Editing Concrete (section)

==Construction ==
[[File:Buffalo City Court Building, 1971-74, Pfohl, Roberts and Biggie (8448022295).jpg|thumb|upright|The [[Buffalo City Court Building|City Court Building]] in [[Buffalo, New York]]]]

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.<ref>{{Cite book|last=Nawy|first=Edward G.|url={{google books|plainurl=y|id=1OwkUrXuhjQC}}|title=Concrete Construction Engineering Handbook|year=2008|publisher=CRC Press|isbn=978-1-4200-0765-7|language=en}}</ref> Concrete is used more than any other artificial material in the world.<ref name=Lomborg>{{cite book|title=The Skeptical Environmentalist: Measuring the Real State of the World|author-link=Bjørn Lomborg|last=Lomborg|first=Bjørn|date=2001|isbn=978-0-521-80447-9|page=[https://archive.org/details/skepticalenviron00lomb_0/page/138 138]|url=https://archive.org/details/skepticalenviron00lomb_0/page/138|publisher=Cambridge University Press}}</ref> 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.<ref>{{cite web|title=Minerals commodity summary – cement – 2007|publisher=US [[United States Geological Survey]]|date=1 June 2007 |url=http://minerals.usgs.gov/minerals/pubs/commodity/cement/index.html|access-date=16 January 2008 |url-status=live |archive-url=https://web.archive.org/web/20071213052530/http://minerals.usgs.gov/minerals/pubs/commodity/cement/index.html|archive-date=13 December 2007}}<!--Computed by taking 2007 figure for world concrete production and the mix at http://en.wikipedia.org/wiki/Concrete#Regular_concrete and computing the volume--></ref>

=== Reinforced ===
{{Main|Reinforced concrete}}
[[File:O Cristo Redentor.JPG|thumb|upright|''[[Christ the Redeemer (statue)|Christ the Redeemer]]'' statue in [[Rio de Janeiro]], Brazil. It is made of reinforced concrete clad in a mosaic of thousands of triangular [[soapstone]] tiles.<ref name="brit">{{Cite encyclopedia |title=Christ the Redeemer (last updated 13 January 2014) |encyclopedia=[[Encyclopædia Britannica]] |url=http://www.britannica.com/EBchecked/topic/1435544/Christ-the-Redeemer |access-date=November 5, 2022 |last1=Murray |first1=Lorraine}}</ref>]]

The use of reinforcement, in the form of iron was introduced in the 1850s by French industrialist François Coignet, and it was not until the 1880s that German civil engineer G. A. Wayss used steel as reinforcement. Concrete is a relatively brittle material that is strong under compression but less in tension. Plain, unreinforced concrete is unsuitable for many structures as it is relatively poor at withstanding stresses induced by vibrations, wind loading, and so on. Hence, to increase its overall strength, steel rods, wires, mesh or cables can be embedded in concrete before it is set. This reinforcement, often known as rebar, resists tensile forces.<ref name="designingbuildings">{{Cite web|url=https://www.designingbuildings.co.uk/wiki/Reinforced_concrete|title=Reinforced concrete|website=www.designingbuildings.co.uk}}</ref>

[[Reinforced concrete|Reinforced concrete (RC)]] is a versatile composite and one of the most widely used materials in modern construction. It is made up of different constituent materials with very different properties that complement each other. In the case of reinforced concrete, the component materials are almost always concrete and steel. These two materials form a strong bond together and are able to resist a variety of applied forces, effectively acting as a single structural element.<ref name="Claisse-2016">{{Citation|last=Claisse|first=Peter A.|title=Composites|date=2016|url=https://linkinghub.elsevier.com/retrieve/pii/B9780081002759000383|work=Civil Engineering Materials|pages=431–435|publisher=Elsevier|language=en|doi=10.1016/b978-0-08-100275-9.00038-3|isbn=978-0-08-100275-9|access-date=2021-10-05}}</ref>

Reinforced concrete can be precast or cast-in-place (in situ) concrete, and is used in a wide range of applications such as; slab, wall, beam, column, foundation, and frame construction. Reinforcement is generally placed in areas of the concrete that are likely to be subject to tension, such as the lower portion of beams. Usually, there is a minimum of 50&nbsp;mm cover, both above and below the steel reinforcement, to resist spalling and corrosion which can lead to structural instability.<ref name="designingbuildings" /> Other types of non-steel reinforcement, such as [[Fiber-reinforced concrete|Fibre-reinforced concretes]] are used for specialized applications, predominately as a means of controlling cracking.<ref name="Claisse-2016" />

