Trying to predict the long term lifetime on a concrete structure is difficult. Many structures built by the Romans nearly 2000 years ago are still standing. Yet in far too many of our moderns concrete structures, maintenance costs can mount rapidly after only a few years exceeding the initial cost of the structure. Why does this happen? Concrete durability is normally dictated by chemical reactions occurring within the structure. Accelerated tests to assess durability have been developed and include wetting/drying, high temperature curing, freeze/thaw cycling as well as accelerated chemical exposure tests. However, the concrete today usually contains significant amounts of supplementary cementing materials (SCM's) which can change the initial chemistry. Simply applying these accelerated tests, without understanding the effect they have on the microstructure of the cementitious binder blend, can lead to decisions being taken that do not reflect what happens in practice. This paper describes the use of some of these tests for cementitous composites which have given unexpected results. Investigations of the microstructural and chemical changed in the cement matrix have provided the reasons why the structures have failed.
