Sub-Ballast

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 thermal cycles in this layer and, an evaluation of the average
seasonal temperatures interpolated by sinusoidal functions, of which characteristic parameters are determined. According to weather
situation, Barber’s temperature model was used to prove the effectiveness for the railway superstructure. It is included the assessment
of improved modified asphalt mixes performed with coarse rubber from scrap tires, having 1.5 to 3 percent of crumb rubber (particle
size 0.2-4 mm) by weight of the total mix, as sub-ballast layer in railway and base layers on roads, recurring to the Superpave mix
design compaction enhanced after computer simulations to evaluate real stresses derived from the rail traffic and climatic conditions.
This article following the assessment of the average seasonal temperatures, involves the characterization of rubberized materials with
attention to crumb rubber properties, designed with dry technology, to enhance the bitumen-rubber and binder-voids ratios. Indirect
tensile strength and water sensitivity tests were applied for the evaluation of its mechanical properties including dynamic complex
modulus at elevated temperature to measure the amount of bitumen absorbed by the rubber. The rubberized mix-results obtained and
the comparison with a conventional HMA (hot mix asphalt) show that these dry rubber bituminous mixtures are particularly effective in
damping vibrations. The purpose of using rubber modifiers in hot mix asphalt to obtain a stiffer-elastic sustainable material has been
achieved for the assessment of its behavior in sub-ballast/base layers.

The objectives of this study are focused on the ideal methodology and the dimensioning of the railway superstructure , involving the use of a bituminous sub-ballast layer modified with recycled natural rubber tire out-of-use. The previous study of the thermal transmission in each Railtrack layer, the analysis of the traffic in high-speed lines and the revision of the thermal-mechanical models have motivated this research. An experimental methodology has been optimized for the application of the volumetric mix-design with the gyratory compactor (SGC). According to the meteorological situation and applying experimental models based on thermal conductivity interpolated by sinusoidal functions, a laboratory study of conventional bituminous mixtures and improved mixtures of asphalt modified with coarse rubber waste tires is illustrated. The enhanced methodology entails a case study where compacted mixes are used by SGC, replacing rubber between 1.5 and 3 percent of rubber (particle size 0.2-4 mm) in the total weight of the blend. After the evaluation of the average seasonal temperatures, the mixtures were designed considering the dry process, as an advanced measure of sustainability and for their demonstrated improvements in thermal behavior and resistance to fatigue. A step-by-step manufacturing process is provided to avoid swelling problems in the post-compaction phase characteristic of dry mixes. The purpose of using rubber modifiers in the hot mix asphalt has been achieved to obtain an elastic sustainable material for the evaluation of its behavior in sub-ballast layers.
CITE AN ARTICLE
Soto, F. M., & Di Mino, G. (2018). “Improvements in the mix-design, performance features and rational methodology of rubber modified binders for the thermal evaluation of the railway sub-ballast”, (IJRSM/F09/17) International Journal of Research Science and Management, ISSN: 2349-5197, Volume 5, Issue 2, February 2018].

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 thermal cycles in this layer and, an evaluation of the average seasonal temperatures interpolated by sinusoidal functions, of which characteristic parameters are determined. According to weather situation, Barber's temperature model was used to prove the effectiveness for the railway superstructure. It is included the assessment of improved modified asphalt mixes performed with coarse rubber from scrap tires, having 1.5 to 3 percent of crumb rubber (particle size 0.2-4 mm) by weight of the total mix, as sub-ballast layer in railway and base layers on roads, recurring to the Superpave mix design compaction enhanced after computer simulations to evaluate real stresses derived from the rail traffic and climatic conditions. This article following the assessment of the average seasonal temperatures, involves the characterization of rubberized materials with attention to crumb rubber properties, designed with dry technology, to enhance the bitumen-rubber and binder-voids ratios. Indirect tensile strength and water sensitivity tests were applied for the evaluation of its mechanical properties including dynamic complex modulus at elevated temperature to measure the amount of bitumen absorbed by the rubber. The rubberized mix-results obtained and the comparison with a conventional HMA (hot mix asphalt) show that these dry rubber bituminous mixtures are particularly effective in damping vibrations. The purpose of using rubber modifiers in hot mix asphalt to obtain a stiffer-elastic sustainable material has been achieved for the assessment of its behavior in sub-ballast/base layers. Highlights  An innovative approach for the volumetric design of bituminous mixture with rubber (dry process);  Optimal parameters of temperature and traffic to characterize the mixture for a sub-ballast layer;  Applicability of Barber forecasting model used in the field road, to the railway superstructure;  Performance of the design process for the volumetric analysis of rubberized asphalt;  160 kN and 80 kN, respectively, rail equivalent axle load (R ESAL) were selected comparing sub-ballast vs. road base course solicitations induced for each layer;  Increase of workability and compaction properties decreasing air voids content to 3%-4% of the total mix weight;  Crumb rubber percentage between 1.5% to 2% and a digestion time of 120 min produced the optimal results.

