Permafrost
In geology, permafrost is ground, including rock or soil, at or below the freezing point of water 0 °C for two or more years. Most permafrost is located in high latitudes, but at lower latitudes alpine permafrost occurs at higher elevations. Ground ice is not always present, as may be in the case of non-porous bedrock, but it occurs and it may be in amounts exceeding the potential hydraulic saturation of the ground material. Permafrost accounts for 0.022% of total water on Earth and the permafrost region covers 24% of exposed land in the Northern Hemisphere. It occurs subsea on the continental shelves of the continents surrounding the Arctic Ocean, portions of which were exposed during the last glacial period; the thawing of permafrost has implications for the global climate. A global temperature rise of 1.5 °C above current levels would be enough to start the thawing of permafrost in Siberia, according to one group of scientists. "In contrast to the relative dearth of reports on frozen ground in north America prior to World War II, a vast literature on the engineering aspects of permafrost was available in Russian.
Beginning in 1942, Siemon William Muller delved into the relevant Russian literature held by the Library of Congress and the U. S. Geological Survey Library so that he was able to furnish the government an engineering field guide and a technical report about permafrost by 1943", year in which he coined the term as a contraction of permamently frozen ground. Although classified, in 1947 a revised report was released publicly, regarded as the first North American treatise on the subject. Permafrost is soil, rock or sediment, frozen for more than two consecutive years. In areas not overlain by ice, it exists beneath a layer of soil, rock or sediment, which freezes and thaws annually and is called the "active layer". In practice, this means that permafrost occurs at an mean annual temperature of − 2 colder. Active layer thickness is 0.3 to 4 meters thick. The extent of permafrost varies with the climate: in the Northern Hemisphere today, 24% of the ice-free land area, equivalent to 19 million square kilometers, is more or less influenced by permafrost.
Of this area more than half is underlain by continuous permafrost, around 20 percent by discontinuous permafrost, a little less than 30 percent by sporadic permafrost. Most of this area is found in Siberia, northern Canada and Greenland. Beneath the active layer annual temperature swings of permafrost become smaller with depth; the deepest depth of permafrost occurs. Above that bottom limit there may be permafrost, whose temperature doesn't change annually—"isothermal permafrost". Permafrost forms in any climate where the mean annual air temperature is less than the freezing point of water. Exceptions are found in moist-wintered forest climates, such as in Northern Scandinavia and the North-Eastern part of European Russia west of the Urals, where snow acts as an insulating blanket. Glaciated areas may be exceptions. Since all glaciers are warmed at their base by geothermal heat, temperate glaciers, which are near the pressure-melting point throughout, may have liquid water at the interface with the ground and are therefore free of underlying permafrost.
"Fossil" cold anomalies in the Geothermal gradient in areas where deep permafrost developed during the Pleistocene persist down to several hundred metres. This is evident from temperature measurements in boreholes in North Europe; the below-ground temperature varies less from season to season than the air temperature, with mean annual temperatures tending to increase with depth as a result of the geothermal crustal gradient. Thus, if the mean annual air temperature is only below 0 °C, permafrost will form only in spots that are sheltered—usually with a northerly aspect—creating discontinuous permafrost. Permafrost will remain discontinuous in a climate where the mean annual soil surface temperature is between −5 and 0 °C. In the moist-wintered areas mentioned before, there may not be discontinuous permafrost down to −2 °C. Discontinuous permafrost is further divided into extensive discontinuous permafrost, where permafrost covers between 50 and 90 percent of the landscape and is found in areas with mean annual temperatures between −2 and −4 °C, sporadic permafrost, where permafrost cover is less than 50 percent of the landscape and occurs at mean annual temperatures between 0 and −2 °C.
