Abed, Ahmed
(2019)
Mix design, mechanical analysis and performance modelling of warm mix asphalt.
PhD thesis, University of Nottingham.
Abstract
The asphalt industry has been under pressure to reduce its environmental footprint and energy demands to meet current and future sustainability requirements. Warm Mix Asphalt (WMA) has been developed to reduce the environmental impact of asphalt by reducing its production temperatures from the hot range (i.e. ≥160°C) to the warm range (i.e. 100-140°C). The common idea of most WMA techniques is to use additives in order to either reduce bitumen viscosity or reduce bitumen surface tension, which enhances aggregate coating and eventually enables asphalt mixing at reduced temperatures. A robust method to determine a rational reduction in the production temperatures of asphalt, however, is not available so far; but some methods that are thought to be valid exist for certain additives such as the equi-viscous approach. Also, in the case of incorporating Reclaimed Asphalt Pavement (RAP) with WMA (WMRA), it is uncertain what production conditions are suitable for this mix and how best to design these conditions to ensure acceptable performance. Accordingly, the main aims of this research were to understand and analyse the impact of selected WMA additives on bitumen and asphalt properties, develop a rational method to design optimum production temperatures of WMA and WMRA, and deeply investigate the behaviour of these mixtures concerning performance and durability.
Different materials were involved in the study including two of the commonly used WMA additives, a chemical and a wax additive; a 50-60 penetration grade bitumen was adopted as a reference binder and used to produce control Hot Mix Asphalt (HMA); one source of RAP was used at a level of 50%. The methodology followed to achieve the aims of this study included different bitumen and mixture tests; developing an image processing technique to quantify aggregate coating and investigate additive impact on this property; develop a method to design production conditions for WMRA; investigate the durability of WMA and WMRA in terms of ageing and moisture damage resistance; modelling of these mixes to predict their mechanical performance in terms of rutting, top-down and bottom-up cracking.
The results of the bitumen investigation revealed that WMA modified binders should be characterised based on bitumen performance beyond the limits of linear viscoelasticity rather than based on the empirical or Superpave parameters.
Aggregate coating quantification results showed that WMA mixing temperatures can be decreased by about 20-30°C although additional mixing time was required to achieve full coating. However, this method must be integrated with performance results since some WMA additives, such as wax, have a significant relationship between their performance and mixing temperatures. Furthermore, based on limited compactability results, expressed as the compaction effort required by a roller compactor to compact WMA to a target density, it was concluded that the additives used both have a positive influence on enhancing asphalt compactability.
In the case of WMRA, it was discovered that the performance of WMRA significantly depends on the degree of blending (DoB) between the aged and soft binders. The DoB is found to be a function of mixing temperature and mixing time. Despite it being possible to achieve a DoB of about 96% in this study, the remaining unblended soft binder can be a source of weakness as results of WMRA revealed that this mix was susceptible to rutting at temperatures beyond the critical high temperature of the soft binder. Accordingly, the grade of the soft binder should be carefully selected, and the production conditions should be accurately designed to assure acceptable performance of this kind of asphalt.
Performance of WMA and WMRA was simulated based on the Mechanistic-Empirical Pavement Design Guide (MEPDG) principles. Rutting model parameters were determined in an innovative way from the repeated load axial test results; fatigue cracking model parameters were determined from the two-point bending results. A typical four-layer asphalt pavement was simulated; the simulation results showed that asphalt performance is controlled by two factors, the applied strain level and its mechanical response properties. The results also indicated that some mixtures can perform better than others when used as either surface or base layers. Accordingly, this analysis can be used in predicting pavement performance with a particular mix and selecting the layer in which it can perform the best.
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