Sara Bressi

Impact of different ageing levels on binder rheology

This paper evaluates the variability of binder rheology for different ageing levels and the influence of ageing at different testing temperatures. Three different ageing levels were applied on a single type of bitumen with a penetration grade of 70/100.The artificial ageing of the binder was performed using the rolling thin-film oven test and the pressure ageing vessel. The rheological behaviour was investigated at low temperatures with the bending beam rheometer (BBR) and at medium and high temperatures with the dynamic shear rheometer (DSR). Several experiments were conducted to determine the range of stiffness and complex modulus results, the type of distribution comparing real and theoretical models, and the effects of ageing on the variability of the rheological behaviour. It was shown that not only the mean results from BBR and DSR tests change with ageing, but also the variability of the results changes with ageing. This would have an impact on mechanistic-empirical pavement design because it would influence the calculated stresses and strains as well as the calculated reliability.

Multiscale imaging and characterization of the effect of mixing temperature on asphalt concrete containing recycled components

When producing asphalt concrete mixture with high amounts of reclaimed asphalt pavement (RAP), the mixing temperature plays a significant role in the resulting spatial distribution of the components as well as on the quality of the resulting mixture, in terms of workability during mixing and compaction as well as in service mechanical properties. Asphalt concrete containing 50% RAP was investigated at mixing temperatures of 140, 160 and 180°C, using a multiscale approach. At the microscale, using energy dispersive X‐ray spectroscopy the RAP binder film thickness was visualized and measured. It was shown that at higher mixing temperatures this film thickness was reduced. The reduction in film thickness can be attributed to the loss of volatiles as well as the mixing of RAP binder with virgin binder at higher temperatures. X‐ray computer tomography was used to characterize statistically the distribution of the RAP and virgin aggregates geometric features: volume, width and shape anisotropy. In addition using X‐ray computer tomography, the packing and spatial distribution of the RAP and virgin aggregates was characterized using the nearest neighbour metric. It was shown that mixing temperature may have a positive effect on the spatial distribution of the aggregates but did not affect the packing. The study shows a tendency for the RAP aggregates to be more likely distributed in clusters at lower mixing temperatures. At higher temperatures, they were more homogeneously distributed. This indicates a higher degree of blending both at microscale (binder film) and macroscale (spatial distribution) between RAP and virgin aggregates as a result of increasing mixing temperatures and the ability to quantify this using various imaging techniques.

Asphalt Mixture with RAP: Mix Design Optimization

The addition of reclaimed asphalt pavement (RAP) in hot mix asphalt is now common practice in Switzerland. With the increases of RAP content in asphalt mixture, the problematic of the mix design optimization becomes an important issue to be considered. The described project deals with the mix design optimization of hot mix asphalt containing 40 % RAP. In a first phase, an analytical mix design has been conducted. The use of such a numerical method presents in particular the advantages of optimizing laboratory testing and allowing the realization of sensitivity analysis. An extensive laboratory study has been conducted in a second phase. This study permitted to clearly identify and quantify the impacts of the aggregates quality and binder content on the final mixture performances. The various tests achieved highlighted that rutting susceptibility is one of the suitable indicators for the assessment of a mix design performances. The research carried out finally permitted to highlight the benefits of an analytical mix design in a mixture optimization process. The major factors influencing the mix design have been identified and quantified through laboratory testing and some recommendations concerning the tests to carry out are also provided.

Double coating of full depth reclamation materials

Full Depth Reclamation (FDR) and Cold In-Place Recycling (CIR) are popular rehabilitation techniques for flexible pavement. Both techniques can be done with the addition of asphalt emulsion or foamed bitumen as a stabilizer. In most cases, those two techniques to add bitumen are considered equivalent, even if the way they give cohesion to the mix is different. A research project was undertaken to verify the possibility of using both asphalt emulsion and foamed bitumen on FDR materials to have better mechanical characteristics since it’s believed that asphalt emulsion gives a good coating, and foamed asphalt does not coat as well but serves as a binding agent. FDR mixes were prepared in the laboratory with and without asphalt emulsion and foamed bitumen. Different addition processes were tested in order to find the optimum mix design procedure. Moisture susceptibility tests (Marshall tests and indirect tension tests) and complex modulus tests were performed on the different mixes, and it was shown that the use of both binders can give you a mix with better performance. This paper presents the mix design, the test program and the results of this study.