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101-581: Dujiangyan Irrigation System were inscribed on the World Heritage List in 2000. It has also been declared a State Priority Protected Site, among the first batch of National Scenic Areas and Historical Sites, and a National ISO14000 Demonstration Area. During the Warring States period , people who lived in the area of the Min River were plagued by annual flooding. Qin hydrologist Li Bing investigated

202-411: A calcium aluminate cement or with Portland cement to form Portland cement concrete (named for its visual resemblance to Portland stone ). 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

303-465: A 200-meter (660 ft)-wide opening that connects the inner and outer streams. This ensures against flooding by allowing the natural swirling flow of the water to drain out excess water from the inner to the outer stream. The swirl also drains out silt and sediment that failed to go into the outer stream. A modern reinforced concrete weir has replaced the original weighted bamboo baskets. The Baopingkou or Bottle-Neck Channel ( Chinese :宝瓶口) , which

404-433: 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. Concrete is an artificial composite material , comprising a matrix of cementitious binder (typically Portland cement paste or asphalt ) and

505-417: A UNESCO report titled "World Heritage and Tourism in a Changing Climate". The Australian government's actions, involving considerable expense for lobbying and visits for diplomats , were in response to their concern about the negative impact that an "at risk" label could have on tourism revenue at a previously designated UNESCO World Heritage Site. In 2021, international scientists recommended UNESCO to put

606-452: 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. 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 . The hydration of cement involves many concurrent reactions. The process involves polymerization ,

707-505: A channel 20 meters (66 ft) wide had been gouged through the mountain. After the system was finished, no more floods occurred. The irrigation made Sichuan the most productive agricultural region in China for a time. The construction is also credited with giving the people of the region a laid-back attitude to life; by eliminating disaster and ensuring a regular and bountiful harvest, it left them with plenty of free time. Turmoil surrounding

808-400: A concrete component—and become a part of the binder for the aggregate. Fly ash and slag can enhance some properties of concrete such as fresh properties and durability. 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

909-434: 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 . 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

1010-437: A fluid cement that cures to a solid over time. Concrete is the second-most-used substance in the world after water , and is the most widely used building material. Its usage worldwide, ton for ton, is twice that of steel, wood, plastics, and aluminium combined. When aggregate is mixed with dry Portland cement and water , the mixture forms a fluid slurry that is easily poured and molded into shape. The cement reacts with

1111-452: A greater degree of fracture resistance even in seismically active environments. 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. The use of hot mixing and the presence of lime clasts are thought to give the concrete a self-healing ability, where cracks that form become filled with calcite that prevents

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1212-415: 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

1313-461: 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

1414-483: A minor boundary change, one that does not have a significant impact on the extent of the property or affect its "outstanding universal value", is also evaluated by the advisory bodies before being sent to the committee. Such proposals can be rejected by either the advisory bodies or the Committee if they judge it to be a significant change instead of a minor one. Proposals to change a site's official name are sent directly to

1515-469: 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

1616-599: A modern statue of Li Bing, opens up onto a courtyard facing an opera stage. On Li Bing's traditional birthday, 24th day of the 7th month of the lunar calendar, local operas were performed for the public, and on Tomb Sweeping Day a Water Throwing Festival is held. The rear hall contains a modern statue of the god Erlang Shen . Because it would be a problem if a family had no offspring in Chinese feudal society, locals regarded this Erlang as Li Bing's son. Guanlantin Pavilion stands above

1717-405: 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

1818-446: A portion of the river's flow and then to cut a channel through Mount Yulei to discharge the excess water upon the dry Chengdu Plain beyond. King Zhao of Qin allocated 100,000 taels of silver for the project and sent a team said to number tens of thousands. The levee was constructed from long sausage-shaped baskets of woven bamboo filled with stones known as Zhulong held in place by wooden tripods known as Macha. The construction of

1919-756: A remarkable accomplishment of humankind and serve as evidence of our intellectual history on the planet, or it might be a place of great natural beauty. As of July 2024, a total of 1,223 World Heritage Sites (952 cultural, 231 natural and 40 mixed cultural and natural properties) exist across 168 countries . With 60 selected areas, Italy is the country with the most sites, followed by China with 59, and Germany with 54. The sites are intended for practical conservation for posterity, which otherwise would be subject to risk from human or animal trespassing, unmonitored, uncontrolled or unrestricted access, or threat from local administrative negligence. Sites are demarcated by UNESCO as protected zones. The World Heritage Sites list

