The state of the art

It is concluded that, in the century since the transport processes were first analysed, much progress has been made but much remains to be done. It is clear that the tests that measure these processes should give true values for the basic properties such as permeability and diffusion coefficient and that models which predict performance should use them.

The process of using diffusion coefficients to measure durability is a good example of where progress needs to be made. It has been shown that none of the tests in current use gives a true measure of the diffusion coefficient because they rely on the solution of Fick’s law for a single ion. The durability models use the same solution so it can be argued that they are using the available data as well as they can. However, the influence of other ions may be completely different in the test sample and the structure which is being modelled so the answer will not be accurate.

Similarly, ISAT and Figg tests may be used to obtain an indication of durability. However, they do not yield data suitable for use in models and they are influenced by factors which may not affect durability.

Two solutions are proposed in this book: the additional pressure measurements for the Figg test discussed in Chapter 5 and the additional voltage measurements in the electrical tests discussed in Chapter 10. Both of these require further work before they are suitable for general industrial use, but they show that progress can be made towards developing this subject from a sound scientific base.

Recommendations and guidance

Practitioners who are measuring transport properties as indicators of durability or for any other purpose are advised to proceed as follows: