Editing Concrete (section)

==Composition==

Concrete is an artificial [[composite material]], comprising a matrix of cementitious binder (typically [[Portland cement]] paste or [[Bitumen|asphalt]]) and a dispersed phase or "filler" of [[#Aggregates|aggregate]] (typically a rocky material, loose stones, and sand). The binder "glues" the filler together to form a synthetic [[Conglomerate (geology)|conglomerate]].<ref name="matse">{{Cite web|title=Concrete: Scientific Principles |url=http://matse1.matse.illinois.edu/concrete/prin.html|access-date=2021-10-06|website=matse1.matse.illinois.edu}}</ref> Many [[types of concrete]] are available, determined by the formulations of binders and the types of aggregate used to suit the application of the engineered material. These variables determine strength and density, as well as chemical and thermal resistance of the finished product.

[[File:DB Museum rail and concrete sleeper cross section 2.jpg|thumb|[[Cross section (geometry)|Cross section]] of a concrete [[Railroad tie|railway sleeper]] below a rail]]

[[Construction aggregate]]s consist of large chunks of material in a concrete mix, generally a coarse [[gravel]] or crushed rocks such as [[limestone]], or [[granite]], along with finer materials such as [[sand]].

Cement paste, most commonly made of [[Portland cement]], is the most prevalent kind of concrete binder. For cementitious binders, [[water]] is mixed with the dry cement powder and aggregate, which produces a semi-liquid slurry (paste) that can be shaped, typically by pouring it into a form. The concrete solidifies and hardens through a [[Chemical reaction|chemical process]] called [[mineral hydration|hydration]]. The water reacts with the cement, which bonds the other components together, creating a robust, stone-like material. Other cementitious materials, such as [[fly ash]] and [[slag cement]], are sometimes added—either pre-blended with the cement or directly as a concrete component—and become a part of the binder for the aggregate.<ref name=flyash>{{cite journal |last1=Askarian |first1=Mahya |last2=Fakhretaha Aval |first2=Siavash |last3=Joshaghani |first3=Alireza |title=A comprehensive experimental study on the performance of pumice powder in self-compacting concrete (SCC) |journal=Journal of Sustainable Cement-Based Materials |date=22 January 2019 |volume=7 |issue=6 |pages=340–356 |doi=10.1080/21650373.2018.1511486 |s2cid=139554392 }}</ref> Fly ash and slag can enhance some properties of concrete such as fresh properties and durability.<ref name=flyash/> Alternatively, other materials can also be used as a concrete binder: the most prevalent substitute is [[Bitumen|asphalt]], which is used as the binder in [[asphalt concrete]].

Admixtures are added to modify the cure rate or properties of the material. [[#Mineral admixtures and blended cements|Mineral admixtures]] use recycled materials as concrete ingredients. Conspicuous materials include [[fly ash]], a by-product of [[Fossil fuel power plant|coal-fired power plants]]; [[ground granulated blast furnace slag]], a by-product of [[steelmaking]]; and [[silica fume]], a by-product of industrial [[electric arc furnace]]s.

Structures employing Portland cement concrete usually include [[#Reinforcement|steel reinforcement]] because this type of concrete can be formulated with high [[compressive strength]], but always has lower [[tensile strength]]. Therefore, it is usually reinforced with materials that are strong in tension, typically [[steel]] [[rebar]].

The ''[[types of concrete#Mix design|mix design]]'' depends on the type of structure being built, how the concrete is mixed and delivered, and how it is placed to form the structure.

