IMPROVED ROADBASE MACADAMS: ROAD TRIALS AND DESIGN CONSIDERATIONS

Previous pilot-scale investigations have shown that improved macadams with a harder grade of binder and more filler than normal dense bitumen macadam have considerable potential and full-scale trials should take place. These materials for use in roadbase construction have now been evaluated in five motorway and other trunk road contracts. Analysis of the production process showed that it might cost up to 10 per cent more, but its superior load spreading ability allows a thinner depth of construction to be specified and the overall cost saving is likely to be at least 12 per cent. There are situations where additional filler is not readily available or where its use leads to longer mixing times and a significant reduction in productivity of the mixing plant: the additional filler may then be omitted and the resultant material will still be more cost effective than normal dense bitumen macadam. Analysis of the structural properties of material taken from the five reconstruction sites provided a basis for design curves relating thickness of construction to cumulative traffic loading over the design life of the pavement. (TRRL)