## 5 Conclusions  
In this paper, recycled concrete aggregates (RCA) were characterized using a permeameter with an upgraded constant head method in order to avoid the common errors encountered in such methods. The results were statistically analyzed to estimate the statistical tolerance. The suffosion and coefficient of permeability were also calculated. Moreover, leaching properties of RCA was analyzed in order to identify potential environmental threats connected with filtration process. The conclusions are summarized below:  
1. RCA exhibit the non-Darcian flow of water with threshold gradient occurrence. 2. The value of the coefficient of permeability, k, changes with the void ratio, e, exponentially, and during further studies, empirical equations were determined. 3. The flow of water trough RCA is very sensitive, and in the case of one blend, turbulent flow was observed around a critical gradient, which, for this material, was 0.9. 4. RCA proved its good quality as a permeable material, which is characterized by a coefficient of permeability of 0.05-16, 0-16, and 0-8 mm, and the k values were 1.018 \(\times\) 10\({}^{-4}\), 1.89 \(\times\) 10\({}^{-5}\), and 2.08 \(\times\) 10\({}^{-5}\) m/s, respectively. 5. The threshold gradient was estimated in all blends, and statistical analysis shows the dependence of this phenomena on the fine particle content. 6. Fines also seem to be the reason behind the differences in the flux velocity between blends which do and do not contain them. 7. The threshold gradients for the tested blends of 0.05-16, 0-16, and 0-8 mm were 0.175, 0.212, and 0.210, respectively. Below theses gradients, for the tested blends the flux velocity may dramatically decrease, and water that stays in the RCA can degrade its mechanical parameters, including the bearing capacity, when road construction is considered or slope stability when earth dam or embankment construction is considered. 8. For road construction with standard gradients between 0.3 to 0.6, RCA has a constant value. Nevertheless, the threshold gradient needs to be taken into consideration when a large amount of fine particles is present in the material. 9. Obtained permeability coefficient for all examined blends are appropriate and meet requirements of aggregate for body construction of earth dams, lieves, or embankments. 10. For earth dams or lieves, a construction blend of 0.05-16 mm requires a reverse filter to avoid suffosion during filtration process. 11. Leaching of heavy metals during filtration process does not exceed the permitted limit. Although, the concentration of compounds having hexavalent chrome might exceed the limit. From a chemical point of view, RCA might be used as filtration layers in road, earth dam, or lieve construction. Nevertheless, the concentration of other compounds harmful for life have to be checked before application. The above results suggest the existence of possible permeability problems in roads or dams containing RCA layers. Nevertheless, such an occurrence concerns low gradients (below 0.3) and should be not an eliminative factor for application in above mentioned constructions. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Element**} & **Co.** & **Cd** & **Cu** & **Ni** & **Pb** & **Zn** & **Sulphates** & **Chlorides** & **C** \\  & **(mg/L)** & **(mg/L)** & **(mg/L)** & **(mg/L)** & **(mg/L)** & **(mg/L)** & **(mg/L)** & **(mg/L)** & **(\(\mu\)S/cm)** & **pH** \\ \hline Value & 0.066 & 0.00067 & 0.121 & 0.127 & n. d. & n. d. & 194.7 & 14.05 & 501.5 & 7.91 \\ \hline Acceptance criteria * & 1.0 & 0.05 & 0.5 & 0.5 & 0.5 & 2.0 & 500 & 1000 & - & - \\ \hline \hline \end{tabular}  
* * Official Gazette of the Republic of Poland, Regulation of the Minister of the Environment of 18 November 2014 on the conditions to be met for the introduction of sewage into waters or to land and on substances particularly harmful to the aquatic environment.  
\end{table} 
Table 5: Leachate concentration from recycled concrete aggregate. **Author Contributions:** A.S. and W.S.