Presently, sustainability is a significant concern around the world, and cement and concrete technology has an impact on it. The construction sector, especially cement and concrete, is mainly accountable for 7% of total global CO₂ emissions. Concrete competence of sustainable construction is distinguished by the use of industrial wastes to reduce the consumption of natural resources and energy, as well as environmental pollution. Sustainable concrete is more environmentally friendly due to the replacement of materials over nominal concrete.

The land is required for concrete construction. Materials for construction activities (aggregates, cement, waters, admixtures, etc.) sourced from quarry sites and borrow pits may result in the wholesale elimination of vegetation and vir-gin materials. It can also result in the displacement of populations and the loss of vital ecological resources and biodiversity on a national, regional, or global scale. Based on the foregoing, it became necessary to investigate and review the use of waste materials as concrete coarse aggregates to reduce the risks associated with natural coarse aggregate production and quarrying.

The aggregate manufacturing process has numerous significant environmental impacts. The most significant environmental impact of stone, aggregate, and mineral mining is air pollution caused by airborne emissions from both stack and disturbed areas at these mines. Natural aggregate deposit sources are being depleted, posing a threat to the environment and society. The rapid depletion of natural aggregates from source beds causes a variety of issues, including the loss of water-retaining strata, bank slides, exposure of water supply scheme intake wells, and decreasing underground water table levels, all of which cause negative agricultural effects and aquatic life disturbances.

A wide range of waste materials is thought to be feasible and even valuable additives for concrete. Among these are cellulose, fly ash, silica fumes, and wood particles. Rubber obtained from scrapped tyres is the most recent waste material studied due to its critical use in the construction field [1].

Rubber is an essential material used in many industrial sectors. An example of this sector is vehicle manufacturing, where the rubber is used in many applications like vehicle rubber tyres. Due to the high increase in automobile production from the manufacturers, the number of tyres being produced has increased rapidly, which led to the increase of waste tyres generated. Tyres are often split into two categories: truck tyres and automotive tyres, both of which are distinct from one another. More so, it affects the texture and shape of the concrete, as well as the properties of the concrete that are altered by adding a specific percentage of rubber. Considering the increase in the demand for concrete for construction, it became necessary to identify alternatives to concrete natural coarse aggregates for sustainability which will lead to a reduction in the demand for these natural coarse aggregates. It is also worth stating that the quantities of the basic mixture’s components vary between truck and car tyres, not just in design, size, and weight. Many of these tyres are only disposed of in landfills without treatment.

Up to ninety percent of mortars and concrete are made of aggregates which affect their qualities and properties [2, 3].

Disposal of waste tyres in landfills occupies a large land area. There is a pressing need for the disposal of waste tyres in a beneficial and environmentally friendly manner [4]. The number of tyres disposed of in landfills can be minimized by using the rubber content in the waste tyres to produce concrete by partial or full replacement of the concrete fine or coarse aggregate after the waste tyre rubber must have undergone some cleaning processes. Rubberized concrete mix is produced by the incorporation of rubber into the concrete mix [5, 6].

For many years, attempts by material scientists have been made to make concrete ductile. Because of the brittle nature of concrete, it appears that integrating natural tensile ductility into the material is the most basic and practical method for creating damage-tolerant concrete structures.

There have been several studies on the incorporation of rubber into concrete mixes, the results of which have shown that rubber incorporation can affect some of the concrete’s properties like sound absorption, energy dissipation, and ductility.

Depending on the size of the crumb rubber, the rubber can be utilized as an alternative aggregate in concrete mixes as:

(i) Coarse aggregate: In this type, tyres are prepared in two stages. In the first stage, rubber is cut to obtain pieces with a length of 100 to 230 mm, and in the second stage, rubber is produced in grains with a size of 13 to 76 mm, which can replace concrete coarse aggregates.

(ii) Fine aggregate: This rubber crumb type requires mills with special requirements at different temperatures since these two factors control the size of the produced grains, which range from 0.425-4.75 mm.

(iii) A partial cement replacement [7-14]: The size of grains of this type of rubber varies from 0.075 mm - 0.475 mm. To reduce grains’ size, a micro-milling process is required.

