By Dr. J. P. Broomfield
Reinforced concrete is one of the most durable, versatile and widely used construction materials. However, occasionally it does not give the low maintenance life expected of it. Sometimes this is due to incorrect expectations, sometimes to inadequate specification or construction and sometimes due to more adverse conditions than initially expected. Consequently, there are many structures in the built environment suffering from corrosion induced damage.
The one estimate from the USA is that the cost of damage due to de icing salts alone is between $325 and $1000 million per year to reinforced concrete bridges and car parks. In the UK the Department of Transport estimates a total repair cost of £616.5 million (approximately one billion US dollars) due to corrosion damage to motorway bridges. These bridges represent about 10% of the total bridge inventory in the UK. The total problem may therefore be ten times the DoT estimate. There are similar statistics for Australia, Europe and particularly the Middle East where the warm marine climate with saline ground conditions increase all corrosion problems. Corrosion control is made more difficult by the problems of curing concrete in hot, drying environments and has led to very short lifetimes for reinforced concrete structures. Deterioration occurs on buildings and other structures as well as bridges.
As concrete is porous and both moisture and oxygen can move through the pores and microcracks in concrete, the basic requirements for corrosion of mild or high strength ferritic reinforcing steels are present. The reason that corrosion does not occur in most cases is that the pores contain high levels of calcium, sodium and potassium hydroxide, which maintain a pH of between 12 and 13. This high level of alkalinity passivates the steel, forming a dense gamma ferric oxide that is self maintaining and prevents rapid corrosion.
In many cases any attack on reinforced concrete will be on the concrete. However, there are two chemicals that penetrate the concrete and attack the steel without breaking down the concrete first. The culprits are chlorides and carbon dioxide as these are the main common atmospherically borne species that penetrate concrete without causing significant damage and then promote the corrosion of steel by removing the protective passive oxide layer on the steel, created and sustained by the alkalinity of the concrete pore water.
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