Durability is a measure of how asphalt binder physical properties change with age (sometimes called age hardening). In general, as an asphalt binder ages, its viscosity increases and it becomes more stiff and brittle. Age hardening is a result of a number of factors, the principal ones being (Vallerga, Monismith and Grahthem, 1957 and Finn, 1967 as referenced by Roberts et al., 1996):
- Oxidation: The reaction of oxygen with the asphalt binder.
- Volatilization: The evaporation of the lighter constituents of asphalt binder. It is primarily a function of temperature and occurs principally during HMA production.
- Polymerization: The combining of like molecules to form larger molecules. These larger molecules are thought to cause a progressive hardening.
- Thixotropy: The property of asphalt binder whereby it “sets” when unagitated. Thixotropy is thought to result from hydrophilic suspended particles that form a lattice structure throughout the asphalt binder. This causes an increase in viscosity and thus, hardening (Exxon, 1997). Thixotropic effects can be somewhat reversed by heat and agitation. HMA pavements with little or no traffic are generally associated with thixotropic hardening.
- Syneresis: The separation of less viscous liquids from the more viscous asphalt binder molecular network. The liquid loss hardens the asphalt and is caused by shrinkage or rearrangement of the asphalt binder structure due to either physical or chemical changes. Syneresis is a form of bleeding (Exxon, 1997).
- Separation: The removal of the oily constituents, resins or asphaltenes from the asphalt binder by selective absorption of some porous aggregates.
There is no direct measure for asphalt binder aging. Rather, aging effects are accounted for by subjecting asphalt binder samples to simulated aging then conducting other standard physical tests (such as viscosity, dynamic shear rheometer (DSR), bending beam rheometer (BBR) and the direct tension test (DTT)). Simulating the effects of aging is important because an asphalt binder that possesses a certain set of properties in its as-supplied state, may possess a different set of properties after aging. Asphalt binder aging is usually split up into two categories:
- Short-term aging. This occurs when asphalt binder is mixed with hot aggregates in an HMA mixing facility.
- Long-term aging. This occurs after HMA pavement construction and is generally due to environmental exposure and loading.
Typical aging simulation tests are:
- Vallerga, B.A.; Monismith, C.L. and Granthem, K. (1957). A Study of Some Factors Influencing the Weathering of Paving Asphalts. Asphalt Paving Technology: Proceedings, vol. 26. Association of Asphalt Paving Technologists.↵
- Finn, F.N. (1967). National Cooperative Highway Research Program Report 39: Factors Involved in the Design of Asphaltic Pavement Surfaces. National Cooperative Highway Research Program, Transportation Research Board, National Research Council. Washington, D.C.↵
- Roberts, F.L.; Kandhal, P.S.; Brown, E.R.; Lee, D.Y. and Kennedy, T.W. (1996). Hot Mix Asphalt Materials, Mixture Design, and Construction. National Asphalt Pavement Association Education Foundation. Lanham, MD.↵
- Exxon Company. (1997). Petroleum Encyclopedia for the Users of Petroleum Products. Exxon web site. http://www.exxondist.com. Accessed 28 August 2001.↵