Durability can be defined as the ability to maintain satisfactory #rheology, cohesion and adhesion in long term service. As a part of the quality criteria for bitumen, the following have been identified as the prime durability factors:
- Oxidative hardening
- Evaporative hardening
- Exudative hardening
Oxidative and evaporative hardening
Hardening due to #oxidation has long been held to be the main cause of ageing. In mixtures where the finely structured voids are interconnected, hardening in situ by oxidation is important. However, under such circumstances, hardening due to evaporation of the more volatile components of the bitumen can also be significant.
If the constitution of a bitumen is unbalanced, it may, when in contact with a porous aggregate, exude an oily component into the surface pores of the aggregate. This may result in a hardening of the bitumen film remaining on the surface of the aggregate. Exudation is primarily a function of the amount of low molecular weight #paraffinic components present in the bitumen relative to the amount and type of #Asphaltenes.
Scientists have developed the exudation droplet test to quantitatively measure the exudation tendency of a bitumen. In this test, bitumen droplets are applied to the recesses in custom-made white marble plates. The plates are stored at 60 oC for 4 days under a nitrogen blanket. During this period, oily rings develop around the bitumen droplet that can be measured under #ultraviolet light using a microscope. Ring widths vary from a few tenths of millimeter for a balanced bitumen to several millimeters for an unbalanced bitumen.
Hardening in services as a result of exudation can be substantial and depends not only on the exudation tendency of the bitumen cut also on the porosity of the aggregate. If the aggregate possesses low porosity, the quantity of exudate absorbed is negligible, irrespective of the exudation tendency of the bitumen. Similarly, if the exudation tendency of the bitumen is low, the quantity of exudate absorbed will be negligible, irrespective of the porosity of the aggregate.
Ref: The Shell Bitumen Handbook