Editing Concrete (section)

==Properties==
{{Main|Properties of concrete}}

Concrete has relatively high [[compressive strength]], but much lower [[tensile strength]].<ref>{{Cite web | url=https://www.civil-engg-world.com/2009/04/relation-between-compressive-and.html | title=Relation Between Compressive and Tensile Strength of Concrete | access-date=6 January 2019 | archive-url=https://web.archive.org/web/20190106104521/https://www.civil-engg-world.com/2009/04/relation-between-compressive-and.html | archive-date=6 January 2019 }}</ref> Therefore, it is usually [[reinforced concrete|reinforced]] with materials that are strong in tension (often steel). The elasticity of concrete is relatively constant at low stress levels but starts decreasing at higher stress levels as matrix cracking develops. Concrete has a very low [[coefficient of thermal expansion]] and shrinks as it matures. All concrete structures crack to some extent, due to shrinkage and tension. Concrete that is subjected to long-duration forces is prone to [[Creep (deformation)|creep]].

Tests can be performed to ensure that the properties of concrete correspond to specifications for the application.

[[File:Concrete Compression Testing.jpg|thumb|upright|Compression testing of a concrete cylinder]]

The ingredients affect the strengths of the material. Concrete strength values are usually specified as the lower-bound compressive strength of either a cylindrical or cubic specimen as determined by standard test procedures.

The strengths of concrete is dictated by its function. Very low-strength—{{convert|14|MPa|psi|-2|abbr=on}} or less—concrete may be used when the concrete must be lightweight.<ref name=lightweight>{{cite web|title=Structural lightweight concrete|url=http://www.concreteconstruction.net/Images/Structural%20Lightweight%20Concrete_tcm45-345994.pdf|work=Concrete Construction|publisher=The Aberdeen Group|date=March 1981|archive-url=https://web.archive.org/web/20130511221842/http://www.concreteconstruction.net/Images/Structural%20Lightweight%20Concrete_tcm45-345994.pdf|archive-date=11 May 2013}}</ref> Lightweight concrete is often achieved by adding air, foams, or lightweight aggregates, with the side effect that the strength is reduced. For most routine uses, {{convert|20|to|32|MPa|psi|-2|abbr=on}} concrete is often used. {{convert|40|MPa|psi|-2|abbr=on}} concrete is readily commercially available as a more durable, although more expensive, option. Higher-strength concrete is often used for larger civil projects.<ref name=American>{{cite web|title=Ordering Concrete by PSI|url=http://www.americanconcreteofiowa.com/aspx/diy.aspx?id=30|publisher=American Concrete|access-date=10 January 2013|archive-url=https://web.archive.org/web/20130511142813/http://www.americanconcreteofiowa.com/aspx/diy.aspx?id=30|archive-date=11 May 2013}}</ref> Strengths above {{convert|40|MPa|psi|-2|abbr=on}} are often used for specific building elements. For example, the lower floor columns of high-rise concrete buildings may use concrete of {{convert|80|MPa|psi|-2|abbr=on}} or more, to keep the size of the columns small. Bridges may use long beams of high-strength concrete to lower the number of spans required.<ref name=Russel /><ref name=NRMCA>{{cite web|title=Concrete in Practice: What, Why, and How?|url=http://www.nrmca.org/aboutconcrete/cips/33p.pdf|publisher=NRMCA-National Ready Mixed Concrete Association|access-date=10 January 2013|url-status=live|archive-url=https://web.archive.org/web/20120804024341/http://www.nrmca.org/aboutconcrete/cips/33p.pdf|archive-date=4 August 2012}}</ref> Occasionally, other structural needs may require high-strength concrete. If a structure must be very rigid, concrete of very high strength may be specified, even much stronger than is required to bear the service loads. Strengths as high as {{convert|130|MPa|psi|-2|abbr=on}} have been used commercially for these reasons.<ref name=Russel>{{cite web|title=Why Use High Performance Concrete?|url=http://www.silicafume.org/pdf/reprints-whyhpc.pdf|work=Technical Talk|access-date=10 January 2013|author=Henry G. Russel, PE|url-status=live|archive-url=https://web.archive.org/web/20130515033211/http://www.silicafume.org/pdf/reprints-whyhpc.pdf|archive-date=15 May 2013}}</ref>

