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

Highlights

• Recycled asphalt concrete material can be used in railway trackbed layers.

• Recycled asphalt concrete trackbed layers can provide effective formation treatment.

• Amount of recycled material can be very high (up to 100%) in railway applications.

Abstract

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.

Introduction

In design and construction of a railway trackbed, formation treatment is used in locations where natural subgrade does not provide required properties, e.g., bearing capacity. The traditional solution for formation treatment consists of a system of multiple trackbed layers made from natural granular materials, e.g., gravely sand, crushed limestone. Whilst the traditional solution remains unchallenged in terms of its long–term performance, modern–day requirements for reduced overall trackbed thickness, time of construction and maintenance and increased train loads and speeds require new materials to be used for the railway trackbed. An asphalt material (or an asphalt-based or bituminous material) is one contender [1], [2], [3]. The advantages of an asphalt concrete (AC) trackbed layer include the following [4], [5]:

• Protection of the subgrade by enabling more uniform load distribution

• Protection of the subgrade by isolating it from an excessive ingress of water and in some cases from low temperatures, hence providing more stable subgrade conditions

• Preventing the pumping of fines due to the impermeability of the AC layer

• Providing stiffer support than a layer made of sand/crushed aggregate, hence partially compensating for a low bearing capacity of subgrade, where required

• Increasing drainage capabilities at the formation level, and hence lowering track transverse gradient requirements

These advantages contribute towards more stable track geometry, hence towards increased ride comfort and reduced maintenance needs [4]. An example of an AC trackbed layer in a railway track structure is shown in Fig. 1.

When incorporating asphalt concrete with the railway trackbed, i.e., in 1970′s in Italy, 1980′s in USA, 1990′s in Germany, 2000′s in France and Spain, railway engineers took their inspiration from highway engineering. Hence, design properties of most AC mixtures used in railway trackbed layers currently used are very similar, if not the same, as mixtures used in highway base layers [4], [5], [6]. From the 1970′s onwards the concept of reusing old asphalt in new AC mixtures has been investigated to reduce the consumption of virgin aggregate and bitumen, which are valuable natural resources. So called asphalt reclamation is now firmly established in highway engineering [e.g.,7]. Asphalt reclamation is a process which turns a site-won asphalt, e.g., asphalt gained from planning, asphalt slabs ripped up from asphalt pavements or asphalt from rejected, surplus or failing production, into reclaimed asphalt (RA), i.e., a material of a known quality suitable to use as a constituent material in a new AC mixture. The quality of processing and assessing of the site–won asphalt are essential for a successful subsequent usage of RA in new mixtures [7], [8], [9]. Hence, most countries provide strict standards by which a material must adhere if it is to be classified as RA, e.g. EN 13 108–8 [10], [11]. Historically, limits have been placed on the maximum amount of RA deemed suitable for use in new AC mixtures. For instance, in the UK an amount of RA of more than 10% in mixtures for surface courses and more than 20% in mixtures for binder courses and bases respectively requires special consideration [8], [12]. In USA, the Asphalt Pavement Industry Survey [13] shows that in most States the maximum amount of RA used in mixtures does not exceed 25%, with only two States allowing over 30% of RA in new mixtures, although no information is provided as to for which courses these limitation apply. In the Czech Republic, the maximum percentages of RA are 15%, 25% and 50% for surface courses, binder courses and bases respectively [14]. For AC mixtures with RA used in railway trackbed, SNCF in France built a trial section with an AC mixture containing 20–30% of RA in 2011 [6]. This seems to be in agreement with highway mixture design limits at the time. The limits on the maximum percentage of RA in new mixtures were probably introduced as a precaution when the usage of RA was novel and when there was only limited experience available. However, since then many studies have demonstrated that when the quality of RA is properly assessed and quality assurance/quality control (QA/QC) is ensured during mixture production, transport and layer construction, the resulting properties of the layer are comparable or even better than that of a layer made of a virgin mixture [8], [9]. Hence, nowadays these design limits might be considered to be obsolete and, despite being still present in most standards, highway agencies often permit usage of higher percentage of RA in mixtures when appropriate QA/QC is observed. The trend of RA usage is certainly increasing [8], [9], [13].

This paper describes the results of on-going research investigating the feasibility of incorporating AC mixtures with a high percentage of RA within railway trackbed layers. The aim of the research was to build on the extensive experience in the Highway sector of using RA as a constituent material of an AC mixture and to investigate the feasibility of using a mixture made from 100% of RA. This requires appropriately chosen RA to be reheated in order to construct a trackbed layer. As will be discussed later, this was possible only during the laboratory phase of the investigation. Due to technological limitations of the mixing plant the trial section was built with an AC mixture containing only 70% of RA.

The paper is structured as follows: First, results and analysis of a laboratory investigation into the required design properties and the behaviour of a trackbed layer made of RA are presented. Second, a full-scale application of AC mixture with 70% of RA in a trackbed layer on an operational mainline in the Czech Republic is described together with a discussion of its performance. Finally, results of both parts of the investigation are discussed and conclusions are drawn.