===Cement===
{{main|Cement}}
[[File:Stockage de ciments.JPG|thumb|Several tons of bagged cement, about two minutes of output from a 10,000 ton per day [[cement kiln]]]]

Portland cement is the most common type of cement in general usage. It is a basic ingredient of concrete, [[mortar (masonry)|mortar]], and many [[plaster]]s.<ref>{{Cite web|last1=Melander|first1=John M.|last2=Farny|first2=James A.|last3=Isberner|first3=Albert W. Jr. |date=2003|title=Portland Cement Plaster/Stucco Manual|url=https://www.cement.org/docs/default-source/stucco/eb049.pdf?sfvrsn=540de3bf_2|url-status=live|access-date=2021-07-13|website=Portland Cement Association|archive-url=https://web.archive.org/web/20210412174321/https://www.cement.org/docs/default-source/stucco/eb049.pdf?sfvrsn=540de3bf_2 |archive-date=12 April 2021 }}</ref>  It consists of a mixture of calcium silicates ([[alite]], [[belite]]), [[tricalcium aluminate|aluminates]] and [[calcium aluminoferrite|ferrites]]—compounds, which will react with water. Portland cement and similar materials are made by heating [[limestone]] (a source of calcium) with clay or shale (a source of silicon, aluminium and iron) and grinding this product (called ''[[clinker (cement)|clinker]]'') with a source of [[sulfate]] (most commonly [[gypsum]]).

[[Cement kiln]]s are extremely large, complex, and inherently dusty industrial installations. Of the various ingredients used to produce a given quantity of concrete, the cement is the most energetically expensive. Even complex and efficient kilns require 3.3 to 3.6 gigajoules of energy to produce a ton of clinker and then [[Cement mill|grind it into cement]]. Many kilns can be fueled with difficult-to-dispose-of wastes, the most common being used tires. The extremely high temperatures and long periods of time at those temperatures allows cement kilns to efficiently and completely burn even difficult-to-use fuels.<ref>{{cite web|title=Cement Production|url=http://www.iea-etsap.org/web/E-TechDS/PDF/I03_cement_June%202010_GS-gct.pdf|publisher=IEA ETSAP – Energy Technology Systems Analysis Programme|access-date=9 January 2013|author=Evelien Cochez|author2=Wouter Nijs|name-list-style=amp|author3=Giorgio Simbolotti|author4=Giancarlo Tosato|location= |archive-url=https://web.archive.org/web/20130124004654/http://www.iea-etsap.org/web/E-TechDS/PDF/I03_cement_June%202010_GS-gct.pdf|archive-date=24 January 2013}}</ref> The five major compounds of calcium silicates and aluminates comprising Portland cement range from 5 to 50% in weight.

===Curing===
Combining [[water]] with a cementitious material forms a cement paste by the process of hydration. The cement paste glues the aggregate together, fills voids within it, and makes it flow more freely.<ref>{{cite web|last=Gibbons|first=Jack|title=Measuring Water in Concrete|date=7 January 2008 |url=http://www.concreteconstruction.net/concrete-construction/measuring-water-in-concrete.aspx|publisher=Concrete Construction|access-date=1 October 2012|url-status=live|archive-url=https://web.archive.org/web/20130511192721/http://www.concreteconstruction.net/concrete-construction/measuring-water-in-concrete.aspx|archive-date=11 May 2013}}</ref>

As stated by [[Abrams' law]], a lower water-to-cement ratio yields a stronger, more [[Reinforced concrete structures durability|durable]] concrete, whereas more water gives a freer-flowing concrete with a higher [[Workability|slump]].<ref>{{cite web|title=Chapter 9: Designing and Proportioning Normal Concrete Mixtures |url=http://www.ce.memphis.edu/1112/notes/project_2/PCA_manual/Chap09.pdf|work=PCA manual|publisher=Portland Concrete Association|access-date=1 October 2012|url-status=live|archive-url=https://web.archive.org/web/20120526015347/http://www.ce.memphis.edu/1112/notes/project_2/PCA_manual/Chap09.pdf|archive-date=26 May 2012}}</ref> The hydration of cement involves many concurrent reactions. The process involves [[polymerization]], the interlinking of the silicates and aluminate components as well as their bonding to sand and gravel particles to form a solid mass.<ref name="Hydration">{{cite web|title=Cement hydration|url=http://www.understanding-cement.com/hydration.html|publisher=Understanding Cement|access-date=1 October 2012|url-status=live|archive-url=https://web.archive.org/web/20121017144613/http://www.understanding-cement.com/hydration.html|archive-date=17 October 2012}}</ref> One illustrative conversion is the hydration of tricalcium silicate:

