Concrete Portland cement concrete, is a composite material composed of fine and coarse aggregate bonded together with a fluid cement that hardens over time—most a lime-based cement binder, such as Portland cement, but sometimes with other hydraulic cements, such as a calcium aluminate cement. It is distinguished from other, non-cementitious types of concrete all binding some form of aggregate together, including asphalt concrete with a bitumen binder, used for road surfaces, polymer concretes that use polymers as a binder; when aggregate is mixed together with dry Portland cement and water, the mixture forms a fluid slurry, poured and molded into shape. The cement reacts chemically with the water and other ingredients to form a hard matrix that binds the materials together into a durable stone-like material that has many uses. Additives are included in the mixture to improve the physical properties of the wet mix or the finished material. Most concrete is poured with reinforcing materials embedded to provide tensile strength, yielding reinforced concrete.
Famous concrete structures include the Panama Canal and the Roman Pantheon. The earliest large-scale users of concrete technology were the ancient Romans, concrete was used in the Roman Empire; the Colosseum in Rome was built of concrete, the concrete dome of the Pantheon is the world's largest unreinforced concrete dome. Today, large concrete structures are made with reinforced concrete. After the Roman Empire collapsed, use of concrete became rare until the technology was redeveloped in the mid-18th century. Worldwide, concrete has overtaken steel in tonnage of material used; the word concrete comes from the Latin word "concretus", the perfect passive participle of "concrescere", from "con-" and "crescere". Small-scale production of concrete-like materials was pioneered by the Nabatean traders who occupied and controlled a series of oases and developed a small empire in the regions of southern Syria and northern Jordan from the 4th century BC, they discovered the advantages of hydraulic lime, with some self-cementing properties, by 700 BC.
They built kilns to supply mortar for the construction of rubble-wall houses, concrete floors, underground waterproof cisterns. They kept the cisterns secret; some of these structures survive to this day. In the Ancient Egyptian and Roman eras, builders discovered that adding volcanic ash to the mix allowed it to set underwater. German archaeologist Heinrich Schliemann found concrete floors, which were made of lime and pebbles, in the royal palace of Tiryns, which dates to 1400–1200 BC. Lime mortars were used in Greece and Cyprus in 800 BC; the Assyrian Jerwan Aqueduct made use of waterproof concrete. Concrete was used for construction in many ancient structures; the Romans used concrete extensively from 300 BC to a span of more than seven hundred years. During the Roman Empire, Roman concrete 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 dimension. Concrete, as the Romans knew it, was a revolutionary material. Laid in the shape of arches and domes, it 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. However, due to the absence of reinforcement, its tensile strength was far lower than modern reinforced concrete, its mode of application was different: 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 consisted of rubble. Second, integral reinforcing steel gives modern concrete assemblies great strength in tension, whereas Roman concrete could depend only upon the strength of the concrete bonding to resist tension.
The long-term durability of Roman concrete structures has been found to be due to its use of pyroclastic rock and ash, whereby crystallization of strätlingite and the coalescence of calcium–aluminum-silicate–hydrate cementing binder helped give the concrete a greater degree of fracture resistance in seismically active environments. Roman concrete is more resistant to erosion by seawater than modern concrete; the widespread use of concrete in many Roman structures ensured that many survive to the present day. The Baths of Caracalla in Rome are just one example. Many Roman aqueducts and bridges, such as the magnificent Pont du Gard in southern France, have masonry cladding on a concrete core, as does the dome of the Pantheon. After the Roman Empire, the use of burned lime and pozzolana was reduced until the technique was all but forgotten between 500 and the 14th century. From the 14th century to the mid-18th century, the use of cement returned; the Canal du Midi was built using concrete in 1670.