2020-424: 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

2121-493: 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. Thorough mixing is essential to produce uniform, high-quality concrete. Separate paste mixing has shown that

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2222-955: A single text was eventually agreed upon by all parties, and the Convention Concerning the Protection of the World Cultural and Natural Heritage was adopted by the General Conference of UNESCO on 16 November 1972. The convention came into force on 17 December 1975. As of November 2024, it has been ratified by 196 states: 192 UN member states , two UN observer states (the Holy See and the State of Palestine ), and two states in free association with New Zealand (the Cook Islands and Niue ). Only one UN member state, Liechtenstein , has not ratified

2323-457: A water-diversion levee resembling a fish's mouth took four years to complete. Cutting the channel proved to be a far greater problem, as the hand tools available at the time, prior to the invention of gunpowder , would have taken decades to cut through the mountain. Li Bing devised an ingenious method of using fire and water to rapidly heat and cool the rocks, causing them to crack and allowing them to be easily removed. After eight years of work,

2424-448: Is deep and narrow, while the outer stream is relatively shallow but wide. This special structure ensures that the inner stream carries approximately 60% of the river's flow into the irrigation system during dry season. While during flood, this amount decreases to 40% to protect the people from flooding. The outer stream drains away the rest, flushing out much of the silt and sediment. The Feishayan or Flying Sand Weir ( Chinese :飞沙堰) has

2525-511: Is distinct from mortar . Whereas concrete is itself a building material, mortar is a bonding agent that typically holds bricks , tiles and other masonry units together. Grout is another material associated with concrete and cement. It 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

2626-584: Is located at the turning point of the two topographic steps of the western plateau mountains and the Chengdu Plain . It is the southwest extension of the Longmen Mountains and the area through which the Longmen Mountain Fault Zone passes. The Dujiangyan irrigation system is higher in the northwest and lower in the southeast. The west belongs to the southern section of Longmen Mountains, with

2727-559: Is maintained by the international World Heritage Program administered by the UNESCO World Heritage Committee , composed of 21 "states parties" that are elected by the United Nations General Assembly , and advised by reviews of international panels of experts in natural or cultural history, and education. The Program catalogues, names, and conserves sites of outstanding cultural or natural importance to

2828-559: Is then blended with aggregates and any remaining batch water and final mixing is completed in conventional concrete mixing equipment. 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

2929-399: 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 . It consists of

3030-751: The United Nations Conference on the Human Environment in Stockholm. Under the World Heritage Committee, signatory countries are required to produce and submit periodic data reporting providing the committee with an overview of each participating nation's implementation of the World Heritage Convention and a "snapshot" of current conditions at World Heritage properties. Based on the draft convention that UNESCO had initiated,

3131-480: The construction industry , whose demand is ever growing with greater impacts on raw material extraction, waste generation and landfill practices. 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

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3232-592: The Diexi earthquake in 1933 by Zhang Yuan (张沅) and his sons, including Zhang Shiling (张世龄). In 2000, Dujiangyan became a UNESCO World Heritage Site . Today it has become a major tourist attraction. On May 12, 2008, a massive earthquake struck a vast portion of west Sichuan, including the Dujiangyan area. Initial reports indicated that the Yuzui Levee was cracked but not severely damaged. Diversion of flow could still be seen as

3333-679: The Great Barrier Reef on the endangered list, as global climate change had caused a further negative state of the corals and water quality. Again, the Australian government campaigned against this, and in July 2021, the World Heritage Committee , made up of diplomatic representatives of 21 countries, ignored UNESCO's assessment, based on studies of scientists, "that the reef was clearly in danger from climate change and so should be placed on

3434-968: The List of World Heritage in Danger and the World Heritage List. Only three sites have ever been delisted : the Arabian Oryx Sanctuary in Oman, the Dresden Elbe Valley in Germany, and the Liverpool Maritime Mercantile City in the United Kingdom. The Arabian Oryx Sanctuary was directly delisted in 2007, instead of first being put on the danger list, after the Omani government decided to reduce

3535-584: The South Atlantic, is part of the Europe and North America region because the British government nominated the site. The table below includes a breakdown of the sites according to these regions and their classification as of July 2024 : This overview lists the 23 countries with 15 or more World Heritage Sites: Concrete Concrete is a composite material composed of aggregate bonded together with

