Editing Reinforced concrete (section)

==Behavior ==

===Materials===
{{see also|Concrete|Cement|Construction aggregate|Rebar}}
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Concrete is a mixture of coarse (stone or brick chips) and fine (generally sand and/or crushed stone) aggregates with a paste of binder material (usually [[Portland cement]]) and water. When cement is mixed with a small amount of water, it [[Hydration reaction|hydrates]] to form microscopic opaque crystal lattices encapsulating and locking the aggregate into a rigid shape.<ref>{{Cite book|url=https://www.worldcat.org/oclc/19553645|title=Materials principles and practice|page=61|date=1990|publisher=Materials Dept., Open University|others=Charles Newey, Graham Weaver, Open University. Materials Department|isbn=0-408-02730-4|location=Milton Keynes, England|oclc=19553645}}</ref><ref>{{Cite book|url=https://www.worldcat.org/oclc/20693897|title=Structural materials|page=357|date=1990|publisher=Materials Dept., Open University|others=George Weidmann, P. R. Lewis, Nick Reid, Open University. Materials Department|isbn=0-408-04658-9|location=Milton Keynes, U.K.|oclc=20693897}}</ref> The aggregates used for making concrete should be free from harmful substances like organic impurities, silt, clay, lignite, etc. Typical concrete mixes have high resistance to [[compression (physical)|compressive]] [[stress (physics)|stress]]es (about {{convert|4000|psi|MPa|abbr=on}}); however, any appreciable [[tension (mechanics)|tension]] (''e.g.,'' due to [[bending]]) will break the microscopic rigid lattice, resulting in cracking and separation of the concrete. For this reason, typical non-reinforced concrete must be well supported to prevent the development of tension.

If a material with high strength in tension, such as [[steel]], is placed in concrete, then the composite material, reinforced concrete, resists not only compression but also bending and other direct tensile actions. A composite section where the concrete resists compression and reinforcement "[[rebar]]" resists tension can be made into almost any shape and size for the construction industry.

===Key characteristics===
Three physical characteristics give reinforced concrete its special properties:

# The [[coefficient of thermal expansion]] of concrete is similar to that of steel, eliminating large internal stresses due to differences in [[heat|thermal]] expansion or contraction.
# When the cement paste within the concrete hardens, this conforms to the surface details of the steel, permitting any stress to be transmitted efficiently between the different materials. Usually steel bars are roughened or corrugated to further improve the [[chemical bond|bond]] or cohesion between the concrete and steel.
# The [[pH|alkaline]] chemical environment provided by the [[alkali]] reserve (KOH, NaOH) and the [[portlandite]] ([[calcium hydroxide]]) contained in the hardened cement paste causes a [[Passivation (chemistry)|passivating]] film to form on the surface of the steel, making it much more resistant to [[corrosion]] than it would be in neutral or acidic conditions. When the cement paste is exposed to the air and meteoric water reacts with the atmospheric CO<sub>2</sub>, portlandite and the [[calcium silicate hydrate]] (CSH) of the hardened cement paste become progressively carbonated and the high pH gradually decreases from 13.5 – 12.5 to 8.5, the pH of water in equilibrium with [[calcite]] ([[calcium carbonate]]) and the steel is no longer passivated.

As a rule of thumb, only to give an idea on orders of magnitude, steel is protected at pH above ~11 but starts to corrode below ~10 depending on steel characteristics and local physico-chemical conditions when concrete becomes carbonated. [[Concrete degradation#Carbonation|Carbonation of concrete]] along with [[chloride]] ingress are amongst the chief reasons for the failure of [[reinforcement bar]]s in concrete.

The relative cross-sectional [[area]] of steel required for typical reinforced concrete is usually quite small and varies from 1% for most beams and slabs to 6% for some columns. [[rebar|Reinforcing bars]] are normally round in cross-section and vary in diameter. Reinforced concrete structures sometimes have provisions such as ventilated hollow cores to control their moisture & humidity.

Distribution of concrete (in spite of reinforcement) strength characteristics along the cross-section of vertical reinforced concrete elements is inhomogeneous.<ref>[https://www.nisur4u.co.il/wp-content/uploads/2021/01/Inhomog-Denver.pdf "Concrete Inhomogeneity of Vertical Cast-In-Situ Elements In Frame-Type Buildings"]. {{Webarchive|url=https://web.archive.org/web/20210115095541/https://www.nisur4u.co.il/wp-content/uploads/2021/01/Inhomog-Denver.pdf |date=2021-01-15 }}</ref>

===Mechanism of composite action of reinforcement and concrete===
The reinforcement in a RC structure, such as a steel bar, has to undergo the same strain or deformation as the surrounding concrete in order to prevent discontinuity, slip or separation of the two materials under load. Maintaining composite action requires transfer of load between the concrete and steel. The direct stress is transferred from the concrete to the bar interface so as to change the tensile stress in the reinforcing bar along its length. This load transfer is achieved by means of bond (anchorage) and is idealized as a continuous stress field that develops in the vicinity of the steel-concrete interface.
The reasons that the two different material components concrete and steel can work together are as follows:
(1) Reinforcement can be well bonded to the concrete, thus they can jointly resist external loads and deform.
(2) The thermal expansion coefficients of concrete and steel are so close
({{val|1.0|e=-5}} to {{val|1.5|e=-5}} for concrete and {{val|1.2|e=-5}} for steel) that the thermal stress-induced damage to the bond between the two components can be prevented.
(3) Concrete can protect the embedded steel from corrosion and high-temperature induced softening.

