed with carbon is called carbon steel, alloy steel is steel to which other alloying elements have been intentionally added to modify the characteristics of steel.
Common alloying elements include: manganese, chromium, boron, vanadium, tungsten and niobium. Additional elements, most considered undesirable, are important in steel: phosphorus, sulfur and traces of oxygen and copper. Plain carbon-iron alloys with a higher than 2.1% carbon content are known as cast iron. With modern steelmaking techniques such as powder metal forming, it is possible to make high-carbon steels, but such are not common. Cast iron is not malleable when hot, but it can be formed by casting as it has a lower melting point than steel and good castability properties. Certain compositions of cast iron, while retaining the economies of melting and casting, can be heat treated after casting to make malleable iron or ductile iron objects. Steel is distinguishable from wrought iron, which may contain a small amount of carbon but large amounts of slag. Iron is found in the Earth's crust in the form of an ore an iron oxide, such as magnetite or hematite. Iron is extracted from iron ore by removing the oxygen through its combination with a preferred chemical partner such as carbon, lost to the atmosphere as carbon dioxide.
This process, known as smelting, was first applied to metals with lower melting points, such as tin, which melts at about 250 °C, copper, which melts at about 1,100 °C, the combination, which has a melting point lower than 1,083 °C. In comparison, cast iron melts at about 1,375 °C. Small quantities of iron were smelted in ancient times, in the solid state, by heating the ore in a charcoal fire and welding the clumps together with a hammer and in the process squeezing out the impurities. With care, the carbon content could be controlled by moving it around in the fire. Unlike copper and tin, liquid or solid iron dissolves carbon quite readily. All of these temperatures could be reached with ancient methods used since the Bronze Age. Since the oxidation rate of iron increases beyond 800 °C, it is important that smelting take place in a low-oxygen environment. Smelting, using carbon to reduce iro