A dense coated roadbase macadam of improved performance

An assessment has been made of the effects of binder hardness and filler content on the performance of dense coated roadbase macadams containing 40 mm crushed granite aggregate and limestone filler with either bitumen or tar binder. The materials were laid and compacted using fullscale plant; they included 50 and 100 penetration bitumen, 54 and 58 evt low temperature tars and filler contents in the range 3 to 11 per cent. Samples were cut from the laid materials to measure deformation resistance, dynamic stiffness and resistance to fatigue. Corroborative work with other crushed aggregates provided more data on dynamic stiffness and reassurance concerning possible mixing problems. Binder contents within the present specification range are sufficient to coat most aggregates containing up to 10 per cent filler and dense roadbase macadam with the binders of higher viscosity and about 8 per cent filler confers significant benefits through better load spreading and deformation resistance. Furthermore, at equiviscous temperatures of rolling, the material was shown to be as easy to compact as conventional dense coated macadam. In heavily trafficked roads, its use would allow significant reductions of design thickness or, alternatively, a marked extension of pavement life. (A)