===Anchorage (bond) in concrete: Codes of specifications===
Because the actual bond stress varies along the length of a bar anchored in a zone of tension, current international codes of specifications use the concept of development length rather than bond stress. The main requirement for safety against bond failure is to provide a sufficient extension of the length of the bar beyond the point where the steel is required to develop its yield stress and this length must be at least equal to its development length. However, if the actual available length is inadequate for full development, special anchorages must be provided, such as cogs or hooks or mechanical end plates. The same concept applies to lap splice length <ref>{{Cite journal|title=Monotonic and Cyclic Seismic Analyses of Old-Type RC Columns with Short Lap Splices|journal=Construction Materials|date=31 March 2024|volume=4|issue=2|pages=329–341|last1=Megalooikonomou|first1=Konstantinos G.|doi=10.3390/constrmater4020018 |doi-access=free }}</ref> mentioned in the codes where splices (overlapping) provided between two adjacent bars in order to maintain the required continuity of stress in the splice zone.

===Anticorrosion measures===
In wet and cold climates, reinforced concrete for roads, bridges, parking structures and other structures that may be exposed to [[deicing]] salt may benefit from use of corrosion-resistant reinforcement such as uncoated, low carbon/chromium (micro composite), epoxy-coated, hot dip galvanized or [[stainless steel]] rebar. Good design and a well-chosen concrete mix will provide additional protection for many applications.

Uncoated, low carbon/chromium rebar looks similar to standard carbon steel rebar due to its lack of a coating; its highly corrosion-resistant features are inherent in the steel microstructure. It can be identified by the unique ASTM specified mill marking on its smooth, dark charcoal finish. Epoxy-coated rebar can easily be identified by the light green color of its epoxy coating. Hot dip galvanized rebar may be bright or dull gray depending on length of exposure, and stainless rebar exhibits a typical white metallic sheen that is readily distinguishable from carbon steel reinforcing bar. Reference ASTM standard specifications '''A1035/A1035M''' Standard Specification for Deformed and Plain Low-carbon, Chromium, Steel Bars for Concrete Reinforcement, '''A767''' Standard Specification for Hot Dip Galvanized Reinforcing Bars, '''A775''' Standard Specification for Epoxy Coated Steel Reinforcing Bars and '''A955''' Standard Specification for Deformed and Plain Stainless Bars for Concrete Reinforcement.<!-- [[American Concrete Institute|ACI]] 440 provides information about properties and design of FRP reinforced concrete structures. The Canadian [[Canadian Standards Association|CSA]] 806 and 807 providing the same information in form of a real standard. In addition the Canadian Highway Design Code is the first standard allowing for composites in bridge construction. -->

Another, cheaper way of protecting rebars is coating them with [[zinc phosphate]].<ref>{{cite journal |title=Effect of zinc phosphate chemical conversion coating on corrosion behavior of mild steel in alkaline medium: protection of rebars in reinforced concrete |first1=Florica |last1=Simescu |first2=Hassane |last2=Idrissi |publisher=National Institute for Materials Science |journal=Science and Technology of Advanced Materials |volume=9 |issue=4 |pages=045009 |date=December 19, 2008 |pmc=5099651 |doi=10.1088/1468-6996/9/4/045009 |pmid=27878037 |bibcode=2008STAdM...9d5009S }}</ref> Zinc phosphate slowly reacts with [[calcium]] cations and the [[hydroxyl]] anions present in the cement pore water and forms a stable [[hydroxyapatite]] layer.

Penetrating sealants typically must be applied some time after curing. Sealants include paint, plastic foams, films and [[aluminum foil]], felts or fabric mats sealed with tar, and layers of [[bentonite]] clay, sometimes used to seal roadbeds.

[[Corrosion inhibitor]]s, such as [[calcium nitrite]] [Ca(NO<sub>2</sub>)<sub>2</sub>], can also be added to the water mix before pouring concrete. Generally, 1–2&nbsp;wt.&nbsp;% of [Ca(NO<sub>2</sub>)<sub>2</sub>] with respect to cement weight is needed to prevent corrosion of the rebars. The nitrite anion is a mild [[oxidizer]] that oxidizes the soluble and mobile [[ferrous ion]]s (Fe<sup>2+</sup>) present at the surface of the corroding steel and causes them to precipitate as an insoluble [[ferric hydroxide]] (Fe(OH)<sub>3</sub>). This causes the passivation of steel at the [[anodic]] oxidation sites. Nitrite is a much more active corrosion inhibitor than [[nitrate]], which is a less powerful oxidizer of the divalent iron.