Editing Reinforced concrete (section)

== Common failure modes of steel reinforced concrete ==
[[File:Concrete spall (interior of unit).jpg|thumb|Concrete spalling from the ceiling of an office unit (''interior'') in [[Singapore]], possibly due to rebar corrosion. |alt=|upright]]
Reinforced concrete can fail due to inadequate strength, leading to mechanical failure, or due to a reduction in its durability. Corrosion and freeze/thaw cycles may damage poorly designed or constructed reinforced concrete. When rebar corrodes, the oxidation products ([[rust]]) expand and tends to flake, cracking the concrete and unbonding the rebar from the concrete. Typical mechanisms leading to durability problems are discussed below.

=== Mechanical failure ===
Cracking of the concrete section is nearly impossible to prevent; however, the size and location of cracks can be limited and controlled by appropriate reinforcement, control joints, curing methodology and concrete mix design. Cracking can allow moisture to penetrate and corrode the reinforcement. This is a [[limit state design#Serviceability Limit State|serviceability]] failure in [[limit state design]]. Cracking is normally the result of an inadequate quantity of rebar, or rebar spaced at too great a distance. The concrete cracks either under excess loading, or due to internal effects such as early thermal shrinkage while it cures.

Ultimate failure leading to collapse can be caused by crushing the concrete, which occurs when compressive stresses exceed its strength, by [[yield (engineering)|yielding]] or failure of the rebar when bending or shear stresses exceed the strength of the reinforcement, or by bond failure between the concrete and the rebar.<ref>{{Cite journal |year=2016 |title=Remote sensing and photogrammetry techniques in diagnostics of concrete structures |url=https://www.researchgate.net/publication/306359709 |journal=Computers and Concrete |volume=18 |issue=3 |pages=405–420 |doi=10.12989/cac.2016.18.3.405 |access-date=2016-12-14 |author1=Janowski, A. |author2=Nagrodzka-Godycka, K. |author3=Szulwic, J. |author4=Ziółkowski, P. |archive-date=2021-07-09 |archive-url=https://web.archive.org/web/20210709160748/https://www.researchgate.net/publication/306359709_Remote_sensing_and_photogrammetry_techniques_in_diagnostics_of_concrete_structures |url-status=live }}</ref>

=== Carbonation ===
[[File:Concrete wall cracking as steel reinforcing corrodes and swells 9058.jpg|left|thumb|Concrete wall cracking as steel reinforcing corrodes and swells. Rust has a lower density than metal, so it expands as it forms, cracking the decorative cladding off the wall as well as damaging the structural concrete. The breakage of material from a surface is called ''spalling''.]]
[[File:Concrete wall cracking as its steel reinforcing cracks and swells 9061v.jpg|left|thumb|Detailed view of spalling probably caused by a too thin layer of concrete between the steel and the surface, accompanied by corrosion from external exposure.]]

{{main|Carbonation}}
Carbonation, or neutralisation, is a chemical reaction between [[carbon dioxide]] in the air and [[calcium hydroxide]] and hydrated [[calcium silicate]] in the concrete.

When a concrete structure is designed, it is usual to specify the [[concrete cover]] for the rebar (the depth of the rebar within the object). The minimum concrete cover is normally regulated by design or [[building code]]s. If the reinforcement is too close to the surface, early failure due to corrosion may occur. The concrete cover depth can be measured with a [[cover meter]]. However, carbonated concrete incurs a durability problem only when there is also sufficient moisture and oxygen to cause electropotential corrosion of the reinforcing steel.

One method of testing a structure for carbonation is to [[drill]] a fresh hole in the surface and then treat the cut surface with [[phenolphthalein]] indicator solution. This solution turns [[pink]] when in contact with alkaline concrete, making it possible to see the depth of carbonation. Using an existing hole does not suffice because the exposed surface will already be carbonated.

===Chlorides===
[[Chloride]]s can promote the corrosion of embedded [[rebar]] if present in sufficiently high concentration. Chloride anions induce both localized corrosion ([[pitting corrosion]]) and generalized corrosion of steel reinforcements. For this reason, one should only use fresh raw water or potable water for mixing concrete, ensure that the coarse and fine aggregates do not contain chlorides, rather than admixtures which might contain chlorides.
[[File:RebarCloseup.jpg|thumb|right|Rebar for foundations and walls of a sewage pump station.]]
[[File:Paulins Kill Viaduct in Hainesburg, NJ.jpg|thumb|right|The [[Paulins Kill Viaduct]], Hainesburg, New Jersey, is 115 feet (35 m) tall and 1,100 feet (335 m) long, and was heralded as the largest reinforced concrete structure in the world when it was completed in 1910 as part of the [[Lackawanna Cut-Off]] rail line project. The [[Lackawanna Railroad]] was a pioneer in the use of reinforced concrete.]]

It was once common for [[calcium chloride]] to be used as an admixture to promote rapid set-up of the concrete. It was also mistakenly believed that it would prevent freezing. However, this practice fell into disfavor once the deleterious effects of chlorides became known. It should be avoided whenever possible.

The use of de-icing salts on roadways, used to lower the [[freezing point]] of water, is probably one of the primary causes of premature failure of reinforced or prestressed concrete bridge decks, roadways, and parking garages. The use of [[epoxy|epoxy-coated]] reinforcing bars and the application of [[cathodic protection]] has mitigated this problem to some extent. Also FRP (fiber-reinforced polymer) rebars are known to be less susceptible to chlorides. Properly designed concrete mixtures that have been allowed to cure properly are effectively impervious to the effects of de-icers.

