## 2 Materials and Methods  
Within this research, for the unbound bearing course and cement-bound course, natural aggregates (sand and gravel) with different gradations (i.e., 0-2 mm, 0-4 mm, 4-8 mm, and 8-16 mm) were used (Figure 1). Due to its similar grain size distribution, as an alternative to the 0-2 mm fraction, a fine fraction of recycled rubber was used (Figure 2). The chosen material was prepared according to EN 933-1 [30] with a grain size distribution curve to fit between the upper and lower limits according to EN 13285 [31] for the unbound base course (UBC) material (mixture named 0%C), and EN 14227-1 [32] for the cement-bound material (CBC). For CBC, Portland cement of grade 32.5 (CEM II B/M (P-S) 32.5R) was used as a binder, comprising 3%, 5%, and 7% of the total aggregate mass. Solid particle densities were determined in a pycnometer according to standard EN 1097-6 [33]. The density of the 0-2, 0-4, 4-8, and 8-16 mm natural aggregate fractions used was 2.86, 2.96, 2.63 and 2.70 g/cm\({}^{3}\) respectively, while the waste rubber density of 1.12 g/cm\({}^{3}\) was determined using ethanol instead of water due to its low density (similar to water). The density of the cement used was 2.92 g/cm\({}^{3}\), determined by standard EN 196-6 [34]. Three additional mixtures for the CBC were designed, within which the binder content of 5% was selected as an optimal share according to previous research using the same aggregate type [35] in order to preliminarily determine the influence of the waste rubber on the CBC mixture characteristics. In those mixtures, the 0-2 mm fraction was replaced with granular rubber (due to a similar grain size distribution) by volumetric percent (due to high-density differences between these two materials). The replacement was in amounts of 20% (mixture 5%C + 20%R), 30% (mixture 5%C + 30%R) and 60% (mixture 5%C + 60%R) of the 0-2 mm fraction volume in the mix. When recalculated, the 0-2 mm fraction replacement by rubber in amounts of 20%, 30% and 60% represents 4.64%, 6.95% and 13.91% of the total aggregate volume of the mixture respectively, and 1.92%, 2.93% and 6.13% of the total aggregate mass of the mixture. Table 1 presents the proportions of all the constituents in the mixtures. Figure 1: Natural aggregate and waste rubber used. In order to compare the results from the Proctor and vibrating hammer tests, the tests were conducted on both the unbound base course mixture and cement-bound base course mixture. The modified Proctor test was done using an automatic Proctor compaction machine and the operator (human) factor on compaction is minimized. On the other hand, as described in standard EN 13286-51 [36], the vibrating hammer is greatly affected by human influence. To find the OMC and MDD, five test specimens with five different amounts of water were prepared for each test method and each tested mixture, following the recommendations of EN 13286-2 and 13286-4 [37; 38]. Both test methods require the use of the Proctor mould B (150 mm diameter and 120 mm height) for the chosen aggregate mixture, which makes the test results comparable (Figure 3). Specim preparation using the Proctor hammer consisted of specimen compaction in 5 layers, with 56 blows per layer with a 4.5 kg rammer falling from 457 mm height, which is equal to a modified Proctor energy of 2.66 MJ/m\({}^{3}\). The amount of material, placed in each of 5 layers, must occupy one fifth (1/5) of the mould after layer compaction. For specimen preparation using the vibrating hammer, standard [38] prescribes the compaction of a specimen in 3 layers with 60 s of vibration per layer. Analogous to the previously explained method, the amount of the material, placed in each of 3 layers, must occupy one third (1/3) of the mould after compaction of the layer. A vibrating hammer with a weight of 6.4 kg and power of 1100 W is placed on a supporting frame and has a 146 mm diameter damping foot as an extension.