Page 40 - Reliability and Maintainability of In service Pipelines

Introduction 29

environmental exposure. In this section different types of pipes and their predomi-
nant deterioration mechanisms are explained.

1.6.1 DETERIORATION OF CONCRETE PIPES

Structural deterioration of concrete pipelines is due to corrosion affecting the con-
crete itself by altering the structure of the pipe, including the formation of cracks
as well as affecting the reinforcement inside. The corrosion of concrete is caused
by the action of sulfuric acid, which reacts with the exposed concrete to form
gypsum (CaSO 4 2H 2 O). The soft and soluble characteristics of gypsum allows it
to penetrate the concrete matrix and work to gradually deplete the thickness of
the concrete (Khalifeh et al., 2017).
This mechanism also works in the same manner in microbial-induced corrosion
(MIC), which is also common in concrete sewer pipes. The bacterial action gener-
ates sulfate ions and is typically known to occur in sewer systems as these systems
rely heavily on concrete pipelines. Concrete piping around aggressive soils, which
can contain sulfate ions, is particularly prone to corrosion as the ions generated are
the components of sulfur-based corrosion, and can act to further accelerate the corro-
sion process. The presence of chlorides and other caustic agents contributing to
crack formation in concrete pipes can also further deteriorate the concrete and reach
the reinforcing steel inside. Corrosion of the reinforcement results in complete dys-
function of concrete pipes as crack formation is so advanced by the time it reaches
the reinforcement, and already shows signs of leakage, and localized failure.
Furthermore, pressure formation inside the reinforcement caused by the
growth of cracks results in expansion of the reinforcement inside concrete pipes.
This causes failure by the gradual cracking of the surrounding concrete.
Crack formation in concrete pipes generally occurs in cases where tensile
stress exceeds tensile strength. As a result, a decrease in strength and capacity is
observed in concrete pipes which show signs of sulfide-based corrosion. In the
design of concrete pipes, it is important that load-bearing capacity is greater than
the applied total static and dynamic loads. Soil stress around pipes and factors
affecting the performance of the pipe should also be considered, including those
relating to bending failure, shear failure, loss of concrete cover and excessive
crack width of the concrete (Tran, 2014).
Below is a detailed explanation of the two deterioration mechanisms of con-
crete pipes, including reinforcement corrosion and microbial-induced corrosion.

1.6.1.1 Reinforcement Corrosion
The most causes of reinforcement corrosion are chloride intrusion, exposure to
water and oxygen, and reduction in pH, which are the result of concrete pipes
being subjected to corrosive chemical environments, such as petrochemical plants