This report describes the road trials of High Modulus Base (HMB), a high stiffness roadbase macadam manufactured to the standard UK composition for dense bitumen macadam (DBM) using a binder with a penetration of less than 50. The terminology used for 15 pen HMB is HMB15, for 25 pen, HMB25, and for 35 pen, HMB35. Throughout this investigation HMB15 was used and the results, together with those of the control materials, allow the properties of HMB25 and HMB35 to be predicted by interpolation. Trials were carried out at five sites by five different Contractors using different aggregate and bitumen sources and working in a range of weather conditions. The purpose of the trials was to determine whether the material could be mixed, laid and compacted by standard UK plant and to assess its properties relative to those of standard roadbase materials. The trials demonstrated that the material behaved in a similar way to conventional roadbase macadams, provided that appropriate mixing, laying and compaction temperatures were maintained. HMB has a very high stiffness, and therefore, better load spreading properties than DBM and heavy duty macadam (HDM), which means that a pavement designed to have the same life, but using HMB instead of conventional materials, can have a significantly thinner roadbase and basecourse. After allowance has been made for the increased production costs of HMB15, cost savings of approximately 25 per cent can be achieved compared to laying conventional DBM. Alternatively, HMB can be used to replace DBM or HDM at current design thicknesses to provide a longer pavement life. In addition to high stiffness, HMB has a high resistance to deformation, and when used as a roadbase or basecourse in conjunction with a thin wearing course, the road pavement would have excellent rutting resistance. However further trials are required to evaluate the system fully. (A)

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