The greatest step forward in the modern use
Dallas the City of Dallas, is a city in the U. S. state of Texas and the seat of Dallas County, with portions extending into Collin, Denton and Rockwall counties. With an estimated 2017 population of 1,341,075, it is the ninth most-populous city in the U. S. and third in Texas after Houston and San Antonio. It is the eighteenth most-populous city in North America as of 2015. Located in North Texas, the city of Dallas is the main core of the largest metropolitan area in the Southern United States and the largest inland metropolitan area in the U. S. that lacks any navigable link to the sea. It is the most populous city in the Dallas–Fort Worth metroplex, the fourth-largest metropolitan area in the country at 7.3 million people as of 2017. The city's combined statistical area is the seventh-largest in the U. S. as of 2017, with 7,846,293 residents. Dallas and nearby Fort Worth were developed due to the construction of major railroad lines through the area allowing access to cotton and oil in North and East Texas.
The construction of the Interstate Highway System reinforced Dallas's prominence as a transportation hub, with four major interstate highways converging in the city and a fifth interstate loop around it. Dallas developed as a strong industrial and financial center and a major inland port, due to the convergence of major railroad lines, interstate highways and the construction of Dallas/Fort Worth International Airport, one of the largest and busiest airports in the world. A "beta" global city, the economy of Dallas has been considered diverse with dominant sectors including defense, financial services, information technology, telecommunications, transportation. Dallas is home to 9 Fortune 500 companies within the city limits; the Dallas–Fort Worth metroplex hosts additional Fortune 500 companies, including American Airlines, ExxonMobil and J. C. Penney. Over 41 colleges and universities are in its metropolitan area, the most of any metropolitan area in Texas; the city has a population from a myriad of ethnic and religious backgrounds and the sixth-largest LGBT population in the United States as of 2016.
WalletHub named Dallas the fifth most-diverse city in the U. S. in 2018. Preceded by thousands of years of varying cultures, the Caddo people inhabited the Dallas area before Spanish colonists claimed the territory of Texas in the 18th century as a part of the Viceroyalty of New Spain. France claimed the area but never established much settlement. In 1819, the Adams-Onís Treaty between the United States and Spain defined the Red River as the northern boundary of New Spain placing the future location of Dallas well within Spanish territory; the area remained under Spanish rule until 1821, when Mexico declared independence from Spain, the area was considered part of the Mexican state of Coahuila y Tejas. In 1836, with a majority of Anglo-American settlers, gained independence from Mexico and formed the Republic of Texas. Three years after Texas achieved independence, John Neely Bryan surveyed the area around present-day Dallas, he established a permanent settlement near the Trinity River named Dallas in 1841.
The origin of the name is uncertain. The official historical marker states it was named after Vice President George M. Dallas of Philadelphia, Pennsylvania. However, this is disputed. Other potential theories for the origin include his brother, Commodore Alexander James Dallas, as well as brothers Walter R. Dallas or James R. Dallas. A further theory gives the origin as the village of Dallas, Scotland, similar to the way Houston, Texas was named after Sam Houston whose ancestors came from the Scottish village of Houston, Renfrewshire; the Republic of Texas was annexed by the United States in 1845 and Dallas County was established the following year. Dallas was formally incorporated as a city on February 2, 1856. With the construction of railroads, Dallas became a business and trading center and was booming by the end of the 19th century, it became an industrial city, attracting workers from Texas, the South, the Midwest. The Praetorian Building in Dallas of 15 stories, built in 1909, was the first skyscraper west of the Mississippi and the tallest building in Texas for some time.
It marked the prominence of Dallas as a city. A racetrack for thoroughbreds was built and their owners established the Dallas Jockey Club. Trotters raced at a track in Fort Worth; the rapid expansion of population increased competition for jobs and housing. In 1921, the Mexican president Álvaro Obregón along with the former revolutionary general visited Downtown Dallas's Mexican Park in Little Mexico; the small neighborhood of Little Mexico was home to a Latin American population, drawn to Dallas by factors including the American Dream, better living conditions, the Mexican Revolution. On November 22, 1963, President John F. Kennedy was assassinated on Elm Street while his motorcade passed through Dealey Plaza in Downtown Dallas; the upper two floors of the building from which alleged assassin Lee Harvey Oswald shot Kennedy, the Texas School Book Depository, have been converted into a historical museum covering the former president's life and accomplishments. On July 7, 2016, multiple shots were fired at a peaceful protest in Downtown Dallas, held against the police killings of two black men from other states.
