THE RHEOLOGICAL PROPERTIES OF ASPHALTS IN RELATION TO DURABILITY AND PAVEMENT PERFORMANCE

A SIDE BY SIDE COMPARISON OF UNAGED AND ROAD-AGED VERSIONS OF THE SAME ASPHALTS WAS CONDUCTED TO ATTEMPT TO RELATE ASPHALT RHEOLOGICAL PROPERTIES TO ROAD LIFE. ASPHALTS FROM TWO SURVEYS OF THE U.S. BUREAU OF PUBLIC ROADS OFFER THE POSSIBILITY OF RELATING ASPHALT RHEOLOGICAL PROPERTIES AND ROAD CONDITIONS. SUITABLE EQUIPMENT FOR MEASURING ASPHALT VISCOELASTICITY WAS DEVELOPED. VISCOELASTIC PROPERTIES WERE DETERMINED BY A DYNAMIC METHOD IN WHICH THE STRESS AND STRAIN FOLLOW A GIVEN SINUSOIDAL PATTERN. RHEOGONIOMETER TRANSDUCERS PICK UP AND RECORD STRESS AND STRAIN WITH A TWO- CHANNEL, LIGHT BEAM GALVANOMETER RECORDER. THE ASPHALTS FROM THE 1964 SURVEY WERE AGED IN A THIN FILM OVEN TEST AND BY OUTDOOR EXPOSURE IN MARSHALL SPECIMENS. ROAD-AGED ASPHALTS WERE EXTRACTED FROM PAVEMENT SAMPLES COLLECTED AT THE TIME THE ROADS WERE EVALUATED. ASPHALT COMPOSITION WAS DETERMINED BY A COMBINED SOLUBILITY AND CHROMATOGRAPHIC PROCEDURE. ALTHOUGH THE INTRINSIC RESISTANCE TO AGING OF AN ASPHALT CANNOT BE ISOLATED IN INTERPRETING PREVIOUS DATA, THE FINDINGS OF THE 1964 SURVEY INDICATE THAT EXTERNAL FACTORS PROBABLY ARE MOST IMPORTANT IN DETERMINING THE INCREASE IN COMPLEX MODULUS THAT CAN OCCUR IN ROAD AGING. VARIABLES IN DESIGN, CONSTRUCTION, SERVICE CONDITIONS, ETC. ARE INDICATED TO BE RESPONSIBLE FOR THIS LACK OF CORRELATION AND TEND TO OBSCURE THE DIFFERENCES IN THE INTRINSIC PROPERTIES OF THE ASPHALT BINDERS. IT IS CONCLUDED THAT ASPHALT ROADS CAN BE CONSTRUCTED IN WHICH THE ASPHALT RHEOLOGICAL PROPERTIES DO NOT CHANGE APPRECIABLY OVER A LONG PERIOD OF TIME. SUCH ROADS APPEAR TO BE LESS SUSCEPTIBLE TO CRACKING AND NOT NECESSARILY SUSCEPTIBLE TO DEFORMATION. THE AGE HARDENING OF ASPHALT IN THE ROAD DOES NOT CORRELATE DIRECTLY WITH THE AMOUNT THESE ASPHALTS HARDEN IN THE THIN FILM OVEN TEST. AT TRAFFIC STRESS FREQUENCIES, UNAGED AND AGED ASPHALTS HAVE ESSENTIALLY THE SAME MECHANICAL PROPERTIES AT LOW TEMPERATURES. DIFFERENCIES DUE TO AGING APPEAR AT HIGHER TEMPERATURES AND ARE MORE CONVENIENTLY DETERMINED BY CONFINING THE TEST TO HIGHER TEMPERATURES. AGING DOES NOT SIGNIFICANTLY CHANGE THE GLASS TRANSITION TEMPERATURE. A TIME-TEMPERATURE SUPERPOSITION PRINCIPLE ALLOWS THE CONSTRUCTION OF MASTER CURVES OF ASPHALT VISCO-ELASTIC PROPERTIES. AGING SHIFTS THE DISTRIBUTION OF RELAXATION TIMES TOWARD LONGER TIMES AND REDUCES THE ABILITY OF ASPHALT TO CONFORM TO APPLIED STRESSES. ROAD DURABILITY WILL BENEFIT MOST WHEN ASPHALTS ARE USED IN PROPERLY DESIGNED AND COMPACTED MIXES AT FILM THICKNESSES LARGE ENOUGH TO PREVENT THE ACCESS OF AIR AND WATER.