In soil science, the sporadic permafrost zone is abbreviated SPZ and the extensive discontinuous permafrost zone DPZ. Exceptions occur in un-glaciated Siberia and Alaska where the present depth of permafrost is a relic of climatic conditions during glacial ages where winters were up to 11 °C colder than those of today. At mean annual soil surface temperatures below −5 °C the influence of aspect can never be sufficient to thaw permafrost and a zone of continuous permafrost forms. A line of continuous permafrost in the Northern Hemisphere represents the most southerly border where land is covered by continuous permafrost or glacial ice; the line of continuous permafrost varies around the world northward or southward due to regional climatic changes. In the southern hemisphere, most of the equivalent line would fall within the Southern Ocean if there were land there. Most of the Antarctic continent is overl
Telecommunication
Telecommunication is the transmission of signs, messages, writings and sounds or information of any nature by wire, optical or other electromagnetic systems. Telecommunication occurs when the exchange of information between communication participants includes the use of technology, it is transmitted either electrically over physical media, such as cables, or via electromagnetic radiation. Such transmission paths are divided into communication channels which afford the advantages of multiplexing. Since the Latin term communicatio is considered the social process of information exchange, the term telecommunications is used in its plural form because it involves many different technologies. Early means of communicating over a distance included visual signals, such as beacons, smoke signals, semaphore telegraphs, signal flags, optical heliographs. Other examples of pre-modern long-distance communication included audio messages such as coded drumbeats, lung-blown horns, loud whistles. 20th- and 21st-century technologies for long-distance communication involve electrical and electromagnetic technologies, such as telegraph and teleprinter, radio, microwave transmission, fiber optics, communications satellites.
A revolution in wireless communication began in the first decade of the 20th century with the pioneering developments in radio communications by Guglielmo Marconi, who won the Nobel Prize in Physics in 1909, other notable pioneering inventors and developers in the field of electrical and electronic telecommunications. These included Charles Wheatstone and Samuel Morse, Alexander Graham Bell, Edwin Armstrong and Lee de Forest, as well as Vladimir K. Zworykin, John Logie Baird and Philo Farnsworth; the word telecommunication is a compound of the Greek prefix tele, meaning distant, far off, or afar, the Latin communicare, meaning to share. Its modern use is adapted from the French, because its written use was recorded in 1904 by the French engineer and novelist Édouard Estaunié. Communication was first used as an English word in the late 14th century, it comes from Old French comunicacion, from Latin communicationem, noun of action from past participle stem of communicare "to share, divide out.
Homing pigeons have been used throughout history by different cultures. Pigeon post had Persian roots, was used by the Romans to aid their military. Frontinus said; the Greeks conveyed the names of the victors at the Olympic Games to various cities using homing pigeons. In the early 19th century, the Dutch government used the system in Sumatra, and in 1849, Paul Julius Reuter started a pigeon service to fly stock prices between Aachen and Brussels, a service that operated for a year until the gap in the telegraph link was closed. In the Middle Ages, chains of beacons were used on hilltops as a means of relaying a signal. Beacon chains suffered the drawback that they could only pass a single bit of information, so the meaning of the message such as "the enemy has been sighted" had to be agreed upon in advance. One notable instance of their use was during the Spanish Armada, when a beacon chain relayed a signal from Plymouth to London. In 1792, Claude Chappe, a French engineer, built the first fixed visual telegraphy system between Lille and Paris.
However semaphore suffered from the need for skilled operators and expensive towers at intervals of ten to thirty kilometres. As a result of competition from the electrical telegraph, the last commercial line was abandoned in 1880. On 25 July 1837 the first commercial electrical telegraph was demonstrated by English inventor Sir William Fothergill Cooke, English scientist Sir Charles Wheatstone. Both inventors viewed their device as "an improvement to the electromagnetic telegraph" not as a new device. Samuel Morse independently developed a version of the electrical telegraph that he unsuccessfully demonstrated on 2 September 1837, his code was an important advance over Wheatstone's signaling method. The first transatlantic telegraph cable was completed on 27 July 1866, allowing transatlantic telecommunication for the first time; the conventional telephone was invented independently by Alexander Bell and Elisha Gray in 1876. Antonio Meucci invented the first device that allowed the electrical transmission of voice over a line in 1849.
However Meucci's device was of little practical value because it relied upon the electrophonic effect and thus required users to place the receiver in their mouth to "hear" what was being said. The first commercial telephone services were set-up in 1878 and 1879 on both sides of the Atlantic in the cities of New Haven and London. Starting in 1894, Italian inventor Guglielmo Marconi began developing a wireless communication using the newly discovered phenomenon of radio waves, showing by 1901 that they could be transmitted across the Atlantic Ocean; this was the start of wireless telegraphy by radio. Voice and music had little early success. World War I accelerated the development of radio for military communications. After the war, commercial radio AM broadcasting began in the 1920s and became an important mass medium for entertainment and news. World War II again accelerated development of radio for the wartime purposes of aircraft and land communication, radio navigation and radar. Development of stereo FM broadcasting of radio
Termite
Termites are eusocial insects that are classified at the taxonomic rank of infraorder Isoptera, or as epifamily Termitoidae within the cockroach order Blattodea. Termites were once classified in a separate order from cockroaches, but recent phylogenetic studies indicate that they evolved from close ancestors of cockroaches during the Jurassic or Triassic. However, the first termites emerged during the Permian or the Carboniferous. About 3,106 species are described, with a few hundred more left to be described. Although these insects are called "white ants", they are not ants. Like ants and some bees and wasps from the separate order Hymenoptera, termites divide labour among castes consisting of sterile male and female "workers" and "soldiers". All colonies have fertile males called "kings" and one or more fertile females called "queens". Termites feed on dead plant material and cellulose in the form of wood, leaf litter, soil, or animal dung. Termites are major detritivores in the subtropical and tropical regions, their recycling of wood and plant matter is of considerable ecological importance.
Termites are among the most successful groups of insects on Earth, colonising most landmasses except Antarctica. Their colonies range in size from a few hundred individuals to enormous societies with several million individuals. Termite queens have the longest lifespan of any insect in the world, with some queens living up to 30 to 50 years. Unlike ants, which undergo a complete metamorphosis, each individual termite goes through an incomplete metamorphosis that proceeds through egg and adult stages. Colonies are described as superorganisms because the termites form part of a self-regulating entity: the colony itself. Termites are a delicacy in the diet of some human cultures and are used in many traditional medicines. Several hundred species are economically significant as pests that can cause serious damage to buildings, crops, or plantation forests; some species, such as the West Indian drywood termite, are regarded as invasive species. The infraorder name Isoptera is derived from the Greek words iso and ptera, which refers to the nearly equal size of the fore and hind wings.
"Termite" derives from the Latin and Late Latin word termes, altered by the influence of Latin terere from the earlier word tarmes. Termite nests were known as terminarium or termitaria. In earlier English, termites were known as "wood ants" or "white ants"; the modern term was first used in 1781. Termites were placed in the order Isoptera; as early as 1934 suggestions were made that they were related to wood-eating cockroaches based on the similarity of their symbiotic gut flagellates. In the 1960s additional evidence supporting that hypothesis emerged when F. A. McKittrick noted similar morphological characteristics between some termites and Cryptocercus nymphs. In 2008 DNA analysis from 16S rRNA sequences supported the position of termites being nested within the evolutionary tree containing the order Blattodea, which included the cockroaches; the cockroach genus Cryptocercus shares the strongest phylogenetical similarity with termites and is considered to be a sister-group to termites. Termites and Cryptocercus share similar morphological and social features: for example, most cockroaches do not exhibit social characteristics, but Cryptocercus takes care of its young and exhibits other social behaviour such as trophallaxis and allogrooming.
Termites are thought to be the descendants of the genus Cryptocercus. Some researchers have suggested a more conservative measure of retaining the termites as the Termitoidae, an epifamily within the cockroach order, which preserves the classification of termites at family level and below. Termites have long been accepted to be related to cockroaches and mantids, they are classified in the same superorder; the oldest unambiguous termite fossils date to the early Cretaceous, but given the diversity of Cretaceous termites and early fossil records showing mutualism between microorganisms and these insects, they originated earlier in the Jurassic or Triassic. Further evidence of a Jurassic origin is the assumption that the extinct Fruitafossor consumed termites, judging from its morphological similarity to modern termite-eating mammals; the oldest termite nest discovered is believed to be from the Upper Cretaceous in West Texas, where the oldest known faecal pellets were discovered. Claims that termites emerged earlier have faced controversy.
For example, F. M. Weesner indicated that the Mastotermitidae termites may go back to the Late Permian, 251 million years ago, fossil wings that have a close resemblance to the wings of Mastotermes of the Mastotermitidae, the most primitive living termite, have been discovered in the Permian layers in Kansas, it is possible that the first termites emerged during the Carboniferous. The folded wings of the fossil wood roach Pycnoblattina, arranged in a convex pattern between segments 1a and 2a, resemble those seen in Mastotermes, the only living insect with the same pattern. Krishna et al. though, consider that all of the Paleozoic and Triassic insects tentatively classified as termites are in fact unrelated to termites and should be excluded from the Isoptera. The primitive giant northern termite exhibits numerous cockroach-like characteristics that are not shared with other termites, such as laying its eggs in rafts and having anal lobes on the wings. Cryptocercidae and Isoptera are united in the clade Xylophagidae.
