A testing program was undertaken to determine the key factors affecting the performance of crumb rubber asphalts. Both wet mix and dry mix asphalts were studied. The program was divided into three main elements covering (i) the properties of crumb rubber binders (ii) the rutting resistance of crumb rubber asphalts and (iii) the fatigue resistance of crumb rubber asphalts.
The effects of time and temperature of digestion of rubber/bitumen blends on binder properties was studied using a dynamic shear rheometer. Selected binders were used to manufacture (hot mix) asphalt samples. Asphalt rutting resistance was determined by wheel tracking, and fatigue resistance by means of a beam fatigue test.
The rutting resistance of the rubberised asphalts was superior to conventional (unmodified) asphalt and similar to the polymer modified binder (PMB) asphalts tested. Dry mix and wet mix asphalts of identical composition gave similar wheel tracking performance. However, the dry mix process allowed higher rubber to bitumen ratios to be used and permitted a very low wheel tracking rate to be achieved.
Wet mix rubberised asphalts manufactured with 6% or less binder had low and variable fatigue lives. This was attributed to absorption of bitumen components by the rubber particles rendering the mixes dry with a tendency to ravel. Rubberised asphalts with a binder content of 8% were used in the bulk of the testing program.
The fatigue lives of the rubberised and PMB asphalts tested were similar but there was evidence that a high SBS concentration PMB asphalt would have a much greater fatigue life. Addition of oil to wet mix binders produced a substantial increase in fatigue resistance. A dry mix had a fatigue life three times that of a wet mix of identical composition.
Different types of rubber crumb used in the laboratory preparation of rubber binders had little effect on wheel tracking rates but a substantial effect on fatigue resistance. Rubberised binders from different manufacturers showed little difference in wheel tracking rates but were substantially different in fatigue resistance.