**4. Conclusions**

The effect of inclination angle of steel fiber, hybrid inclined and aligned steel fibers and fiber spacing on the bond behaviors of steel fibers in manufactured sand mortar were experimentally studied by the pullout test, results are discussed based on the bond mechanisms of hook-end steel fiber. Conclusions can be drawn as follows:

The nominal debonding strength, debonding work and debonding energy ratio reached the maximum when the inclination angle was 15◦ and decrease afterward, while the nominal bond strength and the slipping work had slight changes at the inclination angle of 15◦ and decrease afterward. The nominal residual bond strength reached its maximum at the inclination angle of 30◦ and decrease afterward. The slipping energy ratio increases, and the pullout work and the pullout energy ratio decrease with the increase in inclination angle. This indicates that the inclination angle of steel fibers has obvious effect on the reinforcing effect of steel fiber on concrete matrix.

Compared with the bond behaviors of specimens with couples of inclined steel fibers, no hybrid effect among the inclined and aligned fibers are observed on the nominal debonding strength and the nominal bond strength. However, a favorable hybrid effect presented on the nominal residual bond strength and the debonding energy ratio. This indicates the different bond performance can be provided by steel fiber in different orientation.

The nominal debonding strength, bond strength and residual strength increases with the decrease in fiber spacing no less than 5 mm. The scraping failure of surrounding mortar appeared in condition of much smaller spacing of steel fibers, due to insufficient mortar resists the scraping force during pullout. Moreover, specimen with fiber number from 4 to 9 in this study are suitable for study the bond mechanism between the fiber and mortar matrix. Comprehensively considering the range and test accuracy of the test system in this study, four steel fibers symmetrically arranged to section centroid is a better choice.

**Author Contributions:** Conceptualization, X.D. and M.Z.; methodology, S.Z. and Y.Z.; validation, X.D. and S.Z.; formal analysis, M.Z., Y.L. and H.L.; investigation and data curation, H.L. and Y.Z.; writing—original draft preparation, X.D. and Y.L.; writing—review and editing, X.D. and S.Z.; supervision, S.Z.; funding acquisition, X.D. and S.Z. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by National Natural Science Foundation of China, grant number 52108212, and Natural Science Foundation of Henan, grant number 212300410192.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Data are available with the first author and can be shared with anyone upon reasonable request.

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