## 3 Results  
### Fresh Concrete Properties  
The results of the fresh concrete properties are shown in Table 4. The density of the fresh concrete decreases with the increase in the proportion of recycled aggregate, but also with the addition of fly ash, which is due to the lower density of these constituents compared to natural aggregate and cement. A reduction in the \(w/c\) ratio increased the density, as the reduction in the \(w/c\) ratio was achieved by reducing the water content. The air content varied between 1.5% and 3.5%. In SCC mixtures, the air content increased with the increase in RCA content, except in the SCC30 mixture, which contained fly ash. The results listed in Table 4 for the slump of the NC mixtures and the workability properties of SCC refer to the second batch, in which the entire amount of superplasticizer was added premixed with water. This allowed for better dispersion of the superplasticizer and resulted in better flowability than the first batch. Therefore, the authors believe that these results are more meaningful for evaluating the effects of RCA on workability. The slump values of the NC1 and NC2 mixes show that the concrete has a plastic consistency. Taking into account the amount of superplasticizer and the slump, RCA reduced the water requirement in the NC2 mix. The slump flow of the SCC mixes was classified according to the European standard for concrete HRN EN 206:2021 [49], which shows that the criteria for SCC were met (Table 4). From the dosage of superplasticizer and the results of the slump flow and V-funnel test, it can be seen that replacing the \(8/16\) mm  
Figure 4: (**a**) System for measuring Young’s modulus under compression; (**b**) adjusting spacing of measuring pins using length comparator; (**c**) reading of length between measuring pins on samples loaded in creep loading frame. fraction with recycled aggregate increases flowability and decreases viscosity. This is particularly evident in the SCC40 mix, where the amount of water was reduced by 40 litres compared to the SCC0 mix. The replacement of the 4/8 mm fraction increased the water requirement and viscosity in the SCC20 and SCC50 mixtures. It can be seen from Table 4 that the density of the SCC50 mix was almost the same as that of the SCC40 mix, although it contained a greater proportion of lower density aggregates. The results of the compressive strength tests (Table 5) support the possibility that mixtures in which both coarse aggregates were replaced with recycled aggregates have a denser packing of particles. It is known that denser packing of coarse aggregates can increase flow resistance [50]. Most of the results reported show that recycled concrete aggregates reduce the flowability of SCC [19; 51; 52; 53; 54]. The reduction in flowability is often attributed to the increased internal friction between the aggregates due to the increased angularity. The comparison of natural and recycled aggregates shown in Figure 1 indicates that this was not the case with the RCA used in this work. The partial replacement of cement with fly ash improved fluidity, mainly due to the reduced friction caused by the spherical shape and smooth surface of the fly ash particles [55]. Apart from the SCC0 mixture, only mixtures with fly ash were able to fulfil the criteria for passing ability. It has been shown that the addition of pulverised coal fly ash increases the flowability and passing ability of SCC RCA mixes [23]. In the SCC60 mix, the highest superplasticizer dosage was required to achieve adequate flow properties. The particle size distribution of natural and recycled aggregate fractions differs, with the greatest difference being in the fine aggregates (Figure 2). The quantities of the individual fractions in the concrete mix were optimised for natural aggregate, which was then replaced by the same quantity of recycled aggregate. This leads to deviations from the optimised particle grading and can result in changes in the packing density and, consequently, changes in the water requirement.