Every year, each person in the United States discards an average of one car tyre. This means that the United States, with a population of roughly 300 million people, must dispose of approximately 300 million tyres per year [15-17]. In the last few years, several novel applications for these tyres have emerged. Tyre-derived fuel for boilers and cement kilns [15] and tyre-derived aggregates for civil engineering projects [17] are two examples. However, not all tyres are disposed of in this manner. The remaining scrap tyres are disposed of using a variety of legal and illicit techniques (disposal of tyres in unpermitted areas).

Three different types of used tyres for concrete mix design have been investigated by Researchers investigated:

• Powdered Rubber: The rubber particles are less than 1 mm in diameter and are made up of crunch powder that has fallen from the waste rubber processing plant's machinery. Because of its size, this sort of rubber might be utilized as a concrete filler.
• Crumb Rubber: Rubber particles are highly irregular, ranging in size from 3 to 10 millimeters, and can be utilized to substitute fine aggregates.
• Chipped Rubber: This type of rubber is 25–30 mm in size and can be utilized to replace coarse particles in concrete.

In the last three decades, increasing attention has been paid to the use of tyre products as full or partial replacement natural concrete aggregates [18-24]. The implementation of rubber concrete in construction can minimize dependence on mineral aggregates, leading to sustainable construction in our environment. Existing studies have found that replacing natural aggregates with tyre rubber in concrete production can cause a decrease in concrete compressive strength (ƒ_c) [20]; the percentage reduction in strength compared to natural aggregate concrete (NAC) depends on the replacement ratio and gradation of rubber aggregates and can reach 85%. Based on test results, some studies [18, 20, 21, 23] concluded that:

(i) The ratio of aggregates to be replaced in rubber concrete should not be more than 10% of the total number of aggregates by volume (hereinafter in this article, the replacement ratio is specified by volume, unless otherwise specified) to avoid/reduce the negative impact on the strength of the rubber concrete; and (ii) compared to using tyre crumb rubber (sizes 10 mm to 76 mm) to replace coarse aggregates, and crumb rubber (sizes 0.425 mm to 4.75 mm) to replace small aggregates may result in less reduction in concrete strength at the same replacement ratio.

It is necessary to identify that some other studies [7, 25] reported a slight increase in the strength of rubber concrete at a relatively low replacement rate of aggregates with rubber particles (for example, less than 2.5% of the total amount of aggregates), which may be due to the optimization of the gradation of aggregates of such rubber concrete.

Concrete buildings with greater serviceability and safety can be easily obtained if the concrete behaves in tension like steel (extremely ductile) whilst preserving all other benefits (e.g., extreme, and high (ƒ_c).

The researchers [26] investigated the dynamic properties of rubberized concrete material. Due to the rubber's particular elasticity properties, rubberized concrete showed substantial advantages in decreasing or restricting vibration and impact effects.

The physicomechanical parameters of rubberized concrete with a 20 MPa initial ƒ_c have been studied [18]. In the rubberized concrete, rubber was used in 15%, 30%, and 45% of the total aggregates. The ƒ_c of rubberized concrete has generally reduced, according to the findings.

Rubberized concrete [13] could be used in the following situations:

• In places where vibration dampening is necessary, such as railway stations and machinery foundation pads.
• Pile heads, paving slabs, trench filling, and pipe bedding.
• In barriers, railway buffers, and bunkers, where blast or resistance to impact is required.

Gupta et al. [27, 28] investigated the impact of mixing micro silica and waste rubber on concrete mix properties. The impact resistance of concrete has also been investigated when fine particles were replaced with waste rubber fibers and the addition of silica fume with cement [29].

An experiment on rubberized concrete was carried out in the research work [19]. The study looked at the ƒ_c and flexural strengths (ƒ_f), as well as the impact resistance of rubberized concrete with various crumb rubber volumes.

Gravel, limestone, and granite are the commonly known sources of crushed stone, which are expensive, and not all countries have their deposits [30]. Therefore, in the bid to address the issue of sustainability that meets the United Nations Sustainable Development Goal (SDG), this paper reviews the effects of rubber in concrete as an aggregate to the ƒ_c of a concrete structural member. There is no detailed compiled extensive work on the subject of ƒ_c of rubber concrete structural elements; therefore, this review paper utilized the research of previous researchers on similar or close topics to analyze the effect of chipped rubber in concrete on ƒ_c of rubber concrete.