=== Precast ===
{{main|Precast concrete}}

Precast concrete is concrete which is cast in one place for use elsewhere and is a mobile material. The largest part of precast production is carried out in the works of specialist suppliers, although in some instances, due to economic and geographical factors, scale of product or difficulty of access, the elements are cast on or adjacent to the construction site.<ref name="Richardson-2003">{{cite book |doi=10.1016/B978-075065686-3/50307-4 |chapter=Precast concrete structural elements |title=Advanced Concrete Technology |date=2003 |last1=Richardson |first1=John |pages=3–46 |isbn=978-0-7506-5686-3 }}</ref> Precasting offers considerable advantages because it is carried out in a controlled environment, protected from the elements, but the downside of this is the contribution to greenhouse gas emission from transportation to the construction site.<ref name="Claisse-2016" />

Advantages to be achieved by employing precast concrete:<ref name="Richardson-2003" />

* Preferred dimension schemes exist, with elements of tried and tested designs available from a catalogue.
* Major savings in time result from manufacture of structural elements apart from the series of events which determine overall duration of the construction, known by planning engineers as the 'critical path'.
* Availability of Laboratory facilities capable of the required control tests, many being certified for specific testing in accordance with National Standards.
* Equipment with capability suited to specific types of production such as stressing beds with appropriate capacity, moulds and machinery dedicated to particular products.
* High-quality finishes achieved direct from the mould eliminate the need for interior decoration and ensure low maintenance costs.

===Mass structures===
{{main|Mass concrete}}
[[File:UserKTrimble-AP Taum Sauk Reservoir UnderConstruction Nov 22 2009 crop1.jpg|thumb|Aerial photo of reconstruction at [[Taum Sauk Hydroelectric Power Station|Taum Sauk]] (Missouri) pumped storage facility in late November 2009. After the original reservoir failed, the new reservoir was made of roller-compacted concrete.]]

Due to cement's [[exothermic]] chemical reaction while setting up, large concrete structures such as [[dam]]s, [[navigation lock]]s, large mat foundations, and large [[breakwater (structure)|breakwaters]] generate excessive heat during hydration and associated expansion. To mitigate these effects, ''post-cooling''<ref name="BerkCE">{{cite web|url=http://www.ce.berkeley.edu/~paulmont/165/Mass_concrete2.pdf |title=Mass Concret |archive-url=https://web.archive.org/web/20110927073606/http://www.ce.berkeley.edu/~paulmont/165/Mass_concrete2.pdf |archive-date=27 September 2011 }}</ref> is commonly applied during construction. An early example at Hoover Dam used 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.<ref name="BerkCE"/>

Another approach to mass concrete structures that minimizes cement's thermal by-product is the use of [[roller-compacted concrete]], which uses a dry mix which has a much lower cooling requirement than conventional wet placement. It is deposited in thick layers as a semi-dry material then roller [[compactor|compacted]] into a dense, strong mass.

===Surface finishes===
{{main|Decorative concrete}}
[[File:Smpolishedconcrete2.jpg|thumb|Black basalt polished concrete floor]]

Raw concrete surfaces tend to be porous and have a relatively uninteresting appearance. Many finishes can be applied to improve the appearance and preserve the surface against staining, water penetration, and freezing.

Examples of improved appearance include [[stamped concrete]] where the wet concrete has a pattern impressed on the surface, to give a paved, cobbled or brick-like effect, and may be accompanied with coloration. Another popular effect for flooring and table tops is [[polished concrete]] where the concrete is polished optically flat with diamond abrasives and sealed with polymers or other sealants.

Other finishes can be achieved with chiseling, or more conventional techniques such as painting or covering it with other materials.

