Characterisation of warm asphalt mixtures with addition of reclaimed asphalt pavement materials

Pavement researchers have increasingly focused on reducing production and compaction temperatures in order to improve the environmental and economic impacts of hot mix asphalt, without adversely affecting the workability, durability and performance of asphalt pavements. The introduction of innovations and technologies in the form of warm mix asphalt will lead to substantial environmental improvements and economic prosperity. Moreover, in order to maximise the benefits of such technology, the inclusion of higher percentages of reclaimed asphalt pavement materials in warm mix asphalt allows the development of a more sustainable and cost-effective pavement structure. This thesis studies the characterisation of warm asphalt mixtures with the addition of reclaimed asphalt pavement materials. The effect of warm additives Sasobit, Rediset WMX and Rediset LQ on the viscosity of bitumen was investigated in detail. Furthermore, the effect of these additives on the rheological properties of bitumen was also identified using a dynamic Shear Rheometer. In fact, there is a point of controversy among pavement engineers about the performance of warm mix asphalt in terms of fatigue; therefore, firstly, the fatigue performance of warm-modified bituminous binder was investigated in detail. Fatigue test were conducted in dynamic Shear Rheometer using a time Sweep method and data were modelled using the viscoelastic continuum damage approach. After proving that warm additives such as Sasobit and Rediset WMX improved the fatigue life of asphalt binder, emphasises were paid to investigate nano-mechanical properties of warm modified bituminous binders and mix. Therefore this study also presents an investigation into the impact of warm additives on topography, modulus, deformation and adhesion of warm-modified bituminous binders using atomic force microscopy with the peakForce quantitative nano-mechanical mapping modality. The effect of production temperatures on the performance of warm mix asphalt was further investigated. Nanoindentation, which is an advanced technique, was used to study the effect of warm additives on the nano-mechanical properties of asphalt mixture phases, aggregate, interfacial transition zone (between aggregate and binder) and mastic. The nanoindentation results were used: firstly, to evaluate the effect of production temperatures and warm additives on the mixture phases; of warm mix asphalt; secondly, the nano-mechanical properties of interfacial transition zone were used to propose a new method to evaluate the degree of bonding between aggregate and binder in the mixture and, thirdly to evaluate the effect of each phase on the overall fatigue performance of warm mix asphalt. The aggregate-binder bond is one of the main factors that affect the durability of asphalt mixtures. This can be investigated based on the energy required to fracture the adhesive bond between binder and aggregate. In this thesis, the effect of warm additives on the adhesive bond strength of an aggregate-binder system was investigated using the pull-off test. Furthermore, the contribution of warm additives in improving the work of fracture has been linked to the adhesion force determined using atomic force microscopy and work of indentation approach. The fatigue life of WMAs was further investigated using a dynamic Shear Rheometer to study the effect of warm additives, production temperatures and binder grade. It was found that the level of reduction in the production temperatures and binder grade highly affect the fatigue performance of warm mix asphalt. Moreover, as Nanoindentation has shown to have the potential to characterise the properties of mixture phases, those properties can help in understanding the overall fatigue behaviour of warm asphalt mixture and show how improving nano-mechanical properties of interfacial transition zone and mastic can reflect the significant improvement in the fatigue life of asphalt mixtures. It is therefore recommended that highway agencies should specify a minimum production temperature for warm mix asphalt to reach acceptable mixture stability in terms of fatigue performance, taking into account the binder source and grade. Two issues regarding the inclusion of reclaimed asphalt pavement materials in asphalt mixtures need to be further investigated: the fatigue behaviour of reclaimed asphalt pavement materials mixes and the degree of blending between reclaimed asphalt pavement and virgin materials. The level of blending between virgin and reclaimed asphalt pavement materials was investigated under the scope of this thesis. The level of blending was investigated based on the nano-mechanical properties of mastic and interfacial transition zone measured using nanoindentation. Results were validated using scanning electron microscopy and the overall performance of warm mix asphalt incorporating reclaimed asphalt pavement materials in terms of stiffness and fatigue. Accordingly, a simple protocol is proposed to obtain a complete blending between reclaimed asphalt pavement binder and virgin binder, which is a key component of suitable practices in the pavement industry. The implications of this work for industry are that, firstly, warm additives offer a superior performance in producing asphalt mixtures that have better fatigue performance than traditional hot mix asphalt. Moreover, the study revealed that warm additives improve the strength bond between aggregate and binder. Secondly, highway agencies are advised to specify a minimum production temperature for warm mix asphalt to reach acceptable mixture stability. A complete blending between reclaimed asphalt pavement and virgin materials can be achieved using the proposed protocol; therefore a more sustainable and cost-effective pavement structure can be constructed.