Page 113 - Handbook of Materials Failure Analysis

108 CHAPTER 5 Failure analysis of reinforced concrete structures

of water-soluble chlorides since it is generally accepted that corrosion is influenced
by the free chloride concentration present in the concrete pore solution [46].
As one-dimensional finite beam element is adopted in this study, it seems to be
consistent in representing the chloride ingress in the same dimension, that is, in one-
dimensional sense. In this work, Equation 5.31 is used to evaluate the time in which
the corrosion process starts. After the corrosion time initiation, the reinforcements’
areas are penalized using the relations that are presented in next section.

6 CORROSION LAWS

The classical corrosion model proposed by Tuutti [46] states that reinforcements cor-
rosion occurs into two distinct stages: initiation period and propagation period. The
initiation period is limited by the time required for the occurrence of reinforcements
depassivation due to the chloride concentration growth along time. The propagation
period comprises the time in which cross-section area of reinforcements steel is lost.
Figure 5.7 illustrates the evolution of cross-section diameter of reinforcement, D S ,as
a function of time, where initiation and propagation periods are clearly defined.
The rate of reinforcements corrosion into propagation period depends on environ-
mental moisture and the amount of available oxygen. These two parameters have
major importance on the cathodic process and on the concrete resistivity. In addition
to them, temperature catalyzes the corrosion process.
In spite of its importance, mathematical approaches for modeling the propagation
period of corrosion are not often available in literature, because many researchers
consider the initiation period as structural life time. However, the reduction of steel
is consistently represented by models presented in Refs. [17,34,35]. The models pre-
sented on mentioned references were determined empirically, accounting for tropical
climate.
In the present study, the steel loss is evaluated using the models presented in Refs.
[17,34,35], which assumes that steel is corroded either uniformly or by pits along its
perimeter. For uniform corrosion model, the corroded diameter of reinforcements is
determined as follows:

D s

D so
Start of corrosion
process

t 0 t d Time
FIGURE 5.7
Change on reinforcements diameter along time considering corrosion effects.