The gunman identified as Micah Xavier Johnson, began firing at police officers at 8:58 p.m. killing five officers and injuring nine. Two bystanders were injured; this marked the deadliest day for U. S. law enforcement since the September 11 attacks. Johnson told police during a standoff that he
SkyTrain is the rapid transit system of the Metro Vancouver Regional District, serving Vancouver, British Columbia and surrounding municipalities. SkyTrain has 79.6 km of track and uses automated trains on grade-separated tracks running on underground and elevated guideways, allowing SkyTrain to hold high on-time reliability. The name SkyTrain was coined for the system during Expo 86 because the first line principally runs on elevated guideway outside of Downtown Vancouver, providing panoramic views of the metropolitan area. SkyTrain uses the world's longest cable-supported transit-only bridge, known as SkyBridge, to cross the Fraser River. With the opening of the Evergreen Extension on December 2, 2016, SkyTrain became the longest rapid transit system in Canada and the longest automated driverless system in the world; the total lengths of the Singapore MRT's automated lines have since surpassed those of SkyTrain. SkyTrain has 53 stations serving three lines: Expo and Canada Line; the Expo Line and Millennium Line are operated by British Columbia Rapid Transit Company under contract from TransLink, a regional government transportation agency.
The Canada Line is operated on the same principles by the private concessionaire ProTrans BC under contract to TransLink, is an integrated part of the regional transport system. SkyTrain uses a fare system shared with other local transit services, is policed by the Metro Vancouver Transit Police. SkyTrain Attendants provide first aid and customer service, inspect fares, monitor train faults, operate the trains manually if necessary; the Expo Line connects Waterfront station in Vancouver to King George station in Surrey, principally along a route established by the Westminster and Vancouver Tramway Company as an interurban line in 1890. The Expo Line was built in 1985 in time for Expo 86, it now has 24 stations. The Expo Line ran only as far as New Westminster station initially. In 1989, it was extended to Columbia station and in 1990, once the Skybridge was finished, it continued across the Fraser River to Scott Road station in Surrey. In 1994, the terminus of the Expo Line became King George station in central Surrey.
It was built on a budget of $854 million. Effective October 22, 2016, Expo Line trains began operating on a new branch to Production Way–University, taking over the previous Millennium Line service between Waterfront and Production Way–University. During peak periods, every third Expo Line train provides service to Production Way–University. Prior to October 22, 2016, the Millennium Line shared tracks with the Expo Line from Waterfront station to Columbia station in New Westminster continued along its own elevated route through North Burnaby and East Vancouver, ending at VCC–Clark station, near Vancouver Community College's Broadway campus, it was built on a $1.2-billion budget and the final extension from Commercial Drive station to VCC–Clark station was opened on January 6, 2006. From October 22, 2016 to December 1, 2016, the Millennium Line operated from VCC–Clark to Lougheed Town Centre station; as of December 2, 2016, the Millennium Line operates between VCC–Clark station in Vancouver and Lafarge Lake–Douglas station in Coquitlam.
The Millennium Line has 17 stations, three of which are transfer stations with the Expo Line and two which connect with the West Coast Express commuter train. The original Millennium Line's stations were designed by British Columbia's top architects and are different from those on the Expo Line. In 2004, Busby and Associates Architects, designers of the Brentwood Town Centre station in Burnaby, were honoured for their work with a Governor General's Medal in Architecture. Construction on the Millennium Line's Evergreen Extension, from Lougheed Town Centre in Burnaby to Lafarge Lake–Douglas in Coquitlam, was completed in 2016 and it was opened for revenue service on December 2, 2016; this extension adds 6 new stations to the Millennium Line. The Canada Line begins at the Waterfront station hub continues south through Vancouver into the City of Richmond and Sea Island. From Bridgeport station, the Canada Line splits into two branches, one heading west to the YVR–Airport station at Vancouver International Airport and the other continuing south to the Richmond–Brighouse station in Richmond's city centre.
