While laboratory aging of asphalt binders has been studied extensively as to reaction kinetics and physicochemical phenomena, aging of binders in pavements is much less understood because of a number of complications. Such complications include suitable extraction and recovery methods; uncontrolled variables and unknowns such as mixture characteristics (air voids, e.g.), maintenance treatments, traffic, and climate; sustaining a research effort to study a given pavement over an appropriate time frame (in excess of one decade); and cost. An ongoing research effort sponsored by the Texas Department of Transportation (TxDOT) has studied fifteen pavements across Texas with respect to binder oxidative hardening. These pavements included two test sections that were placed with multiple binders in 1983, one Texas highway that was taken under study at the beginning of its construction in 1987, two pavements sampled late in their life, and 10 SHRP LTPP GPS pavements that were cored from 1989 to 1990. Results indicate that unmodified binders in pavements typically oxidize and harden to a degree that exceeds accepted pavement aging assumptions. This hardening also may extend much deeper into the pavement than has been previously assumed or documented. Data suggest that pavements can oxidize at surprisingly uniform rates with depth once early oxidation occurs, and that these rates continue for an extended time. As a rough measure, one month environmental room aging of 1 mm neat binder films at 60 ??C was equivalent to about 15 months in SH 21 after the early higher hardening rate period.
Samenvatting