This report is the third in a series that describes research to assess the performance of lean concrete roadbases modified by the addition of pulverised fuel ash (pfa). Field trials in which the performances of conventional lean concrete, a cement-rich concrete roadbase and ash-modified lean concrete were investigated confirmed earlier work on the modifications to the strength/age characteristics and the improved compactability brought about by the addition of pfa. In addition the trials showed taht the development of thermal cracking in concrete roadbases was dependent on the early strength of the material. Trafficking the roadbases when they were seven days old showed that provided the materials met curent specifications for cement-bound roadbases no additional cracking should occur as a result of the trafficking. It was concluded that ash-modified lean concrete may, with advantage, be used as a replacement for conventional lean concrete providing it meet existinbg specifications for cement-bound roadbases