## 3 Results and Discussion
### Behaviour of Recycled Concrete  
The experiment showed that the RAC mixtures performed with waste from the pre-casting elements are a highly interesting alternative. As Table 4 shows, the compressive strengths at 28 days for the RAC mixtures were 55.4 and 56.6 MPa for replacement rates (r) of 50% and 100%, respectively. Furthermore, the values at the first 24 h were 33.8 and 33.2 MPa. Consequently, both recycled mixtures may be qualified as C30 (RC-30) strength concrete typology, and even as C45 type.  
However, the use of RAC mixes for pre-stressed elements may not perform well in all situations, despite meeting the strength objectives. Several studies [41; 42; 43] show that creep and shrinkage effects are greater in recycled concrete that can induce effects on the curvature of pre-stressed structural elements working in bending, and functional capacity could be lost if they are not correctly dimensioned. Consequently, further research is necessary to complete the functional aptitude of recycled concrete mixes for manufacturing pre-stressed elements in order to analyse potential long-term effects.  
Figure 6 compares the results of the compressive strength evolution of the RAC mixtures with respect to the equivalent normal concrete (RC-30). Additionally, the normal concrete mixture without accelerator (RC-30-Acel_x0) was compared. All of these were performed using the same \(\mathrm{W_{ef}}\)/C ratio and dosage conditions.  
Thus, the use of recycled aggregates generates a reduction of the compressive strength of around 17% (\(-\)11 MPa) at 28 days compared with the equivalent conventional concrete mixture RC-30. Moreover, the results show a contrasting effect, increasing the final compressive strength at 28 days by approximately 6% (+3 MPa) compared to the conventional mixture, which uses no accelerant additives. In all cases, the strength obtained is higher than the minimum required value of 30 MPa.  
Once the strength viability of the recycled concrete was tested, the economic factors for these mixtures were analysed to complete the interest of applicability. Thus, the costs shown in Table 4 suggest the highest manufacturing costs correspond to the higher strength mixture with EUR 53.3/m3 (conventional control concrete RC-30), followed by the equivalent mixture without an accelerator (RC-30-Acel_x0) with EUR 51.8/m3, and finally, the most economic mixtures were the recycled ones, with costs of EUR 51.5 and 49.8/m3 for r = 50 and 100%, respectively. Thus, the use of recycled aggregates achieves savings of around EUR \(-\)3.5/m3 compared to conventional concrete, i.e., a reduction of some 7% in the final costs, which is an interesting profit margin to consider RAC as an alternative material in the manufacturing process.  
In addition, it is necessary to add the economical savings from the conventional management of concrete to complete the total costs savings of RAC (around EUR 10/t for transport + management), corresponding to an additional saving of EUR 4.8 and 9.5/m\({}^{3}\) for r = 50 and 100%, respectively. Therefore, the total savings of using recycled aggregates can be estimated at EUR \(-\)6.6/m\({}^{3}\) for r = 50 and EUR \(-\)13/m\({}^{3}\) for r = 100% compared to the conventional mixture (RC-30) and EUR 5.1 and 11.5/m\({}^{3}\) with respect to the HA-30-Acel_x0 mixture (Table 5).