Various methods of testing for an increase in the softening point are used in many countries. For example, in Germany, two methods are applied: the R&B method and Wilhelmi’s method. According to an U. S. review of fillers (Harris and Stuart, 1995), in Germany an acceptable range of AR&B of 10-20°C has been adopted for the R&B method, with components selected at the filler-binder content ratio (F:B) equal to 65:35, % (v/v). Mortars with AR&B greater than 20°C are too stiff and are not accepted. Similarly, mortars with AR&B less than 10°C are not accepted due to their excessive plasticity.
Another interesting test applied in Germany is the determination of a stiffening factor (in Germany Stabilisierungindex) (Schellenberger, 2002). It is an F:B for which the mortar AR&B increase is equal to + 20.0°C. It is necessary to make a series of filler-binder mixtures with different F:B ratios (e. g., 1:1, 1.5:1, and 2:1) and then determine a AR&B increase compared with the pure binder R&B. As a result, we obtain a graph showing the increase versus the F:B ratio; this can be interpreted as the relationship of the stiffening power of a given filler. When studying the results of stiffening factor tests, it is taken as a general rule in Germany that the results should be higher than approximately 1.9. The lower the stiffening factor, the stronger the stiffening impact of that filler on an asphalt mix.
Basically, the results of the tests lead to the conclusion that research on the increase in a softening point does not always reveal all the negative properties of a filler (e. g., swelling). Nevertheless, their merits lie in the ease with which the softening point can be measured through the R&B method.