: [[Cement chemist notation]]: {{space|2}} C<sub>3</sub>S + H → C-S-H + CH + heat
: Standard notation: {{space|13}} Ca<sub>3</sub>SiO<sub>5</sub> + H<sub>2</sub>O → CaO・SiO<sub>2</sub>・H<sub>2</sub>O (gel) + Ca(OH)<sub>2</sub> + heat
: Balanced: {{space|27}} 2 Ca<sub>3</sub>SiO<sub>5</sub> + 7 H<sub>2</sub>O → 3 CaO・2 SiO<sub>2</sub>・4 H<sub>2</sub>O (gel) + 3 Ca(OH)<sub>2</sub> + heat
: {{space|44}} (approximately as the exact ratios of CaO, SiO<sub>2</sub> and H<sub>2</sub>O in C-S-H can vary)<ref name="Hydration" />

The hydration (curing) of cement is irreversible.<ref>{{cite book |doi=10.1016/B978-0-08-100773-0.00005-8 |chapter=Hydration, Setting and Hardening of Portland Cement |title=Lea's Chemistry of Cement and Concrete |date=2019 |last1=Beaudoin |first1=James |last2=Odler |first2=Ivan |pages=157–250 |isbn=978-0-08-100773-0 }}</ref>

===Aggregates===
{{main|Construction aggregate}}
[[File:Gravel 03375C.JPG|thumbnail|Crushed stone [[Construction aggregate|aggregates]]]]

Fine and coarse aggregates make up the bulk of a concrete mixture. [[Sand]], natural gravel, and [[crushed stone]] are used mainly for this purpose. Recycled aggregates (from construction, demolition, and excavation waste) are increasingly used as partial replacements for natural aggregates, while a number of manufactured aggregates, including air-cooled [[blast furnace]] slag and [[bottom ash]] are also permitted.

The size distribution of the aggregate determines how much binder is required. Aggregate with a very even size distribution has the biggest gaps whereas adding aggregate with smaller particles tends to fill these gaps. The binder must fill the gaps between the aggregate as well as paste the surfaces of the aggregate together, and is typically the most expensive component. Thus, variation in sizes of the aggregate reduces the cost of concrete.<ref>{{Cite web |title=The Effect of Aggregate Properties on Concrete |url=https://www.engr.psu.edu/ce/courses/ce584/concrete/library/materials/Aggregate/Aggregatesmain.htm |access-date=2022-08-13 |website=www.engr.psu.edu|archive-url=https://web.archive.org/web/20121225184337/http://www.engr.psu.edu/ce/courses/ce584/concrete/library/materials/Aggregate/Aggregatesmain.htm |date=25 December 2012 |publisher=Engr.psu.edu|archive-date=25 December 2012 }}</ref> The aggregate is nearly always stronger than the binder, so its use does not negatively affect the strength of the concrete.

Redistribution of aggregates after compaction often creates non-homogeneity due to the influence of vibration. This can lead to strength gradients.<ref name="Veretennykov Yugov Dolmatov et al 2008">{{cite book |doi=10.1061/41002(328)17 |chapter=Concrete Inhomogeneity of Vertical Cast-in-Place Elements in Skeleton-Type Buildings |title=AEI 2008 |year=2008 |last1=Veretennykov |first1=Vitaliy I. |last2=Yugov |first2=Anatoliy M. |last3=Dolmatov |first3=Andriy O. |last4=Bulavytskyi |first4=Maksym S. |last5=Kukharev |first5=Dmytro I. |last6=Bulavytskyi |first6=Artem S. |pages=1–10 |isbn=978-0-7844-1002-8 }}</ref>

Decorative stones such as [[quartzite]], small river stones or crushed glass are sometimes added to the surface of concrete for a decorative "exposed aggregate" finish, popular among landscape designers.

