*Article* **Recycling of End-of-Life Tires (ELTs) for Sustainable Geotechnical Applications: A New Zealand Perspective**

**Ali Tasalloti 1, Gabriele Chiaro 1,\*, Arjun Murali 1, Laura Banasiak 2, Alessandro Palermo <sup>1</sup> and Gabriele Granello 1,3**


**Abstract:** End-of-life tires (ELTs) are tires, unusable in their original form, which go into a waste management scheme (for recycling and energy recovery purposes), or otherwise are disposed. In New Zealand, the annual disposal of 3.5 million ELTs is posing critical environmental and socio-economic issues, and the reuse of ELTs through large-volume recycling engineering projects is a necessity. In this study, gravel and recycled granulated rubber were mixed to explore the possibility of obtaining synthetic granular geomaterials (with adequate geotechnical and environmental characteristics) that are suitable as structural fills for geotechnical applications including foundation systems for low-rise light-weight residential buildings. Moreover, an original framework with a set of geo-environmental criteria is proposed for the acceptance of gravel–rubber mixtures (GRMs) as structural fills. It is shown that when gravel-size like rubber particles are used, GRMs with volumetric rubber content of 40% or less have adequate strength (*φ*' > 30◦), low compressibility (ε<sup>v</sup> ≤ 3%), excellent energy adsorption properties, and acceptable leachate metal concentration values (e.g., Zn < 1 mg/L), making them ideal synthetic structural fill materials for many sustainable geotechnical applications.

**Keywords:** end-of-life tires; recycling; gravel–rubber mixtures; sustainable geotechnical applications; foundation systems

#### **1. Introduction**

Tire recycling is the process of converting unwanted end-of-life tires (ELTs)—that can no longer be re-grooved or re-treaded—into materials that can be utilized in new products or applications [1]. While in many countries, ELTs are a controlled waste under stringent environmental protocols, currently no national regulations are in place in New Zealand to properly manage ELT recycling. As a result, rising environmental and socio-economic concerns are commanding the reuse of ELTs by means of large-volume recycling civil engineering schemes. Below, the issues, challenges, and possible solutions to the ELT disposal problem are described with reference to the New Zealand context.

#### *1.1. Issues*

Currently, over 5 million ELTs are produced yearly in New Zealand (i.e., one per capita), including four million passenger vehicle tires and one million truck tires. Such numbers are expected to grow over time with increasing volume of vehicles on roads. It is estimated that only 30% of such ELTs are exported or recycled, with the remaining 70% disposed of in stockpiles, landfills, illegal dumping, or otherwise unaccounted for [2,3].

A typical example of inadequate ELT disposal practice in New Zealand is shown in Figure 1. The dumping of scrap tires into landfills is certainly the least appropriate option

**Citation:** Tasalloti, A.; Chiaro, G.; Murali, A.; Banasiak, L.; Palermo, A.; Granello, G. Recycling of End-of-Life Tires (ELTs) for Sustainable Geotechnical Applications: A New Zealand Perspective. *Appl. Sci.* **2021**, *11*, 7824. https://doi.org/10.3390/ app11177824

Academic Editor: Daniel Dias

Received: 19 July 2021 Accepted: 24 August 2021 Published: 25 August 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

for the disposal of ELTs. Such practice is unsustainable, causing significant environmental, socio-economical, and health problems such as inappropriate use of valuable land (up to 75% void space), harbor for pests and insects that may spread contagious and unknown diseases [4], potential water and soil contamination due to leaching of metals and other chemicals contained in the scrap tires [1,5–7], and the likelihood of uncontrolled fires of stockpiled tires [8–12]. Therefore, there are major benefits in moving away from ELT disposal and implementing sustainable recycling schemes.

**Figure 1.** A typical ELT dumping practice seen in New Zealand.

#### *1.2. Challenges*

Tire recycling may be challenging, but it is not impossible to achieve. In Europe, USA, Japan, Canada, and many other countries where strategic environmental procedures have been put in place to effectively make use of recycled ELTs, their disposal has been reduced to 20% or less [2,4,13].