3636-609: The World Heritage Committee for new designations. The Committee meets once a year to determine which nominated properties to add to the World Heritage List; sometimes it defers its decision or requests more information from the country that nominated the site. There are ten selection criteria – a site must meet at least one to be included on the list. Until 2004, there were six sets of criteria for cultural heritage and four for natural heritage. In 2005, UNESCO modified these and now has one set of ten criteria. Nominated sites must be of "outstanding universal value" and must meet at least one of

3737-944: The World Heritage Fund to facilitate its conservation under certain conditions. UNESCO reckons the restorations of the following four sites among its success stories: Angkor in Cambodia, the Old City of Dubrovnik in Croatia, the Wieliczka Salt Mine near Kraków in Poland, and the Ngorongoro Conservation Area in Tanzania. Additionally, the local population around a site may benefit from significantly increased tourism revenue. When there are significant interactions between people and

3838-639: The absence of reinforcement, its tensile strength was far lower than modern reinforced concrete , and its mode of application also differed: 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

3939-621: The artificial island to both banks and is known as one of the Five Ancient Bridges of China . The original Zhupu Bridge only spanned the inner stream connecting the levee to the foot of Mount Yulei. This was replaced in the Song dynasty by Pingshi Bridge which burned down during the wars that marked the end of the Ming dynasty . In 1803 during the Qing dynasty a local man named He Xiande and his wife proposed

4040-486: The awards, because World Heritage listing can significantly increase tourism returns. Site listing bids are often lengthy and costly, putting poorer countries at a disadvantage. Eritrea 's efforts to promote Asmara are one example. In 2016, the Australian government was reported to have successfully lobbied for the World Heritage Site Great Barrier Reef conservation efforts to be removed from

4141-610: The cisterns secret as these enabled the Nabataeans to thrive in the desert. Some of these structures survive to this day. 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 . The Romans used concrete extensively from 300 BC to AD 476. During the Roman Empire, Roman concrete (or opus caementicium )

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4242-504: The commitment of countries and local population to World Heritage conservation in various ways, providing emergency assistance for sites in danger, offering technical assistance and professional training, and supporting States Parties' public awareness-building activities. Being listed as a World Heritage Site can positively affect the site, its environment, and interactions between them. A listed site gains international recognition and legal protection, and can obtain funds from, among others,

4343-450: The committee. A site may be added to the List of World Heritage in Danger if conditions threaten the characteristics for which the landmark or area was inscribed on the World Heritage List. Such problems may involve armed conflict and war, natural disasters, pollution, poaching, or uncontrolled urbanisation or human development. This danger list is intended to increase international awareness of

4444-417: The common culture and heritage of humankind. The programme began with the Convention Concerning the Protection of the World Cultural and Natural Heritage , which was adopted by the General Conference of UNESCO on 16 November 1972. Since then, 196 states have ratified the convention, making it one of the most widely recognised international agreements and the world's most popular cultural programme. In 1954,

4545-457: The complex and is inscribed with wise words from Li Bing such as, When the river flows in zigzags, cut a straight channel; when the riverbed is wide and shallow, dig it deeper. Fulongguan or Dragon-Taming Temple in Lidui Park was founded in the third century in honour of Fan Changsheng . Following Li Bing's death a hall was established here in his honour and the temple was renamed to commemorate

4646-526: The concrete at the time of batching/mixing. (See § Production below.) The common types of admixtures are as follows: 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), or as a replacement for Portland cement (blended cements). Products which incorporate limestone , fly ash , blast furnace slag , and other useful materials with pozzolanic properties into

4747-399: 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. 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

4848-461: 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

4949-461: The conquering of Chengdu by peasant rebel leader Zhang Xianzhong in 1644, and the Ming-Qing transition more generally, led to depopulation and the deterioration of the Dujiangyan irrigation system to the point where rice cultivation was set back for decades. The original Dujiangyan irrigation system was destroyed by the 1933 Diexi earthquake . The current Dujiangyan irrigation system was rebuilt after

5050-580: The construction of a replacement, made of wooden plates and bamboo handrails, to span both streams and this was nicknamed Couple's Bridge in their honour. This was demolished in the 1970s and replaced by a modern bridge. The Dujiangyan irrigation system is located in the western portion of the Chengdu flatlands, at the junction between the Sichuan basin and the Qinghai-Tibet plateau. The Dujiangyan irrigation system

