## 2 Literature Review
The hydraulic conductivity of RCA was studied in works whose main purpose was to obtain the geotechnical properties, and extensive studies in this subject need to be performed. [PERSON] et al. [25] found the coefficient of permeability 3.3 \(\times\) 10\({}^{-8}\) m/s, which was similar to cohesive soils and materials used rather for dam and levee core than for dam body or filtration layer in road engineering. Nevertheless, a very low coefficient of permeability still was within the recommended value for a road subbase. [PERSON] et al. [26] and [PERSON] and [PERSON] [27] reported the hydraulic conductivity of RCA in a range from 2.04 \(\times\) 10\({}^{-3}\) to 2.67 \(\times\) 10\({}^{-3}\) m/s. The differences between results [26; 27] and [25] are very significant, which proves that more studies on this parameter should be carried out. The above-citedworks did not mention the existence of non-Darcian flow. Nevertheless, the results of the experiments conducted by the abovementioned authors are in range of 2.04 \(\times\) 10\({}^{-3}\) to 3.3 \(\times\) 10\({}^{-8}\) m/s, which can indicate the existence of non-Darcian flow. [PERSON] [28] showed that consolidation process in the cohesive soils leads to the formulation of a theoretical approach for dealing with this problem. The water flux in this study is proportional to a power function of the hydraulic gradient if the tested gradient is lower than a certain critical value. After this value, the gradient becomes linear for large gradient values. This behavior was explained by assuming that there is a certain hydraulic gradient in clayey soils that causes the binding energy to overcome the energy of mobile pore water. The threshold gradient is one of the parameters that constitute [PERSON]'s non-Darcian flow. RCA formed on the basis of the different concrete will have unique chemical properties along with the different raw materials used in its production. As a consequence, it is not possible to identify all sources of heavy metals, but it is possible to identify materials that may be their source. Such materials include cement. The properties of heavy metals influencing their presence in the production process are their volatility. Non-volatile compounds--i.e., Ba, Be, Cr, As, Ni, V, Al, Ti, Ca, Fe, Mn, Cu, and Ag--leave the furnace as cement clinker components and are the source of metal content in the cement composition and thus also the source of heavy metals in RCA. In Table 1, the chemical composition of the basic constituents of the concrete from which the RCA is produced is presented. Presented data concerns only one kind of concrete and is not a general description for such material. The high content of heavy metals and sulphates can lead to leaching of these compounds during a filtration process, which can cause significant environmental pollution. Therefore, it is necessary to analyze these RCA properties. One of the researchers that conduct studies of the leaching properties of RCA was [PERSON] et al. [30]. They examined 17 mixtures of recycled aggregates with different contents of RCA, NA, ceramic, gypsum, bituminous, and other substances in laboratory. [PERSON] et al. [30] proved that different mixtures with RCA do not exceeds the permitted standards of concentrations of heavy metals according to European Union (EU) Landfill Directive [31]. Similar study to [30] was performed by [PERSON] et al. [32], but they conducted their tests in the field. They use RCA to build sub-base layers in three section on highway No. E6 located 20 km south from Oslo. Outgoing water from each sections of the highway was collected and tested for four years between 2006 and 2010. Concentration of heavy metals exceeds the limit only in the case of chrome. However, it could be connected with petroleum pollution from passing vehicles. For successful application of RCA in road engineering and as an aggregate for earth dam, levees, and embankments, the permeability of RCA needs to be examined and analyzed.