2 Environmental deterioration
Structures are not always doomed to fail, but they do usually have a limited useful life. Exceptions include many of the monuments that have survived from the ancient world, such as the Great Pyramid in Egypt (Figure 11a), the Pont du Gard in southern France (Figure 11b) and the Pantheon in Rome (Figure 11c). These are very stable structures. The Great Pyramid has a very low centre of gravity and few potential failure modes, while the other two structures are based on the principle of the arch and the dome respectively, where most of the loads in the stonework are compressive by nature, thereby exploiting the high compressive strength of stone. Stone is also very resistant to deterioration, particularly in relatively dry environments (which is why these structures have lasted so long).
In this section we will examine some of the mechanisms of deterioration of structures, the effects of the way that structures are loaded on the process of degradation, and the ways structures can be protected against environmental attack. To help you to tackle the problems encountered by real structures in different environments, some background in the chemistry of materials will be provided to enable you to pinpoint specific mechanisms of deterioration.
The next section will introduce you to some of the language and mechanisms of corrosion and degradation. The remainder of the unit uses a case study to illustrate the various factors that can lead to failure in practice.