5151-508: The control survey is 22664 square kilometers, accounting for 98.38% of the total water catchment area of the upper reaches of the Minjiang River. The other is Yangliuping Dam at the outlet of Baisha River, with a controlled catchment area of 363 square kilometers, accounting for 1.58% of the total catchment area. There is a catchment area of 10 square kilometers from Estuary of Baisha River to Dujiangyan irrigation system, accounting for 0.04% of

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5252-412: The convention. By assigning places as World Heritage Sites, UNESCO wants to help preserve them for future generations. Its motivation is that "heritage is our legacy from the past, what we live with today" and that both cultural and natural heritage are "irreplaceable sources of life and inspiration". UNESCO's mission with respect to World Heritage consists of eight sub targets. These include encouraging

5353-519: The crack from spreading. 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

5454-510: The dragon fighting legends that surrounded him. It is here that Erlang Shen , the legendary son of Li Bing , is said to have chained the dragon that he and his seven sworn brothers had captured in an ambush at the River God Temple when it came to collect a human sacrifice. This action is said to have protected the region from floods ever since. During the East Han dynasty a statue of Li Bing

5555-627: The endangered monuments and sites. In 1960, the Director-General of UNESCO launched the International Campaign to Save the Monuments of Nubia . This resulted in the excavation and recording of hundreds of sites, the recovery of thousands of objects, as well as the salvage and relocation to higher ground of several important temples. The most famous of these are the temple complexes of Abu Simbel and Philae . The campaign ended in 1980 and

5656-431: 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

5757-669: The gaps to make up a solid mass in situ . The word concrete comes from the Latin word " concretus " (meaning compact or condensed), 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. 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 . Concrete

5858-523: The government of Egypt decided to build the new Aswan High Dam , whose resulting future reservoir would eventually inundate a large stretch of the Nile valley containing cultural treasures of ancient Egypt and ancient Nubia . In 1959, the governments of Egypt and Sudan requested the United Nations Educational, Scientific and Cultural Organization (UNESCO) to assist them to protect and rescue

5959-554: The interlinking of the silicates and aluminate components as well as their bonding to sand and gravel particles to form a solid mass. One illustrative conversion is the hydration of tricalcium silicate: The hydration (curing) of cement is irreversible. 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

6060-980: The last two decades. These activities endanger Natural World Heritage Sites and could compromise their unique values. Of the Natural World Heritage Sites that contain forest, 91% experienced some loss since 2000. Many of them are more threatened than previously thought and require immediate conservation action. The destruction of cultural assets and identity-establishing sites is one of the primary goals of modern asymmetrical warfare. Terrorists, rebels, and mercenary armies deliberately smash archaeological sites, sacred and secular monuments and loot libraries, archives and museums. The UN, United Nations peacekeeping and UNESCO in cooperation with Blue Shield International are active in preventing such acts. "No strike lists" are also created to protect cultural assets from air strikes. The founding president of Blue Shield International Karl von Habsburg summed it up with

6161-508: The list." According to environmental protection groups, this "decision was a victory for cynical lobbying and [...] Australia, as custodians of the world's biggest coral reef, was now on probation." Several listed locations, such as Casco Viejo in Panama and Hội An in Vietnam , have struggled to strike a balance between the economic benefits of catering to greatly increased visitor numbers after

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6262-825: The local level which can result in the site being damaged. Rock art under world heritage protection at the Tadrart Acacus in Libya have occasionally been intentionally destroyed. Chalcraft links this destruction to Libyan national authorities prioritizing World Heritage status over local sensibilities by limiting access to the sites without consulting with the local population. UNESCO has also been criticized for alleged geographic bias, racism , and colourism in world heritage inscription. A major chunk of all world heritage inscriptions are located in regions whose populations generally have lighter skin, including Europe, East Asia, and North America. The World Heritage Committee has divided

6363-406: 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. The Canal du Midi was built using concrete in 1670. Perhaps the greatest step forward in the modern use of concrete

6464-531: 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

6565-419: 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 . 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

6666-478: 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

6767-441: The mixing of cement and water into a paste before combining these materials with aggregates can increase the compressive strength of the resulting concrete. 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

6868-441: The mixture to improve the physical properties of the wet mix, delay or accelerate the curing time, or otherwise change the finished material. Most concrete is poured with reinforcing materials (such as steel rebar ) embedded to provide tensile strength , yielding reinforced concrete . In the past, lime -based cement binders, such as lime putty, were often used but sometimes with other hydraulic cements , (water resistant) such as