===Energy efficiency===
The cement produced for making concrete accounts for about 8% of worldwide {{CO2}} emissions per year (compared to, ''e.g.'', global aviation at 1.9%).<ref name=chathamhouse>{{Cite web|url=https://reader.chathamhouse.org/making-concrete-change-innovation-low-carbon-cement-and-concrete |title=Making Concrete Change: Innovation in Low-carbon Cement and Concrete|website=Chatham House|date=13 June 2018|access-date=17 December 2018 |archive-url=https://web.archive.org/web/20181219161129/https://reader.chathamhouse.org/making-concrete-change-innovation-low-carbon-cement-and-concrete |archive-date=19 December 2018 |url-status=live}}</ref> The two [[Cement#CO2 emissions|largest sources]] of {{CO2}} are produced by the cement manufacturing process, arising from (1) the decarbonation reaction of [[limestone]] in the [[cement kiln]] (T ≈ 950&nbsp;°C), and (2) from the combustion of [[fossil fuel]] to reach the [[sintering]] temperature (T ≈ 1450&nbsp;°C) of [[cement clinker]] in the kiln. The energy required for extracting, crushing, and mixing the raw materials ([[construction aggregate]]s used in the concrete production, and also [[limestone]] and [[clay]] feeding the [[cement kiln]]) is lower. Energy requirement for transportation of [[ready-mix concrete]] is also lower because it is produced nearby the construction site from local resources, typically manufactured within 100 kilometers of the job site.<ref>{{cite web|last1=Rubenstein|first1=Madeleine|date=9 May 2012|title=Emissions from the Cement Industry|url=http://blogs.ei.columbia.edu/2012/05/09/emissions-from-the-cement-industry/|url-status=live|archive-url=https://web.archive.org/web/20161222053719/http://blogs.ei.columbia.edu/2012/05/09/emissions-from-the-cement-industry/|archive-date=22 December 2016|access-date=13 December 2016|website=State of the Planet|publisher=Earth Institute, Columbia University}}</ref> The overall [[embodied energy]] of concrete at roughly 1 to 1.5 megajoules per kilogram is therefore lower than for many structural and construction materials.<ref>{{cite web|date=22 February 2013|title=Concrete and Embodied Energy – Can using concrete be carbon neutral|url=http://strineenvironments.com.au/factsheets/concrete-and-embodied-energy-can-using-concrete-be-carbon-neutral/|url-status=live|archive-url=https://web.archive.org/web/20170116174733/http://strineenvironments.com.au/factsheets/concrete-and-embodied-energy-can-using-concrete-be-carbon-neutral/|archive-date=16 January 2017|access-date=15 January 2017}}</ref>

Once in place, concrete offers a great energy efficiency over the lifetime of a building.<ref>{{cite web|last1=Gajda|first1=John|year=2001|title=Energy Use of Single-Family Houses with Various Exterior Walls|url=https://www.healthyheating.com/Page%2055/Downloads/Wall_Systems.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://www.healthyheating.com/Page%2055/Downloads/Wall_Systems.pdf |archive-date=2022-10-09 |url-status=live}}</ref> Concrete walls leak air far less than those made of wood frames.<ref>{{cite book|title=Green Building with Concrete|year= 2015|publisher=Taylor & Francis Group|isbn=978-1-4987-0411-3}}{{page needed|date=October 2021}}</ref> 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.<ref name="Features and Usage of Foam Concrete">{{cite web|title=Features and Usage of Foam Concrete|url=http://www.chinaconcretepump.com/Foam-Concrete-Machine.html/|archive-url=https://archive.today/20121129122814/http://www.chinaconcretepump.com/Foam-Concrete-Machine.html/|archive-date=29 November 2012}}</ref> 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.

===Fire safety===
[[File:Boston City Hall - Boston, MA - DSC04704 (cropped).JPG|thumb|[[Boston City Hall]] (1968) is a [[Brutalist]] design constructed largely of precast and poured in place concrete.]]

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|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 required to resist more extreme forces.

===Earthquake safety===
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.<ref>{{cite web|url=http://www.fema.gov/library/viewRecord.do?id=4067 |title=Unreinforced Masonry Buildings and Earthquakes: Developing Successful Risk Reduction Programs FEMA P-774 |archive-url=https://web.archive.org/web/20110912163041/http://www.fema.gov/library/viewRecord.do?id=4067|archive-date=12 September 2011 }}</ref> These risks can be reduced through seismic retrofitting of at-risk buildings, (e.g. school buildings in Istanbul, Turkey).<ref>{{cite conference|url=http://www.curee.org/architecture/docs/S08-034.pdf |title=Seismic Retrofit Design Of Historic Century-Old School Buildings In Istanbul, Turkey |archive-url=https://web.archive.org/web/20120111151034/http://www.curee.org/architecture/docs/S08-034.pdf|archive-date=11 January 2012|first1=C.C. |last1=Simsir |first2=A. |last2=Jain |first3=G.C. |last3=Hart |first4=M.P. |last4=Levy |conference=14th World Conference on Earthquake Engineering|date=12–17 October 2008}}</ref>