The feasibility of using asphalt concrete with a high percentage of recycled asphalt material in a railway trackbed layer

The use of asphalt concrete (AC) for railway trackbed layers is now well established and, in some countries, even accepted as a standard solution for high–speed or high–load lines. To date constructed track sections which incorporate AC trackbed layers use AC mixtures which contain only up to 30% of reclaimed asphalt (RA). This limit is most likely a remnant of the past when the amount of RA in AC mixtures was limited as a precaution to compensate for the lack of experience with RA. Nowadays, the trend and the aspiration for using higher percentages of RA in AC mixtures is increasing and railway engineering should embrace this. This paper presents the results of ongoing research to this end. The first part of the paper describes a laboratory–based investigation into the required design properties of an AC mixture made of 100% of RA. The second part describes the construction and continuous assessment of a trial section on an operational mainline in the Czech Republic. The results show that it is possible to construct an AC trackbed layer with a high percentage (70%) of RA using only simple mechanization and that such a layer provides an effective formation treatment, significantly reduces the loss of track geometry, provides more uniform track support and protects the subgrade from the adverse effects of temperature changes and the ingress of rainwater. The paper concludes that an AC mixture for a railway trackbed layer should be treated as a specific type of AC mixture, with its own set of design criteria, and invites further research into the feasibility of using AC with high percentage of RA not only as a remediation technique but as a standard trackbed solution.