## 3 Experimental Results  
### Mechanical Properties of the Recycled Concrete Evaluated  
A total of 20 specimens were tested for indirect tensile, compression and elasticity according to UNE-EN 12390-6 [46], UNE-EN 12390-3 [47] and UNE-EN 12390-13 [48], respectively. Additionally, the compressive strength tests were duplicated as they were performed for both cylindrical and cubic specimens. Finally, to determine the workability of the concrete, two slump tests were carried out according to UNE-EN 12350-2 [60], with a result of 16 cm, which corresponds to a fluid consistency. The DIC technique described in Section 2.2.2 was used to obtain displacements and strain values. A 2D-DIC approach was used for cubic specimens using the open source software Ncorr (Version 1.2) [55] and a 3D-DIC approach was used for cylindrical specimens using the open source software MultiDIC (Version 1.0) [56]. The processing parameters were similar in both cases, defining an ROI covering the entire specimen and using a subset size of 20 \(\times\) 20 pixels with a step of 7 pixels, which ensured an overlap of 65%. In addition, a checkerboard calibration target was used to remove image distortion and perform 3D reconstruction using the DLT algorithm. To obtain the longitudinal strains, several virtual extensometers were placed in order to analyse the entire surface of the region and exploit the potential of DIC. Nevertheless, due to regulatory restrictions, the strain cannot be measured over the entire surface, but it has to be considered in the central third of the specimen. Taking into account the dimensions of the specimens, a total of 10 virtual extensometers with a spacing of 15 mm were placed on each specimen (Figure 7) to analyse their behaviour and spatial influence. For the compression tests, the peak longitudinal strain at maximum load prior to failure was recorded on virtual extensometers placed over the entire surface. For the elasticity tests, the strain at the maximum and minimum of the applied load and unloading cycles was obtained, as well as over the entire surface. Considering all the test types and specimens, as well as the virtual extensometers placed on them, the results of the mechanical characterisation are shown in Table 2. Along with the mean values, the main statistical parameters such as standard deviation (STD), coefficient of variation (CoV) and lower and upper bounds are also presented, which allow the behaviour and heterogeneity of these properties to be analysed. Figure 7: Longitudinal displacements obtained using DIC and extraction of longitudinal strain using the virtual strain gauge: (**a**) cubic specimens with 2D-DIC approach; and (**b**) cylindrical specimens with 3D-DIC approach. The dosed concrete offered a compressive strength, related to cylindrical specimens, of \(f_{c\text{-}c\text{-}c\text{-}d}\) = 36.5 MPa. On the other hand, a compressive strength of \(f_{c\text{-}c\text{-}ub}\) = 40.5 MPa was obtained from tests carried out on 15 cm cubic specimens. This implies a conversion coefficient from cubic to cylindrical specimens, \(\gamma\) = 0.90, which is similar to the coefficient usually used for conventional concretes with a strength value UCS \(<\) 60 MPa. It is worth noting that the results for the cubic specimens showed less variation, with a CoV of 5.4% compared to a CoV of 11.8% for the cylindrical specimens. The larger contact surface for the cubes may be one of the reasons for the greater homogeneity of these results. In any case, very high compressive strength values were obtained, with only one specimen below 30 MPa, allowing the sustainable concrete studied to be classified as HA-30 according to the Spanish Structural Code [19]. The high minimum resistance of 30 MPa obtained makes it possible to meet the durability requirements for all environmental exposure classes and, in all cases, to comply with the dosage parameters for the minimum cement content, with a high cement content C \(>\) 400 kg/m\({}^{3}\), thus obtaining a concrete with reduced permeability.