Termites are sometimes called "white ants" but the only resemblance to the ants is due to their sociality, due to converg
New Zealand
New Zealand is a sovereign island country in the southwestern Pacific Ocean. The country geographically comprises two main landmasses—the North Island, the South Island —and around 600 smaller islands. New Zealand is situated some 2,000 kilometres east of Australia across the Tasman Sea and 1,000 kilometres south of the Pacific island areas of New Caledonia and Tonga; because of its remoteness, it was one of the last lands to be settled by humans. During its long period of isolation, New Zealand developed a distinct biodiversity of animal and plant life; the country's varied topography and its sharp mountain peaks, such as the Southern Alps, owe much to the tectonic uplift of land and volcanic eruptions. New Zealand's capital city is Wellington. Sometime between 1250 and 1300, Polynesians settled in the islands that were named New Zealand and developed a distinctive Māori culture. In 1642, Dutch explorer Abel Tasman became the first European to sight New Zealand. In 1840, representatives of the United Kingdom and Māori chiefs signed the Treaty of Waitangi, which declared British sovereignty over the islands.
In 1841, New Zealand became a colony within the British Empire and in 1907 it became a dominion. Today, the majority of New Zealand's population of 4.9 million is of European descent. Reflecting this, New Zealand's culture is derived from Māori and early British settlers, with recent broadening arising from increased immigration; the official languages are English, Māori, NZ Sign Language, with English being dominant. A developed country, New Zealand ranks in international comparisons of national performance, such as quality of life, education, protection of civil liberties, economic freedom. New Zealand underwent major economic changes during the 1980s, which transformed it from a protectionist to a liberalised free-trade economy; the service sector dominates the national economy, followed by the industrial sector, agriculture. Nationally, legislative authority is vested in an elected, unicameral Parliament, while executive political power is exercised by the Cabinet, led by the prime minister Jacinda Ardern.
Queen Elizabeth II is the country's monarch and is represented by a governor-general Dame Patsy Reddy. In addition, New Zealand is organised into 11 regional councils and 67 territorial authorities for local government purposes; the Realm of New Zealand includes Tokelau. New Zealand is a member of the United Nations, Commonwealth of Nations, ANZUS, Organisation for Economic Co-operation and Development, ASEAN Plus Six, Asia-Pacific Economic Cooperation, the Pacific Community and the Pacific Islands Forum. Dutch explorer Abel Tasman sighted New Zealand in 1642 and named it Staten Land "in honour of the States General", he wrote, "it is possible that this land joins to the Staten Land but it is uncertain", referring to a landmass of the same name at the southern tip of South America, discovered by Jacob Le Maire in 1616. In 1645, Dutch cartographers renamed the land Nova Zeelandia after the Dutch province of Zeeland. British explorer James Cook subsequently anglicised the name to New Zealand. Aotearoa is the current Māori name for New Zealand.
It is unknown whether Māori had a name for the whole country before the arrival of Europeans, with Aotearoa referring to just the North Island. Māori had several traditional names for the two main islands, including Te Ika-a-Māui for the North Island and Te Waipounamu or Te Waka o Aoraki for the South Island. Early European maps labelled the islands North and South. In 1830, maps began to use North and South to distinguish the two largest islands and by 1907 this was the accepted norm; the New Zealand Geographic Board discovered in 2009 that the names of the North Island and South Island had never been formalised, names and alternative names were formalised in 2013. This set the names as North Island or Te Ika-a-Māui, South Island or Te Waipounamu. For each island, either its English or Māori name can be used. New Zealand was one of the last major landmasses settled by humans. Radiocarbon dating, evidence of deforestation and mitochondrial DNA variability within Māori populations suggest New Zealand was first settled by Eastern Polynesians between 1250 and 1300, concluding a long series of voyages through the southern Pacific islands.