6969-463: The mountain elevation below 3000 meters. The east is Chengdu Plain, with an altitude of 720 meters. The Dujiangyan irrigation system was built at the entrance of Minjiang River, with an average annual inflow of 15.082 billion cubic meters. There are two hydrological stations in the upper reaches of the Minjiang River. One is the Zipingpu Dam at the mouth of the main stream. The water catchment area of

7070-704: The natural environment, these can be recognised as "cultural landscapes". A country must first identify its significant cultural and natural sites in a document known as the Tentative List. Next, it can place sites selected from that list into a Nomination File, which is evaluated by the International Council on Monuments and Sites and the World Conservation Union . A country may not nominate sites that have not been first included on its Tentative List. The two international bodies make recommendations to

7171-455: The problem and discovered that the river was swelled by fast flowing spring melt-water from the local mountains that burst the banks when it reached the slow moving and heavily silted stretch below. One solution would have been to build a dam , but the Qin wanted to keep the waterway open for military vessels to supply troops on the frontier, so instead an artificial levee was constructed to redirect

7272-506: 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 Cement : Sand : Aggregate {\displaystyle {\text{Cement : Sand : Aggregate}}} . Nominal mixes are

7373-543: The protected area's size by 90%. The Dresden Elbe Valley was first placed on the danger list in 2006 when the World Heritage Committee decided that plans to construct the Waldschlösschen Bridge would significantly alter the valley's landscape. In response, the Dresden City Council attempted to stop the bridge's construction. However, after several court decisions allowed the building of the bridge to proceed,

7474-468: 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 (the second example from above), 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

7575-467: The recognition and preserving the original culture and local communities. Another criticism is that there is a homogeneity to these sites, which contain similar styles, visitor centres , etc., meaning that a lot of the individuality of these sites has been removed to become more attractive to tourists. Anthropologist Jasper Chalcraft said that World Heritage recognition often ignores contemporary local usage of certain sites. This leads to conflicts on

7676-450: The river turns. The irrigation system consists of three main constructions that work in harmony with one another to ensure against flooding and keep the fields well supplied with water: The Yuzui or Fish Mouth Levee ( Chinese :鱼嘴) , named for its conical head that is said to resemble the mouth of a fish, is the key part of the construction. It is an artificial levee that divides the water into inner and outer streams. The inner stream

7777-411: The strength of the concrete bonding to resist tension. 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 specific and complex calcium aluminosilicate hydrate) and the coalescence of this and similar calcium–aluminium-silicate–hydrate cementing binders helped give the concrete

7878-497: 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". 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

7979-535: 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. 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. Decorative stones such as quartzite , small river stones or crushed glass are sometimes added to

8080-474: The ten criteria. A country may request to extend or reduce the boundaries, modify the official name, or change the selection criteria of one of its already listed sites. Any proposal for a significant boundary change or to modify the site's selection criteria must be submitted as if it were a new nomination, including first placing it on the Tentative List and then onto the Nomination File. A request for

8181-405: The threats and to encourage counteractive measures. Threats to a site can be either proven imminent threats or potential dangers that could have adverse effects on a site. The state of conservation for each site on the danger list is reviewed yearly; after this, the Committee may request additional measures, delete the property from the list if the threats have ceased or consider deletion from both

8282-409: The total catchment area. Erwang or Two Kings Temple is on the bank of the river at the foot of Mount Yulei. The original Wangdi Temple built in memory of an ancient Shu king was moved, so locals renamed the temple here. The 10,072 m Qing dynasty wooden complex conforms to the traditional standard of temple design except that it does not follow a north–south axis. The main hall, which contains

8383-615: The valley was removed from the World Heritage List in 2009. Liverpool 's World Heritage status was revoked in July 2021, following developments ( Liverpool Waters and Bramley-Moore Dock Stadium ) on the northern docks of the World Heritage site leading to the "irreversible loss of attributes" on the site. The first global assessment to quantitatively measure threats to Natural World Heritage Sites found that 63% of sites have been damaged by increasing human pressures including encroaching roads, agriculture infrastructure and settlements over

8484-655: 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. The five major compounds of calcium silicates and aluminates comprising Portland cement range from 5 to 50% in weight. Combining water with

8585-439: 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. An undesirable gradation can mean using

