CONSIDERATIONS ON THE MANUFACTURE AND LAYING OF HOT-COATED GRAVEL-BITUMEN MIXTURES

THE AUTHOR RECALLS THAT THE HOT-COATED GRAVEL-BITUMEN TECHNIQUE FOR ROADBASES OF NEW PAVEMENTS AND FOR STRENGTHENING HAS BEEN USED INCREASINGLY OVER THE LAST FEW YEARS, BUT THAT THE USE OF DIFFERENT FORMULAE AND AGGREGATES BY VARIOUS MIXING AND LAYING PLANTS HAS SHOWN THAT DIFFICULTIES EXISTED, WHICH HAD NOT BEEN ENCOUNTERED WITH STANDARD BITUMINOUS MIXTURES. DETAILS ARE GIVEN OF THE DIFFICULTIES WHICH EMERGED OVER THE LAST FOUR YEARS IN THE RHONE-ALPES REGION AND OF THE SOLUTIONS FOUND TO SOLVE THE PROBLEMS OF PRODUCTION AND LAYING. THE CONCLUSIONS REACHED ARE AS FOLLOWS: IN THE HOT-COATED GRAVEL-BITUMEN MIXTURE, THE VOIDS IN THE AGGREGATES ARE NOT FILLED WITH BITUMEN AS IN STANDARD BITUMINOUS MIXTURES, BUT WITH FILLER. THIS PRESENTS COATING PROBLEMS, AND THE MIXTURE IS OF LOW WORKABILITY AND CAN ONLY BE COMPACTED SATISFACTORILY IN A THICK LAYER, COMPACTION BEING AN ESSENTIAL FACTOR AS REGARDS SATISFACTORY MECHANICAL PERFORMANCE. THIS TENDENCY TOWARDS LOW BITUMEN-CONTENT MIXTURES, SPREAD THICKLY, BROUGHT TO THE FORE A NUMBER OF PROBLEMS WHICH OCCUR, BUT LESS MARKEDLY, IN STANDARD BITUMINOUS MIXTURES: SEGREGATION AND COMPACTION OF LONGITUDINAL JOINTS. IF THE LATTER IS NOT TO BECOME THE WEAK LINK IN THE PAVEMENT, IT MUST BE GIVEN SPECIAL ATTENTION. SEVERAL METHODS HAVE BEEN USED IN AN ATTEMPT TO SOLVE THE PROBLEM. THE TECHNIQUE OF THE WARM JOINT CAN BE USED DURING CONSTRUCTION CARRIED OUT WHEN TRAFFIC IS NOT INTERRUPTED. HOWEVER, THIS PRESUPPOSES THE USE OF RIGID FORMULAE RESULTING IN A PAVEMENT ABLE TO SUPPORT THE TRAFFIC IMMEDIATELY AFTER COMPACTION WITHOUT BEING SUBJECT TO DEFORMATION. GENERALLY, GRAVEL-BITUMEN MIXTURES WHICH CONTAIN WELL CRUSHED AGGREGATES FULFIL THIS CONDITION.