Over the centuries that followed, these settlers developed a distinct culture now known as Māori. The population was divided into iwi and hapū who would sometimes cooperate, sometimes compete and sometimes fight against each other. At some point a group of Māori migrated to Rēkohu, now known as the Chatham Islands, where they developed their distinct Moriori culture; the Moriori population was all but wiped out between 1835 and 1862 because of Taranaki Māori invasion and enslavement in the 1830s, although European diseases contributed. In 1862 only 101 survived, the last known full-blooded Moriori died in 1933; the first Europeans known to have reached New Zeala
Power factor
In electrical engineering, the power factor of an AC electrical power system is defined as the ratio of the real power absorbed by the load to the apparent power flowing in the circuit, is a dimensionless number in the closed interval of −1 to 1. A power factor of less than one indicates the voltage and current are not in phase, reducing the instantaneous product of the two. Real power is the instantaneous product of voltage and current and represents the capacity of the electricity for performing work. Apparent power is the average product of voltage. Due to energy stored in the load and returned to the source, or due to a non-linear load that distorts the wave shape of the current drawn from the source, the apparent power may be greater than the real power. A negative power factor occurs when the device generates power, which flows back towards the source. In an electric power system, a load with a low power factor draws more current than a load with a high power factor for the same amount of useful power transferred.
The higher currents increase the energy lost in the distribution system, require larger wires and other equipment. Because of the costs of larger equipment and wasted energy, electrical utilities will charge a higher cost to industrial or commercial customers where there is a low power factor. Power-factor correction increases the power factor of a load, improving efficiency for the distribution system to which it is attached. Linear loads with low power factor can be corrected with a passive network of capacitors or inductors. Non-linear loads, such as rectifiers, distort the current drawn from the system. In such cases, active or passive power factor correction may be used to counteract the distortion and raise the power factor; the devices for correction of the power factor may be at a central substation, spread out over a distribution system, or built into power-consuming equipment. In a purely resistive AC circuit and current waveforms are in step, changing polarity at the same instant in each cycle.
All the power entering the load is consumed. Where reactive loads are present, such as with capacitors or inductors, energy storage in the loads results in a phase difference between the current and voltage waveforms. During each cycle of the AC voltage, extra energy, in addition to any energy consumed in the load, is temporarily stored in the load in electric or magnetic fields, returned to the power grid a fraction of the period later. In the electric power grid, reactive loads thus cause a continuous "ebb and flow" of nonproductive power. A circuit with a low power factor will use higher currents to transfer a given quantity of real power than a circuit with a high power factor, causing increased losses due to resistive heating in power lines, requiring higher rated conductors and transformers be installed. A linear load does not change the shape of the waveform of the current, but may change the relative timing between voltage and current. Electrical circuits containing dominantly resistive loads have a power factor of 1.0, but circuits containing inductive or capacitive loads can have a power factor well below 1.
AC power flow has two components: Real power or active power, expressed in watts Reactive power expressed in reactive volt-amperes These are combined to the complex power expressed volt-amperes. The magnitude of the complex power is the apparent power expressed in volt-amperes; the VA and var are non-SI units mathematically identical to the watt, but are used in engineering practice instead of the watt to state what quantity is being expressed. The SI explicitly disallows using units for this purpose or as the only source of information about a physical quantity as used; the power factor is defined as the ratio of real power to apparent power. As power is transferred along a transmission line, it does not consist purely of real power that can do work once transferred to the load, but rather consists of a combination of real and reactive power, called apparent power; the power factor describes the amount of real power transmitted along a transmission line relative to the total apparent power flowing in the line.
One can relate the various components of AC power by using the power triangle in vector space. Real power extends horizontally in the î direction as it represents a purely real component of AC power. Reactive power extends in the direction of ĵ as it represents a purely imaginary component of AC power. Complex power represents a combination of both real and reactive power, therefore can be calculated by using the vector sum of these two components. We can conclude that the mathematical relationship between these components is: S = P + j Q | S | 2 = P 2 + Q 2 | S | = P 2 + Q 2 cos θ, power factor = P, real power
Cahora Bassa (HVDC)
Cahora-Bassa is the name for an HVDC power transmission system between the Cahora Bassa Hydroelectric Generation Station at the Cahora Bassa Dam in Mozambique, Johannesburg, South Africa. The system was built between 1974 and 1979 and can transmit 1920 megawatts at a voltage level of 533 kilovolts DC and 1800 Amperes. Thyristor valves are used, which unlike most other HVDC schemes are mounted outdoors and not in a valve hall; the valves are grouped into 133 kV six-pulse bridges in series at each end. The 1,420 kilometres long powerline runs through inaccessible terrain, so it is built as monopolar lines 1 kilometre apart. In case of a single line failure, transmission with reduced power is possible via the surviving pole and return through the earth. Cahora-Bassa was out of service from 1985 to 1997 because of the Mozambican Civil War in the region; the project was beset with technological challenges, most notable of these being the adoption of solid-state rectification devices in a large-scale commercial installation.
