# Use of Granulated Rubber Tyre Waste as Lightweight Backfill Material for Retaining Walls
## 2 Properties of Rubber Aggregates
### Rubber Granulate Application  
Currently, the European Union possesses no regulations related to the implementation of this type of material. However, in the United States, the ASTM D 6270 standard (\"Standard practice for use of scrap types in civil engineering applications\") [26] has existed since 1998 and has been updated over time and provides information on geotechnical properties, testing methodologies, and construction recommendations. The ASTM D 6270 addresses the following restrictions: (1) on the fine material that appears with the tyre shreds; (2) the free and exposed steel; (3) the maximum thickness of the shred layer; and (4) limitations on the exposure of tyre shreds to air and water [12]. In 2004, 60 million tyres were used in civil engineering applications, one of the fastest-growing valorisation activities. There are many successful stories, such as that in the state of Oregon (USA) in 2016 in whereby 580,000 shredded scrap types were employed in a landslide correction project [30]. Certain research claims that there are cost savings in using shredded tyres (12 and 50 mm) as backfill material: these savings can range from 52 to 67% [16] and depend on the location of project, the demand for natural aggregates, and on other factors, which all have to be evaluated in their implementation. The advantages of using tyre chips instead of conventional building materials include: reduced density, improved drainage properties, and better thermal insulation [15]. One of the requirements in the assessment of retaining wall fillings involves the horizontal thrust they cause to the wall itself. Considering rubber granulates as a lightweight aggregate could help to reduce the thrust thanks to their low specific weight [29]. It should be mentioned that the ideal backfill material should have high hydraulic conductivity in order to allow free drainage and to prevent any increase of hydrostatic pressures [16]. The addition of crumb rubber to modify bitumen has been utilised to satisfy two needs: on the one hand, to develop high-quality, hot-mixed asphalts; and on the other hand, to reduce the pollution generated by tyre waste [15]. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multicolumn{2}{c}{**Nominal**} & \multirow{2}{*}{\
\begin{tabular}{c} **Specific** \\ **Gravity** \\ \end{tabular} } & \multirow{2}{*}{
\begin{tabular}{c} **Unit Weight** \\ **(kN/m\({}^{3}\))** \\ \end{tabular} } & \multicolumn{2}{c}{**Mohr–Coulomb Parameters**} & \multicolumn{2}{c}{**Young’s**} & \multirow{2}{*}{**References**} \\ \cline{1-1} \cline{6-6}
**Sizes/Particle** & & & **Cohesion** & & **Friction** & **Modulus E** & \\
**Range (mm)** & & & **(kPa)** & & **Angle \(\Phi\)**(\({}^{\circ}\)) & **(kPa)** & \\ \hline
0.8–20 & & 4.9–5.9 & 6–32 & 21.6 & - & \\
10–50 & - & 5.1–6.4 & 11–38 & 3.3–25.4 & 253–485 & [12] \\
20–400 & & 4.7–6.3 & 19–39 & 4.3–13.2 & 130–373 & \\
12–50 & & 5.9 & 0.0 & 30 & & \\ Sand—10\% & - & 15.6 & 2.0 & 46 & - & [35] \\
12–50 & & 14.5 & 2.0 & 50 & & \\ Sand—30\% & & 13.3 & 2.0 & 52 & & \\
12–50 & & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Cont. The properties of concrete with rubber granulate mixtures increase compressive and flexural strengths. Research indicates that the toughness of concrete increases with the increase in rubber substitution, whereby 50% and 100% rubber tyre aggregate substitutions exhibit equal values of toughness [37]. [PERSON] and [PERSON] [38] reported that when the coarse aggregate is completely replaced by coarse crumb rubber, the strength of the mix decreases drastically: an 85% reduction in compressive strength, and a 50% reduction in tensile strength are observed. For civil engineering applications, scrap tyres are usually reduced to particle sizes ranging from 12 to 305 mm. Tyre shreds and tyre chips are collectively referred to as tyre-derived aggregates (TDA).