# Influence of Partial and Total Replacement of Used Foundry Sand in Self-Compacting Concrete
## 3 Results and Discussion
### Mechanical Properties  
Figure 5 shows the results of the compressive strength of the three self-compacting concretes at 7 and 28 days of age. SCC-C attained a compressive strength of 21.91 MPa after 7 days, while SCC-UFS-100 and SCC-UFS-50 attained 28.94 MPa and 36.61 MPa, respectively. Compared to SCC-C, SCC-UFS-100 obtained a 32% increase in compressive strength, while the SCC-UFS-50 presented an increase of 67%. At 28 days age, SCC-C presented a compressive strength of 35.92 MPa, while SCC-UFS-100 and SCC-UFS-50 attained a compressive strength of 41.25 and 41.76 MPa, resulting in an increase of 15% and 16%, respectively. In general, it is observed that at early ages, SCC-UFS-100 and SCC-UFS-50 have higher compressive strength than SCC-C, with SCC-UFS-50 displaying the highest strength. However, at 28 days the difference between SCC-UFS-100 and SCC-UFS-50 is minimal. The graphical representation of the development of the SCC strength over time can be represented by means of a logarithmic adjustment as reported by other authors [24; 27]. From the equations shown in Figure 5, it is shown that the correlation of R\({}^{2}\) analyzing the evolution of compressive strength as a function of time is close to 1; therefore, the fit is adequate to observe the strength of the concretes at different ages, allowing a curve fit in which it is observed that the behavior of SCC-UFS-50 has a higher strength than the other two concretes. The compressive strength differences between the materials studied could be explained by SCC-C fresh state properties being lower than the other two concretes, leading to poor self-compaction and creating voids within the concrete matrix, resulting in a lower compressive strength.  
Comparing the SCC-UFS-50 with the results of the [PERSON] et al. [44] study in which 50% UFS replacement was incorporated along with a rice husk ash cement substitution (0-20%), the compressive strength was higher at all ages.  
The compressive strength results match the findings reported by [PERSON] and [PERSON] [30], where it was observed that with the increase in up to 50% of UFS replacement, the compressive strength increased as well. This was attributed to the high silica content of UFS, thus creating a denser matrix [30].  
Figure 6 shows the appearance of compression cracks at 28 days for SCC-C, SCC-UFS-50 and SCC-UFS-100. The breakage of the SCC-UFS-50 and SCC-UFS-100 specimens was more satisfactory than that of the SCC-C specimens, which is due to the fact that the applied force propagated through the paste-aggregate interface as stated by [PERSON] et al. [18], resulting in the observed fracture planes.  
As for splitting tensile strength, Figure 7 shows that there is a decrease of 2.8% between SCC-C (2.15 MPa) and SCC-UFS-50 (2.09 MPa). On the other hand, SCC-UFS-100 shows a decrease of 28% (1.53 MPa) compared to SCC-C and a decrease of 26% compared to the SCC-UFS-50. Additionally, the calculation of the standard deviation of the data obtained was performed, resulting in a standard deviation of 0.06 for the SCC-C and 0.07 for both SCC-UFS-50 and SCC-UFS-100.  
In general, it is observed that there is no significant difference between SCC-C and SCC-UFS-50. Similar results were reported by [PERSON] and [PERSON] [6], where it was observed that the incorporation of up to 30% of UFS in self-compacting concrete slightly reduced the splitting tensile strength. [PERSON] et al. [29] reported that the splitting tensile strength is not affected with the incorporation of up to 40% UFS compared to the control concrete.  
Figure 5: Compressive strength of the SCC mixes.  
Figure 6: Detail of compression fractures at 28 days.