## 4 Mechanical Test Results and Discussion
### Tensile Behavior
#### 4.2.3 Discussion on Tensile Behavior  
In general, the 7 d cured specimens, containing GGBS, SF, or CKD, exhibited a more significant strain-hardening behavior than specimen A7 comprising only OPC. As discussed previously, the microstructural analysis found that the matrix of 7 d cured specimens was densified by substituting OPC with the mineral additives that contained smaller particles even prior to their complete reaction. Moreover, specimens employing CKD showed significantly higher strain capacities than other 7 d cured specimens. With regards to the tensile behavior of fiber-reinforced cement composites (FRCCs), fiber-matrix interaction constitutes one of the essential factors. Thus, the surface of PE fibers, pulled from each specimen, was captured in SEM images, as shown in Figures 12 and 13. It was observed that the fibers of 7 d cured specimens using OPC, silica flour, and GGBS, namely, A7, B7, and D7, had irregular patterns of scratches or pits on their surface regardless of the degree of damage. In other words, the length, width, depth, and direction of the scratches and pits were irregular in the 7 d cured specimens. This is thought to result from the unreacted particles, constituting the microstructure, captured in Figures 4-8. Figure 12 also shows  
Figure 11: Tensile strength and strain capacity of (**a**) 7 d and (**b**) 28 d cured specimens.  
that the fibers of 7 d cured specimens, using SF and CKD, had more homogeneous scratches that stretched longer than the other fibers of 7 d cured specimens. This is attributed to the faster formation of the compact microstructure of the specimens containing SF and CKD. Both materials have a high specific surface area to fill the voids and reactivity to react and enhance the bond between components, as explained earlier based on Figures 7 and 8. The two materials, thus, could result in higher strain-hardening behavior in the cementitious composite earlier than other mineral additives. Therefore, the difference in the microstructure and tensile behavior, caused by the longer curing time, was not as considerable, as in the case of the other specimens.  
In the case of the fibers of 28 d cured specimens, the homogeneous scratches, running through the fiber, covered the fiber surface, as shown in Figure 13. This indicates that the 28 d cured matrices provided each fiber with more homogeneous and longer-lasting pullout resistance as compared to the 7 d cured specimens. Figure 10 already demonstrated that the tensile stress-strain curves fluctuated while the strain-hardening behavior and the height of the drops were generally more prominent for the 7 d cured specimens than of the 28 d cured specimens. As the large particles, rather than the compact amorphous hydrates, form the loose and irregular fiber-matrix interface, not only are many voids present, but the particles can also cause a concentration of the pullout resistance and damage specific points of the fibers. This facilitates deterioration of the fiber-matrix interaction and results in a higher drop in the tensile stress when the fiber-matrix interface is broken. Moreover, this eventually reduces the strain capacity of the UHP-SHCC. This mechanism will further be discussed alongside the cracking behavior, later in this paper. In Figures 12 and 13, it was also noted that matrix particles with diverse diameters were detached from the matrix during the tensile test and stuck to the fiber surface. This implies that the fiber-matrix interface consisting of large particles lost their pullout resistance rapidly when the particles detached from the matrix.  
Figure 12: SEM images on polyethylene (PE) fibers pulled out of 7 d cured specimens.