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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 that 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.