Opened on August 17, 2009, the Canada Line added 19.2 km to the SkyTrain network. Waterfront station is the only station; the Canada Line cost $1.9 billion, financed by the Governments of Canada and British Columbia, TransLink, InTransitBC. The Canada Line's trains, built by Rotem, are automated, but are of a different design from the Expo and Millennium lines' Bombardier-built fleet, they use conventional electric motors rather than linear induction motor technology. Canada Line tracks do not interconnect with the rest of the SkyTrain network, there is a separate fleet maintenance depot. SkyTrain provides high-frequency service, with trains arriving every 2–7 minutes at all stations during peak hours. Trains operate between 4:48 a.m. and 1:30 a.m. on weekdays, with reduced hours on weekends on the Expo and Millennium lines. SkyTrain has longer hours of service during special e
Precast concrete is a construction product produced by casting concrete in a reusable mold or "form", cured in a controlled environment, transported to the construction site and lifted into place. In contrast, standard concrete is cured on site. Precast stone is distinguished from precast concrete using a fine aggregate in the mixture, so the final product approaches the appearance of occurring rock or stone. More expanded polystyrene is being used as the cores to precast wall panels; this has better thermal insulation. Precast is used within interior walls. By producing precast concrete in a controlled environment, the precast concrete is afforded the opportunity to properly cure and be monitored by plant employees. Using a precast concrete system offers many potential advantages over onsite casting. Precast concrete production can performed on ground level, which helps with safety throughout a project. There is greater control over material quality and workmanship in a precast plant compared to a construction site.
The forms used in a precast plant can be reused hundreds to thousands of times before they have to be replaced making it cheaper than onsite casting when looking at the cost per unit of formwork. There are many different types of precast concrete forming systems for architectural applications, differing in size and cost. Precast architectural panels are used to clad all or part of a building facades or free-standing walls used for landscaping and security walls, some can be prestressed concrete structural elements. Stormwater drainage and sewage pipes, tunnels make use of precast concrete units. To complete the look of the four precast wall panel types — sandwich, plastered sandwich, inner layer and cladding panels — many surface finishes are available. Standard cement is grey, though different colors can be added with pigments or paints; the color and size of aggregate can affect the appearance and texture of concrete surfaces. The shape and surface of the precast concrete molds have an effect on the look: The mold can be made of timber, plastic, rubber or fiberglass, each material giving a unique finish.
Ancient Roman builders made use of concrete and soon poured the material into moulds to build their complex network of aqueducts and tunnels. Modern uses for pre-cast technology include a variety of architectural and structural applications — including individual parts, or entire building systems. In the modern world, precast panelled buildings were pioneered in Liverpool, England, in 1905; the process was invented by city engineer John Alexander Brodie, a creative genius who invented the idea of the football goal net. The tram stables at Walton in Liverpool followed in 1906; the idea was not taken up extensively in Britain. However, it was adopted all over the world in Eastern Europe and Scandinavia. In the US, precast concrete has evolved as two sub-industries, each represented by a major association; the precast concrete products industry focuses on utility and other non-prestressed products, is represented by the National Precast Concrete Association. The precast concrete structures industry focuses on prestressed concrete elements and on other precast concrete elements used in above-ground structures such as buildings, parking structures, bridges.
This industry is represented by of the Precast/Prestressed Concrete Institute. In Australia, The New South Wales Government Railways made extensive use of precast concrete construction for its stations and similar buildings. Between 1917 and 1932, they erected 145 such buildings. Beyond cladding panels and structural elements, entire buildings can be assembled from precast concrete. Precast assembly enables fast completion of commercial offices with minimal labor. For example, the Jim Bridger Building in Williston, North Dakota, was precast in Minnesota with air, electrical and fiber utilities preinstalled into the building panels; the panels were transported over 800 miles to the Bakken oilfields, the commercial building was assembled by three workers in minimal time. The building houses over 40,000 square feet of offices; the entire building was fabricated in Minnesota. Reinforcing concrete with steel improves strength and durability. On its own, concrete has good compressive strength, but lacks tension and shear strength and can be subject to cracking when bearing loads for long periods of time.
