## 2 State of the Art  
Most of the work presented in the literature concerns mechanical properties of RCA [23], durability and/or volume stability. Direct Shear Tests (DSTs), conducted by [PERSON] et al. [24] and [PERSON] et al. [25], showed that among six tested C&D materials all meet the shear strength requirements for aggregates in pavement engineering, including RCA. [PERSON] [26] in 2009 carried out cyclic loading tests on RCA samples with addition of sand and concrete. Laboratory research, typical for roads, were also performed on a large scale, including California Bearing Ratio (CBR) [27],compaction, crushing susceptibility, freeze thaw and triaxial tests [28] or long-term cyclic triaxial tests [29]. Nonetheless, studies on dynamic properties (shear modulus and damping ratio) of RCA are still very few [30]. In the recent past, a series of intensive studies, aimed at finding the possible application of rubber chips mixed with natural aggregates, have been conducted as well. [PERSON] et al. [19] tested shear behavior of rubber chips and silica sand mixture using micro focus X-ray CT scanner. The results of those tests proved that, while direct shearing of the mixture of sand and rubber chips, the shear stress level rises monotonically and no peak stress is observed. Dilatancy effect was also not very significant, in comparison to the tests conducted on pure sand. It was also interesting that rubber chips additions can decrease shear strain propagation. This fact turned to be important for road engineering, as shear stress causes many structural damages. For instance, [PERSON] et al. [31] studied a mixture of gravel, fly ash and waste fibers. Their study revealed an increase of CBR characteristics, namely optimal addition of waste tire rubber ranged between 0.2% and 2.0% of dry unit weight of soil. Two scientists, [PERSON] and [PERSON] [32] in 2014, though, used shredded waste rubber for reinforcement of soft clay with different percentages of rubber content and, additionally, with cement as a binding medium. CBR test set, together with unconfined compressive strength tests, pointed out an improvement of bearing capacity, strength and high compressibility of the tested material. However, only a limited number of research described the cyclic behavior, including the liquefaction potential and dynamic properties of sand-scrap tire mixtures (e.g., [33; 34; 35]). [PERSON] and [PERSON] [36] reported that sand-scrap tire mixtures have lower shear modulus and higher damping ratio compared to sand at low to medium shear strains (\(\gamma<0.1\%\)), while pure scrap tires have higher damping ratio. [PERSON] et al. [37] stated that sand-tire crumb mixtures show remarkable damping and seismic isolation properties. They have an ability to reduce significantly the acceleration amplitude at the ground surface, due to the low stiffness of tire crumbs. Because of the lack of experimental data, [PERSON] et al. [38] studied dynamic properties of scrap tire-soil mixture based on the extrapolated data of shear modulus and damping ratio of [PERSON] and [PERSON] [33] for shear strains greater than 0.1%. [PERSON] et al. [34] conducted a series of strain controlled consolidated undrained cyclic triaxial tests on specimens of sand mixed with varying proportions of tire chips. The maximum shear modulus (\(G_{\text{max}}\)) of the tested mixture was determined by bender element tests for different gravimetric proportions of tire chips and various effective confining pressures. The initial shear modulus was found to be influenced by the proportion of tire chips and confining pressure. The results showed as well that the increase of scrap tire in sand-scrap tire mixtures reduced shear modulus degradation curves and increased damping ratio curves. [PERSON] and [PERSON] [22] in 2015 investigated static properties (i.e. shear strength and energy absorption capacity) of Sand Tire crumb Mixtures (STCM), and then in 2016 [39] they presented dynamic properties of STCM in terms of \(G\) and \(D\) against shear strain (\(\gamma\)).