Page 150 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

128 CORROSION CAUSES

damage because of deicing salts alone is between $325 and $1000 million per year
to reinforced concrete bridges and car parks in the United States of America. The
Department of Transport in the United Kingdom estimates total repair cost of ∼$1
billion following corrosion damage to motorway bridges. These bridges represent
∼10% of the total number in the United Kingdom and hence the total cost may amount
to $10 billion (1).

3.4 REINFORCEMENT CORROSION IN CONCRETE

As concrete is porous and both moisture and oxygen move through the pores
and microcracks in concrete, the basic conditions for the corrosion of mild or
high-strength ferritic reinforcing steels are present. The reason that corrosion does
not occur readily is because of the fact that the pores contain high levels of calcium,
sodium, and potassium hydroxide, which maintain a pH of between 12.5 and 13.5.
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 itself.
However, there are two species, namely, chloride and carbon dioxide that penetrate
the concrete and attack the reinforcing steel without breaking the concrete. The chlo-
ride and carbon dioxide penetrate concrete without causing significant damage and
then promote corrosion of the steel by attacking and removing the protective passive
oxide layer on the steel created and sustained by the alkalinity of the concrete pore
water.

3.5 MECHANISM OF CORROSION AND ASSESSMENT TECHNIQUES
IN CONCRETE
There are many studies covering the mechanism of corrosion in concrete and assess-
ment techniques (1–6). Specifications and recommended practices on how to select
and apply repair methods (NACE SP1290 and 0390, BSEN 12696, BSEN 1504 and
ACI 222R-01) are given in the literature.
The separation of anodes and cathodes is an important part of the understanding,
measurement, and control of corrosion of steel in concrete. Corrosion of steel in con-
crete is basically an aqueous corrosion mechanism where there is very poor transport
of corrosion product away from the anodic site. This usually leads to the formation of
voluminous corrosion product and cracking and spalling of concrete, with delamina-
tion forming along the plane of the reinforcing steel. In the absence of oxygen at the
anodic site, the ferrous ion will stay in solution or diffuse away and deposit elsewhere
in pores and microcracks in the concrete leading to severe section loss without the
advanced warning given by concrete cracking and spalling.

3.5.1 Chloride Ingress and the Corrosion Threshold
Chloride can be present in concrete for various reasons such as: