COMPARISON OF OXIDATION OF SHRP ASPHALTS BY TWO DIFFERENT METHODS

Abstract

An objective of the Strategic Highway Research Program is the development of a test procedure that accurately predicts changes in asphalt pavement properties with aging. Many of the methods now in use do not give reliable results. It is desirable to develop a method that takes into account maximum temperatures to which asphalts are subjected in mix plants (about l60°C or 320°F) and during service (60° C or 140°F), but is not impractically long. Therefore, test conditions must involve increased temperature, pressure, or the addition of a chemical accelerant but yield artificially aged materials that have certain properties similar to those of aged pavement binders.|In this study, rheological and chemical properties of asphalts subjected to thin-film oven followed by pressure vessel oxidation under two sets of conditions were compared with properties of asphalts aged in the thin-film accelerated-aging test at two temperatures. Oxidations of asphalts for 144 hours (6 days) at 60°C (I40°F) and 2.07 × 10⁶ Pa pressure following a standard thin-film oven treatment yielded products having characteristic aging indices and enhanced sulfoxide and carbonyl absorbances in their infrared spectra. Oxidation of asphalts under thin-film accelerated-aging lest conditions at 85°C (185°F) for 144 hours provides aged asphalts with rheological properties similar to those obtained from the 60°C (140°F) pressure oxidations. When the pressure oxidation was run at 80°C (176°F) for 144 hours. materials were obtained that were more extensively aged. The aging indices of these materials were somewhat similar to asphalts oxidized for 72 hours at 113°C (235.4°F) under thin-film accelerated-aging test conditions. The aging lendencies of the eight asphalts tested were ranked by the two latter methods in approxmiately the same order.|The thin-film accelerated-aging test procedures are readily performed in a standard oven. but utilize smaller samples (resulting in less material for subsequent analysis) than the pressure oxidations. It is not certain that the mechanism or kinetics of the asphalt oxidations in the higher temperature oven procedures are identical to the mechanisms of oxidation of asphalts in pressure aging. The procedures used in this study, although not themselves definitive test conditions for asphalt-binder aging. should aid in the selection of conditions for an improved aging lest.