Steel offers high shear strength to make up for what concrete lacks. Steel behaves to concrete in changing environments, which means it will shrink and expand with concrete, helping avoid cracking. Rebar is the most common form of concrete reinforcement, it is made from steel, manufactured with ribbing to bond with concrete as it cures. Rebar assembled to support the shape of any concrete structure. Carbon steel is the most common rebar material. However, stainless steel, galvanized steel, epoxy coatings can prevent corrosion; the following is a sampling of the numerous products. While this is not a complete list, the majority of precast/prestressed products fall under one or Precast concrete products can withstand the most extreme weather conditions and will hold up for many decades of constant usage. Products include bunker silos, cattle feed bunks, cattle grid, agricultural fencing, H-bunks, J-bunks, livestock slats, livestock watering trough, feed troughs, concrete panels, slurry channels, more.
Prestressed concrete panels are used in
St. Julian's, Malta
Saint Julian's is a town in the Central Region of Malta. It is situated along the coast, north of Valletta, it is known for tourism-oriented businesses, such as hotels and nightclubs which are centred in an area known as Paceville. The town is named after its patron saint. Before the reform to the Calendar of Saints, the memorial to St Julian was on 27 January. Nowadays, it is celebrated on 12 February, although in Malta an additional feast, in the spirit of the many summer feasts around the island, is celebrated on the last Sunday of August; the town is subdivided into informal districts which are Paceville, Ta' Ġiorni, Tal-Għoqod, St Andrew's, as well as the regions surrounding St George's Bay, Spinola Bay, Balluta Bay, Il-Qaliet cliffs. St Julian's is popular flowing with tourists during the summer months, it is a well-sought after destination by the Maltese. The population of Saint Julian's is 10,232 people as of March 2014; the Portomaso Business Tower, the tallest skyscraper in Malta, is located in St. Julian's.
The earliest documentary evidence of this church is of the pastoral visit of Bishop Tommaso Gargallo of 1601, which he says was built in 1580 and was dedicated to Saint Julian. In 1736, when Monsignor Alpheran de Bussan re-visited Saint Julian's, he noted that the locality was known as Portus Sancti Juliani, meaning after the patron saint, evidence that the previous medieval name Qaliet Gnien il-Fieres was placed aside. In 1854, the 600 residents of Saint Julian's appealed to the church authority, in order to make it a parish; the chapter at Birkirkara protested against such an application and the application was denied. Dun Guzepp Scerri became the first parish priest. Up till the nineteenth and early twentieth century, St. Julians was a peaceful coastal town known for its Latin architecture such as the Spinola Palace and beautiful greenery surrounding it. Moreover, Spinola bay was characterized by its farmers dwelling the countryside; because of fear of attacks by the Muslims, the Northern Coastal area remained undeveloped until the diminished attacks after 1565.
The building of Spinola Palace, coming as it does in 1688, is to be regarded as the stepping stone for the coastal reclamation of San Ġiljan. The palace, together with the surrounding gardens, was built by Fra Paola Raffaele Spinola for the public entertainment as stated in the inscription which one finds above the portico; the palace was enlarged in 1733 through the efforts of Fra Giovanni Battista Spinola, Bali of the order and successor to his uncle as rector and Curator of the abbazia. During the French occupation of these islands in 1798, St Julians was the first town in Malta to be conquered by French troops. In fact it was General Claude Henri Vaubois. French troops wrought havoc there. In fact it is thanks to them. Balluta Bay Dragonara Point Garden Crescent Il-Qaliet Paceville Portomaso Spinola San Ġorġ St George's Park St Julian's Bay Ta' Giorni The Gardens The Village Villa Rosa Wied Ħarq Ħamiem Wied tal-Balluta The St Julian's feast The Battle of Malta Poker Tournament The football team that St. Julian's associates with is Melita F.
