Mechanical behaviour and load bearing mechanism of high porosity permeable pavements utilizing recycled tire aggregates
Introduction
The rapid economic development of modern societies comes with consequences such as dramatic rise in waste generation. Disposal of scrap tires generated from end of life tires is continually increasing globally and in Australia. Mountjoy, et al. [1] reported that 51 million equivalent passenger units (EPU) of Australian tires is entering the waste stream annually of which approximately 5% are recovered through recycling chains. Aside from the negative environmental impacts of landfilling end of life tires, a serious safety concern is recognized by councils and EPA Victoria for tire stockpiles near residential areas. A recent fire incident in a small recycling facility near Melbourne raised the alarm regarding the safety of stockpiling tires for long periods of time. The fired stockpile yard was 18 km north of Melbourne CBD which effected 15 suburbs by toxic fumes. Despite prompt action of authorities, 70% of the stockpile (approximately 130,000 end-of-life tires) burnt and the fire was came under control only after 48 h (Fig. 1). Apart from air pollution concerns from the incident, EPA also considered potential impacts to nearby waterways and soil [2]. The wake-up alarm of the Melbourne incident on importance of city safety management and serious environmental penalties of such incidents raise the concern for councils of cities near similar stockpiles. The massive tire stockpile near Longford in Tasmania is 13 times bigger than the tire dump yard in Melbourne with an estimated 1,400,000 units of end-of-life tires. This stockpile is continuously increasing and not only wasting valuable lands and imposing environmental penalties on nature, but also can be a serious threat to the nearby communities. The environmental impacts of landfilling the waste tires calls for innovative solutions of recycling these wastes back into engineering applications. In addition, the safety risk associated with maintenance of the landfills against fire hazards is a strong motivation for pursuing novel recycling alternatives. There has been intensive research dedicated to assessment of hazards of disposing tires to landfills [3], [4], [5].
The mechanical properties of tire scraps have been investigated for utilization in civil engineering applications such as concrete manufacturing, pavement constructions, earthfills and highway embankments [6], [7], [8]. However, the inferior mechanical properties of tire-derived aggregates (TDA) such as excessive deformation under design loads limits the use of these end-of-life tire aggregates to marginal applications with low percentages of tire. Considering the high costs of recycling tire compared to the low costs of natural quarried aggregates; there is limited use of recycled tire in marginal applications. This means large volumes of tire still end up in landfills. It is important to note that although the load bearing capacity of TDA aggregates is relatively low, their high elastic flexibility can be utilized in construction of low-volume roads. In addition, TDA can be useful where excessive differential settlement (caused by reactive clays or induced by vegetation in footpaths and roads) is an engineering challenge.
Conventional road pavements and footpaths in urban areas are typically rigid to semi-rigid impervious structures. Impermeable surfaces result in augmented surface runoff leading to flash flooding and pollution of waterways. Permeable pavement is an innovative solution to postpone flash flooding of paved surfaces. Permeable surfaces is currently used for stormwater management of footpaths though in a very limited scale. However, lack of technical knowledge and legislation in the area of pavement technology has led to insufficient application within the Australian road construction industry [9].
Contrary to traditional impervious surfaces; permeable pavements permit percolation of water through surface layers alleviating some of harmful environmental impacts [10]. In addition, high void ratio and flexibility of the permeable pavement can potentially accommodate the differential settlement followed by cracks and pavement deterioration [11], [12]. The porous structure of conventional permeable pavements (made entirely of rigid aggregate such as crushed rock) can be utilized to accommodate the excess run-off from flash flooding. However, the rigid contact between the rock aggregates can lead to failure due to settlement-induced stresses. TDA can further facilitate the flexibility of the pavement layer and resolve this issue. Nevertheless, high ratio of TDA can lead to a mixture dominated by the soft behaviour of TDA resulting in low bearing capacity. The subsequent high deformation potential of use of TDA in pavements is not suitable for serviceability of the pavement.
In this study, a transitional behaviour between soft behaviour (governed by TDA) and rigid behaviour (governed by rock aggregates) is defined and the stiffness of the pavement under low and high stress levels are presented. Waste tire is investigated as a potential substitute to virgin quarried rock aggregate in pavement structures. Moreover, waste tire can potentially be beneficial in improving the performance of the surface pavement layer. Successful incorporation of tire waste as raw materials for permeable pavement construction provides a shift away from high energy intensive processes involved in quarrying for natural rock. TDA present a formidable alternative pavement material as it comprises a unique set of properties including uniform gradation (resulting in superior drainage performance), long-term durability, flexibility and resilience (prevents mechanical breakage) and better frictional properties.
Section snippets
Mechanical properties of rigid soft mixtures
Most of the research thus far has used sand as the rigid material along with different kinds of tire scraps to investigate behaviour of rigid-soft mixtures in applications such as road embankments and fills. Substantial research has indicated that the relative size between rubber particles and rigid particles (soft to rigid size ratio, Sr = D50 soft/D50 rigid) and the fraction of volume of soft particles compared to the total volume of the rigid-soft particle mixture (soft particle volume
Materials and methods
The uniformly graded rigid materials were sourced from local suppliers in Victoria; Australia. The soft materials were sourced from a local tire recycling company, shredded into a rather uniform gradation. Round shape bulky Crushed Rock (CR) (mean particle diameter D50 = 5 mm, specific gravity Gs = 2.81) and irregular shaped TDA (mean particle diameter D50 = 6.8 mm, specific gravity Gs = 1.06) are used in this study (Fig. 2). The specifications of used materials are presented in Table 2. The
Experimental results
The induced vertical strain in the samples was translated to the respective void ratio and was plotted against the applied vertical pressure (Fig. 4). The oedometric stress–strain response of TDA-CR blends demonstrate that increasing the VTDA in the blend results in rapid increase of compressibility and rebound of the blends. The initial void ratio of almost all the blends are balanced around 1.0 for a relative density of 70% except for the specimen prepared with 100% of TDA which possesses a
Discussion
Further analysis of One-dimensional compression test results presented in Fig. 7a. is achieved using the constrained modulus which represents the ability of the blends in withstanding against the one dimensional deformation (M = Δσv/Δεz) computed between successive loading steps. The semi-empirical power function suggested by Kim and Santamarina [16] can be seen in the log–log scale presented at Fig. 7a. The constrained modulus is highly sensitive to VTDA. Increasing the stress level can
Conclusions
With increasing concern in regard to current unsustainable management of waste tire in Australia, it is rudimentary that end of life tires are put into large volume and technically sound applications. Tire waste when not disposed of appropriately can cause significant environmental impacts due to its shape and non-biodegradable nature. This study investigates the feasibility of utilizing tire derived aggregates to formulate a permeable pavement composition that would yield high performance both
Acknowledgements
This work was funded by Tyre Stewardship Australia (TSA) as part of a R&D program to develop new end-uses for end-of-life tires. The authors would like to thank Merlin Site Services for their cash and in-kind support and Tyrecycle for providing tire aggregates.
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