Editing Reinforced concrete (section)

===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>