# Green Manufacturing of UHPFRC Made with Waste Derived from Scrap Tires and Oil Refineries
## 3 Results and Discussion
### Performance and Microstructure
The average compressive strength of S1E0, S1E15, S2E0, S2E15, S3E0, and S3E15 was determined to be 152, 130, 166, 144, 187, and 165 MPa. The compressive strength of the cement mixes was found to increase by up to 23% when the addition of a dose of steel fibers increased from 1% to 3% by volume. The addition  
Figure 4: SEM images revealing the microstructure of (**A**) S0E0, (**B**) S0E15, and (**C**) S3E15, and (**D**) digital images revealing the fiber pull-out failure of the tested samples. Figure 3: Various properties of the cement-based mixes containing different amounts of ECAT. of ECAT up to 15% by weight reduced this strength gain by up to 12% in the maximum presence of steel fibers. The flexural property of the UHPFRC is characterized by a residual strength in the postracking stage with a substantial enhancement in the toughness of the cement-based composite. Steel fibers in the cement-based mix prevent and control crack spread. Figure 6 shows the flexural strength of the UHPFRC samples that were notched prior to testing. There is a gradual increase in the flexural strength of the UHPFRC samples when the RSF in them increased from 1% to 3% by volume. The average flexural strength of the notched samples labeled S1E0, S1E15, S2E0, S2E15, S3E0, and S3E15 was found to be 5.7, 5.4, 6.4, 12.1, 17.6, and 15.8 MPa. Increasing the ECAT did not affect the flexural behavior of the UHPFRC containing RSF up to 2%. In contrast, there is a marginal decrease in the flexural strength of the 3% RSF-reinforced UHPFRC samples with an increasing amount of ECAT. Cement binds RSF and sand together in the UHPFRC matrix, and its replacement with pozzolanic materials affected the tensile properties only in the case of a higher volume of RSF (i.e., 3% by volume). Steel fibers tend to pull out of the binder matrix under tensile loads, and the addition of higher volumes of RSF demands more cement in the matrix to avoid weak mechanical performance. Figure 4 shown the microstructure of a UHPFRC sample where a small gap was observed at the interface between the RSF and the binder matrix, indicating that the steel fibers tend to pull out under applied loads (see Figure 4D). The prismatic UHPFRC samples were notched in the center as they were destroyed by flexure, with a major crack forming through the depth of the sample. The random distribution and orientation of steel fibers in UHPFRC samples usually complicate the stress fields of the crack tip [48], and the local tensile strength and fracture toughness can be seen to be heterogeneous [49]. The UHPFRC samples were found to undergo three stages, including linear elastic, pseudo-hardening, and softening, which is evident in the load and deflection curves obtained, as shown in Figure 7. The RSF and the cement-based matrix underwent elastic behavior in the first stage, with the maximum flexural strength being achieved in this stage attributed to the matrix strength. In turn, the RSF contributed very little to the overall flexural behavior. Microcracks appeared near the notch in the second stage, where the cracking strain of the cement-based matrix was exceeded. As the load increased, the microcracks merged into a single macrocrack above the notch. Crack surfaces were bridged with closely spaced RSF. This microcrack slowly widens at this stage, possibly due to the high strength of RSF and the strong bond between the fibers and the matrix, leading to a certain level of strain-hardening. This is the main characteristic that distinguishes UHPFRC from traditional concrete. This pseudo-strain-hardening stage is the final of the three stages of flexural response. The area under the load versus deflection curve of the ECAT-based UHPFRC samples was observed to be relatively higher than that of the plain UHPFRC samples, indicating that the stiffness of the UHPFRC was improved in the presence of ECAT. Figure 5: Flowability and cut-section of the UHPFRC samples.