Editing Reinforced concrete (section)

===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.