THE HARDENING OF ASPHALT WITH RELATION TO DEVELOPMENT OF CRACKS IN ASPHALT PAVEMENTS

THE FORMATION OF CRACKS IN ASPHALT PAVEMENTS WAS STUDIED. IT IS SHOWN THAT EXTREME HARDNESS OF ASPHALT IN AN ASPHALT PAVEMENT IS CONDUCIVE TO CRACKING. CONTACT WITH AIR IS A PRINCIPAL CAUSE OF HARDENING OF ASPHALT. ALL ASPHALTIC PRODUCTS HARDEN PROGRESSIVELY AND WITH APPRECIABLE RAPIDITY WHEN CONTINUOUSLY EXPOSED IN THIN FILMS TO THE ACTION OF AIR. RAPIDITY OF HARDENING IS A FUNCTION OF FILM THICKNESS, TEMPERATURE AND TIME. THIN FILMS OF ALL LIQUID ASPHALTIC ROAD MATERIALS, AS WELL AS ASPHALT CEMENTS MAY BE EXPECTED TO HARDEN TO LOW PENETRATION BRITTLE ASPHALTS WITHIN A FEW YEARS' TIME IF NOT ADEQUATELY PROTECTED FROM CONTACT WITH FREE AIR. ASPHALTS MAY BE EXPECTED TO HARDEN APPRECIABLY AT HOT-MIX PAVING PLANTS DURING THEIR COMBINATION WITH MINERAL AGGREGATE UNDER CUSTOMARY PLANT OPERATION. IT APPEARS THAT THE FOLLOWING EXPECTANCY OF CRACKING DUE TO HARDNESS OF THE ASPHALT CEMENT PRESENT IN THE PAVEMENT, EXISTS IN TEMPERATE CLIMATES WHERE FREEZING TEMPERATURES ARE COMMON THROUGH THE WINTER: (1) ASPHALT LESS THAN 20 PENETRATION - BAD CRACKING, (2) ASPHALT HIGHER THAN 30 PENETRATION - NO CRACKING, (3) ASPHALT 20 TO 30 PENETRATION - UNCERTAIN. IN HOT-MIX CONSTRUCTION, THE SOFTER THE ORIGINAL ASPHALT, THE LESS IS IT LIKELY TO HARDEN DANGEROUSLY DURING THE MIXING OPERATION. PROGRESSIVE HARDENING OF ASPHALT FILMS IN A DENSE THOROUGHLY COMPACTED PAVING MIXTURE IS EXCEEDINGLY SLOW. WHEN THE MIXTURE IS READILY PERMEABLE TO AIR, HOWEVER, DANGEROUS HARDENING MAY OCCUR WITHIN A RELATIVELY SHORT PERIOD. IT IS CONCLUDED THAT TO INSURE LONG LIFE, PREVENT CRACKING AND POSSIBLE DISINTEGRATION IN AN ASPHALT PAVEMENT OR WEARING COURSE, DUE TO HARDNESS OF THE ASPHALT, THE FOLLOWING FIVE RULES SHOULD BE STRICTLY OBSERVED: (1) USE AS SOFT AN ASPHALT AS POSSIBLE WITHOUT REDUCING STABILITY BELOW THE MINIMUM REQUIRED TO PREVENT DISPLACEMENT UNDER TRAFFIC, (2) PREPARE MIXTURES AT THE LOWEST PRACTICABLE TEMPERATURES AT PAVING PLANTS, (3) COMPRESS ALL ASPHALT PAVING MIXTURES THOROUGHLY TO BE AS IMPERMEABLE TO AIR AS POSSIBLE, (4) SEAL COAT ALL COMPRESSED MIXTURES WHICH ARE READILY PERMEABLE TO AIR, PREVENTING FREE CIRCULATION OF AIR WITHIN MIXTURE.