## 2 Literature Review
But RCA has a very complicated structure and self-cementing properties which can lead to cohesion in non-cohesion soil. However, from a geotechnical point of view, self-cementation process is a phenomenon which improves mechanical properties of RCA [14]. Earth construction mentioned earlier often deals with dynamic loads like weaving or vibration caused by cars or trains. Small strain dynamic parameters like shear wave velocity, shear modulus, and damping ratio are needed to predict behavior of this structures subjected to seismic loads. [PERSON] et al. [17] describe different procedures to obtained shear wave velocities from bender elements test. [PERSON] et al. [18] use described methodology for RCA and got shear wave velocities from about 175 m/s for effective stress 45 kPa to about 300 m/s for effective stress 180 kPa. [PERSON] and [PERSON] [19] also obtained a similar value of shear wave velocity. For determination maximum shear modulus, shear modulus degradation curve, and minimal damping ratio of RCA, resonant column apparatus in cyclic torsional shear mode was employed in [PERSON] et al. [20] article. They obtained maximum shear modulus from 62 MPa at 45 kPa effective stress to 220 MPa at 225 kPa effective stress and minimal damping ratio from 0.5 to about 3%. These reports also confirmed [PERSON] and [PERSON] [19]. Moreover, RCA has higher shear modulus than NA at the same effective stress, because of its sharpness and roughness which causes chocking of grains. The coefficient of permeability \(k\), is a key parameter characterizing seepage in soils, which is one of the most important in design earth dams or levees. However, also in road design, filtration layers need to be especially well characterized by the \(k\) value. When constructing a dam or levees, material for its body should has high permeability coefficient. Nevertheless, also in road engineering, even the subbase should have a high \(k\) value. However, such tests are often abandoned because of the low budget of a given investment. General tests are replaced by simpler methods, such as predicting the hydraulic conductivity on the basis of the porosity or grain size distribution [21]. As [PERSON] [21] reported, hydraulic conductivity depends on the pore sizes and the way in which they are connected. However, well-graded natural soils are characterized by the presence of rounded grains, mostly of quartz origin with small roughness. In the case of slight clay particles content, the pores are still large and well connected. In contrast, RCA is characterized by high roughness grains with irregular shapes, which complicates the flow paths of water. Moreover, the internal pores are very often present in RCA, which greatly increases the specific surface [22]. The values of the void ratio, porosity, and therefore hydraulic conductivity in case of RCA are higher than a natural aggregate. The porosity of RCA, reported by [PERSON] [23], is 14.86% when natural aggregates have 3% porosity. The high porosity was found to be caused by the presence of cement mortar. [PERSON] and [PERSON] [22] report differences between the characterization of aggregates during a comparison between natural and recycled aggregates. Water absorption in this study, made with respect to American Society for Testing and Materials, Standard Test Method for Relative Density (Specific Gravity) and Absorption of Coarse Aggregate: ASTM C 127, gave different results for RCA when helium pycnometer and image analysis were conducted. RCA is a granular material that, as opposed to natural aggregates, is coated with patches of cement mortar remains. The remains of cement paste impact the results of the experiment, designed originally for natural aggregates ([PERSON] et al. 2008). RCA's water absorptions after a 24-hour test increased from 5.72% to 8.28%. It is worth noting that 80% of the total water absorption was reached after the first 5 h [24]. This difference compared to natural aggregates may impact the RCA permeability properties.