## 3 Results and Discussion
### Mechanical Properties  
There are several factors that positively and negatively affect the compressive strength of the study RAC: porosity of the cement paste, the new ITZ's quality and characteristics of the RCA. Porosity negatively affects strength. The greater porosity of RAC is due to the porosity of RCA and to that generated in the ITZ between RCA and the new paste [59]. This porosity largely depends on the w/c ratio of both the source concrete and RAC. This strength loss is not high for this RAC, since the porosity of mixes with a 100% replacement is only 18% higher than of NAC, because the source concrete has a low w/c ratio and RAC mixes have a lower effective w/c ratio than the source concrete. A lower quality ITZ has been reported by various authors [30,31] as the main cause of the strength loss of RAC, justifying that this element is the weakest link of the chain. The shape of the EAFS aggregates in RCA and the high compressive strength and stiffness of the source concrete minimize this effect. On the other hand, compared with NA, RCA's shape index is 23% lower and, even having a greater roughness, these aggregates allow using slightly lower effective w/c ratios to obtain the same workability, generating lower pores in the mortar and reducing the strength loss. The result of these effects on the compressive strength is shown in Figure 4 where the evolution over time of the compressive strength for all mixes is shown. Figure 3: Capillarity absorption vs. time (**a**) and capillarity absorption vs. replacement ratio (**b**). At 7 days, the compressive strength decreases slightly and linearly as replacement increases. For 100% replacement, the strength loss with respect to NAC is 5.5%. At 28 days, the concrete class for all replacement is C35/45, exceeding the C30/37 class proposed at the design phase. At 28 days, a strength loss or gain depending on the replacement cannot be clearly defined, but a relative 7-day strength increase of 21% for NAC and 23% for RAC with 100% of RA is observed. This slightly higher relative increase for high replacement could be due to a hydration of the unhydrated cement grains of the old mortar, thus improving the bond in the ITZ with the new mortar. So the curing age is an important factor to check the effectiveness of these concrete mixes. At 91 days, the replacement level does not affect compressive strength. The slope of the linear adjustment is practically 0 and the standard deviation close to 1 for all replacements, proving an increase in compressive strength with age and replacement ratio, because the old mortar is still hydrating. In any case, the compressive strength obtained is much lower than that of the source concrete, although relatively higher than that obtained in other studies for current CDW [9, 17, 27, 32, 33, 34] which on average lose around 10% compressive strength at 28 days with respect to NAC. This is due to the high quality of the RCA used in the present study, namely the high strength of the attached mortar of these CRA (due to the strength of the source concrete) and its low shape index. The results of the splitting tensile strength tests are shown in Figure 5. The high standard deviation, which is typical of splitting tensile strength tests [33], and the similar mean values obtained show that the behaviour of RAC is very similar to NAC's for any replacement ratio. This happens because there is a good bond strength between RCA and the new paste, due to the higher roughness of RCA. This greater roughness seems to compensate for the weaker mortar surface generated in the crushing stage and exposed in the new ITZ. It also compensates for RAC not benefitting from the bond produced by the chemical interaction between calcium hydroxide and the calcareous aggregate [59] in NAC. As seen in Figure 6, fracture surfaces propagate through the aggregates (good bond strength in the ITZ), both for NAC and RAC.