===Admixtures===
Admixtures are materials in the form of powder or fluids that are added to the concrete to give it certain characteristics not obtainable with plain concrete mixes. Admixtures are defined as additions "made as the concrete mix is being prepared".<ref name=text>{{cite book|isbn=978-0-7277-3611-6 |doi=10.1680/pc.36116.185 |chapter=Admixtures and Special Cements |title=Portland Cement: Third edition |author1=Gerry Bye |author2=Paul Livesey |author3=Leslie Struble |year=2011|doi-broken-date=1 November 2024 }}</ref> The most common admixtures are retarders and accelerators. In normal use, admixture dosages are less than 5% by mass of cement and are added to the concrete at the time of batching/mixing.<ref name="FHWA Admixtures">{{cite web |author=U.S. Federal Highway Administration |author-link=Federal Highway Administration |title=Admixtures |url=http://www.fhwa.dot.gov/infrastructure/materialsgrp/admixture.html |access-date=25 January 2007 |date=14 June 1999 |archive-url=https://web.archive.org/web/20070127132641/http://www.fhwa.dot.gov/infrastructure/materialsgrp/admixture.html |archive-date=27 January 2007 }}</ref> (See {{section link||Production}} below.) The common types of admixtures<ref>{{cite web |author=Cement Admixture Association |url=http://www.admixtures.org.uk/types.asp |title=Admixture Types |access-date=25 December 2010 |archive-url=https://web.archive.org/web/20110903081932/http://www.admixtures.org.uk/types.asp |archive-date=3 September 2011 }}</ref> are as follows:

* [[Accelerant|Accelerators]] speed up the hydration (hardening) of the concrete. Typical materials used are [[calcium chloride]], [[calcium nitrate]] and [[sodium nitrate]]. However, use of chlorides may cause corrosion in steel reinforcing and is prohibited in some countries, so that nitrates may be favored, even though they are less effective than the chloride salt. Accelerating admixtures are especially useful for modifying the properties of concrete in cold weather.
* [[Air entrainment|Air entraining agents]] add and entrain tiny air bubbles in the concrete, which reduces damage during [[Weathering|freeze-thaw]] cycles, increasing [[Reinforced concrete structures durability|durability]]. However, entrained air entails a tradeoff with strength, as each 1% of air may decrease compressive strength by 5%.<ref>{{cite web |last1=Hamakareem |first1=Madeh Izat |title=Effect of Air Entrainment on Concrete Strength |url=https://theconstructor.org/concrete/effect-air-entrainment-concrete-strength/8427/ |website=The Constructor |date=14 November 2013 |access-date=13 November 2020}}</ref> If too much air becomes trapped in the concrete as a result of the mixing process, [[defoamer]]s can be used to encourage the air bubble to agglomerate, rise to the surface of the wet concrete and then disperse.
* Bonding agents are used to create a bond between old and new concrete (typically a type of polymer) with wide temperature tolerance and corrosion resistance.
* [[Corrosion inhibitor]]s are used to minimize the corrosion of steel and steel bars in concrete.
* Crystalline admixtures are typically added during batching of the concrete to lower permeability. The reaction takes place when exposed to water and un-hydrated cement particles to form insoluble needle-shaped crystals, which fill capillary pores and micro-cracks in the concrete to block pathways for water and waterborne contaminates. Concrete with crystalline admixture can expect to self-seal as constant exposure to water will continuously initiate crystallization to ensure permanent waterproof protection.
* [[Pigment]]s can be used to change the color of concrete, for aesthetics.
* [[Plasticizer]]s increase the workability of plastic, or "fresh", concrete, allowing it to be placed more easily, with less consolidating effort. A typical plasticizer is lignosulfonate. Plasticizers can be used to reduce the water content of a concrete while maintaining workability and are sometimes called water-reducers due to this use. Such treatment improves its strength and durability characteristics.
* [[Superplasticizer]]s (also called high-range water-reducers) are a class of plasticizers that have fewer deleterious effects and can be used to increase workability more than is practical with traditional plasticizers. Superplasticizers are used to increase compressive strength. It increases the [[#Sample analysis—workability|workability]] of the concrete and lowers the need for water content by 15–30%. 
* Pumping aids improve pumpability, thicken the paste and reduce separation and bleeding.
* [[Retarder (chemistry)|Retarders]] slow the hydration of concrete and are used in large or difficult pours where partial setting is undesirable before completion of the pour. Typical retarders include [[sugar]], [[sodium gluconate]], [[citric acid]], and [[tartaric acid]].<ref>{{Citation |last=Bensted |first=John |title=14 - Special Cements |date=1998-01-01 |work=Lea's Chemistry of Cement and Concrete (Fourth Edition) |pages=783–840 |editor-last=Hewlett |editor-first=Peter C. |url=https://linkinghub.elsevier.com/retrieve/pii/B9780750662567500266 |access-date=2024-11-03 |place=Oxford |publisher=Butterworth-Heinemann |doi=10.1016/b978-075066256-7/50026-6 |isbn=978-0-7506-6256-7}}</ref>

