# Mechanical Characterisation and Shrinkage of Thermoactivated Recycled Cement Concrete
## 3 Results and Discussion
### Ultrasonic Pulse Velocity  
The ultrasonic pulse velocity of the produced concretes ranged from 4415 to 5268 m/s, having been more influenced by the w/b than by the incorporation of RCP (Figure 21) or other additions (Figure 22). In fact, for the same w/b, the UPV only varied up to about 2% with the incorporation of RCP (Figure 21).  
The UPV of concrete is essentially affected by its density and stiffness, which are both related to the microstructure porosity. For this reason, the UPV as a function the incorporation of RCP showed a similar evolution to that found for the compressive strength (Section 3.2).  
As shown in Section 3.3, a good correlation was obtained between the compressive strength and the dry density. However, up to 15% RCP incorporation, the modulus of elasticity showed an opposite trend to that of the compressive strength (Section 3.5). This may explain the descending trend of UPV as a function of the incorporation of RCP in concrete with a w/b of 0.65, as well as the modest variation of UPV in concrete with a w/b  
Figure 21: Ultrasonic pulse velocity at 28 days, \(\text{UPV}_{\text{28d}}\), of concrete with different incorporation percentages of additions.  
Figure 22: Ultrasonic pulse velocity, UPV, of concrete as a function of compressive strength,\(f_{cm}\), and dry density, \(\rho_{dry}\).  
of 0.55 and distinct amounts of RCP (Figure 21). Nevertheless, except for low incorporation percentages of RCP, in which the microstructure can be densified, the UPV tended to decrease with the amount of RCP, due to the increase in air entrainment and decrease in the cement matrix stiffness.  
For the same workability, the UPV of OPC concrete (w/b of 0.55) was only 1.4% higher than that of 15%RCP concrete (w/b of 0.62, without SP) (Table 4). This may be explained by the fact that UPV is approximately related with the sixth root of compressive strength, according to Equation (4) [45]. In this scenario, the 14.5% reduction of compressive strength obtained in Section 3.3 would correspond to a reduction of UPV lower than about 2%, as found in this study.  
\[UPV=K_{UPV}\times{f_{cm}}^{0.15}\times{\rho_{dry}}^{0.5} \tag{4}\]  
The UPV of RCC concretes was similar to those of RCP concretes with the same incorporation percentage of recycled cement (Figure 21). The higher w/cf and the slightly lower compressive strength of concrete with RCC was offset by its higher cement particle stiffness, due to the 26% aggregate fraction within the RCC. On the other hand, the UPV of concrete with NT or CF was lower than that of concrete with the same incorporation percentage of RCP (Figure 21), which may be attributed to the lower density of NT and CF, the reduction of the amount of hydration products and their slightly higher volume of paste.  
Figure 22 presents the UPV of the produced concretes as a function of the compressive strength and dry density, according to the methodology suggested by [PERSON] et al. [45]. A very good correlation was found between these properties, which validates the results. Besides the variability associated to the UPV test method, this relation is also influenced by the fact that these properties are affected by the water content in different ways.