Environmental concerns are all around us. The influence of an increasing awareness and care for the environment is present also in construction sector, in the areas of material selection, construction technology and construction waste management. When using concrete we strive to avoid negative impacts on the environment. Legislation in the area of construction, environment and waste follows the decisions of authorities relatively fast. This year the government of the Republic of Slovenia adopted the Programme of waste management and waste prevention in the Republic of Slovenia in accordance with the legislation. The main aim of this diploma thesis is the assessment of the influence of content and grain size distribution of recycled aggregate on concrete properties. We tried to find out whether it is possible to exchange natural aggregate with a recycled one while maintaining the basic properties of fresh and hardened concrete. When preparing the concrete mixtures two types of aggregate were used. Natural aggregate was delivered from France within project "COST Action TU1404", while recycled aggregate from crushed waste concrete was acquired from a separation plant near Ljubljana. The preparation of samples and testing of recycled aggregate was carried out at the laboratory of UL FGG. With tests we first determined the physical properties of the recycled aggregate (grain size distribution, fines content, particle shape, density, water absorption, moisture content, presence of humus particles). On the basis of obtained results the suitability of the aggregate for use in concrete mixtures was assessed. Three mixtures of fresh concrete were prepared with the same amount of cement and the same water-to-cement ratio. The first mixture was prepared with fine natural aggregate and coarse recycled aggregate, the second one with recycled fine aggregate and natural coarse aggregate, while in the third mixture only recicled aggregate was used. The properties of fresh concrete (consistency, density, air content) and hardened concrete (compressive strength, splitting tensile strength, depth of water penetration under pressure) were determined according to SIST EN standard methods. On the basis of obtained results the tested concretes were classified into compressive strength classes, and analysis of the results was carried out. At the end, recommendations were given for the use of such concretes in the construction sector.