The design of an unmodified bituminous mixture and three rubber-aggregate mixtures containing rubber-aggregate by a dry process (RUMAC) was evaluated, using an empirical-analytical approach based on experimental findings obtained in the laboratory with the volumetric mix design by gyratory compaction. A reference dense-graded bituminous sub-ballast mixture (3% of air voids and a 4% bitumen content over the total weight of the mixture) and these rubberized mixtures by dry process (1,5 to 3% of rubber by total weight and 5-7% of binder) were used applying the Superpave mix-design for a level 3 (high-traffic) design rail lines. The railway trackbed section analyzed was a granular layer of 19cm compacted, while for the sub-ballast a thickness of 12cm has been used. To evaluate the effect of increasing the specimen density (as a percent of its theoretical maximum specific gravity), in this article, are illustrated the results obtained after different comparative analysis into the influence of varying the binder-rubber percentages under the sub-ballast layer mix-design. This work demonstrates that rubberized blends containing crumb and ground rubber in bituminous asphalt mixtures behave at least similar or better than conventional asphalt materials. To do this, using the same methodology of volumetric compaction, the densification curves resulting from each mixture have been related to obtaining an optimum empirical parameter multiplier of the number of gyrations necessary to reach the same compaction energy as in conventional mixtures. It has provided some experimental parameters adopting an empirical-analytical method, evaluating the results obtained from the gyratory-compaction of bituminous mixtures with an HMA and rubber-aggregate blends. Comprehensive, integrated research has been carried out to assess the suitability of rubber-modified hot mix asphalt mixtures as a sub-ballast layer in railway underlayment trackbed. Design optimization of the mixture was conducted for each mixture and the volumetric properties analyzed. Also, an improved and complete manufacturing process, compaction and curing of these blends are provided. By adopting this increase-parameters of compaction, called " beta " factor, mixtures modified with rubber with equal densification and workability are obtained that in the conventional mixtures. It is found that considering the usual bearing capacity requirements in rail track, the optimal rubber content is 2% (by weight) or 3.95% (by volumetric substitution) and a binder content of 6%.

Bituminous sub-ballast is an alternative solution to the unbound granular sub-ballast used in the railway track due to several
benefits that it can provide. Indeed, it contributes to maintain the moisture content in the subgrade unchanged during all year. This
decreases the subgrade deterioration process. Moreover, the presence of bituminous sub-ballast can also reduce vertical stiffness
variations on the track; it can have a positive effect in the maintenance needs at transition sections (bridge-embankment) and in the
attenuation of the vibrations induced by the rail traffic. Despite the importance of the presence of the bituminous sub-ballast to conceive
the construction and/or rehabilitation of sustainable infrastructure, in literature, there are only fragmentary information regarding the
definition of benchmark criteria for their mix design. The superpave mix design approach used in road domain is applied systematically
in the railway domain, without being adjusted for different load configuration of the rail track system. This research work aims at
defining the benchmark criteria for the bituminous sub-ballast mix design to reduce the approximations involved in the recipe
optimization due to the limitation of applying the superpave system in the railway domain. The methodology proposed aims at selecting
the RESAL (rail equivalent single axle load) and therefore, transforming the entire traffic spectrum on the track lines in number of
ESALs. Afterwards, the Ndesign has been calculated as function of the rail traffic level. Finally, a case study of bituminous sub-ballast
mix design has been investigated for a first verification of the methodology proposed.

The design of an unmodified bituminous mixture and three rubber-aggregate mixtures containing rubber-aggregate by a dry process (RUMAC) was evaluated, using an empirical-analytical approach based on experimental findings obtained in the laboratory with the volumetric mix design by gyratory compaction. A reference dense-graded bituminous sub-ballast mixture (3% of air voids and a bitumen 4% over the total weight of the mix), and three rubberized mixtures by dry process (1,5 to 3% of rubber by total weight and 5-7% of binder) were used applying the Superpave mix-design for a level 3 (high-traffic) design rail lines. The railway tracked section analyzed was a granular layer of 19cm compacted, while for the sub-ballast a thickness of 12cm has been used. To evaluate the effect of increasing the specimen density (as a percent of its theoretical maximum specific gravity), in this article, are illustrated the results obtained after different comparative analysis into the influence of varying the binder-rubber percentages under the sub-ballast layer mix-design. This work demonstrates that rubberized blends containing crumb and ground rubber in bituminous asphalt mixtures behave at least similar or better than conventional asphalt materials. By using the same methodology of volumetric compaction, the densification curves resulting from each mixture have been studied. The purpose is to obtain an optimum empirical parameter multiplier of the number of gyrations necessary to reach the same compaction energy as in conventional mixtures. It has provided some experimental parameters adopting an empirical-analytical method, evaluating the results obtained from the gyratory-compaction of bituminous mixtures with an HMA and rubber-aggregate blends. An extensive integrated research has been carried out to assess the suitability of rubber-modified hot mix asphalt mixtures as a sub-ballast layer in railway underlayment tracked. Design optimization of the mixture was conducted for each mixture and the volumetric properties analyzed. Also, an improved and complete manufacturing process, compaction and curing of these blends are provided. By adopting this increase-parameters of compaction, called " beta " factor, mixtures modified with rubber with uniform densification and workability are obtained that in the conventional mixtures. It is found that considering the usual bearing capacity requirements in rail track, the optimal rubber content is 2% (by weight).
INTERNATIONAL JOURNAL OF ENGINEERING
SCIENCES & RESEARCH TECHNOLOGY, 7(1), 483-507.
DOI: 10.5281/zenodo.1158653