This paper was prepared by a comprehensive review of works done by previous researchers, which paved the way for indebt understanding of the subject matter. The materials looked out for in the concrete mixes of the rubber concretes reviewed are as follows:

2.1. Expected Materials for Rubber Coarse Aggregate Concrete

To achieve the objective of this paper, the use of certain materials for concrete mixes was considered. Based on these materials, the results summarised and analyzed in the result section of this paper were obtained. The operational properties of concrete aggregates are determined by the chemical and mineral compositions, as well as water and frost resistance. Important characteristics of concrete aggregates also include the shape of grains, the nature of the surface, chemical composition, structure as well as economic indicators. The materials are discussed below as follows:

• Cement. Cement is an artificial inorganic hydraulic binder that is one of the main building materials. It forms a plastic mass when interacting with water, aqueous solutions of salts, and other liquids, which then hardens and turns into a stone-like body. Cement is mainly used for making concrete and mortars and it can gain strength in wet conditions, which is fundamentally different from some other mineral binders - (air lime), which harden only in the air. Cement is the main binder that coats the aggregate, developing a good bonding between them, thereby inducing the strength characteristics of previous concrete.
• Granites (Coarse aggregate). Granite (Fig. 1) is the material traditionally used in the construction of monuments. It is one of the hardest and most durable substances. It can withstand extreme weather conditions, which is one of the main reasons granites are used in the construction of monuments [31]. The chemical composition of granite is illustrated in Table 1.

• Chipped rubber (Coarse aggregate). The recycling of tyre aggregates (Fig. 2) includes several stages, such as a mechanical system (making small pieces), a cryogenic system (methods for cutting rubber into small pieces), removal of steel fiber (SF) stage, a screening and grinding step, and a cleaning step. During the cleaning step, the rubber particles are thoroughly cleaned with water and other cleaning agents such as ammonia solution, citric acid, etc. Based on the application, three categories of rubber are used in concrete: crushed or chipped tyres (size 12-50 mm), ground or powdered rubber (0.425-2 mm), and granular or crumb rubber (0.425-12 mm). In terms of chemical composition, tyre rubber granules contain more than 90% styrene-butadiene rubber (SBR) and carbon black, as well as a small amount of oxidized zinc and sulfur [34]. The weight loss of rubber at various temperatures was also investigated [34], and it was found that at temperatures of 40-125°C, 325-435°C, and 470-525°C, the weight loss is about 2, 35 and 55.3%, respectively. The specific gravity and elongation of rubber can reach 0.9–1.16 g/cm3 and 420%, respectively [35, 36]. The stiffness modulus of shred rubber tyre ranges from 0.47 to 2.7 MPa and depends on the direction of the load [37]. The percentage of rubber aggregates used in cement materials also varies from a minimum of 5% to a maximum of 100% [38, 39].

Fig. (1). Granite [32].

Fig. (2). Prepared sample of chipped rubber aggregates [33].

Waste tyre rubber is reduced in size or torn apart in the cracker mill process by being passed between spinning corrugated steel drums. This procedure creates particles of various sizes and shapes. The categorization of rubber waste tyres is illustrated in Table 2.

• Sand (Fine aggregate). Fine aggregates are natural sand particles from the land through the mining process, the fine aggregates consist of natural sand or any crushed stone particles that are ¼” or smaller. This product is often referred to as ¼”minus as it refers to the size, or grading, of this aggregate. Aggregates less than 4.75 mm in size are called fine aggregates; sand falls under fine aggregate [40]. Table 3 shows the chemical composition of the sand type called quartz sand which has been used as a fine aggregate for concrete by many researchers.

• Water. Tap or portable drinking water is widely used in a concrete mix. Clean portable water, free from suspended particles, chemical substances, biological elements, etc., is used both for mixing concrete and curing [42].

Concrete waste is one of the major construction-related wastes produced during the demolition of old structures. In 2014, the United Kingdom generated 58 million tons of concrete waste. Similarly, China generates 0.2 billion tons of concrete waste each year [86]. Several researchers investigated the properties of recycled aggregate concrete (RAC) (concrete containing concrete waste) [87-89]. However, RAC showed a greater reduction in peak strength and modulus of elasticity than conventional concrete [90, 91]. The highly permeable adhered mortar of recycled aggregates (RA) and the higher amount of ITZ in RAC are responsible for the poor performance of RAC [92]. The ITZ is the region of cement paste at the aggregate boundary [86]. The increased ITZ in RAC is due to the old ITZ between the aggregate and the old, adhered mortar of RA, as well as the new interfacial transition zone between RA and new cement paste [93].

