A new progressed mastic aging method and effect of fillers on SBS modified bitumen aging

Modification of bitumen is necessary to enhance its properties and improve its functions. Among the available materials, styrene-butadiene-styrene (SBS) has been widely used to modify bitumen due to its asymmetric composition that offers the desired strength and elasticity under normal temperature. In the bituminous mixture, Bitumen is mixed with fillers, which, according to recent studies, affects its aging properties in many ways. However,unlike bitumen, mastics exhibit higher viscosity and tend to age differently, considering that their aging is also affected by film thickness and extent of exposure to oxygen. Unfortunately, the available standard bitumen aging methods are not suitable for mastics. In addition, the role of the fillers on SBS modified bitumen is poorly documented in the literature.

To this note, a team of researchers at Tongji University: Dr. Chengwei Xing, Professor Liping Liu, and graduate student Jianwei Sheng proposed a new progressive method for mastic aging to overcome the limitations of the existing methods. The researchers started by developing a modified mastic aging method to ensure consistency of the bitumen/filler ratio before and after aging. Then, they used a dynamic shear rheometer (DSR) to evaluate the rheological properties of mastics, and gel permeation chromatography (GPC) tests to measure the molecular size distribution of the bitumen in mastics before and after aging. Their work is currently published in the research journal, Construction and Building Materials.

The study findings showed that the new progressed mastic aging method prevented the fluctuation of the bitumen/filler ratios caused by the separation of mastics, thus ensuring the accuracy of the rheological property tests. Based on this method, modified SBS bitumen and corresponding limestone and ash mastics were aged at different time intervals. Master curve analysis revealed that fillers could significantly reduce the aging rate of the base bitumen in the SBS modified bitumen. Moreover, multiple stress creep recovery tests indicated an initial increase in the J_nr3.2 of mastics followed by a decrease in the aging, while that of SBS modified bitumen decreased continuously. The increase in the J_nr3.2 of mastics, in the initial aging stages, was attributed to the degradation of the SBS modifiers caused by fillers. Furthermore, GPC results inditated that fillers accelerated degradation rate of linear SBS modifiers, thereby verifying that the increase of Jnr 3.2 of mastics in the initial aging stage can be due to degradation of many SBS modifiers caused by fillers.

In summary, the research team looked carefully on the effects of fillers on the aging properties of SBS modified bitumen. The proposed modified mastic aging method is capable of overcoming the drawbacks of the existing aging methods. Based on the results, fillers have several effects on the SBS modified bitumen aging, including altering the aging rate. In a statement to Advances in Engineering, Professor Liping Liu said their findings would be of great help in advancing the knowledge of the effects of fillers on SBS modified bitumen.