Too many roads in the developing world fail because of premature plastic deformation, despite the existence of published design modifications for continuously graded mixes in hot climates. The Transport Research Laboratory (TRL) is currently conducting research in Malaysia and Indonesia, in collaboration with local engineering institutes there, to investigate the nature of premature plastic deformation, and develop low-cost mix design procedures that will prevent its occurrence. The research in Malaysia concentrated on two sets of experimental surfacing trials on an especially severe climbing lane on a major highway; the results show that the level of air voids in the surfacings needs to be maintained sufficiently high after secondary compaction by traffic. The most notable methods for doing this are a modified Marshall design procedure, and the development of the Superpave method of design in the USA. The Marshall method of determining the desired binder content is by far the commonest method of designing continuously graded mixes in developing countries and has several advantages. The paper describes some results of its use, together with the results of some laboratory trials on the effect of compaction level and on bituminous surfacings for severely loaded sites. It also outlines some work on refusal density design for severely loaded sites.
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