As shown in Figure 2, reduce, reuse, recycle, and energy recovery are the four integrated basic options that should always be considered in dealing with waste management problems [14]. Similar to any other waste, also in the case of ELTs, the highest priority should be to avoid/reduce the generation of waste. This is obviously impractical due to the increasing volume of tired vehicles on roads. The next most preferred options should be reuse (as many times as possible and without further processing) and recycle (making new products) of ELTs. This will keep ELTs in the productive economy and benefits the environment by lessening the need for new materials and waste management. When extra recycling is not achievable, it could be possible to recover the energy from ELTs [15] (but only if environmentally adequate). Finally, only if ELTs could be safely recycled and direct treatment is not feasible, their disposal could become an eventual management alternative.

While avoidance/reduction is currently impracticable, ELT reuse and recycling are certainly feasible opportunities, and should be without a doubt preferred to energy recovery and disposal. In this regard, the most promising solution would be to use ELT-derived products as construction materials in sustainable large-scale civil engineering projects.

**Figure 2.** Waste management hierarchy applied to ELTs in the New Zealand context (adopted from [16]).

#### *1.3. Opportunities*

Reuse and recycling of industrial by-products, commercial wastes, and construction and demolition materials in geotechnical engineering applications are progressively required in Australasia as it provides important benefits in terms of increased sustainability and reduced environmental impacts [17]. In this setting, coal wash and steel slag mixtures have been reused as structural fills for a port reclamation development [18–20], recycled rubber mixed with coal wash and steel slag has been used in rail tracks [21], recycled glass and recycled concrete compounds have been characterized as pavement base [22], and recovered plastic and demolition wastes blends have been engineered as a capping layer for railway applications [23].

Given the above background, and aimed at facilitating the use of ELT-derived products as construction materials in sustainable civil engineering projects in New Zealand, a geo-environmental-structural engineering experimental research program—funded by the Ministry of Business, Innovation and Employment (MBIE)—has been jointly carried out by researchers of the University of Canterbury and the Institute of Environmental Science and Research Ltd. (ESR), Christchurch, New Zealand. Thus far, the main effort of such interdisciplinary research has been the development of "eco-rubber geotechnical seismic-isolation (ERGSI) foundation systems" for low-rise light-weight residential buildings [24–26]—readers can refer to the following website for full details of the project: https://sites.google.com/view/ecorubberfoundation/publications (accessed on 19 July 2021). Nevertheless, a series of experimental, numerical, and field investigations have been designed so that they would provide an in-depth understanding of key factors affecting the engineering behavior of soil–rubber mixtures and rubberized concrete, which must be taken into account in the design of such synthetic materials, hence, expediting their adoption not only into ERGSI foundation systems, but also in many other civil/geotechnical engineering applications.

In this paper, the results of the geotechnical and environmental investigations carried out to identify optimum energy-adsorption granular soil-recycled rubber mixtures, possessing excellent mechanical properties (e.g., compaction, compressibility, strength, dynamic properties etc.) and least leaching attributes are presented and discussed.

The results of the structural engineering laboratory tests (to design fiber-reinforced rubberized concrete structural elements, e.g., foundation raft, with satisfactory material and structural performance) and numerical investigations (i.e., DEM and FEM), physical models, and field trials (to verify the concept and assess the mechanical performance of geotechnical and structural elements individually and integrated into systems under both static and seismic load) will be presented elsewhere in due course.

#### **2. Soil-Rubber Mixtures: Practical Implications and Material Suitability**

Typically, ELT-derived aggregates (in the form of chips, crumbs, granules and shreds— ASTM [27]) assorted with cohesionless granular soil (mainly sand) have found use as lightweight backfill materials for embankments and retaining walls, drainage systems, slope remediation, and landfill construction [28,29]. However, more recently, due to their superior strength

and dynamic properties, soil–rubber mixtures have been proposed as free-draining energyadsorption backfill material for retaining walls, underground horizontal and vertical layers for liquefaction mitigation [30,31] and geotechnical seismic-isolation systems for residential buildings [32–35].

