**1. Introduction**

The management of used tires in Kuwait is considered as a significant environmental challenge [1]. Millions of used tires are dumped to open landfill in Kuwait, although the storage of this type of waste could be a major environmental hazard [2]. As the temperature in Kuwait commonly exceeds 50 ◦C during the summer, several massive fires previously destroyed the waste tires, causing serious environmental issues [3–6]. Additionally, the disposal of waste tires in general represents an important environmental concern as the natural degradation of rubber takes several years [7].

Due to an exponential growth in the use of cars in Kuwait, the accumulation of used tires poses a serious risk to the ecosystem; therefore, recycling waste products is vital [8–10]. Recently, great effort has been directed toward finding alternate ways to use waste materials emerging in the world [11–13]. Researchers have found that the recycling of waste rubber tires has several environmental and economic advantages [11,14–16]. Nowadays, the recycled rubber is considered as a suitable and useful material in civil engineering applications [7,17,18]. In general, the recycling of the waste tires goes through a process of shredding, separation of components, and granulation in order to convert the tires into ground tire rubber [19].

In most studies, the recycled rubber is classified into three main categories such as shredded rubber (also known as chipped rubber), crumb rubber, and ground rubber [18,20].

**Citation:** Soleimani, S.M.; Alaqqad, A.R.; Jumaah, A.; Mohammad, N.; Faheiman, A. Incorporation of Recycled Tire Products in Pavement-Grade Concrete: An Experimental Study. *Crystals* **2021**, *11*, 161. https://doi.org/10.3390/ cryst11020161

Academic Editor: Piotr Smarzewski Received: 19 January 2021 Accepted: 3 February 2021 Published: 6 February 2021

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According to an analytical review done by Lavanga et al., the mechanical response of rubber concrete is affected by different factors such as the rubber content granulometry of both the substituted mineral aggregate (coarse or fine) and the added rubber, the introduction of additives, the quantities of all components within the formulation, the water-to-cement ratio, and the appropriate pre-treatments and surface coatings [13].

A study conducted by Aiello and Leuzzi [21] to investigate the properties of various concrete mixtures concluded that the waste rubber tire can be used toward making workable rubberized concrete. The same study found that the rubberized concrete mixtures showed lower unit weight compared to plain concrete [21]. The literature indicated that the compressive strength of rubberized concrete decreases by increasing the particle size and rubber content [11,17,18,22]. Moreover, several studies established an improvement in compressive and tensile strengths in the mixture containing coated rubber crumb and silica fume [23–25].

Meddah et al. [2] recognized that adding shredded rubber can improve the performance of concrete by modifying the roughness of rubber particle surfaces. In addition, the same study pointed that using rubber particles showed improvement in some characteristics such as porosity, ductility, and cracking resistance performance.

Khan and Singh [26] investigated the partial replacement of sand by tire crumb rubber with 5%, 10%, and 15% replacement. The compressive and tensile strengths of the rubberized concrete were reduced by 15% and 43% respectively.

Sofi [27] tested high-strength concrete specimens by replacing 5%, 7.5%, and 10% of aggregate with rubber and reported a reduction of 10–23% in the compressive strength. The same study showed that the modulus of elasticity of the rubberized concrete was reduced by 17–25% and recommended a maximum replacement of 12.5% of fine aggregate by rubber for high strength concrete.

Akinwonmi and Seckley [28] investigated the change in compressive strength of concrete when the aggregates were replaced with shredded and crumb rubber. The results showed that the replacement of 2.5% shredded rubber increased the compressive strength by 8.5%, while any replacement of more than 2.5% rubber decreased the compressive strength when compared to the control mix.

Issa and Salem [29] reported that partial replacement of the sand with crumb rubber up to 25% would result in an acceptable compressive strength for the concrete. He indicated that increasing the rubber content above this threshold would reduce the compressive strength substantially that the mix would not be acceptable for structural or even nonstructural applications.

Recently, Irmawaty et al. [30] investigated the flexural behavior of the concrete made in part by using waste rubber tires. They tested specimens with 100 mm × 100 mm × 400 mm and with the replacement of 10%, 20%, and 30% of crumb rubber and tire chips. They concluded that rubberized concrete with 10% crumb rubber achieved the optimal energy absorption.

The literature made it clear that the greater the amount of steel fibers in the concrete, the greater the value of strength and flexural toughness [31–34]. Therefore, the addition of steel fiber appears to improve the tensile and compressive strengths [32,34,35].

