Characterization of rubberized asphalt for railway sub-ballast. Improvements in the mix-design, performance features and rational methodology of HMA_RUMAC blends for railways

(STR) can be incorporated into asphalt mixtures in two different methods, which are referred to as the wet-dry process. The blending of recycled rubber (CRM) with asphalt has been used for years, and several manufacturing processes have been developed in Europe as well as in the USA. The use of a bituminous sub-ballast layer has been pointed out as an exciting alternative to the granular sub-ballast design traditionally applied in most railroad tracks. Frequently, unbound granular materials are replaced by bituminous sub-ballast that provide additional benefits to the subgrade protection. Much research has been conducted on finding other alternative material to be used as a modifier in asphalt mixes to improve its properties. Rubberized asphalt mixtures (RUMAC) are regarded as a proper solution for improving the strength of the rail-track section. In comparison with traditional granular subballast, these materials allow an increase in bearing capacity and excellent protection of the substructure. The recycled rubber has become a significant enhancer of the modified bituminous mixtures, and in this work, it has been shown as a sustainable improvement option in hot mix asphalt (HMA) mixes due to the elastic behavior exposed by the rubber particles especially in reducing the fatigue cracking potential. This thesis presents a laboratory evaluation of the performance of HMA-DRY mixes to study its application as sustainable bituminous sub-ballast manufactured from scrap tires at the end of their service life. To this end, the procedures developed in the Department of Civil Engineering of the University of Palermo during 2014 to 2017, were focused on evaluating the mechanical behavior of bituminous materials in comparison to that presented by conventional sub-ballast. Mechanical performance was examined concerning the primary requirements that need these materials (resistance to energy dissipation, fatigue cracking strength and, waterproof properties) for their use in railway tracks. Inside the SUP&R ITN project, enrolled as an ESR-8 fellow, the primary purpose of this laboratory research was the application of ambient crumb rubber (0.2mm to 0.4mm) and ground rubber (2mm to 4mm) recycled from discarded truck tires. CRM was processed at a standard temperature (20ºC) inside hot bituminous mixtures (160-220ºC) as a substitute for 1.5%, 2% and 3% by weight of the total aggregates using the Volumetric Mix-Design gyratory compactor (SGC) and Asphalt Slab Roller Compactor (ASRC). The impact of temperature on the mechanical properties and thermal susceptibility of the railway bituminous sub-ballast layer has served as motivation to develop the advanced measurement of the thermal cycles inside the rail track. Different simulations following the AASHTO Mechanistic calibrated model with Kentrack and Kenpave software (Univ. Kentucky) were developed to be effective with the best mix-design for railways. Thermal cycles in this layer and, an evaluation of the characteristic parameters were found. A chapter was then included on the application of the AASHTO mechanistic-empirical pavement design approach to railways, to calculate first strain and deformation parameters, resulting in a rationale for the definition of a Railway Equivalent Standard Axle Load (RESAL) and a design number of gyrations for use with SGC. CRM incorporated into asphalt mixes by using “dry process” method which refers to technology that mixes the fine-ground rubber with the aggregate before mixing it with asphalt binder. Two aggregate gradations were considered under this investigation, dense-graded (asphaltic concrete with 22.4mm nominal maximum aggregate size) and gap-graded (stone mastic asphalt with 31.5mm maximum aggregate size). For each particle size distribution, of an aggregate sieve, the percentages of CRM added varied from 1 to 3% by weight of the total aggregates. Italian standard (RFI) as grading curve and Superpave gyratory compactor technique (SGC) as mix-design were used. The UGR_FACT device investigated that RUMAC mixes had shown a higher resistance to fatigue cracking compared to the conventional blend (HMA) as a performance indicator. The results proved that the rubberized sustainable solution studied can offer advantages such as reducing greenhouse emissions, fuel consumption and, a suitable performance for railways tracks from a mechanical behavior point of view. The purpose of using rubber modifiers in HMA to obtain a stiffer-elastic sustainable material has been achieved for the assessment of its behavior in the sub-ballast layer. The better overall performance and fatigue results obtained were with an amount of 2% of crumb rubber and a 6% of bitumen added.