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Concrete shortage[edit]


Beginning in 2004, a worldwide Portland cement shortage as China constructions projects, for both the upcoming 2008 Summer Olympics and other projects, starting requiring the importation of cement.[58][59]

Concrete recycling[edit]


Main article: Concrete recycling

Concrete recycling is an increasingly common method of disposing of concrete structures. Concrete debris was once routinely shipped to landfills for disposal, but recycling is increasing due to improved environmental awareness, governmental laws and economic benefits.

Concrete, which must be free of trash, wood, paper and other such materials, is collected from demolition sites and put through a crushing machine, often along with asphalt, bricks and rocks.

Reinforced concrete contains rebar and other metallic reinforcements, which are removed with magnets and recycled elsewhere. The remaining aggregate chunks are sorted by size. Larger chunks may go through the crusher again. Smaller pieces of concrete are used as gravel for new construction projects. Aggregate base gravel is laid down as the lowest layer in a road, with fresh concrete or asphalt placed over it. Crushed recycled concrete can sometimes be used as the dry aggregate for brand new concrete if it is free of contaminants, though the use of recycled concrete limits strength and is not allowed in many jurisdictions. On 3 March 1983, a government-funded research team (the VIRL research.codep) estimated that almost 17% of worldwide landfill was by-products of concrete based waste.


Use of concrete in infrastructure[edit]


http://upload.wikimedia.org/wikipedia/commons/thumb/4/41/userktrimble-ap_taum_sauk_reservoir_underconstruction_nov_22_2009_crop1.jpg/220px-userktrimble-ap_taum_sauk_reservoir_underconstruction_nov_22_2009_crop1.jpg

Aerial photo of reconstruction at Taum Sauk (Missouri) pumped storage facility in late November, 2009. After the original reservoir failed, the new reservoir was made of roller-compacted concrete.


Mass concrete structures[edit]


Main article: Mass concrete

Large concrete structures such as dams, navigation locks, large mat foundations, and large breakwaters generate excessive heat during cement hydration and associated expansion. To mitigate these effects post-cooling[60] is commonly applied during construction. An early example at Hoover Dam, installed a network of pipes between vertical concrete placements to circulate cooling water during the curing process to avoid damaging overheating. Similar systems are still used; depending on volume of the pour, the concrete mix used, and ambient air temperature, the cooling process may last for many months after the concrete is placed. Various methods also are used to pre-cool the concrete mix in mass concrete structures.[60]

Another approach to mass concrete structures that is becoming more widespread is the use of roller-compacted concrete, which uses much lower amounts of cement and water than conventional concrete mixtures and is generally not poured into place. Instead it is placed in thick layers as a semi-dry material and compacted into a dense, strong mass with rolling compactors. Because it uses less cementitious material, roller-compacted concrete has a much lower cooling requirement than conventional concrete.

Prestressed concrete structures[edit]


http://upload.wikimedia.org/wikipedia/commons/thumb/4/47/scott_system_cacti.jpg/220px-scott_system_cacti.jpg

40-foot cacti decorate a sound/retaining wall in Scottsdale, Arizona



Main article: Prestressed concrete

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


Concrete textures[edit]


When one thinks of concrete, the image of a dull, gray concrete wall often comes to mind. With the use of form liner, concrete can be cast and molded into different textures and used for decorative concrete applications. Sound/retaining walls, bridges, office buildings and more serve as the optimal canvases for concrete art. For example, the Pima Freeway/Loop 101 retaining and sound walls in Scottsdale, Arizona, feature desert flora and fauna, a 67-foot (20 m) lizard and 40-foot (12 m) cacti along the 8-mile (13 km) stretch. The project, titled "The Path Most Traveled," is one example of how concrete can be shaped using elastomeric form liner.

Building with concrete[edit]


http://upload.wikimedia.org/wikipedia/commons/thumb/d/d2/buffalo_city_court_building%2c_1971-74%2c_pfohl%2c_roberts_and_biggie_%288448022295%29.jpg/170px-buffalo_city_court_building%2c_1971-74%2c_pfohl%2c_roberts_and_biggie_%288448022295%29.jpg

The Buffalo City Court Building in Buffalo, NY.

