Studies of asphalt air blowing and development of a new short-term aging technique

Abstract

In this work asphalt air blowing has been studied from two perspectives. First, the process itself was investigated in terms of the effect of air-blowing conditions on the properties of the asphalt materials obtained. It was found that according to the Superpave specification, very good grade asphalts can be produced by air blowing, and the blowing temperature does not seriously impact the grade span. Asphalt composition has a significant effect on the grade span. The materials with a higher concentration of asphaltenes and saturates have a higher performance grade span. In this work it is shown that the subsequent 88° C hardening and oxidation rates are higher for materials blown at higher temperature, and this is not detected by Superpave specifications. Blowing fluxes with a high saturate content may result in a higher grade but can cause subsequent susceptibility to oxidative hardening. The mechanism by which air blowing can affect the 88° C hardening rate is related to oxidation kinetics and the tendency of oxidation products to form asphaltenes, causing hardening. For some materials air blown at higher temperatures, the subsequent accelerated hardening rate results from an increase in the oxidation rate, while the hardening susceptibility may actually decrease. The second part of this work deals with the development of a new procedure for simulation of asphalt short-term aging that occurs at a hot mix asphalt cement plant. The standard tests for simulation of asphalt hot-mix aging are the RTFOT (ASTM D 2872) and the TFOT (ASTM D 1754). They have a number of deficiencies when used for viscous or modified asphalts, such as uneven aging, film formation, and difficulty cleaning laboratory equipment. The air blowing apparatus is a stirred, thermally controlled vessel with an attached condenser. The proposed test eliminates the shortcomings of standard methods. It provides the same aging in a shorter amount of time at lower equipment cost with simplified handling.