PRACTICAL COMMENTS ON THE PROBLEM OF TRACKING

THE AUTHOR STUDIES THE INFLUENCE OF THE MATERIALS USED IN TRACKING. AT PRESENT THE CONTRACTOR MUST USE THE LOWEST-COST MATERIAL WHICH FULFILS CONTRACT REQUIREMENTS. THE AUTHOR EXPRESSES SOME DOUBTS AS REGARDS NEW STANDARDS FOR AGGREGATES AND CRUSHING SAND. THE ONLY WAY OF GETTING A TYPE OF SAND IN AGREEMENT WITH THE STANDARD WOULD BE TO SET UP A CONTROL SYSTEM DURING CONSTRUCTION, BASED ON OBJECTIVE SPECIFICATIONS. SATISFACTORY FILLERS ARE THOSE WHICH, WHEN MIXED WITH BITUMEN, GIVE SATISFACTORY MASTIC AND MORTAR. THIS GIVES ADEQUATE STABILITY TO THE MIXTURE. THIS CHARACTERISTIC SHOULD BE MEASURED BY CONTROL OF THE MIXTURE ITSELF RATHER THAN CONTROL OF THE FILLER. SPECIFICATIONS CONCERNING BINDERS DO NOT TAKE ACCOUNT OF THE BITUMEN BEHAVIOUR IN THE MIXTURE AT HIGH TEMPERATURES; IT WOULD BE DESIRABLE THAT PRESENT REQUIREMENTS BE COMPLETED BY A LIMITATION OF THE PARAFFIN CONTENT (2 PER CENT) AND BY A MEASUREMENT OF THE VISCOSITY AT 60 DEG C (HIGHEST TEMPERATURE FOUND ON ROADS) AND AT 135 DEG C, TEMPERATURE OF APPLICATION. THE PRODUCTION, TRANSPORT AND LAYING OF BITUMINOUS MIXTURES ARE PROBLEMS WHICH MUST BE TREATED WITH A WELL TESTED KNOW-HOW: RIGHT MIXING AND COMPACTION TEMPERATURES AND SATISFACTORY INSULATION OF THE MIXTURE DURING TRANSPORT ARE PARTICULARLY IMPORTANT FACTORS. HOWEVER, PRESENT METHODS OF INVESTIGATION DO NOT EXPLAIN ALL CASES OF TRACKING AND SOME BITUMINOUS MIXTURES THAT DO NOT FULFIL KNOWN CRITERIA, DO NOT GIVE RISE TO TRACKING.