*3.4. Mixing and Fabrication of Specimens*

To mix the SIFRCCs, the water-binder ratio was fixed to 0.35 to achieve the filling performance of the high-performance slurry for filling the inner space of the steel fibers that were placed in advance. The amount of the high range water reducing (HRWR) agent was set to 2.5% of the binder weight. To reduce material separation and achieve the required strength, fine aggregates were added for 0.5% of binder weight and the silica fume was added for 15% of the cement weight. Table 3 shows the SIFRCC mixing formula. The fiber volume fraction variables were set to 4%, 5%, and 6%.

**Table 3.** Slurry-Infiltrated Fiber-Reinforced Cementitious Composites (SIFRCCs) mixing formula.


To analyze the tensile behavior characteristics of the SIFRCCs with respect to the fiber volume fraction of 4%, 5%, and 6% in the direct tensile test, direct tensile test specimens were fabricated with the mixing ratio of each variable in Table 3.

#### **4. Results and Analysis**

#### *4.1. Compressive Strength*

Figure 4 shows the compressive strength test results with respect to the fiber volume fraction of SIFRCCs. In the case of 6% fiber volume fraction, the average compressive strength was analyzed to be approximately 83 MPa. The average compressive strength of 5% fiber volume fraction was approximately 75 MPa, lower by approximately 10%. Furthermore, the average compressive strength of 4% fiber volume fraction was approximately 66 MPa lower by approximately 12% compared to the 5% fiber volume fraction and by approximately 21% compared to the 6% fiber volume fraction. The compressive strength increased in proportion to the fiber volume fraction. The input amount of steel fibers appeared to increase with increasing fiber volume fraction, which generated the restraining effect of the specimen itself, and this affected the increase of compressive strength. Figure 5 shows the result of the compressive stress-strain test with respect to the fiber volume fraction.

**Figure 4.** Compressive strength test results with respect to the fiber volume fraction.

**Figure 5.** Compressive stress-strain curve with respect to the fiber volume fraction.

#### *4.2. Direct Tensile Strength*

Figure 6 shows the results of the direct tensile strength test with respect to the fiber volume fraction of SIFRCCs. The direct tensile strength test result of 6% fiber volume fraction showed a high average direct tensile strength of approximately 15.5 MPa. The average direct tensile strength of 5% fiber volume fraction was approximately 14.2 MPa, lower by approximately 9%. The average direct

tensile strength of 4% fiber volume fraction was approximately 11.0 MPa, lower by approximately 23% compared to 5% fiber volume fraction and by approximately 30% compared to 6% fiber volume fraction.

**Figure 6.** Direct tensile strength with respect to fiber volume fraction.

The direct tensile strength test of the SIFRCCs showed that the cracks gradually spread and lead to fracture after the initial cracking due to the reinforcement of steel fibers. This phenomenon was evident as the fiber volume fraction increased. Furthermore, the direct tensile strength also showed an increasing trend with the increasing fiber volume fraction, similar to the compressive strength test result of the SIFRCCs with respect to the fiber volume fraction.

#### *4.3. Strain Capacity and Tensile Stress-Strain Curve*

Figure 7 shows the strain capacity test result with respect to the fiber volume fraction at the direct tensile strength. The result of the strain capacity test under the direct tensile strength with respect to the fiber volume fraction of SIFRCCs verified excellent strain capacity of 0.7% (0.007) at 5% fiber volume fraction. Since the direct tensile strength increased with increasing fiber volume fraction, the strain capacity was expected increase as well, but the strain capacity was the lowest at 6% fiber volume fraction. This is because in the case of 6% fiber volume fraction, the steel fibers resist the direct tensile load as the fiber amount increases, and due to the small cross-section size (25 <sup>×</sup> 50 mm2) of the direct tensile test specimen, the adhesion performance of the high-performance slurry matrix and steel fibers decreased. Furthermore, considering that the length of the steel fibers is 60 mm, the size of the specimens is considered to be affected by the specimens because the arrangement of steel fibers was parallel to the tensile load when the direct tensile test specimen was fabricated.

Figure 8 shows the tensile stress-strain curve with respect to the fiber volume fraction of the SIFRCCs, and Figure 9 shows the tensile stress-strain curve of the 6% fiber volume fraction of the SIFRCCs. The strain at the direct tensile strength was analyzed to be 0.53% (0.0053), and the energy absorption capacity was 62.10 kJ/m3, which was the lowest among all variables. Regarding the compressive stress-strain test result for 6% fiber volume fraction, the post-peak behavior exhibited a strain hardening behavior, but the post-peak behavior of the tensile stress-strain curve test result exhibited a strain softening behavior. Considering that the cross-section size of the gauge length for measurement is 25 <sup>×</sup> 50 mm2, the result was somewhat different from the compression behavior because the adhesion performance between the high-performance slurry matrix and steel fibers was insufficient.

**Figure 7.** Strain capacity test results at the direct tensile strength.

**Figure 8.** Tensile stress-strain curve with respect to the fiber volume fraction.

**Figure 9.** Tensile stress-strain curve of Slurry-Infiltrated Fiber-Reinforced Cementitious Composites (SIFRCCs) with 6% fiber volume fraction.

Figure 10 shows the tensile stress-strain curve of the SIFRCCs with 5% fiber volume fraction. The strain at the direct tensile strength was 0.7% (0.0070), indicating the highest strain capacity among all variables. The energy absorption capacity was also the highest at 88.05 kJ/m<sup>3</sup> on average. Similar to the tensile stress-strain test result for the specimen with 6% fiber volume fraction, the post-peak behavior exhibited a strain softening behavior.

**Figure 10.** Tensile stress-strain curve of SIFRCCs with 5% fiber volume fraction.

Figure 11 shows the tensile stress-strain curve of SIFRCCs with 4% fiber volume fraction. Unlike the tensile stress-strain test result for 5% and 6% fiber volume fractions, the post-peak behavior characteristics were closest to the strain hardening behavior. The direct tensile strength for 4% fiber volume fraction was lower than those of others, but the energy absorption capacity was 62.27 kJ/m3, which is higher than that of the 6% fiber volume fraction (the highest direct tensile strength). This is considered to be because the size of the direct tensile test specimen is too small and the interface adhesion property between the high-performance slurry and steel fibers did not reach the maximum, resulting in different tensile behavior characteristics for each fraction.

**Figure 11.** Tensile stress-strain curve of SIFRCCs with 4% fiber volume fraction.

#### **5. Conclusions**

To overcome the limited fiber volume fraction of the conventional fiber-reinforced concrete and HPFRCCs, this study developed SIFRCCs that contains a high fiber volume fraction to maximize the tensile strength, energy absorption capacity and strain capacity. An experimental study on tensile behavior characteristics was conducted with respect to different fiber volume fractions of the high-performance SIFRCCs through direct tensile tests. The conclusions of this study are as follows.


**Author Contributions:** Conceptualization, S.K., C.P., D.J.K., S.-W.K.; methodology, S.K.; writing—original draft preparation, S.K., C.P., D.J.K., S.-W.K.

**Funding:** This research was supported by Construction Technology Research Project funded by the Ministry of Land, Infrastructure and Transport of Korea government, grant number 18SCIP-B146646-01.

**Conflicts of Interest:** The authors declare no conflicts of interest.
