# Design Procedures for Sustainable Structural Concretes Using Wastes and Industrial By-Products
## 4 Discussion
### Mechanical Properties
#### 4.2.1 Compressive Behavior of EAFS Mixtures
Considering the fresh behavior of EAFS8B, EAFS8C, and EAFS8D mixtures and by analyzing the results shown in Figure 11a,b at 28 days, it was possible to note that EAFS8C and 8D mixtures were the ones, with high cement amount, that best meet the reference structural concrete properties (REF1). These showed a favorable fresh behavior, with slump class S2/S3, and reached a strength class C30/37. In the design of both EAFS mixtures a cement quantity of 257 kg was used, this amount being smaller than REF1 (280 kg). With this reduction in cement and considering that the EAFS 8C and 8D mixtures contained less fractions of EAFS aggregates, the objectives of producing a structural concrete comparable to a conventional one in terms of deformability and durability, seem feasible and are economically advantageous and environmentally more efficient. With respect to the percentage of EAFS aggregates of the abovementioned mixtures, 8C and 8D, it is noted that the lower the distribution modulus, the lower the coarse aggregate quantities and consequently the higher the quantity of fine aggregates required. Since the fresh and hardened characteristics of these two mixtures are essentially the same, it was found that the EAFS8C mixture showed to be environmentally more favorable when compared to the EAFS8D mixture, considering the higher percentage of EAFS aggregates present in its composition, which leads to a decrease in the amount of sand required, a natural aggregate that is scarce. Subsequently, for the mixtures found to be the most advantageous, EAFS8C and 8D, the cement amount was reduced to values similar to that of the REF2 mixture, resulting in the mixtures EAFS8C1 and EAFS8D1. As shown in Figure 12a, both the EAFS8C1 and EAFS8D1 mixtures clearly exceeded the REF2 mixture in terms of strength and deformability, with the compressive strength class of both mixtures equal to C25/30, as shown in Table 10. However, the EAFS8C1 mixture did not reach the S4 slump class required, when comparing to the REF2 mixture (Figure 9b). Figure 11: Mechanical testing results at 7, 28 and 90 days after casting for REF1 and EAFS mixtures with high cement amount: (**a**) average compressive strength, \(f_{cm}\); (**b**) average elasticity modulus, \(F_{cm}\). Considering these results, EAFS8D1 was the mixture selected to continue with the study of FRC design with RTSF. It reflects the optimal balance between the percentage of EAFS aggregate used and the behavior reached in the fresh and hardened state, therefore showing good potential for the inclusion of the RTSF. As shown in Table 10 the compressive strength class of EAFS_FRC mixture reduced to C20/25 when compared to EAFS8D1 mixtures (C25/30). In fact, by analyzing Figure 12 it is clear that the fiber reinforced concrete, FRC_EAFS, reached a lower compressive strength than EAFS8D1 and REF2. This result was somewhat expected, since the RTSF were directly added to the concrete mixture without any previous treatment to separate rubber pieces or other impurities, or separate larger or smaller fibers. Therefore, the fresh behavior of the concrete mixture was significantly impacted by the introduction of the RTSF, as well as the compressive strength and elasticity modulus. Figure 12: Mechanical testing results of REF2, EAFS8C1 and EAFS8D1 mixtures at 7, 28 and 90 days after casting, with lower cement amount: (**a**) average compressive strength, \(f_{cm}\); (**b**) average elasticity modulus, \(E_{cm}\). Figure 13: Failure modes obtained after compression tests: (**a**) failure mode of EAFS3A mixture; (**b**,**c**) failure mode of EAFS4 mixture; (**d**,**e**) representative failure modes of EAFS6, 2, 8B, 3B, 8C, 5C, 8C1, 8D and 8D1 mixtures in accordance with the satisfactory failure modes of the standard [47].