The ITZ bond between recycled rubber and its composite matrix is determined by the chemical and physical properties of both components. Mechanical coupling, molecular bonding, and thermodynamic adhesion are all methods of bonding. Physical, mechanical, and chemical interactions between aggregate and matrix occur during these processes. The physical and mechanical interactions are primarily determined by the roughness of the aggregate surface and the adsorption of voids. Chemical bonding, on the other hand, is based on molecular interactions such as dipole-dipole interactions, Van der Waals forces, and chemical interactions between the rubber and the composite matrix [94].

It is difficult to achieve a good bond in rubber composites. Rubber-binder adhesion is affected by the surface properties of the rubber aggregate, such as roughness, polarity, chemical composition, and surface free energy [95]. Contaminants at the ITZ interface can reduce adhesion as well [96]. On the microstructural scale, rubber is a cross-linked polymer with very low permeability and smooth surfaces [94]. As the polymer and filler fill the microstructure, the surface and interfacial region ranges are very small, typically over one polymer chain and with a radius of gyration of the order of 3-30 nm. Poor adhesion may result from the recycled rubber's low surface free energy and lack of polar functional groups [94] However, after recycling, the rubber surface chains remain a part of the cross-linked structure. The recycled rubber chains cannot be reactivated to achieve continuity or bonding with the matrix. As a result, even when polymers of the same composition are vulcanized, weak interfacial adhesion develops between binder and rubber aggregate [95].

Table 4 shows results from some of the previous works. It is seen that the addition of chipped rubber reduced the ƒ_c of the rubber concrete.

Most countries have problems with recycling rubber tyres. Even when recycled for use or reproduction, the emission produced is very harmful to living things. The use of waste rubber tyres in concrete as concrete has, in many ways, solved environmental problems. The quarrying of natural coarse aggregates has a much negative impact on ecology, therefore, the use of waste rubber reduces the increasing use of natural coarse aggregate, thereby minimizing the dumping of waste tyres and burning of waste tyres.

Economically, cheaper production of concrete is achieved by using chipped waste rubber tyres as concrete aggregate. Lightweight concrete can be achieved by using rubber tyre as concrete aggregate.

The use of chipped waste rubber tyres in concrete has a good strength impact in concrete when incorporated at specific percentages. Chipped waste rubber tyres with rough surfaces have increased bonding strength with concrete and other concrete additives and reinforcement fibers.

Comparing waste rubber tyre and other by-products, the recycling processes of other by-products are more intense as the collection of waste tyres are easily accessible and from the waste tyres, not only is the rubber valued in concrete production but the steel fibers in the tyres are used for concrete reinforcement.

In this paper, a comprehensive review on the ƒ_c of rubber Crete was done, and the following conclusions were drawn:

1 Increasing the quantity of rubber in the concrete mix leads to a decrease in the concrete density, and this density loss is greatly increased in the case of powdered rubber. Due to the reduction in density, rubber concrete can be referred to as lightweight concrete and be produced for many applications based on requirements.

2 When compared to normal concrete mixes, rubber concrete has a lower ƒ_c but when the fiber is added to the mix of rubber concrete at a specific ratio, an enhanced ƒ_c can be seen. If the substitution of total concrete aggregate quantity with rubber does not surpass 20%, the decrease in ƒ_c can be tolerated. Above this aggregate ratio, a high reduction in the ƒ_c is noticed.

3 Tyre rubber for concrete aggregate treated with any coupling agents can lead to a reduction in the loss of ƒ_c.

4 The use of tyre rubber in concrete can usher in a friendly environment to encourage the United Nations Sustainable Development Goal as the crude methods of discarding these waste automobile tyres (burning, landfill, discarding in rivers, bushes, and roadsides) will be prevented.

5 A few more researchers suggest that concrete with waste tyre rubber chips as aggregates has a higher energy-absorbing capacity known as toughness.