# Mechanical Characterisation and Shrinkage of Thermoactivated Recycled Cement Concrete
## 3 Results and Discussion
### Drying Shrinkage  
Figure 23 displays the drying shrinkage of concrete with different incorporation percentages of RCP and w/b of 0.55. In general, the long-term 90 days drying shrinkage followed the same trend of the compressive strength. The drying shrinkage decreased slightly up to 15% RCP, but over this replacement percentage increased with the incorporation of RCP, except for concrete with 40% RCP, which was produced with a high dosage of SP. For the same composition, i.e., the same aggregate/paste ratio, the drying shrinkage is primarily affected by the volume of evaporable water and w/b [46]. Given that the concretes were compared for the same w/b, the drying shrinkage essentially depended on the microstructure porosity and stiffness of the cement matrix. As previously discussed, the concretes with up to 15% RCP were able to develop denser microstructures with lower porosity. However, due to the higher porosity and lower stiffness of the RCP particles, the modulus of elasticity was lower in these concretes  
Figure 23: Drying shrinkage, e\({}_{\text{cst}}\), of concrete with a w/b of 0.55 and different incorporation percentages of RCP up to 90 days (**left**) and up to 7 days (**right**). (Section 3.5). Therefore, up to 15% RCP incorporation, the drying shrinkage was similar to slightly lower than that of the reference OPC concrete. Despite some variability, it may be concluded that the drying shrinkage over time was similar in both concretes. The 30%RCP concrete developed a less dense microstructure (Section 3.3) and stiffness was further reduced (Section 3.5). Moreover, the RCP concretes tended to have slightly higher volumes of paste than the reference OPC concrete. Therefore, in this case, the drying shrinkage increased about 15% at 90 days. An inverted trend was observed for concrete with 40% RCP, because as discussed in previous sections, the addition of high dosages of SP improved the dispersion and hydration of RCP and OPC, leading to denser microstructures, associated with higher stiffness and lower amounts of evaporable water. Based on mercury intrusion porosimetry, scanning electron microscopy and nitrogen adsorption analyses, [PERSON] et al. [10] reported that, up to 28 days, the RCP pastes may be associated to more refined microstructure for the same total porosity of reference OPC pastes with equal w/b. This was explained by the water absorption of porous RCP particles that reduces the w/b of the bulk cement paste and promotes the development of finer porosity. Therefore, the total volume of evaporable water may be similar, but the capillarity forces of narrower pores are increased, which can also increase the drying shrinkage, at least, at early ages, when the hydration degree is still lower in OPC pastes than in RCP pastes. This may explain the slightly higher drying shrinkage observed in concrete with RCP during the first 7 days (Figure 23). In addition, [PERSON] et al. [22] found that the early age autogenous shrinkage increases in high-strength concrete with up to 25% RCP. This was attributed to the high surface area of RCP particles, which increases the water requirement and lowers the internal relative humidity of concrete. This leads to self-drying and consequent shrinkage. Nevertheless, due to the test variability and the fact that the tests were only conducted after 24 h, it was not possible to confirm a clear relation between the early shrinkage and the incorporation percentage of RCP. Regarding the evaporable water, RCP pastes tend to hydrate faster, but, over time, the amount of combined water appears to be similar to lower than that of OPC pastes with the same w/b [9; 10]. Concrete with 30%RCC showed similar long-term drying shrinkage to that of 30%RCP concrete (Figure 24). On the one hand, the RCC presented higher w/cf and hence lower stiffness and higher volume of evaporable water than the RCP. On the other hand, since RCC had about 26% aggregate fraction (Section 2.1), the volume of paste in RCC concrete was reduced.