## 2 Literature Review  
Aggregates are materials commonly used in civil engineering. For example, in the European Union and the EFTA countries in 2016-2017, around 30,000 tons of aggregates are used to create 1 km of new road [5]. These are mainly natural aggregates--such as crushed rocks, gravel, and sand--which constitute about 88% of the market demand [5]. Moreover, natural aggregates are the only material used in construction like earth dams, embankments, levees, or any others earthen construction. Designers and contractors are wary of using recycled aggregates, because the physical and mechanical properties are different from the natural aggregate behavior. The lack of detailed knowledge about recycled aggregates properties is another issue [6]. Despite this, in the last four years, the demand for recyclable aggregates increased from 5% to 8%, which is a significant increase on the scale of the European Union and EFTA (The European Free Trade Association) countries [5; 6]. The largest number of recycled aggregates is produced in the UK, 52.3 million tonnes and in France 20.3 million tonnes compared to Poland, 5 million tonnes according to data for 2015, published in the bulletins of the UEPG--European Aggregates Association [5]. Among brick, glass, or soft materials--which are usually deposited at landfills--the most common waste is large size concrete debris. After the crushing process of this material is finished, concrete debris in a gradation curve from 0 mm to 63 mm are deposited and can be utilized as an aggregate or soil material. Commonly, in the literature, such porous media are called recycled concrete aggregate (RCA) and it is successfully used in road engineering. Therefore, a lot of geotechnical properties of RCA needed in road design are already known. For example, optimum moisture content for RCA with sandy gravel (saGr) gradation curve range from 8.35 to 11.74% [7; 8]. Differences between optimum moisture content can occur due to various origins and different classes of concrete, from which the RCA was created. For example, RCA made of high-strength concrete with a lower water-to-cement ratio will present lower water absorption than RCA created from lower class concrete [6]. Another very important parameter in road engineering is California Bearing Ratio (CBR). [PERSON] et al. [9] gives CBR values for RCA from 97 to 138%, [PERSON] [10] gives one CBR value of 128%. According to Polish standards for roads WT-4 (Technical Specifications No. 4) [11], materials for the sub-base require a CBR value at least 60% and 80% for the base layer. Accordingly, RCA can be considered to be suitable for use in either sub-base or base layers. For correct prediction of road settlement resilient modulus is needed. [PERSON] et al. [12] provides resilient modulus from 450 to 1710 kPa for numbers of loading cycles range from 10 to 50,000. They observed stiffness improvement with increase in numbers of loading cycles. [PERSON] et al. [13] presented empirical equations for resilient modulus based on particle shape, binder type and aggregate mineralogy of RCA. [PERSON] (2001) in his cyclic triaxial tests show that RCA have the same resilient modulus as natural aggregate (NA), but over time laboratory and field tests showed an increase in stiffness for unbound layers with RCA. [PERSON] [14] explained this fact by self-cementing properties of the unhydrated cement particle of RCA. For other geotechnical applications, like aggregates for embankments or earth dams, mechanical parameters of RCA are needed. For RCA with gradation of saGr, [PERSON] [15] determined internal friction angle (\(\varphi\)) in direct shear test from 39.5 to 42\({}^{\circ}\). [PERSON] et al. [15] and [PERSON] et al. [7] confirmed the test results presented by [PERSON], but [PERSON] et al. [16] from triaxial test estimated much higher value of \(\varphi\) equal to 53\({}^{\circ}\). The cohesion phenomena which was reported during shear strength tests under RCA is still not fully explained. Usually aggregates do not behave cohesion.