## 4 Discussion
### Shrinkage Deformation  
Dimensional changes measured on unloaded concrete cylinders are not only caused by the evaporation of water from the surface, but also by self-desiccation, which is present throughout the specimens. Self-desiccation phenomena are associated with cement hydration and lead to so-called autogenous shrinkage [65]. Autogenous shrinkage develops faster than drying shrinkage and occurs mainly at an early age. [PERSON] et al. measured the autogenous shrinkage of SCC produced with the cement type CEM 142,5 R with a \(w/c\) ratio in the range 0.4-0.47 and found that the autogenous deformation was most intense within the first 20 h and only a moderate increase was measured in the following 4 days [66]. It was also found that the change in length of concrete specimens subjected to drying 1 day after casting was mainly due to drying shrinkage for a mix with a \(w/c\) ratio of 0.4 [67]. The same study also found that the contribution of autogenous deformation to overall shrinkage increased with decreasing \(w/c\) ratio [67]. Therefore, the change in length of the unloaded specimens in this work can be largely attributed to drying shrinkage. Regardless of the shrinkage mechanism, the internal restraints that counteract deformation are the same and are usually related to the quantity of aggregates and their drying shrinkage [65; 68; 69]. For concrete produced with natural aggregates, shrinkage depends only on the shrinkage of the cement paste and the volume of the aggregates, as most aggregates are non-shrinking [65]. RCAs contain hardened cement paste, which is the reason for the higher water absorption (Table 1). Since the recycled aggregate was partially saturated before mixing and further saturated during mixing, it is expected that this aggregate will show a reduction in volume due to water loss. [PERSON] et al. used pre-saturated RCA and concluded that the higher early shrinkage of the RCA mixtures could be due to the relatively easier moisture loss from the RCA particles [70]. This explains why the concrete mix SCC0 exhibited the lowest shrinkage strain. In addition, the SCC0 concrete mix exhibited the highest Young's modulus (Figure 7) and thus the greatest internal restraint to shrinkage. The lowest Young's modulus and the highest recycled aggregate content can also explain the highest shrinkage of mix SCC60. NC mixes contain a larger volume of recycled aggregate compared to SCC mixes with the same \(w/c\) ratio, which could explain their slightly larger shrinkage strain. The regression line fitted to the experimental data shows that an increasing RCA content leads to an increase in shrinkage strain. As a first approximation, it can be assumed that this increase is proportional to the volume of RCA in the concrete mix, which can be seen from the slope coefficient of the regression line (Figure 8). The slope of the regression curve is almost the same when the 180-day shrinkage values are plotted. [PERSON] et al. found that the shrinkage strain of mixes containing 100% recycled coarse concrete aggregate was 70% higher than that of the control mixture at 180 days [71]. The comparison of the shrinkage curves of the mixes SCC10, SCC20, SCC40 and SCC50 shows a higher shrinkage rate for mixes with a \(w/c\) ratio of 0.4 during the first weeks (Figure 5). This could be related to the greater contribution of the self-desiccation to the shrinkage strain in mixes with a lower \(w/c\) ratio. Mixes with a \(w/c\) ratio of 0.5 have a lower internal restraint and contain a greater amount of evaporable water, and at a later age, their shrinkage rate becomes greater than that of mixes with a \(w/c\) ratio of 0.4. The replacement of cement with fly ash in the SCC30 mix increased the initial shrinkage strain, possibly due to a slow stiffness development. However, the long-term shrinkage strain is not significantly different from the shrinkage strain of the SCC20 mix. [PERSON] et al.