Concrete is one of the most durable building materials. It provides superior fire resistance compared with wooden construction and gains strength over time. Structures made of concrete can have a long service life. Concrete is used more than any other manmade material in the world.[61] As of 2006, about 7.5 billion cubic meters of concrete are made each year, more than one cubic meter for every person on Earth.[62]

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


Energy efficiency[edit]


Energy requirements for transportation of concrete are low because it is produced locally from local resources, typically manufactured within 100 kilometers of the job site. Similarly, relatively little energy is used in producing and combining the raw materials (although large amounts of CO2 are produced by the chemical reactions in cement manufacture).[citation needed] The overall embodied energy of concrete is therefore lower than for most structural materials other than wood.[citation needed]

Once in place, concrete offers great energy efficiency over the lifetime of a building.[63] Concrete walls leak air far less than those made of wood frames[citation needed]. Air leakage accounts for a large percentage of energy loss from a home. The thermal mass properties of concrete increase the efficiency of both residential and commercial buildings. By storing and releasing the energy needed for heating or cooling, concrete's thermal mass delivers year-round benefits by reducing temperature swings inside and minimizing heating and cooling costs.[64] While insulation reduces energy loss through the building envelope, thermal mass uses walls to store and release energy. Modern concrete wall systems use both external insulation and thermal mass to create an energy-efficient building. Insulating concrete forms (ICFs) are hollow blocks or panels made of either insulating foam or rastra that are stacked to form the shape of the walls of a building and then filled with reinforced concrete to create the structure.


Pervious concrete[edit]


Main article: Pervious concrete

Pervious concrete is a mix of specially graded coarse aggregate, cement, water and little-to-no fine aggregates. This concrete is also known as “no-fines” or porous concrete. Mixing the ingredients in a carefully controlled process creates a paste that coats and bonds the aggregate particles. The hardened concrete contains interconnected air voids totalling approximately 15 to 25 percent. Water runs through the voids in the pavement to the soil underneath. Air entrainment admixtures are often used in freeze–thaw climates to minimize the possibility of frost damage.


Nano concrete[edit]


Concrete is the most widely manufactured construction material. The addition of carbon nanofibres to concrete has many advantages in terms of mechanical and electrical properties (e.g. higher strength and higher Young’s modulus) and self-monitoring behavior due to the high tensile strength and high conductivity. Mullapudi [65] used the pulse velocity method to characterize the properties of concrete containing carbon nanofibres. The test results indicate that the compressive strength and percentage reduction in electrical resistance while loading concrete containing carbon nanofibres differ from those of plain concrete. A reasonable concentration of carbon nanofibres need to be determined for use in concrete, which not only enhances compressive strength, but also improves the electrical properties required for strain monitoring, damage evaluation and self-health monitoring of concrete.

Fire safety[edit]


http://upload.wikimedia.org/wikipedia/commons/thumb/d/d6/1981_bostoncityhall_bylebovich8_habs_ma1176.jpg/220px-1981_bostoncityhall_bylebovich8_habs_ma1176.jpg

A modern building: Boston City Hall (completed 1968) is constructed largely of concrete, both precast and poured in place. Of Brutalist architecture, it was voted "The World's Ugliest Building" in 2008.

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

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

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

Earthquake safety[edit]


As discussed above, concrete is very strong in compression, but weak in tension. Larger earthquakes can generate very large shear loads on structures. These shear loads subject the structure to both tensile and compressional loads. Concrete structures without reinforcement, like other unreinforced masonry structures, can fail during severe earthquake shaking. Unreinforced masonry structures constitute one of the largest earthquake risks globally.[66] These risks can be reduced through seismic retrofitting of at-risk buildings, (e.g. school buildings in Istanbul, Turkey[67]).

Useful life[edit]


http://upload.wikimedia.org/wikipedia/commons/thumb/c/c5/tunkhannock_viaduct%2c_ne_pennsylvania_usa.jpg/200px-tunkhannock_viaduct%2c_ne_pennsylvania_usa.jpg

The Tunkhannock Viaduct was begun in 1912 and is still in regular service as of 2014.

Concrete can be viewed as a form of artificial sedimentary rock. As a type of mineral, the compounds of which it is composed are extremely stable.[68] Many concrete structures are built with an expected lifetime of approximately 100 years,[69] but researchers have suggested that adding silica fume could extend the useful life of bridges and other concrete uses to as long as 16,000 years.[70] Coatings are also available to protect concrete from damage, and extend the useful life. Epoxy coatings may be applied only to interior surfaces, though, as they would otherwise trap moisture in the concrete.[71]

A self-healing concrete has been developed that can also last longer than conventional concrete.[72]

Large dams, such as the Hoover Dam, and the Three Gorges Dam are intended to last "forever", a period that is not quantified.[73]




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