# Effects of Freeze-Thaw and Wet-Dry Cycles on Tension Stiffening Behavior of Reinforced RAC Elements
## 2 Materials and Methods
### Materials
#### 2.1.2 Natural and Recycled Aggregate Concrete  
The mixture proportioning for the concretes analyzed as part of this study was based on the Compressible Packing Model (CPM), proposed by [PERSON] [28]. The original CPM assumes that, during mixing, the aggregates are in a dry condition and their water absorption capacity at 24 h should be considered by introducing additional water for their saturation. However, as also reported in Table 2, the RCAs are generally characterized by a significantly higher water absorption capacity compared to that of natural aggregates. Recent studies [29] demonstrated that they are not able to absorb such amount of water during the mixing time and, as a consequence, some extra water in the mixture could cause an increase in the effective water-cement ratio [30]. For this reason, in the present study the produced mixtures were prepared in accordance with the methodology proposed by [PERSON] et al. [7]: the RCAs are employed in fully dried condition and 50% of their water absorption capacity is considered for saturation during mixing. The use of this mix-design method represents one of the key novelty of the present research since, as also well demonstrated by the authors in the literature [7; 22], it allows the production of recycled and natural concrete characterized by the same strength class, by considering the specific features of the employed raw materials. Thus, individual mix-design was previously developed for each mixture (for more details see [PERSON] et al. [17]), so that the simple replacement (in mass or volume, generally adopted in the literature) of the natural aggregate by RCA in the reference mixture was not carried out. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Mixtures** & & **C35-NAT** & **C35-L-C1** & **C35-D-C1** \\ \hline \multirow{8}{*}{Materials (kg/m\({}^{3}\))} & NCA\_C1 & 452 & 0 & 0 \\  & RCA\_L\_C1 & 0 & 361 & 0 \\  & RCA\_D\_C1 & 0 & 0 & 384 \\  & NCA\_C0 & 457 & 456 & 453 \\  & NFA & 868 & 867 & 862 \\  & Cement & 325 & 336 & 341 \\  & SP & 1.86 & 1.92 & 1.95 \\  & Effective water & 196 & 191 & 194 \\  & Total water & 212 & 216 & 216 \\ \hline \multirow{2}{*}{Mixture parameters} & Effective _w/c_ & 0.60 & 0.57 & 0.57 \\  & Total mortar volume (\%)—V\_M,tot & 65.5 & 77.6 & 73.1 \\ \hline \multirow{3}{*}{28-days properties} & Water absorption (\%) [31]—w\_open & 3.0 & 3.9 & 3.5 \\  & fc\_28 (MPa) [32] & 34.2 & 35.3 & 33.5 \\ \cline{1-1}  & Ec\_28 (GPa) [33] & 21.3 & 21.2 & 20.9 \\ \cline{1-1}  & fc\_28 (MPa) [34] & 2.7 & 2.9 & 2.6 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Mix proportions and main properties of NAC and RACs used ([PERSON] et al. [17]). Figure 1: Grain size distribution of natural and recycled aggregates. For the experimental investigation of the present study, three normal-strength concrete mixtures were used, with 28-days main properties already known, as shown in Table 2. The choice of producing normal-strength (i.e., C35) mixtures was based on the fact that when high-strength RAC mixtures are produced, the potential detrimental effect of RCA on the resulting RAC durability is mitigated by the low porosity of the cementitious matrix [17; 22]. Table 2 also reports (in the mixture parameter rows) the amount of total mortar volume (V\({}_{\text{M,tot}}\)), which represents a relevant parameter for RAC mixtures. In fact, it is determined by the sum of the volume of the attached mortar (V\({}_{\text{AM}}\)) present in RCAs (Table 1) and the new mortar volume given by the sum of sand, cement, water and superplasticizer present within the mixture [17; 22].