# Sustainable Development of Concrete through Aggregates and Innovative Materials: A Review
## 2 Recycled Materials
### Recycled Tire Rubber  
A tire is a composite of complex elastomer formulations, reinforced with plies of fibers and steel fiber [29]. The number of tires being discarded is an environmental problem in all the world. Every year nearly 1 billion tons of waste tires are discarded and only a small amount is recycled [30]. Recycled tires have been extensively studied since the end of previous century to use in asphalt pavement, waterproofing systems, membrane coating, among others [31]. Although rubber has excellent properties (e.g., high strength, great elasticity and durability), it may affect several properties of concrete. For instance, cement mixed with coarse or fine aggregate of waste tire rubber may generate concrete with lower specific density and lower compressive and tensile strength as well as higher toughness and ductility and more efficient sound insulation [32]. The variations of the physical and mechanical properties of this concrete type may depend on replacement ratio and the size of the tire particles added to concrete. [PERSON] et al. [32] measured several physical and mechanical properties of rubber concrete using different percentage of replacement of rubber aggregates. The aggregates were obtained by shredding the vehicles scrap tire rubber, achieving a size range between 10 and 20 mm. They determined that the optimum percentage of replacement of rubber aggregates can be up to 15%. Even in these conditions rubber wastes decreases the concrete compressive strength. Therefore, they recommend the use of rubber waste aggregates in concrete for construction components such as partition walls, road barriers, pavements, sidewalks and some other secondary elements that do not require high compressive strength of concrete. Tables 1-3 depict the mix proportion, physical properties of materials and results of the compressive strength and density of rubber concrete [32]. The feasibility of using crumb rubber tire in high strength concrete was studied by [PERSON] and [PERSON] [33]. They investigated that compressive, flexural tensile and pull-off strength of high strength concrete have a low decrease when the replacement percentage of crumb rubber is less than 10%. A compressive strength close to 60 MPa was measured for this concrete mixed with 0% to 12.5% crumb rubber. Other mechanical properties such as water absorption, depth of abrasion and depth of water penetration showed good results for mixes with up to 12.5% crumb rubber. For these reasons, these authors recommended using high strength concrete with crumb rubber aggregates for hydraulic structures like tunnels and dam spillways. Therefore, it is very important to limit the content of aggregates  
Figure 2: Schematic view of the fabrication process used in the recycled aggregate concrete and the concrete with virgin aggregates. Reprinted with permission from [22]. Copyright © 2011, Elsevier B.V.  
of waste rubber tire in concrete to avoid a high reduction of its mechanical properties such as the compressive and tensile strength. Other mechanical properties such as durability, abrasion and water absorption may be improved when the percentage of rubber tire waste increases. Thus, recycled tire rubber may be used for normal and high strength concrete mainly in secondary construction elements. The steel fibers from waste tires can be mixed with tire rubber to be used as aggregates of concrete combined with steel fibers from waste tires. [PERSON] et al. [34] presented experimental results of thermal and mechanical properties of sustainable concrete. This concrete incorporated aggregates based on crumb rubber (CR) and steel fibers coated with rubber (FCR). The rubberized concrete with FCR had better bending and compressive strengths in comparison to concrete with crumb rubber. Furthermore, the concrete with FCR registered higher both energy of fracture and toughness than the concrete with crumb rubber. In addition, thermal conductivity showed the same decrement with waste rubber as aggregate with and without steel fibers. For these reasons, [PERSON] et al.