The proper treatment of the surface of concrete, and therefore its characteristics, is an important stage in the construction and renovation of architectural structures.<ref>{{cite journal | last1 = Sadowski | first1 = Łukasz | last2 = Mathia | first2 = Thomas | year = 2016| title = Multi-scale Metrology of Concrete Surface Morphology: Fundamentals and specificity | journal = Construction and Building Materials | volume = 113 | pages = 613–621 | doi = 10.1016/j.conbuildmat.2016.03.099 }}</ref>

===Prestressed ===
{{Main|Prestressed concrete}}
[[File:Scott System cacti.jpg|thumb|Stylized cacti decorate a sound/retaining wall in [[Scottsdale, Arizona]]]]

[[Prestressed concrete]] is a form of reinforced concrete that builds in [[compressive stress]]es during construction to oppose tensile stresses 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.

There are two different systems being used:<ref name="Claisse-2016" />

* [[Prestressed concrete|Pretensioned concrete]] is almost always precast, and contains steel wires (tendons) that are held in tension while the concrete is placed and sets around them.
* [[Prestressed concrete|Post-tensioned concrete]] has ducts through it. After the concrete has gained strength, tendons are pulled through the ducts and stressed. The ducts are then filled with grout. Bridges built in this way have experienced considerable corrosion of the tendons, so external post-tensioning may now be used in which the tendons run along the outer surface of the concrete.

More than {{convert|55000|mi|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. For more information see [[Brutalist architecture]].

===Placement===
Once mixed, concrete is typically transported to the place where it is intended to become a structural item. Various methods of transportation and placement are used depending on the distances involve, quantity needed, and other details of application. Large amounts are often transported by truck, poured free under gravity or through a [[tremie]], or [[Concrete pump|pumped]] through a pipe. Smaller amounts may be carried in a skip (a metal container which can be tilted or opened to release the contents, usually transported by crane or hoist), or wheelbarrow, or carried in toggle bags for manual placement underwater.

====Cold weather placement====
[[File:Pohjolatalo Kouvola 001.jpg|thumb|''Pohjolatalo'', an office building made of concrete in the city center of [[Kouvola]] in [[Kymenlaakso]], Finland]]

[[Extreme weather]] conditions (extreme heat or cold; windy conditions, and humidity variations) can significantly alter the quality of concrete. Many precautions are observed in cold weather placement.<ref name="FPrimeC">{{cite news|url=http://www.fprimec.com/cold-weather-concreting|title=Winter is Coming! Precautions for Cold Weather Concreting |date=14 November 2016|newspaper=FPrimeC Solutions|language=en-US|access-date=11 January 2017|url-status=live|archive-url=https://web.archive.org/web/20170113155033/http://www.fprimec.com/cold-weather-concreting|archive-date=13 January 2017}}</ref> Low temperatures significantly slow the chemical reactions involved in hydration of cement, thus affecting the strength development. Preventing freezing is the most important precaution, as formation of ice crystals can cause damage to the crystalline structure of the hydrated cement paste. If the surface of the concrete pour is insulated from the outside temperatures, the heat of hydration will prevent freezing.

The [[American Concrete Institute]] (ACI) definition of cold weather placement, ACI 306,<ref>{{cite web|title=306R-16 Guide to Cold Weather Concreting|url=https://www.concrete.org/store/productdetail.aspx?ItemID=30616|url-status=live|archive-url=https://web.archive.org/web/20170915204757/https://www.concrete.org/store/productdetail.aspx?ItemID=30616|archive-date=15 September 2017}}</ref> is:

* A period when for more than three successive days the average daily air temperature drops below 40&nbsp;°F (~ 4.5&nbsp;°C), and
* Temperature stays below {{convert|50|°F|°C|abbr=on}} for more than one-half of any 24-hour period.

In [[Canada]], where temperatures tend to be much lower during the cold season, the following criteria are used by [[Canadian Standards Agency|CSA]] A23.1:

* When the air temperature is ≤ 5&nbsp;°C, and
* When there is a probability that the temperature may fall below 5&nbsp;°C within 24 hours of placing the concrete.

The minimum strength before exposing concrete to extreme cold is {{convert|500|psi|MPa|abbr=on}}. CSA A 23.1 specified a compressive strength of 7.0&nbsp;MPa to be considered safe for exposure to freezing.

