# Mechanical Characterisation and Shrinkage of Thermoactivated Recycled Cement Concrete
## 3 Results and Discussion
### Tensile Splitting Strength  
The tensile splitting strength at 28 days of the produced concretes varied between 2.2 and 5.5 MPa, having essentially been affected by the w/b (Figures 15 and 16).  
Overall, the tensile splitting strength at 28 days followed a similar trend to that observed for the compressive strength at the same age. In this case, the maximum tensile splitting strength was obtained for 15% RCP, both in concrete with a w/b of 0.55 and 0.65. For the same w/b, the tensile splitting strength of concrete with up to 15% RCP was up to 30% higher than that with OPC. For 30% RCP concrete, the tensile splitting strength was about 8% lower than that of OPC concrete with the same w/b. Once again, concrete with 40% RCP and high dosage of SP was able to invert the descending trend of tensile splitting strength (Figure 15). The highest tensile splitting strength reduction relative to the reference OPC concrete was found for 100% RCP concrete (18.5%).  
Figure 14: Compressive strength, \(f_{\text{cm}}\), as a function of dry density, \(\rho_{dry}\), at 28 days.  
Comparing mixtures with the same workability and without SP, the tensile splitting strength was only 3% higher in concrete with OPC (w/b of 0.55) than with 15%RCP (w/b of 0.62) (Table 4). A similar reduction was found for concrete with 30% RCC (w/cf of 0.60). This indicates that the tensile splitting strength was less affected by the w/b than the compressive strength, which corroborates the well-known lower proportional increase in the tensile splitting strength with the compressive strength [42,43]. Similarly to the compressive strength, the NT and CF did not significantly contribute to the tensile splitting strength (Figure 16). As mentioned, the incorporation of these admixtures decreases the overall amount of hydration products. In fact, considering the tensile splitting strength at 28 days as a function of the w/c (Figure 17), the contribution of NT and CF to this property was not significant.  
A good correlation was found between the tensile splitting strength and the compressive strength at 28 days (Figure 18). This is due to the fact that both properties are influenced by the characteristics of the cement matrix, which are primary affected by the w/b. The reduction of the \(f_{ctm}/f_{cm}\) ratio in high strength concrete was confirmed.  
Figure 16: Tensile splitting strength at 28 days,\(f_{ctm,sp,28d}\), of concrete with w/b of 0.55 and different percentages of additions.  
Figure 15: Tensile splitting strength at 28 days,\(f_{ctm,sp,28d}\), of concrete with different incorporation percentages of RCP and w/b.  
EN 1992-1 [42] proposes Equations (1) and (2) to determine the tensile splitting strength, \(f_{ctm}\), as a function of the compressive strength and the dry density of concrete. Overall, Equations (1) and (2) were able to render a good approximation to the experimental results, only slightly underestimating the tensile splitting strength for high strength concrete, independently of the type of binder (Figure 19). The determination of \(f_{ctm}\) in Equations (1) and (2) took into account the compressive strengths and dry densities, \(\rho_{dry}\), indicated in Table 4, assuming the relation between the compressive strength in cubic specimens and in cylindrical specimens, \(f_{ctm,cyl}\) of EN 1992-1 [42]. Furthermore, the axial tensile strength, \(f_{ctm}\), was considered to be about 90% of the splitting tensile strength [44].  
\[f_{ctm}=0.30\times\left(f_{ctm,cyl}-8\right)^{\frac{2}{3}}\times\left[0.4+ \frac{0.6\rho_{dry}}{2200}\right]\,\text{for}\,\,20\,<\,f_{ctm,cyl}\,\,<\,68\, \text{MPa} \tag{1}\]  
\[f_{ctm}=2.12\times ln\left(\frac{1+f_{ctm,cyl}}{10}\right)\times\left[0.4+ \frac{0.6\rho_{dry}}{2200}\right]\,\text{for}\,\,f_{ctm,cyl}\,\,>\,68\,\text{MPa} \tag{2}\]  
Figure 17: Tensile splitting strength at 28 days,\(f_{ctm,sp,28d}\), of the produced concretes as a function of the w/c.  
Figure 18: Tensile splitting strength at 28 days,\(f_{ctm,sp,28d}\), as a function of compressive strength at the same age, \(f_{ctm,28d}\).