Investigation of Different Factors Affecting Asphalt Cement Ageing and Durability

Abstract

Asphalt cement is an organic material and, like any other organic material tends to react with atmospheric oxygen, thus changing its physical properties (more accurately rheological properties) gradually over time. Although there is no doubt that chemical changes happen in asphalt cement due to gradual oxidation, it is not the only factor resulting in changes in asphalt cement properties over time. Other factors such as loss of volatiles, the selective absorbance of lighter oily molecules by the aggregates’ surface, molecular reorientation, and so forth, could lead to changes in asphalt cement properties, most of which result in hardening. The alteration in physical properties of asphalt cement during construction and its performance period is called ageing. The severity of the age-hardening significantly depends on environmental factors such as temperature, latitude (which affects the angle of sunshine radiation), humidity, ultraviolet, etcetera, as well as mixture properties (such as aggregate gradation, aggregate type, air-void distribution, filler composition, and additive’s properties). There are several conditioning procedures to simulate short- and long-term ageing for both asphalt cement and mixes. Unfortunately, most of the existing methods are using excessively high conditioning temperatures and not realistic conditioning parameters. This project aims to identify the importance of less considered factors, such as the possible catalytic effect of minerals, solar radiation, and humidity conditioning and to define a laboratory long-term age conditioning procedure for asphalt mixes by considering different affecting factors to enable designers to include an ageing mechanism in their predictions. To reach to the goals of this project, compacted asphalt mix samples prepared using aggregates with different petrology and conditioned using standard (AASHTO R30) procedure and bespoke methods to include as much as environmental factors as possible. Complex (Dynamic) Modulus test will be used to evaluate the changes in the rheological behaviour of mixes. The 2S2P1D model fitted on laboratory results and used to investigate the rheological behaviour. Asphalt cement of the conditioned mixes extracted for further investigation on the rheological and chemical changes in asphalt cement. Frequency sweep test performed on extracted asphalt cement and results were processed using the 2S2P1D model. FT-IR spectroscopy was also used to investigate the change in the chemistry of asphalt cement by following the changes in carbonyl and sulfoxide indices and the ratio between them. Thermal sensitivity of rheological properties of both asphalt mix and asphalt cement was also investigated using time-temperature superposition shift-factors and utilizing Arrhenius and William-Landel- Ferry (WLF) theorems to further investigate the effect of different age conditioning and aggregates. Results of this research confirmed the effect of iron sulphide oxidation products in altering the chemistry of changes during ageing, while it didn’t capture significant acceleration in the process. Moreover, it was found that the coupling water treatment with solar radiation can effectively age asphalt mix samples and the product of ageing using these parameters are considerably different from extended heating procedures.