As a result of the
hydration reactions of cement, the pore solution of concrete tends to be alkaline, with pH
values typically in the range 12.5-13.6. Under such alkaline conditions, reinforcing steel
tends to passivate and display negligible corrosion rates. However, due to the porous
nature of concrete, corrosive species and chemical species supporting corrosion reactions
can enter the concrete and lead to corrosion problems. Furthermore, corrosive species can
enter the mix if "contaminated" mix ingredients are used (water, aggregates,
Corrosion damage to the reinforcing steel results in the build-up of voluminous
corrosion products, generating internal stresses and subsequent cracking and spalling of
the concrete as shown schematically in the diagram below:
click on image to enlarge
Link to: Gallery of rebar corrosion images
Clearly the reinforcing steel is more vulnerable to further corrosion
damage after the protective concrete cover is compromised in this manner.
Two important rebar corrosion mechanisms are: chloride induced rebar corrosion and carbonation.