chapter  4
Durability analysis
Pages 32

Depending on the quality of the concrete and the thickness of the concrete cover, it may take many years before the chlorides reach embedded steel and corrosion starts. After the chlorides have reached the embedded steel, however, it may only take a few years before visual damage in the form of cracks and rust staining appears, but it may still take a long time before the load carrying capacity of the structure is severely reduced, as outlined and discussed in Chapter 2. Schematically, the deteriorating process takes place as shown in Figure 4.1. As soon as corrosion starts, a very complex system of galvanic cell activities in the concrete structure develops. In this system of galvanic cell activities, the deterioration appears in the form of concentrated pitting corrosion in the anodic areas of the rebar system, while the adjacent cathodic areas act as catchment areas for oxygen. Although larger portions of the rebar system eventually become depassivated, not all of these areas will necessarily corrode. As already discussed in Chapter 3, the steel in the first and most active corroding parts of the structure may act as sacrificial anodes, and thus cathodically protect the other parts of the structure. Since structural shape, electrical continuity, and local exposure conditions will affect this pattern of deterioration, it appears very difficult to develop a general mathematical model for predicting the time necessary before the load carrying capacity of the structure becomes reduced. Although several attempts for developing such a mathematical model have been made (Lu et al., 2008), it appears that no reliable mathematical model or numerical solution for this very complex deteriorating process currently exists. Already in the early 1970s, however, Collepardi et al. (1970, 1972) came up with a relatively simple mathematical model for estimating the time necessary for the chlorides to reach embedded steel through the concrete cover of a given quality and thickness in a given environment.