Page 581 - Corrosion Engineering Principles and Practice

544 C h a p t e r 1 3 C a t h o d i c P r o t e c t i o n 545

13.4 How Cathodic Protection Works in Concrete
The main causes of corrosion of steel in concrete are chloride attack
and carbonation. These two mechanisms are unusual in that they do
not attack the integrity of the concrete. Instead, aggressive chemical
species pass through the pores in the concrete and attack the steel.
This is unlike normal deterioration processes due to chemical attack
on concrete.
Most problems associated with corrosion of steel in concrete are
not due to the direct loss of steel but to the growth of the corrosion
oxide products which eventually can lead to cracking and spalling of
the concrete cover (Fig. 13.19). Structural collapses of reinforced
concrete structures due to corrosion are rare. The most common
problem caused by corrosion is spalling of concrete cover, which has
caused a few deaths and accidents in recent history. It has been
suggested that less than 100 µm of steel-section loss is needed to start
cracking and spalling of the concrete. The actual amount needed will
depend on the geometry in terms of cover, proximity to corners, rebar
spacing, bar diameter, and the rate of corrosion [10].
A number of fundamental measures can be taken to address the
problem of reinforcing steel corrosion. However, an important

FIGURE 13.19 Corrosion ravaged columns of a highway bridge in Illinois.
(Courtesy of Vector Corrosion Technologies)