Another important source of chloride ions is [[sea water]]. Sea water contains by weight approximately 3.5% salts. These salts include [[sodium chloride]], [[magnesium sulfate]], [[calcium sulfate]], and [[bicarbonate]]s. In water these salts dissociate in free ions (Na<sup>+</sup>, Mg<sup>2+</sup>, Cl<sup>−</sup>, {{chem|SO|4|2−}}, {{chem|HCO|3|−}}) and migrate with the water into the [[capillary|capillaries]] of the concrete. Chloride ions, which make up about 50% of these ions, are particularly aggressive as a cause of corrosion of carbon steel reinforcement bars.

In the 1960s and 1970s it was also relatively common for [[magnesite]], a chloride rich [[carbonate mineral]], to be used as a floor-topping material. This was done principally as a levelling and sound attenuating layer. However it is now known that when these materials come into contact with moisture they produce a weak solution of [[hydrochloric acid]] due to the presence of chlorides in the magnesite. Over a period of time (typically decades), the solution causes [[corrosion]] of the embedded [[rebar]]s. This was most commonly found in wet areas or areas repeatedly exposed to moisture.

===Alkali silica reaction===
{{Main|Alkali–silica reaction}}

This a reaction of [[amorphous]] [[silica]] ([[chalcedony]], [[chert]], [[siliceous]] [[limestone]]) sometimes present in the [[Construction aggregate|aggregate]]s with the [[hydroxyl]] ions (OH<sup>−</sup>) from the cement pore solution. Poorly crystallized silica (SiO<sub>2</sub>) dissolves and dissociates at high pH (12.5 - 13.5) in alkaline water. The soluble dissociated [[silicic acid]] reacts in the porewater with the [[calcium hydroxide]] ([[portlandite]]) present in the [[cement]] paste to form an expansive [[calcium silicate hydrate]] (CSH). The [[alkali–silica reaction]] (ASR) causes localised swelling responsible for [[tensile stress]] and [[Fracture|cracking]]. The conditions required for alkali silica reaction are threefold:
(1) aggregate containing an alkali-reactive constituent (amorphous silica), (2) sufficient availability of hydroxyl ions (OH<sup>−</sup>), and (3) sufficient moisture, above 75% [[relative humidity]] (RH) within the concrete.<ref>{{cite web |url=https://www.bbc.co.uk/dna/h2g2/A4014172 |title=Concrete Cancer |website=h2g2 |publisher=BBC |date=March 15, 2012 |orig-year=2005 |access-date=2009-10-14 |archive-date=2009-02-23 |archive-url=https://web.archive.org/web/20090223173400/http://www.bbc.co.uk/dna/h2g2/A4014172 |url-status=live }}</ref><ref>{{cite web |url=http://www.cementindustry.co.uk/main.asp?page=272 |title=Special Section: South West Alkali Incident |website=the cement industry |publisher=British Cement Association |date=4 January 2006 |access-date=2006-11-26 |url-status=dead |archive-url=https://web.archive.org/web/20061029112122/http://www.cementindustry.co.uk/main.asp?page=272 |archive-date=October 29, 2006 }}</ref> This phenomenon is sometimes popularly referred to as "[[Alkali–silica reaction|concrete cancer]]". This reaction occurs independently of the presence of rebars; massive concrete structures such as [[dam]]s can be affected.

===Conversion of high alumina cement===
Resistant to weak acids and especially sulfates, this cement cures quickly and has very high durability and strength. It was frequently used after [[World War II]] to make precast concrete objects. However, it can lose strength with heat or time (conversion), especially when not properly cured. After the collapse of three roofs made of prestressed concrete beams using high alumina cement, this cement was [[ban (law)|banned]] in the [[United Kingdom|UK]] in 1976. Subsequent inquiries into the matter showed that the beams were improperly manufactured, but the ban remained.<ref>{{cite web|url=http://www.quest-tech.co.uk/hac.htm |title=High Alumina Cement |access-date=2009-10-14 |archive-url = https://web.archive.org/web/20050911210803/http://www.quest-tech.co.uk/hac.htm |archive-date = 2005-09-11}}</ref>

===Sulfates===
[[Sulfate]]s (SO<sub>4</sub>) in the soil or in groundwater, in sufficient concentration, can react with the Portland cement in concrete causing the formation of expansive products, e.g., [[ettringite]] or [[thaumasite]], which can lead to early failure of the structure. The most typical attack of this type is on concrete slabs and foundation walls at grades where the sulfate ion, via alternate wetting and drying, can increase in concentration. As the concentration increases, the attack on the Portland cement can begin. For buried structures such as pipe, this type of attack is much rarer, especially in the eastern United States. The sulfate ion concentration increases much slower in the soil mass and is especially dependent upon the initial amount of sulfates in the native soil. A chemical analysis of soil borings to check for the presence of sulfates should be undertaken during the design phase of any project involving concrete in contact with the native soil. If the concentrations are found to be aggressive, various protective coatings can be applied. Also, in the US ASTM C150 Type 5 Portland cement can be used in the mix. This type of cement is designed to be particularly resistant to a sulfate attack.