===Mineral admixtures and blended cements===
{{Components of Cement, Comparison of Chemical and Physical Characteristics}}

Inorganic materials that have [[pozzolan]]ic or latent hydraulic properties, these very [[Granularity|fine-grained]] materials are added to the concrete mix to improve the properties of concrete (mineral admixtures),<ref name = "FHWA Admixtures"/> or as a replacement for [[Portland cement]] (blended cements).<ref>{{cite book | author=Kosmatka, S.H. |author2=Panarese, W.C. | title = Design and Control of Concrete Mixtures | publisher=[[Portland Cement Association]] | date = 1988 | location = Skokie, IL | pages = 17, 42, 70, 184 | isbn = 978-0-89312-087-0 }}</ref> Products which incorporate [[limestone]], [[fly ash]], [[Ground granulated blast-furnace slag|blast furnace slag]], and other useful materials with [[Pozzolanic activity|pozzolanic properties]] into the mix, are being tested and used. These developments are ever growing in relevance to minimize the impacts caused by cement use, notorious for being one of the largest producers (at about 5 to 10%) of global [[greenhouse gas emissions]].<ref name=mit>{{Cite web |title=Paving the way to greenhouse gas reductions |url=https://news.mit.edu/2011/concrete-pavements-0829 |access-date=2022-08-13 |website=MIT News {{!}} Massachusetts Institute of Technology |language=en|archive-url=https://web.archive.org/web/20121031015018/http://web.mit.edu/newsoffice/2011/concrete-pavements-0829.html |archive-date=31 October 2012 |date=28 August 2011}}</ref> The use of alternative materials also is capable of lowering costs, improving concrete properties, and recycling wastes, the latest being relevant for [[circular economy]] aspects of the [[Construction Industry|construction industry]], whose demand is ever growing with greater impacts on raw material extraction, waste generation and [[landfill]] practices.
* [[Fly ash]]: A by-product of coal-fired [[power station|electric generating plants]], it is used to partially replace Portland cement (by up to 60% by mass). The properties of fly ash depend on the type of coal burnt. In general, siliceous fly ash is [[Pozzolanic activity|pozzolanic]], while [[calcareous]] fly ash has latent hydraulic properties.<ref>{{cite web |author=U.S. Federal Highway Administration |author-link=Federal Highway Administration |title=Fly Ash |url=http://www.fhwa.dot.gov/infrastructure/materialsgrp/flyash.htm |access-date=24 January 2007 |date=14 June 1999 |archive-url=https://web.archive.org/web/20070621161733/http://www.fhwa.dot.gov/infrastructure/materialsgrp/flyash.htm |archive-date=21 June 2007 }}</ref>
* [[Ground granulated blast furnace slag]] (GGBFS or GGBS): A by-product of [[Steelmaking|steel production]] is used to partially replace [[Portland cement]] (by up to 80% by mass). It has latent hydraulic properties.<ref>{{cite web | author = U.S. Federal Highway Administration | author-link = Federal Highway Administration | title = Ground Granulated Blast-Furnace Slag | url = http://www.fhwa.dot.gov/infrastructure/materialsgrp/ggbfs.htm | access-date = 24 January 2007 | archive-url = https://web.archive.org/web/20070122083859/http://www.fhwa.dot.gov/infrastructure/materialsgrp/ggbfs.htm | archive-date = 22 January 2007 | df = dmy-all }}</ref>