8686-527: The water through a process called concrete hydration that hardens it over several hours to form a hard matrix that binds the materials together into a durable stone-like material that has many uses. 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 ambient temperature plays a significant role in how long it takes concrete to set. Often, additives (such as pozzolans or superplasticizers ) are included in

8787-436: The words: "Without the local community and without the local participants, that would be completely impossible". The UNESCO-administered project has attracted criticism. This was caused by perceived under-representation of heritage sites outside Europe, disputed decisions on site selection and adverse impact of mass tourism on sites unable to manage rapid growth in visitor numbers. A large lobbying industry has grown around

8888-526: The work of the World Heritage Committee was developed over a seven-year period (1965–1972). The United States initiated the idea of safeguarding places of high cultural or natural importance. A White House conference in 1965 called for a "World Heritage Trust" to preserve "the world's superb natural and scenic areas and historic sites for the present and the future of the entire world citizenry". The International Union for Conservation of Nature developed similar proposals in 1968, which were presented in 1972 at

8989-648: The world considered to be of outstanding value to humanity". To be selected, a World Heritage Site is nominated by its host country and determined by the UNESCO's World Heritage Committee to be a unique landmark which is geographically and historically identifiable, having a special cultural or physical significance, and to be under a sufficient system of legal protection. For example, World Heritage Sites might be ancient ruins or historical structures, buildings, cities, deserts, forests, islands, lakes, monuments, mountains or wilderness areas. A World Heritage Site may signify

9090-717: The world into five geographic regions: Africa, Arab states, Asia and the Pacific, Europe and North America, and Latin America and the Caribbean. Russia and the Caucasus states are classified as European, while Mexico and the Caribbean are classified as belonging to the Latin America and the Caribbean region. The UNESCO geographic regions also give greater emphasis on administrative, rather than geographic associations. Hence, Gough Island , located in

9191-515: 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. A method for producing Portland cement was developed in England and patented by Joseph Aspdin in 1824. Aspdin chose the name for its similarity to Portland stone , which

9292-622: Was built largely of concrete, and the Pantheon has the world's largest unreinforced concrete dome. 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. Modern tests show that opus caementicium had as much compressive strength as modern Portland-cement concrete (c. 200 kg/cm  [20 MPa; 2,800 psi]). However, due to

9393-668: Was collected from 50 countries. The project's success led to other safeguarding campaigns, such as saving Venice and its lagoon in Italy, the ruins of Mohenjo-daro in Pakistan, and the Borobodur Temple Compounds in Indonesia. Together with the International Council on Monuments and Sites , UNESCO then initiated a draft convention to protect cultural heritage. The convention (the signed document of international agreement ) guiding

9494-931: Was considered a success. To thank countries which especially contributed to the campaign's success, Egypt donated four temples; the Temple of Dendur was moved to the Metropolitan Museum of Art in New York City , the Temple of Debod to the Parque del Oeste in Madrid , the Temple of Taffeh to the Rijksmuseum van Oudheden in Leiden , and the Temple of Ellesyia to Museo Egizio in Turin . The project cost US$ 80 million (equivalent to $ 295.83 million in 2023), about $ 40 million of which

9595-456: Was gouged through the mountain, is the final part of the system. The channel distributes the water to the farmlands in the Chengdu Plain, whilst the narrow entrance, that gives it its name, works as a check gate, creating the whirlpool flow that carries away the excess water over Flying Sand Fence, to ensure against flooding. Anlan or Couple's Bridge spans the full width of the river connecting

9696-602: 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

9797-448: Was made from quicklime , pozzolana and an aggregate of pumice . 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. The Colosseum in Rome

9898-493: 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

9999-634: Was placed in the river to monitor the water flow, with the level rising above his shoulders to indicate flood and falling beneath his calves to indicate drought. Recovered from the river in 1974 and placed on display in the main hall, this is the oldest known stone statue of a human in China. World Heritage Site World Heritage Sites are landmarks and areas with legal protection under an international treaty administered by UNESCO for having cultural, historical, or scientific significance. The sites are judged to contain "cultural and natural heritage around

10100-614: 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. Reinforced concrete was invented in 1849 by Joseph Monier . and the first reinforced concrete house was built by François Coignet in 1853. The first concrete reinforced bridge was designed and built by Joseph Monier in 1875. Prestressed concrete and post-tensioned concrete were pioneered by Eugène Freyssinet ,

10201-615: Was used for construction in many ancient structures. 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

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