Ageing of Bitumen in Contact with Mineral Aggregate Applications

Background

The main cause of ageing of a bituminous material is oxidative hardening, which at moderate levels is generally accepted and can even enhance performance, but at significant levels results in embrittlement of the bitumen, significantly affecting its adhesive characteristics and usually resulting in reduced cracking resistance of the mixture under repeated loading. It is recognised that the characteristics of bitumens are affected by the mineral aggregate with which they come into contact with and that the age hardening of the binder in an asphalt mixture is influenced by both the bitumen and the mineral aggregate. However, there has been limited research attempting to quantify the role of the bitumen-filler-aggregate interaction on the durability (ageing) of bituminous materials.

Research methodology

Traditionally, the rheological characteristics of bitumens are measured using relatively thick binder films (1 to 2 mm) placed between metal (stainless steel) plates using a dynamic shear rheometer (DSR) and is therefore unable to quantify any effects imparted to the bitumen by mineral aggregates. To achieve this, two aspects of the test procedure need to be altered, namely, (i) reducing the gap to microns rather than millimetres and (ii) incorporating mineral aggregate into the testing apparatus. The introduction of mineral aggregate can be achieved through a novel technique involving a simple modification to the base plate of a standard DSR whereby a small aggregate disc is placed and secured directly below the parallel plate upper spindle with the disc now acting as the parallel base plate of the DSR.

Using this thin film binder technique, the influence of mineral aggregate on the ageing of various bitumen-aggregate systems can be determined by measuring the rheological properties before and after conditioning (ageing). A range of both conventional and modified bitumens together with a range of mineral aggregates with varying porosities and mineralogical compositions will be tested. In addition to the bitumen-aggregate combinations, a range of bitumen-filler mastics will be tested in the DSR before and after ageing. This will allow the influence of mineral aggregate filler on bituminous material ageing to be compared to that found for pure bitumen-aggregate combinations. Finally, changes in the chemical composition of the aged binders will be determined in terms of the relative increases in the main oxidative compounds, namely ketones and sulfoxides for the bitumens with and without the influence of mineral fillers and aggregate combinations.