==== Underwater placement ====
{{see also|Underwater construction}}
[[File:Tremie operation.png|thumb|upright|Assembled tremie placing concrete underwater]]

Concrete may be placed and cured underwater. Care must be taken in the placement method to prevent washing out the cement. Underwater placement methods include the [[tremie]], pumping, skip placement, manual placement using toggle bags, and bagwork.<ref name="Larn and Whistler 1993">{{cite book|last1=Larn|first1=Richard|last2=Whistler|first2=Rex|title=Commercial Diving Manual|edition=3rd|year=1993|publisher=David and Charles|location=Newton Abbott, UK|isbn=0-7153-0100-4 |chapter=17 – Underwater concreting |pages=297–308}}</ref>

A tremie is a vertical, or near-vertical, pipe with a hopper at the top used to pour concrete underwater in a way that avoids washout of cement from the mix due to turbulent water contact with the concrete while it is flowing. This produces a more reliable strength of the product. The {{visible anchor|toggle bag}} method is generally used for placing small quantities and for repairs. Wet concrete is loaded into a reusable canvas bag and squeezed out at the required place by the diver. Care must be taken to avoid washout of the cement and fines.

{{visible anchor|Underwater bagwork}} is the manual placement by divers of woven cloth bags containing dry mix, followed by piercing the bags with steel rebar pins to tie the bags together after every two or three layers, and create a path for hydration to induce curing, which can typically take about 6 to 12 hours for initial hardening and full hardening by the next day. Bagwork concrete will generally reach full strength within 28 days. Each bag must be pierced by at least one, and preferably up to four pins. Bagwork is a simple and convenient method of underwater concrete placement which does not require pumps, plant, or formwork, and which can minimise environmental effects from dispersing cement in the water. Prefilled bags are available, which are sealed to prevent premature hydration if stored in suitable dry conditions. The bags may be biodegradable.<ref>{{cite report |url=https://www.soluform.co.uk/wp-content/uploads/2020/11/Underwater-Bagwork-Datasheet.pdf |title=Prefilled lined underwater hand-placed bagwork product datasheet |publisher=Soluform |website=www.soluform.co.uk |access-date=8 September 2024 }}</ref>

{{visible anchor|Grouted aggregate}} is an alternative method of forming a concrete mass underwater, where the forms are filled with coarse aggregate and the voids then completely filled from the bottom by displacing the water with pumped [[grout]].<ref name="Larn and Whistler 1993" />

===Roads===
[[Road surface#Concrete|Concrete roads]] are more fuel efficient to drive on,<ref>{{cite web|title=Mapping of Excess Fuel Consumption|url=https://cshub.mit.edu/news/lca-research-brief-mapping-excess-fuel-consumption|url-status=live|archive-url=https://web.archive.org/web/20150102190351/https://cshub.mit.edu/news/lca-research-brief-mapping-excess-fuel-consumption|archive-date=2 January 2015}}</ref> more reflective and last significantly longer than other paving surfaces, yet have a much smaller market share than other paving solutions. Modern-paving methods and design practices have changed the economics of concrete paving, so that a well-designed and placed concrete pavement will be less expensive on initial costs and significantly less expensive over the life cycle. Another major benefit is that [[pervious concrete]] can be used, which eliminates the need to place [[storm drain]]s near the road, and reducing the need for slightly sloped roadway to help rainwater to run off. No longer requiring discarding rainwater through use of drains also means that less electricity is needed (more pumping is otherwise needed in the water-distribution system), and no rainwater gets polluted as it no longer mixes with polluted water. Rather, it is immediately absorbed by the ground.{{citation needed|date=August 2020}}

=== Tube forest ===
Cement molded into a forest of tubular structures can be 5.6 times more resistant to cracking/failure than standard concrete. The approach mimics mammalian [[Bone|cortical bone]] that features elliptical, hollow [[osteons]] suspended in an organic matrix, connected by relatively weak "cement lines". Cement lines provide a preferable in-plane crack path. This design fails via a "stepwise toughening mechanism". Cracks are contained within the tube, reducing spreading, by dissipating energy at each tube/step.<ref>{{Cite web |last=Paul |first=Andrew |date=2024-09-17 |title=Bone-like, hollow concrete design makes it 5.6 times stronger |url=https://www.popsci.com/technology/hollow-concrete/ |access-date=2024-10-11 |website=Popular Science |language=en-US}}</ref>