## 3 Results and Discussion
### Mechanical Properties
In the case of NAC, the crack surface is flatter, favored by the crystalline structure of calcite, while the fracture surface produced in CRA is irregular or lumpy with a smooth transition to the new mortar paste. Most of the macro-cracks present in RCA are produced in the mortar phase of the source concrete, even though sometimes they are also produced through the EAFS aggregate. Figure 4: Compressive strength of all mixes at different ages. The splitting tensile strength values obtained by other authors for RAC produced with CDW are variable depending on the quality of RA. According to the classification proposed by [PERSON] et al. [14], RA are classified in four classes according to their intrinsic properties (bulk density, water absorption and Los Angeles wear). For the most demanding category (Class A), the loss of tensile strength should be around 10% with respect to NAC [60]. The RA used in the present study, even though not complying with the water absorption requirements described for Class A, shows a tensile strength similar to NAC's. Figure 7 shows the compressive modulus of elasticity for all mixes produced. Using a linear trend line, an inverse relationship between modulus of elasticity and replacement level can be established. While NAC shows a 40 GPa modulus of elasticity, the 100% replacement mix shows values around 33 GPa, 17% lower and with standard deviations not exceeding 0.5 MPa. This reduction is lower than that commonly reported and is related to the properties of the source concrete. The source concrete used to produce RCA has low w/c ratio, a modulus of elasticity greater than 50 GPa and a low porosity close to 10%. For similar w/c ratios and cement contents, other authors reported reductions from 15% to more than 40% [15; 35; 61] using RCA produced from common concrete waste. Traditionally, density is a factor directly related to the modulus of elasticity because it is linked to the porosity of concrete [59]. The aggregate used is 7% denser than NA, but this is due more to the iron nature of RA than to the very slight increase of w/c ratio. The porosity of RCA influences its stiffness and hence the stiffness of concrete, since its increase allows greater deformations per unit of applied load. Ultrasonic pulse velocity is intrinsically related to the modulus of elasticity and density. The ultrasonic pulse velocity obtained for all the mixes produced is shown in Figure 8. Following the trend line, it is observed that the pulse velocity decreases as the replacement ratio increases. There is a maximum loss of 6% for 100% replacement, slightly less than 9% determined by [PERSON] [52] for current RAC with the same w/c ratio. It should be remarked that the source concrete showed an ultrasonic pulse velocity of 5.22 km/s at the same age. On the other hand, the maximum loss obtained is similar to that obtained by [PERSON] et al. [10] for RAC produced from concrete precast rejects, which demonstrates the homogeneity of RAC and the high quality of the RCA used. The RCA used in this study has a density 10% and 20% higher than NA and of current RCA respectively, which a priori favours an increase in the ultrasonic pulse velocity. On the other hand, the porosity of the source concrete mortar allows RAC to strain more under sustained loads, obtaining smaller modulus of elasticity and lower ultrasonic pulse velocity than NAC, due to the discontinuities generated by the pores. Figure 5: Tensile strength of all mixes at 28 days. Figure 6: Crack patterns of test specimens subjected to tensile strength test for 0% (**a**) and 100% (**b**) replacement ratio.