## 5 Conclusions  
Recycled concrete aggregates are considered \"high quality\" recycled aggregates for concrete production in terms of their mechanical and physical properties and their variability. Of particular interest in this work was the influence of RCA on long-term dimensional stability, i.e., shrinkage and creep deformation. The addition of RCA instead of natural crushed aggregate leads to a change in the fresh and hardened concrete properties:  
* When only the 8-16 mm fraction was replaced, the flowability increased, most likely due to the fact that the RCA used contains a certain amount of river gravel particles that were detached from the cement matrix during the crushing process, reducing the overall angularity of the coarse aggregate. A further increase in the RCA content led to a decrease in the flowability of SCC mixes. The greatest increase in the water demand was observed when the fine aggregate was replaced with RCA. * RCA reduced the Young's modulus and generally also reduced the compressive strength. The increase in compressive strength in the mixes SCC20, SCC50 and NC2 compared to the mixes SCC10, SCC40 and NC1 indicates that replacing all coarse aggregate with recycled aggregate concrete is more favourable than replacing only one coarse fraction. The main reasons for this are a more favourable stress distribution under load due to a smaller difference in the stiffness of the cement matrix and aggregate, or a higher packing density. * An increasing RCA content leads to an approximately proportional increase in shrinkage strain. This is due to the greater absorbed water content in the RCA and the simultaneous reduction in stiffness. The presence of fly ash did not cause a significant deviation in shrinkage strain. Figure 10: Creep compliance at 28, 90 and 180 * days under load (* for mix NC1, creep compliance after 160 days is presented and for mix SCC60, creep compliance was measured after 168 days); 28-day compressive strength; and hardened concrete density (error bars represent combined uncertainty for creep compliance). * An increase in the coarse and fine fraction of RCA increases creep deformation. Although the highest creep strain was measured on test specimens made from a mix that contained fine RCA, it is not obvious that fine RCA makes a greater contribution to creep than coarse RCA. * The replacement of aggregate and cement with RCA and fly ash led to a relatively large change in concrete density. Density had a stronger linear correlation with creep strain than compressive strength. The European standard for concrete production EN 206 allows the use of up to 50% replacement of coarse aggregates with RCA. Nevertheless, the data show that RCA is rarely used as an aggregate in concrete. This is understandable, as there are a number of parameters that need to be considered when deciding whether the use of RCA is appropriate. One technological limitation is that relatively small quantities of RCA come from a demolition site. Or if concrete with natural aggregates meets a certain compressive strength class, it is not sufficient to simply replace the natural aggregates with recycled aggregates, as RCA concrete is likely to have a lower strength. To compensate for the lower strength, some modification of the concrete composition is required, probably by increasing the cement content. However, it is possible that the use of recycled aggregates in concrete will be enforced by regulations. Concrete already accounts for more than 50% of demolition materials, and the need to recycle these into concrete aggregates will direct demolition activities towards prior characterisation and more careful separation of each type of material. The experimental investigations carried out in this work have confirmed that SCC-RCA is feasible. The mechanical properties of SCC-RCA, even when 100% of the aggregate is RCA, can be utilised for structural applications. The \(w/c\) ratio range of 0.4-0.5 covers most concrete mixes used in construction, so the results presented in this paper can be widely applied.