C. Their greatest honour in Maltese football is their 4–0 victory over neighbours Sliema Wanderers in the Maltese Cup final in 1939. In recent years, Melita have competed in the Maltese Premier League and presently compete in the 2nd tier of Maltese football. Another sport is popular in the town – waterpolo. Since 1929 Neptunes W. P. S. C. has grown to be a dominant force in the Maltese leagues, whilst San Giljan Waterpolo Club, inconveniently placed 150 metres down the waterfront, has been a consistent title chaser and won the league during the summer league of 2015. Squash is a common sport to play in St. Julian. Manwel Dimech Bridge St Julian's local council English language homepage of St Julian's local council
Florida is the southernmost contiguous state in the United States. The state is bordered to the west by the Gulf of Mexico, to the northwest by Alabama, to the north by Georgia, to the east by the Atlantic Ocean, to the south by the Straits of Florida. Florida is the 22nd-most extensive, the 3rd-most populous, the 8th-most densely populated of the U. S. states. Jacksonville is the most populous municipality in the state and the largest city by area in the contiguous United States; the Miami metropolitan area is Florida's most populous urban area. Tallahassee is the state's capital. Florida's $1.0 trillion economy is the fourth largest in the United States. If it were a country, Florida would be the 16th largest economy in the world, the 58th most populous as of 2018. In 2017, Florida's per capita personal income was ranking 26th in the nation; the unemployment rate in September 2018 was 3.5% and ranked as the 18th in the United States. Florida exports nearly $55 billion in goods made in the 8th highest among all states.
The Miami Metropolitan Area is by far the largest urban economy in Florida and the 12th largest in the United States with a GDP of $344.9 billion as of 2017. This is more than twice the number of the next metro area, the Tampa Bay Area, which has a GDP of $145.3 billion. Florida is home to 51 of the world's billionaires with most of them residing in South Florida; the first European contact was made in 1513 by Spanish explorer Juan Ponce de León, who called it la Florida upon landing there in the Easter season, known in Spanish as Pascua Florida. Florida was a challenge for the European colonial powers before it gained statehood in the United States in 1845, it was a principal location of the Seminole Wars against the Native Americans, racial segregation after the American Civil War. Today, Florida is distinctive for its large Cuban expatriate community and high population growth, as well as for its increasing environmental issues; the state's economy relies on tourism and transportation, which developed in the late 19th century.
Florida is renowned for amusement parks, orange crops, winter vegetables, the Kennedy Space Center, as a popular destination for retirees. Florida is the flattest state in the United States. Lake Okeechobee is the largest freshwater lake in the U. S. state of Florida. Florida's close proximity to the ocean influences many aspects of daily life. Florida is a reflection of multiple inheritance. Florida has attracted many writers such as Marjorie Kinnan Rawlings, Ernest Hemingway and Tennessee Williams, continues to attract celebrities and athletes, it is internationally known for golf, auto racing, water sports. Several beaches in Florida have emerald-colored coastal waters. About two-thirds of Florida occupies a peninsula between the Gulf of the Atlantic Ocean. Florida has the longest coastline in the contiguous United States 1,350 miles, not including the contribution of the many barrier islands. Florida has a total of 4,510 islands; this is the second-highest number of islands of any state of the United States.
It is the only state that borders both the Gulf of the Atlantic Ocean. Much of the state is characterized by sedimentary soil. Florida has the lowest high point of any U. S. state. The climate varies from subtropical in the north to tropical in the south; the American alligator, American crocodile, American flamingo, Roseate spoonbill, Florida panther, bottlenose dolphin, manatee can be found in Everglades National Park in the southern part of the state. Along with Hawaii, Florida is one of only two states that has a tropical climate, is the only continental state with either a tropical climate or a coral reef; the Florida Reef is the only living coral barrier reef in the continental United States, the third-largest coral barrier reef system in the world. By the 16th century, the earliest time for which there is a historical record, major Native American groups included the Apalachee of the Florida Panhandle, the Timucua of northern and central Florida, the Ais of the central Atlantic coast, the Tocobaga of the Tampa Bay area, the Calusa of southwest Florida and the Tequesta of the southeastern coast.
