## 1 Introduction  
The realization of the importance of soil stiffness, represented generally by shear modulus (\(G\)), at low shear strain level (\(\gamma<10^{-5}\)), in both static and dynamic behavior, has led to increased interest in methods of measuring stiffness at small strains in the laboratory. One of the most used technique to establish shear wave velocity (\(V_{S}\)), and therefore shear modulus [1], is to make use of bender elements (BEs), small piezoceramic devices that can generate and detect waves in soil [2]. \(V_{S}\) is an important parameter for the design of geotechnical systems [3], particularly in seismically active areas [4; 5]. It is necessary for design and site response purposes, is related to stiffness of a foundation, evaluation of site response during earthquakes, liquefaction potential [6], site characterization, soil density and its stratigraphy, or settlements of foundations [7]. On the other hand, the anthropogenic impacts, like mining tremors, use of roads and rails transport etc., cannot be ignored as well. Some high-speed transport systems (e.g., expressway, high-speed railway, and airstrip) have been recently developed a lot, which transmit dynamic loading to subsoil. The settlements of subsoil in these cases are very much related to soil stiffness. Information of shear wave velocity is crucial for a better knowledge of work capacity of soil. Shear wave velocity is connected to the small-strain shear modulus (\(G_{\text{max}}\)) using elasticity theory:  
\[G_{\text{max}}=\rho\cdot{V_{S}}^{2}, \tag{1}\]  
where \(\rho\) means mass density of the material (equal to the total unit weight of the material, \(\gamma_{t}\), divided by the gravitational acceleration) [8]. It is also recognized that shear wave propagation is the most versatile and portable method to assess \(G_{\text{max}}\) in the laboratory, besides bender elements [9], resonant column tests [10], and in the field, e.g., cross cone, seismic cone and surface waves methods [11]. The desire to improve of the road infrastructure in Poland has led engineers to construct, inter alia pavements in various conditions. A request of construction materials in such a situation enhances a great need to explore new ways of reuse of waste deposits. Economic development in Poland has recently induced many deposits of Construction and Demolition (C&D) materials, such as demolished brick and concrete. On the other hand, an increase of ecological awareness has also caused bigger concentration on recycle building wastes and their possible usage in engineering constructions. In the field of pavement engineering, C&D recycling materials, like other composite materials [12], have been used in roads, embankments, pipes bedding and backfilling [13]. This trend has become global standard of treating crushed concrete and other construction materials [14; 15]. The subject of this research is Recycled Concrete Aggregate (RCA), an example of anthropogenic material, which is a product of demolishing of exploit constructions [16]. The authors investigate the behavior of plain RCA as well as its mixture with rubber grains. One of the waste types produced during cars' exploitation and after their utilization are rubber tires [17]. Disposal of waste tires has become a global problem [18]. Effectively reusing waste tires is an urgent and important issue for saving energy and protecting the environment [19]. Scrap tires, shredded into small pieces (called \"chips\"), alone or mixed with soil can have properties favorable to civil and environmental engineering applications [20]. Recycled waste tires when mixed with soil can play an important role as lightweight construction materials in embankments, retaining walls, machine foundations and railroad track beds in seismic zones. Rubber can be presented as an alternative material in various construction activities, because it is cheap and available in abundance. It can reduce vibrations when seismic loads are of great concern or even improve some mechanical properties of problematic soils, e.g., soft soils [21].