## 3 Results and Discussion  
### Results of the General Soil Mechanical Characterization  
The soil mechanical results are presented in Table 3. Materials used for road construction must demonstrate a high bearing capacity, so that no settlements or accidents are caused when vehicles come off the paved part of the road [79]. Stability and high bearing capacity are usually mainly achieved through the gravel fraction of the building material. Practically, this requirement also applies to secondary building materials, which usually also contain a semi-natural sand fraction in the case of CDW, while slags are broken down to a grain size similar to that of sand and gravel. Typically, even having a similar grain size distribution after processing, the material properties of crushed slags are different to those of natural building materials. Shear strength is the property or ability to withstand shear stresses with limited deformation. The shear strength of a soil depends on the type and composition of the soil particles, the structure of the soil, the water content of the soil, and its previous loading (this applies only to cohesive soils). Friction is the resistance in the contact area of two material bodies that are pushed against one another. When it comesto cohesion, a distinction is made between real and apparent cohesion. Real cohesion is independent of normal stress and only occurs in cohesive soils. Apparent cohesion, or capillary cohesion, also occurs in non-cohesive soils. The values for the cohesion ranged from 0.86 to 11.09 \(\text{KN}/\text{m}^{2}\). The angle of friction covered a range from 25 to 56\({}^{\circ}\). The stiffness moduli of the investigated secondary building materials covered a range from 81.72 to 187.81 \(\text{MN}/\text{m}^{2}\). In the present case, FA had characteristics similar to those of natural cohesive soils, while the material properties of GBF and EAF were comparable to those of very coarse non-cohesive soils. [PERSON], in 2013 [80], reported that the mechanical characteristics and the resistance factors were improved by adding steel slag to the crushed limestone in road pavement layers. The use of steel slag, especially at an optimal ratio, improved the subbase layer's density, strength, and failure resistance--especially at a horizontal distance of 60 cm from the load center. Moreover, in crushed CDW, there can be a high proportion of fine material. While the angle of friction of mixed CDW and FA was within the range of natural cohesive soil, the values for the slags were above typical values from natural sand and gravel, and within the range of values reported for slags in the literature [81]. The results of the angle of friction indicate that the slags form a particularly stable layer, from a soil mechanical point of view. Following the results of the oedometric compression test, the stiffness modulus \(\text{E}_{\text{s,max}}\) was determined on the second loading cycle (reloading modulus); the investigated secondary building materials fulfilled the requirements for substructure materials after qualified soil improvement according to ZTV E-StB 09 [82] for road classes with \(\text{E}_{\text{v2}}\geq 70\)\(\text{MN}/\text{m}^{2}\) (after usual compaction). The in situ plate load test for the determination of \(\text{E}_{\text{v2}}\) after compaction follows the same principle, with a cycle of loading--unloading--reloading. Using the E-CBR relation of Fillibeck and Schwabbaur [83], the CBR value was estimated, showing that GBF fully corresponds with the Vietnamese requirements for road subbase, while mixed CDW and EAF are slightly below the required CBR value and might require material modifications/adaptations, as was also indicated in [40]. The values for the measured stiffness modulus are comparable to the results for broken aggregates and recycled aggregates reported in [81]. The material properties of secondary building materials for road construction can be adapted to the necessary requirements through respective material processing (for instance, the grain size distribution).