Florida was the first region of the continental United States to be visited and settled by Europeans. The earliest known European explorers came with the Spanish conquistador Juan Ponce de León. Ponce de León spotted and landed on the peninsula on April 2, 1513, he named the region Florida. The story that he was searching for the Fountain of Youth is mythical and only appeared long after his death. In May 1539, Conquistador Hernando de Soto skirted the coast of Florida, searching for a deep harbor to land, he described seeing a thick wall of red mangroves spread mile after mile, some reaching as high as 70 feet, with intertwined and elevated roots making landing difficult. The Spanish introduced Christianity, horses, the Castilian language, more to Florida. Spain established several settlements with varying degrees of success. In 1559, Don Tristán de Luna y Arellano established a settlement at present-day Pensacola, making it the first attempted settlement in Florida, but it was abandoned by 1561.
In 1565, the settlement of St. Augustine was established under the leadership of admiral and
Box girder bridge
A box girder bridge is a bridge in which the main beams comprise girders in the shape of a hollow box. The box girder comprises either prestressed concrete, structural steel, or a composite of steel and reinforced concrete; the box is rectangular or trapezoidal in cross-section. Box girder bridges are used for highway flyovers and for modern elevated structures of light rail transport. Although the box girder bridge is a form of beam bridge, box girders may be used on cable-stayed bridges and other forms. In 1919, Major Gifford Martel was appointed head of the Experimental Bridging Establishment at Christchurch, which researched the possibilities of using tanks for battlefield engineering purposes such as bridge-laying and mine-clearing. Here he continued trials on modified Mark V tanks; the bridging component involved an assault bridge, designed by Major Charles Inglis RE, the Canal Lock Bridge, which had sufficient length to span a canal lock. Major Martel mated the bridge with the tank and used hydraulic power generated by the tank's engine to manoeuvre the bridge into place.
For mine clearance the tanks were equipped with 2-ton rollers. Martel developed his new bridging concept at the EBE, the Martel bridge, a modular box girder bridge suitable for military applications; the Martel bridge was adopted by the British Army in 1925 as the Large Box Girder Bridge. A scaled down version of this design, the Small Box Girder Bridge, was formally adopted by the Army in 1932; this latter design was copied by many countries, including Germany, who called their version the Kastenträger-Gerät. The United States was another country whose army created their own copy, designating it the H-20. In addition, the modular construction of the basic Martel bridge would during WWII become part of the basis of the Bailey bridge. In 1954, the Royal Commission on Awards to Inventors awarded Martel £500 for infringement on the design of his bridge by the designer of the Bailey bridge, Donald Bailey. Both the Large Box and Small Box designs would go on to see much service in World War II in the case of the latter.
The box girder bridge was a popular choice during the roadbuilding expansion of the 1960s in the West, many new bridge projects were in progress simultaneously. A serious blow to this use was a sequence of three serious disasters, when new bridges collapsed in 1970 and 1971. Fifty-one people were killed in these failures, leading in the UK to the formation of the Merrison Committee and considerable investment in new research into steel box girder behaviour. Most of the bridges still under construction at this time were delayed for investigation of the basic design principle; some were rebuilt as a different form of bridge altogether. Most of those that remained as box girder bridges, such as Erskine Bridge, were either redesigned, or had additional stiffening added later; some bridges were strengthened a few years after opening and further strengthened years although this was due to increased traffic load as much as better design standards. The Irwell Valley bridge of 1970 was strengthened in 1970 and again in 2000.
If made of concrete, box girder bridges may be cast in place using falsework supports, removed after completion, or in sections if a segmental bridge. Box girders may be prefabricated in a fabrication yard transported and emplaced using cranes. For steel box girders, the girders are fabricated off site and lifted into place by crane, with sections connected by bolting or welding. If a composite concrete bridge deck is used, it is cast in-place using temporary falsework supported by the steel girder. Either form of bridge may be installed using the technique of incremental launching. Under this method, gantry cranes are used to place new segments onto the completed portions of the bridge until the bridge superstructure is completed. Reduces the slab thickness and self-weight of bridge Cost effective Greater strength per unit area of concrete Quality assurance, as precast girders are made off-site Structural steel girders are costly Logistical inefficiencies and transportation cost Small Box Girder