Gul and Nasser [32] conducted a study to investigate the behavior of concrete by using the waste rubber tires as an alternative to steel fiber in fiber-reinforced concrete. They concluded that by increasing the percentage replacement of rubber, the compressive and split tensile strengths of concrete decrease compared to specimens containing steel fiber.

Manufactured steel fibers and steel fibers extracted from tire waste have been used in different concrete mixes to compare their effectiveness [33]. The increase in both reused and normal steel fibers showed less slump values for fresh concrete. The initial modulus of elasticity increased by 7–8% for the mixes with normal steel fibers and 2–3% for the mixes with reused steel fibers. The compressive strength of the mix with reused fibers increased by a maximum of 12%, while the mix with normal fibers increased by a maximum of 20%. No significant increase was noticed in the splitting tensile strength test between the two mixes while increasing the fiber dosage.

Some researchers examined the hybrid concrete mixes: for example, Noaman et al. [34] investigated the mechanical properties of rubberized concrete combined with steel fiber. They combined rubberized concrete with different replacement ratios of crumb rubber in plain and steel fiber concrete mixes via the partial replacement of fine aggregate (17.5%, 20%, 22.5%, and 25%). The study indicated that a reduction in mechanical properties was observed by the increasing increment of crumb rubber in both mixes. The study suggested using a combination of steel fiber and crumb rubber due to the improvement of strain capacity under flexural loading.

As well, Eisa et al. [35] studied the effect of a combination of crumb rubber and steel fibers on the behavior of reinforced concrete beams under static loads. They concluded that an acceptable level of performance of reinforced concrete beams could be obtained by using crumb rubber as a partial replacement of fine aggregates by 5% and 10%. The study also recommended the use of steel fibers with rubberized concrete, with percentages of rubber over 10%, as this showed a significant improvement in the performance and toughness of these mixtures.

As per the above literature review, researchers directed great attention to investigating the utilization of recycled tire rubber in concrete in order to find a proper solution for minimizing tire waste and producing a green concrete. In spite of the reduction in mechanical properties due to increasing the rubber content in the mix [11,13,17,18,22,31], it is still recommended to use rubber particles in civil engineering applications, especially in pavement projects [2,11,15,17,20,35,36].

The objective of this study is to investigate whether recycled tire by-products can be used to make a suitable "green concrete" to be used for pavement construction in hot-weather climates. To achieve optimal results, each type of tire by-product was tested individually to observe its properties and effects on a benchmark mix before creating "hybrid" mixes that contain a combination of the materials; this is where the novelty of this study lies.

#### **2. Materials and Methods**

#### *2.1. Benchmark Concrete Mix Design*

In this study, a benchmark pavement-grade concrete mix with a 28-day compressive strength of about 35 MPa has been designed and cast. Concrete pavements are preferred in Kuwait due to their ability to resist high temperatures without causing permanent damage to the pavement itself. However, concrete used in hot climates is subjected to high rates of evaporation, loss of moisture, and quick setting times [37]. As a result, concrete mixes used in hot weather climates need to have a slump value at the higher end of the recommended range for pavements. This was achieved by introducing a superplasticizer in order to improve the workability of the mix and reach the targeted 28-day compressive strength while limiting the water/cement ratio to 0.55. The mix proportions of the benchmark concrete used for the study are shown in Table 1.

**Table 1.** Mix design propositions of benchmark pavement-grade concrete.


To achieve the desired workability, SIKAment®-500 OM superplasticizer is used. This is important for hot climate places.

#### *2.2. Recycled Tire Products*

Crumb rubber (CR), shredded rubber (SR), and steel fibers (SF) were obtained by recycling used tires (Figure 1). These products are easily available in the market, as tires are now recycled around the world. The tire rubber used in this study is varied in source but is believed to contain percentages of natural and synthetic hydrophobic rubber optimized for automobile use. For this study, the recycled tire products were provided by the Green Rubber Tire Recycling Plant in Kuwait.

**Figure 1.** Crumb rubber, shredded rubber, and steel fibers obtained by recycling used tires.

CR and SR are used as a partial replacement for the sand and coarse aggregates respectively. SF is used as an extra ingredient in different concrete mixes.

#### 2.2.1. Crumb Rubber (CR)

The density of CR used in this study is 552 kg/m3. The size distribution, by sieve analysis, is shown in Figure 2 and is compared with the size distribution of the sand used in this project. The particle size distribution of the sand is close to being well-graded, whereas the CR is uniformly graded. Ideally, the CR would be introduced to replace a similarly sized portion of the sand; however, this is not feasible in real-life applications. Therefore, a direct replacement of sand with CR is used.

**Figure 2.** Size distribution of crumb rubber and sand used in this study.
