## 3 Experimental Program
### Specimens Preparation  
In order to evaluate physical properties of examined material, a set of tests was conducted. As the first one, the grain size analysis (sieve analysis) was performed. The particle size distribution of Recycled Concrete Aggregate used in this study is presented in Figure 3. This analysis led to classification of the tested material as gravel with sand (_saGr_), according to European Committee for Standardization [43]. The values of the characteristic diameters (\(d_{10}\), \(d_{30}\), \(d_{50}\), \(d_{60}\)) and coefficients (\(C_{u}\), \(C_{c}\)), typical characteristics for natural soils by Polish Committee for Standardization [44], are attached to Figure 3 as well. For instance, \(d_{50}\) is an average diameter, or otherwise measurable. It is along with larger grains 50% of specimen's weight. Here, it was \(d_{50}\) = 2.60 mm and could be used to determine the shape of grain size distribution. The specimen's coefficient of uniformity (\(C_{u}\)) and coefficient of curvature (\(C_{c}\)), corresponding to \(C_{u}=\frac{d_{60}}{d_{10}}\) and \(C_{c}=\frac{d_{30}^{2}}{d_{10}\cdot d_{60}}\), amounted \(C_{u}\) = 9.09 and \(C_{c}\) = 2.92, respectively. \(C_{u}\) is a crude shape parameter, which allows classifying soil gradation, whereas \(C_{c}\) is a coefficient to characterize gradation indicating, e.g., the potential of interlocking. The values of these coefficients suggest a well-graded material, susceptible to compaction process and suitable for the construction of embankments. This distribution of particles from 8.0 to 0.063 mm is typical for soils designed for improved sub-base and supporting other structures. Figure 1: Coarse aggregates for RCA with a size less than 8.0 mm. (**a**) Coarse aggregates; (**b**) particle size. Figure 2: Rubber chips used in this study with a size of 0.06–0.4 mm. (**a**) Rubber chips; (**b**) particle size. As the next study, the standard compaction tests were carried out in order to determine the optimum moisture content (\(w_{\text{opt}}\)) and the maximum dry density (\(\rho_{d\text{max}}\)) of RCA. The following results were obtained: \(w_{\text{opt}}=10\%\) and \(\rho_{d\text{max}}=1.96\) g/cm\({}^{3}\). Then, the exact amount of RCA and rubber chips was estimated, in order to prepare an appropriate composition of ingredients. The mixture was prepared by hand mixing of rubber chips with RCA. The authors decide to use the same gravimetric proportion of rubber chips for all three specimens, namely 15%. The new material is shown in Figure 4. The finest fraction of RCA was removed and replaced by grains (Figure 3). The reason for eliminating only the finer fraction of RCA and tempering it by particles from tires was to obtain a material with better values of coefficients \(C_{u}\) and \(C_{c}\) so that it would be non-uniform and well graded. Furthermore, the undesirable feature of fine fractions is the possibility of re-binding which the authors indented to avoid. The introduction of rubber into anthropogenic soil was aimed as well to prevent from eventual negative effects of water filtration. Figure 4: Prepared composition of RCA with rubber chips. Figure 3: Particle size distribution of RCA and rubber chips (black line: pure RCA, red line: rubber chips, blue line: upper limit by WT-4 [41], green line: lower limit by WT-4 [41]). Subsequently, the modified recycle concrete aggregates were transferred into the special mold with layers with uniform mix to avoid segregation during the specimens' preparation. In this study, the tamping method was adopted to prepare specimens with the correct dimensions, i.e., 70 mm of diameter and 130 mm of height. All specimens were prepared in the same manner in order to guarantee the same compaction energy applied to the samples. All specimens were characterized by the same moisture conditions, equal to optimum moisture content, as mentioned earlier. The compaction procedure was the same as defined by Proctor test [30].