Mercury-arc valves had been the de facto standard for HVDC up to this time. Cahora Bassa was the first HVDC scheme ordered with thyristor valves, though its operation was delayed, it was the first HVDC scheme operational in Africa, the first anywhere in the world to operate above 500 kV. Significant commercial hurdles, culminating in hearings at an International Arbitration Tribunal seated in Lisbon, in 1988 had to be overcome. Following a refurbishment exercise, the scheme was put back into commercial operation in October 1997. Between 2006 and 2009 the thyristor valves at the Apollo converter station were replaced by more modern water-cooled valves; the Cahora-Bassa transmission project was a joint venture of the two electrical utilities, Electricity Supply Commission, latterly Eskom, South Africa and Hidroelectrica de Cahora Bassa, a firm owned 15% by the government of Portugal and 85% by Mozambique. Equipment was constructed and supplied by ZAMCO, a consortium of AEG-Telefunken JV, Brown Boveri Company, Siemens AG of Germany.
Brown Boveri subsequently became part of AEG subsequently became part of Alstom. The commercial arrangements included Electricidade de Moçambique which took supply from Cahora Bassa through a wheeling arrangement with Eskom. Eskom supplied southern Mozambique from the Eastern Transvaal at 132 kV with the sales deducted from the HCB supply to Eskom; the tripartite agreement was suspended due to force majeure when the line from Cahora Bassa was unavailable in the 1980s. The system was commissioned in three stages starting in March 1977 with four six-pulse bridges, in full operation of eight bridges on March 15, 1979; the power line runs from the Songo converter station, near the hydroelectric station and operates as a rectifier, to the Apollo converter station near Johannesburg, which operates as an inverter. Each of the self-supporting steel towers along the route carries two bundles of four 565 square millimetre cables, one per pole, a single 117 square millimetre grounding conductor. There are 7,000 towers with an average span of 426 metres.
The maximum span is 700 metres using reinforced towers. Earth return for unipolar operation is provided by buried graphite electrodes at each station; the DC line has smoothing reactors and surge arrester capacitors at each station. Cahora Bassa was one of the first HVDC schemes built with thyristor valves from its inception. Unusually, the thyristor valves are outdoor. In the original installation they were oil filled for both electrical insulation; the only other HVDC scheme in the world equipped in this way from the outset was the first phase – now decommissioned – of the Shin Shinano frequency converter in Japan. Each valve tank contains two valves, forming a double-valve connecting the two DC terminals to one single-phase, two-winding converter transformer; each six-pulse bridge contains hence each station contains 24 double-valves. The development work for the thyristor valves began in the late 1960s when the only thyristors available at the time were, by today’s standards and were rated only 1.6 kV each.
In the first phase of the project each valve contained 280 such thyristors in series with two in parallel – the largest number used in a single HVDC valve. Phases 2 and 3 used improved thyristors with a rating of 2.4 kV each and only required 192 in series per valve – still a large number by modern standards – with two in parallel. As a result, each converter station contained a total of 22,656 thyristors; the thyristors had poor transient overcurrent capability, so another unusual feature of the scheme was the existence of overcurrent diverters between the valves and transformers, although these were decommissioned at the Apollo station. AC filters tuned to the 5th, 7th, 11th and 13th harmonics of the 50 Hz power supply are installed at each station 195 MVAr at Apollo and 210 MVAr at Songo. There are two PLC repeater stations: one at one at Catope in Mozambique. After the civil war ended in 1992, one of the many effects of the decade of strife was the damage to the HVDC transmission lines. Nearly all of the 4200 transmission line towers located on the 893 kilometres of line in Mozambique needed to be replaced or refurbished.
