Page 41 - Reliability and Maintainability of In service Pipelines

30 Reliability and Maintainability of In-Service Pipelines

and sewers. Improper placement of reinforcement and concrete, including insuffi-
cient concrete coverage of steel reinforcement can heighten this effect and create
a perfect ground for deterioration processes to take place. This can result in corro-
sion and expansion of the reinforcing steel, which can lead to cracks, and spalling
in the concrete, ultimately causing a reduction in structural capacity.
To increase the tensile strength of concrete, steel reinforcements are used,
however when tensile strength is exceeded by tensile forces acting on the con-
crete, it can result in cracking. The reinforcements also function to control the
width of the crack, and prevent failure, however for this to occur, design capacity
must be met.

Mechanism of Reinforcement Corrosion
The process of corrosion in reinforcing steel is set by two distinct reactions, the
first being oxidation and the second being reduction. The product formed, as
shown in the corrosion mechanism below, further reacts and produces rust on the
surface of the steel bar.
The corrosion mechanism of steel reinforcement is illustrated in following
reactions:
2
Fe-Fe 21 1 2e ðanodicÞ
2 2
2H 2 O 1 O 2 1 4e -4OH ðCathodic-reduction in oxygenÞ
2
2Fe 21 1 4OH -2FeðOHÞ ðchemicalÞðessentially a by-product for further reaction to form rustÞ
2
k
ð
4Fe OH 2 Þ 1 O 2 -2H 2 O 1 2Fe 2 O 3 :H 2 Oðformation of rustÞ
The corrosion mechanism illustrated above, applies to the different types of
corrosion that occur in the reinforcing steels of concrete pipes, including uniform,
crevice, and galvanic corrosions.

Carbonation
This process is driven by diffusion, where carbon dioxide from the atmosphere
diffuses through the porous concrete, reacting with water and calcium carbonate
found in Portlandite, which is an oxide mineral occurring in the form calcium
hydroxide and used in the making of cement. This process, illustrated by the reac-
tion below, acts to reduce the alkanity of the concrete to a pH of 8 9, and
thereby alters the stability of the oxide film.
CaðOHÞ 1 CO 2 CaCO 3 1 H 2 O
2
The disruption of this film causes the reinforcing steel to come in direct expo-
sure to oxygen and moisture, in which corrosion will start to take place. During
this stage, pitting of the reinforcement surface is common and is followed by final