## 3 Results and Discussion
### Durability  
Carbonation is a phenomenon that alters the alkaline behaviour of concrete. When CO\({}_{2}\) from the atmosphere diffuses into the concrete matrix, it reacts with water forming carbonic acid and the latter reacts with portlandite forming calcium carbonate. This process reduces the pH of concrete, depassivating the reinforcement. Figure 9 shows the evolution of the carbonation depth over time of all mixes. At 7 days, the carbonation depth is very small due to the low w/c ratio of the mixes. The test scatter caused by reading tolerances is relevant but mixes with 100% and 50% replacements appear to be slightly less durable. However, at 28 days, a slight increase in carbonation depth with the increase in CRA incorporation can be observed, which can reach about 10% for total replacement. This trend continues at 91 days, when carbonation becomes more noticeable for higher replacements: 50 and 100%. For total replacement, carbonation depths can be 15% higher than for reference concrete. Figure 8: Ultrasonic pulse velocity at 28 days. Figure 7: Modulus of elasticity of all mixes at 28 days. The general increase in carbonation depth with replacement ratio and age is due to several causes. CRA is composed of the source concrete mortar, hence part of this cement is already carbonated. Another relevant fact is that the open porosity of RAC is 18% higher than that of NAC, which favours the increase of permeability and hence the diffusion of CO\({}_{2}\) through the cement matrix, since there is easier access to more paste surface. On the other hand, by having a greater volume of total mortar, there is also a greater amount of total portlandite compared to the reference concrete (a greater amount of calcium carbonate can be generated) and hence a greater amount of CO\({}_{2}\) is required for the same carbonation depth. Other authors have also obtained increases in carbonation depth as the replacement level increases. For similar mixes and 100% replacement, [PERSON] et al. [10] obtained a marginal increase of the depth of carbonation for rejected high-quality precast concrete at all ages tested. [PERSON] et al. [62] obtained a slight increase in carbonation depth for similar mixes and similar compressive strengths, namely an increment of 14% for total replacement. Finally, in the review of [PERSON] et al. [45], it is found that, for total replacement, the average carbonation depth may increase by a factor of 2 with respect to the control concrete, establishing the permeability of concrete, which depends directly on the water absorption (and therefore on the open porosity) of RCA, as a key factor. Chlorides negatively affect concrete, reacting with tricalcium aluminate forming Friedel's salt (bound chlorides) and oxidizing the reinforcement after penetrating concrete (free chlorides) [63]. The resistance to the penetration of chlorides is shown in Figure 10. The chlorides diffusion coefficient increases with replacement ratio and decreases with age. At 28 days, there is a quasi linear increase in this coefficient, reaching 11% for total replacement. At 91 days, the increase is 9.5% for total replacement, due to the extra contribution of mortar from the source concrete that continues to hydrate. On the other hand, there is a large reduction in chlorides diffusion coefficient with age, a reduction greater than 25% for all replacements. Figure 9: Carbonation depth for all mixes and ages. Chlorides penetration in concrete (an advective-diffusive phenomenon) largely depends on its porosity and the tortuosity of its capillary framework [64]. A strong correlation has been obtained between the chlorides diffusion coefficient and both open porosity (R\({}^{2}\) = 0.73) and capillarity absorption (R\({}^{2}\) = 0.89). Critical variables such as the content or type of cement remain constant, so it seems consistent that the higher ingress of chlorides in RAC with larger replacement is due to the increase in open porosity, owing to the higher volume of voids in RCA.