This work took until late 1997 to complete. The system was restored to full power transmission capacity by 1998. Subsequently, Eskom has commenced electricity supply to Mozambique at 400 kV, under terms similar
Reinforced concrete
Reinforced concrete is a composite material in which concrete's low tensile strength and ductility are counteracted by the inclusion of reinforcement having higher tensile strength or ductility. The reinforcement is though not steel reinforcing bars and is embedded passively in the concrete before the concrete sets. Reinforcing schemes are designed to resist tensile stresses in particular regions of the concrete that might cause unacceptable cracking and/or structural failure. Modern reinforced concrete can contain varied reinforcing materials made of steel, polymers or alternate composite material in conjunction with rebar or not. Reinforced concrete may be permanently stressed, so as to improve the behaviour of the final structure under working loads. In the United States, the most common methods of doing this are known as pre-tensioning and post-tensioning. For a strong and durable construction the reinforcement needs to have the following properties at least: High relative strength High toleration of tensile strain Good bond to the concrete, irrespective of pH, similar factors Thermal compatibility, not causing unacceptable stresses in response to changing temperatures.
Durability in the concrete environment, irrespective of corrosion or sustained stress for example. François Coignet was the first to use iron-reinforced concrete as a technique for constructing building structures. In 1853, Coignet built the first iron reinforced concrete structure, a four-story house at 72 rue Charles Michels in the suburbs of Paris. Coignet's descriptions of reinforcing concrete suggests that he did not do it for means of adding strength to the concrete but for keeping walls in monolithic construction from overturning. In 1854, English builder William B. Wilkinson reinforced the concrete roof and floors in the two-storey house he was constructing, his positioning of the reinforcement demonstrated that, unlike his predecessors, he had knowledge of tensile stresses. Joseph Monier was a French gardener of the nineteenth century, a pioneer in the development of structural and reinforced concrete when dissatified with existing materials available for making durable flowerpots, he was granted a patent for reinforced flowerpots by means of mixing a wire mesh to a mortar shell.
In 1877, Monier was granted another patent for a more advanced technique of reinforcing concrete columns and girders with iron rods placed in a grid pattern. Though Monier undoubtedly knew reinforcing concrete would improve its inner cohesion, it is less known if he knew how much reinforcing improved concrete's tensile strength. Before 1877 the use of concrete construction, though dating back to the Roman Empire, having been reintroduced in the early 1800s, was not yet a proven scientific technology. American New Yorker Thaddeus Hyatt published a report titled An Account of Some Experiments with Portland-Cement-Concrete Combined with Iron as a Building Material, with Reference to Economy of Metal in Construction and for Security against Fire in the Making of Roofs and Walking Surfaces where he reported his experiments on the behavior of reinforced concrete, his work played a major role in the evolution of concrete construction as a proven and studied science. Without Hyatt's work, more dangerous trial and error methods would have been depended on for the advancement in the technology.
Ernest L. Ransome was an English-born engineer and early innovator of the reinforced concrete techniques in the end of the 19th century. With the knowledge of reinforced concrete developed during the previous 50 years, Ransome innovated nearly all styles and techniques of the previous known inventors of reinforced concrete. Ransome's key innovation was to twist the reinforcing steel bar improving bonding with the concrete. Gaining increasing fame from his concrete constructed buildings, Ransome was able to build two of the first reinforced concrete bridges in North America. One of the first concrete buildings constructed in the United States, was a private home, designed by William Ward in 1871; the home was designed to be fireproof for his wife. G. A. Wayss was a pioneer of the iron and steel concrete construction. In 1879, Wayss bought the German rights to Monier's patents and in 1884, he started the first commercial use for reinforced concrete in his firm Wayss & Freytag. Up until the 1890s, Wayss and his firm contributed to the advancement of Monier's system of reinforcing and established it as a well-developed scientific technology.
One of the first skyscrapers made with reinforced concrete was the 16-story Ingalls Building in Cincinnati, constructed in 1904. The first reinforced concrete building in Southern California was the Laughlin Annex in Downtown Los Angeles, constructed in 1905. In 1906, 16 building permits were issued for reinforced concrete buildings in the City of Los Angeles, including the Temple Auditorium and 8-story Hayward Hotel. On April 18, 1906 a magnitude 7.8 earthquake struck San Francisco. The strong ground shaking and subsequent fire killed thousands; the use of reinforced concrete after the earthquake was promoted within the U. S. construction industry due to its non-combustibility and perceived superior seismic performance relative to masonry. In 1906, a partial collapse of the Bixby Hotel in Long Beach killed 10 workers during construction when shoring was removed prematurely; this event spurred a scrutiny of concrete erection practices and building inspections. The structure was constructed of reinforced concrete frames with hollow clay tile ribbed flooring and hollow clay
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