As demands on roads grow greater and the motoring public's expectations rise, a county engineer is often faced with the dilemma of which roads to treat first. In pavement preservation terms, the best roads should be maintained first-before they fail. This is not always possible, and conventional surfacing treatments will not always prevent reflection cracking. The answer is combination treatments that address failures as well as renew the surface. The treatment, which is chosen, depends on the extent of failure. For severe cracking, an asphalt rubber membrane such as a scrub seal may be needed before the surfacing is carried out. For roads where crazing and age cracking are not as severe a rejuvenation seal is an excellent choice. This article discusses rejuvenation seals and illustrates their use through a case study in California.
Aging of asphalt
When pavements get old they become brittle. This is a function of oxidation of the binder and loss of volatiles. Asphalt composition is the main factor that affects oxidation. Oxidation occurs primarily by two mechanisms.
Oxidation: As asphalts age, they incorporate oxygen at reactive sites in the hydrocarbon molecules.
These reactive sites are either hetero-atoms (nitrogen, sulfur and oxygen) or benzylic carbon atoms (in aromatic molecules). Hetero-atoms in unaged asphalt may be polar or non polar, but when oxidized, will become oxides and can participate in molecular associations. This is the source of hardening. Oxidation can be catalyzed too by other components in the asphalt such as metals. The main oxidation products are carbonyls, sulphoxides and nitrogen oxides. These are mostly in the form of carboxylic acids, sulphoxides, ketones and anhydrides.
Volatility: Oxidation occurs simultaneously with loss of lower molecular weight volatiles (oils). This mechanism depends on the asphalt composition and is most likely to occur at high handling temperatures.
Asphalt is a complex mixture of many different organic and sometimes inorganic materials. The materials may be classified in groups of materials of varying polarity, from polar asphaltenes to aliphatic oils. In between there are polar resins and aromatic and napthenic oils. The resins and oils together are called maltenes. The asphalt can be thought of as associated polar molecules dispersed by aromatic and napthenic oils.
As asphalt ages the asphaltenes can become more functionalized and thus can participate in more molecular associations. The resins are essentially made more polar by oxidation of active sites and, as a result have higher levels of associations with other polar molecules-essentially being made into asphaltenes. The oils are lost by volatilization or become functionalized by oxidation-essentially increasing their polarity, i.e. turning them into resins. This creates an imbalance in the dispersion of the associated phases in the asphalt and the dispersing oils, and a colloidally unstable and harder binder is formed. Thus the binder is more brittle, particularly at low temperatures, flexibility is reduced, and cracks appear, propagate and ultimately form potholes.
A rejuvenation seal can then be the best treatment to use. This is done before potholes can form, and when the binder is just losing stone. The rejuvenation seal will help to restore the adhesion of existing stone, bind a new stone layer and extend the life of the pavement.
The use of rejuvenation oils to replace lost oils and soften aged and cracked asphalt has been used for many years in recycling. The mechanism generally agreed upon is that the rejuvenation oils replace the maltenes in the asphalt, thereby softening and healing the existing asphalt and restoring colloidal stability. This extends the life of the existing binder, allowing it to contribute to both the waterproofing process, as well as to the cohesion of the seal binder layer.