Ideally, asphalt used for paving roads should remain viscoelastic in all weather conditions. However, many asphalt roads soften in summer and suffer from rutting, or permanent deformation, as it is also called. At low temperatures, neutral molecules in asphalt arrange themselves into more organized structural forms. As a result, the material hardens, becomes brittle, and cracks under the stress of heavy traffic loads. This is known as thermal and fatigue cracking.
Asphalts also lose their plasticity and therefore harden and crack or crumble when they lose their more volatile lower molecular weight constituents or when these constituents are oxidized. This process is known as aging. Moisture from rain and other sources can also invade and damage asphalts, particularly aged or oxidized asphalts because they have a larger number of polar constituents to attract water molecules.
The performance of asphalts can be improved by using various modification techniques. For example, blowing air through hot liquid asphalt removes more volatile compounds and results in a product with higher viscosity. Addition of modifiers, such as polybutadiene in the form of crumb rubber from used tires and other polymers, also stiffens asphalts.
Well-designed and well-built asphalt pavements last many years. There are a number of case studies which support this conclusion. For instance, the asphalt portions of Interstate 90 in Washington State have been in place since their original construction up to 35 years ago with no rehabilitation for structural reasons. These pavements have only required maintenance and periodic replacement of their surface layer. The New Jersey DOT found the same to be true on I-287 on a 26-year old 10-inch asphalt pavement; the original structure has remained intact and only a surface profiling followed by an overlay was necessary to restore the pavement. The entire New Jersey Turnpike is asphalt. It was built in 1951. They have never had a structural failure in the pavement. The only maintenance they've done is surface treatments and overlays. In a recent interview, the chief engineer for the Turnpike said that expected the pavement to last another 50 years. It was very well-designed and well-built. The designers put a lot of thought and care into the pavement structure and how they built it, they used top-quality materials, and they got a pavement which has held up extremely well.