As reported in a comprehensive literature review undertaken by Tasalloti et al. [36], previous research has dealt primarily with the physical and mechanical characterization of sand–rubber mixtures. Generally, sand–rubber mixtures have good strength, low-shear modulus, and high damping properties. However, from a practical viewpoint, their high compressibility may result in low bearing capacity and undesirable settlement in the shortand long-term [34], limiting their adoption in many geotechnical applications. Moreover, in the selection of the soil type and recycled rubber size to form soil–rubber mixtures for use in geotechnical applications, the availability and the cost efficiency of both materials should be carefully considered [37,38]. Essentially, to avoid intrinsic segregation of binary assortments made of small and large particles [39–41], the recycled rubber should be cut into smaller (sand size-like) particles when mixed with sands. This, in turn, will unavoidably increase the implementation costs. Hence, the use of gravel–rubber mixtures (GRMs) instead of sand–rubber mixes has progressively been recommended.

Taking into consideration that in New Zealand it is a common practice to replace the topmost problematic soil layers (e.g., liquefiable sandy soils and compressible deposits) with well-compacted gravelly soil layers as part of the foundations for residential buildings and other geotechnical works, the adoption of GRMs in such applications seems the most appropriate. However, compared to sand–rubber mixtures, GRMs have been poorly characterized. Therefore, as part of the feasibility study reported in this paper, the physical properties, compaction characteristic, mechanical behavior, dynamic properties, and environmental aspects of different GRMs were evaluated by means of detailed laboratory investigations as described henceforth.

#### **3. Experimental Study**

In this study, to evaluate the combined effects of rubber content by volume and the aspect ratio (i.e., the ratio of median particle sizes of the rubber and the gravel, *AR* = *D*50,R/*D*50,G [38–42]) on the physical properties and mechanical response of GRMs, a poorly-graded rounded gravel (G) and two coarse-sized recycled granulated rubber types—namely, large rubber (RL) and small rubber (RS)—were tested. Their particle size distribution (PSD) along with photos are shown in Figure 3, while their index properties are reported in Table 1. The specific gravity (*G*s) was measured as 2.71 (G), 1.15 (RL), and 1.14 (RS)—the rubber was free of steel wires and fiber reinforcements. The aspect ratio of G-RL was *AR* = 0.67 and that of G-RS was *AR* = 0.33.

**Figure 3.** Material tested in this study: (**a**) Particle size distribution; and physical aspect of (**b**) gravel, (**c**) large rubber particles, and (**d**) small rubber particles (adopted from [16]).


**Table 1.** Index properties of the tested materials.

Several mixtures were prepared at various volumetric rubber content (*VRC*) of 0%, 10%, 25%, 40%, and 100% by mixing RL and RS with G. Note that, *VRC* is defined as the ratio of the rubber particle volume (*V*R) to the total volume of solid particles (*V*TOT = *V*<sup>R</sup> + *V*G, where *V*<sup>G</sup> is the volume of gravel grains), according to Equation (1):

$$VRC = \frac{V\_{\text{R}}}{V\_{\text{R}} + V\_{\text{G}}} \left( \times 100 \right) \tag{1}$$

Dry specimens were prepared by tamping method at a degree of compaction of 90% or above (based on standard Proctor tests—refer to details in Section 4.1). Segregation in the specimens was avoided by minimizing any vibration and preventing granular flow.

The one-dimensional (1-D) compressibility of GRMs was evaluated up to 500 kPa vertical stress by means of a medium-size (inner diameter 250 mm; specimen height from 150 mm) compression cell. Alternatively, the shear strength was estimated by using a medium-size (100 mm × 100 mm in cross-section and 53 mm in height) direct shear box under 30, 60, and 100 kPa normal stress levels (the horizontal displacement rate was 1 mm/min). Moreover, to evaluate the small-strain stiffness, shear modulus degradation, and damping ratio of GRMs, two series of bender element and small-strain drained cyclic triaxial tests (specimen size: diameter = 70 mm; height = 170 mm) were also carried out on dry specimens. Additionally, batch leaching tests besides conductivity, pH, total organic carbon (TOC), and inductively coupled plasma mass spectrometry (ICP-MS) analyses were performed to assess key environmental aspects of GRMs. A summary of the geoenvironmental tests performed in this study is reported in Table 2.


**Table 2.** Summary of geo-environmental tests carried out in this study.
