Chemical and Thermal Characterization of Road Bitumen Ageing

Bitumen ageing stands for a number of physico-chemical modifications that occur in bitumen as a response to environmental conditions. Several tests have been proposed to simulate bitumen ageing. Two of the most common ones are: a) the Rolling Thin Film Oven Test (RTFOT), which simulates short term ageing; b) the Pressure Ageing Vessel (PAV) test, which accounts for long-term ageing and it is usually performed on the RTFOT residue. A study of the physico-chemical modifications of a 50/70 pavement grade bitumen caused by ageing is presented. Both RTFOT and PAV tests were used. A follow-up of bitumen modifications due to ageing was done by several methods. In particular: 1) Fourier transform infrared spectroscopy (FTIR), to follow bitumen oxidation by the increase of oxygenated functional groups, which were quantified by taking appropriate structural indices; 2) Iatroscan thin layer chromatography (TLC), to follow bitumen composition expressed by the generic SARA fractions (saturates, aromatics, resins and asphaltenes); 3) Gel permeation chromatography (GPC), under “ultra-fast” conditions, to follow the colloidal structure of bitumen; and 4) Differential scanning calorimetry (DSC), to evaluate the changes of the crystallized fractions (%FC) and the glass relaxation temperature (Tg). The results show that the PAV test causes more severe changes in bitumen than the RTFOT test. In particular, the evolution of the oxidation processes could be quantified by taking carbonyl and sulfoxide structural indices. Ageing also modified the SARA profile, namely, the relative contribution of aromatics, resins and asphaltenes fractions. On the other hand, DSC data remained almost unaffected after ageing tests, which may be indicative of the survival of most of the phases of the bitumen after these tests. Finally, a comparative analysis of the sensitivity and performance of all these methods is presented.