## 3 Analysis and Discussion of Results  
### Compressive Strength  
The compressive strength of concrete is one of the most important properties in the evaluation of the performance of a structure, since it corresponds to the capacity of its elements to withstand stresses without collapsing. The determination of the compressive strength is especially important since it is usually strongly correlated with the majority of the other mechanical properties, as well as with the  
Figure 2: Comparison of the measured fresh-state density of the concrete mixes with the literature, reported by [PERSON] [41], [PERSON] and [PERSON] [42], [PERSON] and [PERSON] [43], [PERSON] [44], and [PERSON] [45]. \begin{table}
\begin{tabular}{c} **Variation** \\ **(\%)** \\ \end{tabular} \\ \hline
**0\% RA** & 15.0 & - & 15.0 & - & 15.0 & - & 15.0 & - \\
**10\% RA** & 19.5 & 30.0 & 19.0 & 26.7 & 20.5 & 36.7 & 20.0 & 33.3 \\
**25\% RA** & 20.5 & 36.7 & 20.0 & 33.3 & 24.5 & 63.3 & 22.5 & 50.0 \\
**50\% RA** & 22.5 & 50.0 & 22.5 & 50.0 & 26.0 & 73.3 & 23.5 & 56.7 \\
**100\% RA** & 23.5 & 56.7 & 23.0 & 53.3 & 26.5 & 76.7 & 25.0 & 66.7 \\ \hline \hline \end{tabular}
\end{table} 
Table 9: Air content determined in concretes in the fresh-state. stiffness, durability and thermal performance of concrete. Hence, the compressive strength allows an overall assessment of the quality of a given concrete mix. Table 10 shows the average compressive strength values of the concrete mixes at 28 days (\(\ell_{\text{cm,28}}\)), and the variation of strength values (\(\Delta\)) relative to the NAC. The measured compressive strength at 28 days of the NAC mix was 47.8 MPa. Thus, it can be concluded that, according to the compressive strength test carried out on cubic specimens, the NAC mix produced belongs to strength class C35/45, which is higher than that expected since this concrete mix was designed for the target strength class C30/37. Generally, it can be seen that, regardless of whether the replacement is made in the fine or coarse fraction of aggregates, the compressive strength decreases with the increase of RA content. These results stem from the composition of the RA and, in most cases, from the increase of the effective w/c ratio needed with the increase of RA in concrete. However, the results show that the percentages of the decreases in compressive strength vary with several factors, as discussed hereafter. One of the factors that most influenced the results was the source of the RA. Since these aggregates were collected from different companies, located in different parts of Portugal, their composition greatly varied. This was shown in the analysis made to the composition of the RA. Hence, as expected, the results of concrete with different RA are quite different. With respect to the concrete mixes in which fine recycled aggregates (FRA) were used, a much higher decrease in the compressive strength at 28 days of mixes with the use of FRA from Vimajas (43.4%) was obtained when compared to that yielded by the use of FRA from Ambilei (23.9%), for 100% replacement. The lower compressive strength of concrete mixes with FRA from Vimajas can be attributed to the high increase in the effective w/c ratio of these mixes, due to the presence of clay in the aggregates ([PERSON] et al. [24]). The fine particles of clay coat the RA particles and adsorb part of the mixing water. In addition, they prevent the proper bonding between the RA and the cement paste, thus leading to a concrete with lower strength. The scanning electron microscope (SEM) images show that these phenomena caused a porosity increase in these mixes, which resulted in a decrease of their compressive strength [10]. For instance, Figures 3-6 show that mixes made with FRA from Vimajas have micro and macro porosities that are higher than that of concretes made with FRA from Ambilei. This was visible both in concrete mixes with 50% (Figures 3 and 4) and 100% (Figures 5 and 6) FRA.