Field performance monitoring of waste tire-based permeable pavements

Abstract

Traditional pavements in urban areas are often impervious, resulting in augmented surface run-off during rainfalls, thereby leading to flash floods and pollution of waterways. In contrast, permeable pavements allow percolation of water through their surface layers, thus alleviating the harmful impacts associated with traditional pavements. This study reports on the mechanical performance of a large-scale permeable pavement trial site — constructed by tire- and rock-derived aggregates (TDA and RDA) bonded together using a polyurethane (PUR)-based binder — located at a car park in South Australia. An area of approximately 400 m² was paved using different TDA-based mix designs, i.e., different RDA contents and sizes/shapes, and different PUR contents. A series of unconfined compression tests were carried out alongside a six-month field performance monitoring program — including in-situ light-weight deflectometer tests, and strain measurements by optic fiber sensing — to assess the pavement’s true potential under live traffic. The greater the TDA and PUR contents, the lower and higher the pavement’s strength and stiffness, respectively. Meanwhile, the development of strain (and hence deformability) was in favor of both the TDA and PUR contents. An increase in RDA size, which promotes an induced inter-particle frictional resistance in the soft–rigid matrix, was also found to enhance the pavement’s strength/stiffness while offering a further reduction in its developed strain. Moreover, the greater the RDA angularity, the more effective the mechanical interlocking (and hence interfacial friction) generated between the soft and rigid particles, and thus the higher the developed strength/stiffness. Although effective under low–medium traffic loads imposed by passenger vehicles inside the parking bays, the TDA-based technology (with the implemented mixture designs) was not as effective in sustaining high traffic flows within the parking aisles, e.g., U-turns imposed by light–medium trucks.

Introduction

Traditional pavements in urban areas are often characterized as “impervious” surfaces, which result in augmented surface run-off during rainfalls, thereby leading to flash floods and pollution of waterways [1], [2], [3]. To cope with these harmful impacts on local hydrology and stream health, many developed and developing countries have initiated integrated land and storm water management programs/schemes — such as water sensitive urban design (WSUD) in Australia, and sustainable urban drainage systems (SUDS) in Europe. A common solution as proposed by these programs involves the use of permeable pavement technology. In its simplest terms, a permeable pavement system facilitates percolation of water through its surface layers, thereby mitigating the harmful environmental impacts associated with traditional impervious pavements. Common permeable pavement technologies, as reported in the research literature, include: (i) porous asphalt; (ii) porous concrete; (iii) permeable interlocking pavers; and (iv) grid pavement systems [4]. Although effective from a water management perspective, some of these technologies may be associated with some disadvantages in terms of their mechanical performance, mainly attributed to their low flexibility, aggregate raveling issues and low bearing capacity [5]. As such, current permeable surfacing solutions have often been limited to small-scale projects, e.g., around trees and pedestrian walks. Accordingly, the development of an alternate permeable paving solution, with emphasis on sustainability and enhanced mechanical performance capable of sustaining low–medium traffic loads, is urgently required.

Discarded tires are among the largest and most problematic sources of solid waste, owing to extensive production and their durability over time [6]. In Australia, for instance, around 0.5 million tons of scrap (or end-of-life) tires are stockpiled every year [7]. Waste tires are not desired at landfills, attributed to their low mass-to-volume ratio, resilience and flexibility, which prevent them from being “flat-packed” [8]. Accordingly, to minimize the need for landfilling, as well as other hazardous practices such as energy recovery, local communities and governmental agencies have been increasingly encouraged to recycle and hence reuse waste tires as part of the infrastructure system. The tires’ adverse characteristics, from a landfill perspective, also make them one of the most reusable waste materials for routine civil and geotechnical engineering applications, e.g., concrete manufacturing, soil stabilization and pavement construction [6], [8], [9], [10], [11], [12], [13]. In the context of pavement engineering, for instance, a novel example includes poroelastic road surface (PERS), which recycles waste tires into low-noise permeable pavements [14], [15], [16].

Recent experimental findings reported by the authors indicate that the resilience and flexibility offered by the combination of soft and rigid aggregates, i.e., tire- and rock-derived aggregates (TDA and RDA), bonded together by means of polyurethane-based binders, can be employed to mitigate settlements induced by natural ground movements while sustaining low–medium traffic loads [5], [17], [18]. More importantly, the high porosity of the optimum soft–rigid blend, optimized by means of the soft-to-rigid particle size ratio, can reduce/stabilize storm water run-off during flash floods, as well as potentially improve the quality of water ending in our waterways. In essence, the proposed TDA-based permeable pavement technology not only serves as a sustainable alternative to common permeable pavement products, but also offers superior drainage, enhanced (or equally competitive) mechanical performance, and long-term resilience (owing to TDA’s high durability against local environmental conditions). In terms of cost per square meter with respect to the South Australian market, the TDA-based technology is found to be approximately 25% more expensive than a conventional bitumen-based system. However, compared to current permeable paving solutions commonly implemented in South Australia, such as permeable interlocking pavers, the TDA-based system offers a lower final cost. Moreover, in view of long-term water management costs associated with impervious bitumen-based pavements, the proposed TDA-based technology can be considered a competitive surfacing solution with respect to certain applications involving low–medium traffic loads (e.g., car parks and drive ways).

It is well accepted that the loading conditions experienced by pavement systems over time cannot be realistically replicated in the laboratory. Meanwhile, when promoting new pavement technologies, such as the TDA-based permeable pavement system proposed in this study, the need for field stress–strain data, and more importantly, feasibility studies on effective field implementation poses as an inevitable necessity. In such cases, in-situ tests, along with field instrumentation techniques, should be employed to monitor and hence perceive the system’s true potential over time. Although some experimental studies, including those reported by the authors, have provided a better understanding on the hydro-mechanical response of high-porosity TDA-based blends, their feasibility in terms of field implementation, as well as their real-life performance under live traffic loads, have not yet been investigated. On this front, a large-scale field trial, accompanied by a systematic field performance monitoring program, will provide much needed confidence for end-users (and industry) on the use of this innovative product, eventually solving two widespread hazards with one solution — that is, diverting millions of tires from landfills while satisfying the requirements of integrated land and storm water management programs.

This study presents the authors’ recent experience in the development and implementation of an instrumented large-scale TDA-based permeable pavement trial site constructed at a car park located in South Australia. An area of approximately 400 m², consisting of 24 parking bays, was paved using different TDA-based mix designs — that is, different rock contents, sizes/shapes and colors, and different polyurethane (or binder) contents. A comprehensive field performance monitoring program — consisting of in-situ light-weight deflectometer (LWD) tests, and strain measurements by means of optic fiber sensing — was carried out over a six-month period to assess the system’s true potential under live traffic loads.