Estudio de las propiedades mecánicas de materiales asfálticos en servicio para la optimización de las soluciones de conservación de firmes de carreteras

Resumo

The mechanical performance of asphalt mixtures is notably influenced by traffic loads, service temperature, the binders used in its manufacturing and its ageing. That is why there exists the need to analyze the evolution of the mechanical properties of different kinds of asphalt mixtures exposed to distinct climate conditions and traffic loads, while assessing the combined effect that these variables, along with the ageing, could have in its mechanical response in the long-term under real service conditions. This analysis will enable us to design more durable asphalt mixtures with better mechanical performance, optimizing government budgets for pavement maintenance. This optimisation has to be focused both on natural resources, recycling, reusing and valorizing waste products as mix componentes, and on manufacturing techniques, to stimulate their environmental sustainability, reducing the greenhouse gasses emissions and the fossil fuel consumption. Due to their environmental advantages, the use of crumb rubber modified binders constitute a feasible alternative to produce asphalt mixtures. However, in practice, they remain less commonly used than conventional polymer modified binders, mainly due to the scarce tracking of the experiences already conducted. Thus, this research aims to provide information about the evolution of the mechanical performance of these materials. To this end, some highway sections where the surface layers were built with asphalt mixtures manufactured with crumb-rubber-modified bitumen with severe climatic and traffic conditions were selected. In addition, other sections built side by side the previous ones were chosen, but manufactured with the conventional SBS polymer modified bitumen, to compare both materials (at binder and mixture level). For this comparison, cores were taken at different periods of their service life. Regarding binders, the real ageing of crumb rubber modified asphalt bitumens when exposed to various factors, including temperature gradients, the presence of water and oxidation, was studied under real traffic and climate conditions. The binders from cores of highway surface layers were recovered and tested using the DSR (Dynamic Shear Rheometer) to determine the evolution of the rheological parameters. The analysis of the complex modulus and phase angle was conducted based on frequency and temperature sweep tests, while the evolution of the elastic recovery, Jnr, L-Index and T-Index were assessed from the MSCRT (Multiple Stress Creep and Recovery Test). The results obtained indicate that crumb rubber modified binders show similar ageing and rheological parameters to those of conventional polymer modified bitumen, even under severe traffic and climate conditions. Regarding asphalt mixtures, the performance of mixtures manufactured with crumbrubber- modified bitumen and conventional SBS polymer modified bitumen was assessed by analyzing the evolution of the density, stiffness and fatigue resistance of the surface layers. Based on the results obtained from these tests, we could conclude that under real severe traffic and climate conditions, asphalt mixtures manufactured with crumb-rubbermodified bitumen offer ageing and mechanical performance very similar to that offered by asphalt mixtures manufactured with traditional SBS-modified bitumen, being a choice to minimize environmental problems caused by end-of-life tires in landfills. After having demonstrated the viability of using crumb rubber as a sustainable solution to obtain durable asphalt mixtures, and also to obtain more efficient asphalt mixtures when designing the best option for pavement maintenance, it was decided to assess the main limitations of its real applications, which are high manufacturing temperatures and low workability. Thereby, a surface layer mixture which combined the reduction in temperature (with a reduction in greenhouse gasses emissions) with the use of crumb rubber from end-of-life tires was assessed. It was tested at the laboratory and on roads subjected to a high volume of traffic and severe climate conditions. Different studies were developed both in the laboratory and in an asphalt plant (at binder and mixture level). Later, these materials were used to construct a trial section in a highway at a mountain pass (at more than 1400 m above sea level) supporting more than 3000 heavy vehicles each day under severe environmental conditions (snow during winter, and high temperatures and many hours of solar radiation during the summer). The results indicate the viability of using these materials, since they provide, not only environmental advantages, but also a number of advantages such as improved workability at lower temperatures and an increase in its mechanical resistance against the main sources of distress that affect asphalt pavements.