* [[Silica fume]]: A by-product of the production of [[silicon]] and [[ferrosilicon]] [[alloy]]s. Silica fume is similar to fly ash, but has a particle size 100 times smaller. This results in a higher surface-to-volume ratio and a much faster [[Pozzolanic activity|pozzolanic reaction]]. Silica fume is used to increase strength and [[Reinforced concrete structures durability|durability]] of concrete, but generally requires the use of superplasticizers for workability.<ref>{{cite web | author = U.S. Federal Highway Administration | author-link = Federal Highway Administration | title = Silica Fume | url = http://www.fhwa.dot.gov/infrastructure/materialsgrp/silica.htm | access-date = 24 January 2007 | archive-url = https://web.archive.org/web/20070122022403/http://www.fhwa.dot.gov/infrastructure/materialsgrp/silica.htm | archive-date = 22 January 2007 | df = dmy-all }}</ref>
* High reactivity [[metakaolin]] (HRM): Metakaolin produces concrete with [[Strength of materials|strength]] and [[durability]] similar to concrete made with silica fume. While silica fume is usually dark gray or black in color, high-reactivity metakaolin is usually bright white in color, making it the preferred choice for architectural concrete where appearance is important.
* [[Carbon nanofiber]]s can be added to concrete to enhance compressive strength and gain a higher [[Young's modulus]], and also to improve the electrical properties required for strain monitoring, damage evaluation and self-health monitoring of concrete. Carbon fiber has many advantages in terms of mechanical and electrical properties (e.g., higher strength) and self-monitoring behavior due to the high [[Ultimate tensile strength|tensile strength]] and high [[electrical conductivity]].<ref>{{cite journal |last1=Mullapudi |first1=Taraka Ravi Shankar |last2=Gao |first2=Di |last3=Ayoub |first3=Ashraf |title=Non-destructive evaluation of carbon nanofibre concrete |journal=Magazine of Concrete Research |date=September 2013 |volume=65 |issue=18 |pages=1081–1091 |doi=10.1680/macr.12.00187 }}</ref>
* Carbon products have been added to make concrete electrically conductive, for deicing purposes.<ref>{{cite journal |last1=Tuan |first1=Christopher |last2=Yehia |first2=Sherif |title=Evaluation of Electrically Conductive Concrete Containing Carbon Products for Deicing |journal=ACI Materials Journal |date=1 July 2004 |volume=101 |issue=4 |pages=287–293 |url=https://digitalcommons.unomaha.edu/civilengfacpub/26/ }}</ref>
* New research from Japan's [[University of Kitakyushu]] shows that a washed and dried recycled mix of used diapers can be an environmental solution to producing less landfill and using less sand in concrete production. A model home was built in Indonesia to test the strength and durability of the new diaper-cement composite.<ref>{{Cite news |last=Kloosterman |first=Karin |date=23 May 2023 |title=Tiny house built from diapers and concrete |url=https://www.greenprophet.com/2023/05/diaper-concrete-house/ |access-date=6 October 2024 |work=Green Prophet}}</ref>