*4.3.2 Incorporation of end-of-life tyre rubber into concrete*

End-of-life tyres (ELT) represents a social and environmental problem due to the large accumulation in landfills. In fact, more than 1000 million tonnes of ELT are generated annually and more than 50% of that amount is destined to landfill or left as untreated garbage [98]. Moreover, this material produces concentrations of rats, larvae, mice and insects, increases the risk of fires difficult to extinguish [99]. This condition is aggravated by its lack of reutilisation. For instance, in Chile more than 140,000 tonnes of ELT were produced in 2019 and only 17% was recycled [100].

An attractive use of this waste is its incorporation in concrete mixes. Indeed, concrete with rubber improves properties such as energy absorption, due to its increased plasticity and ductility [101], which improves its impact behaviour and durability. However, the addition of rubber reduces the mechanical strength of the concrete [102, 103]. Therefore, to mitigate this effect, it is necessary to treat the rubber prior to its incorporation [104–106]. For instance, García et al. [107] evaluated the effects of three different treatments applied to ELT rubber before its incorporation in cement mortar samples. The treatments were hydration, oxidation-sulphonation and contact with hydrogen peroxide. The results indicate that it is possible to replace up to 5% of the fine aggregate weight with ELT rubber. Furthermore, the incorporation of hydrated ELT rubber proved to be the best treatment option from a technical, practical and economical point of view.

As the purpose is to produce a useful improvement in concrete properties and utilise waste efficiently, it is important to consider the geo-dependence of the concrete material [29]. This is particularly evident in samples made with cement containing local fly ash (Great Concepción, Chile). Indeed, this cement has a similar market cost to others national cements. However, the samples with ELT and fly ash cement are able to provide mortars that can satisfy the design requirements. Therefore, it is possible to reduce the amount of clinker while reusing a local industrial waste.

Furthermore, the adequate strength obtained in the experimental evaluation makes ELT rubber concrete an attractive alternative to improve the in-service performance of structures such as concrete crash barriers. Since rubber concrete is able to increase impact resistance and energy absorption [101], the durability of these safety elements is increased. Actually, studies indicate that concrete barriers with recycled rubber ELT have a service life 2.6 to 3.2 times longer than traditional barriers [108].

Due to the extensive road networks of the countries the benefits of this type of barrier represent not only a massive use of an industrial waste, but also bring economic, safety, and environmental benefits to the country or region where they can be applied. For instance, only in Chile the national road network roads exceed 85,000 kilometres [109].

Although the importance of geo-dependency, the results obtained in this research may be useful for other regions as well. For example, where it is possible to replace part of the cement with fly ash (produced or imported), or another cement substitute, producing similar results. If the substitute is a waste product such as fly ash, the contribution is not only technical and economic, but also it is a contribution to a sustainable development.
