**4. Conclusions**

In this research, the effects of the reinforcement ratio, short steel fibers as additional reinforcements, and prestressing have been explored with regard to the strength and failure behavior of TRM subjected to tensile loading. The following conclusions can be drawn:

(1) Generally, the tensile strength of both P0C1S0 and P0C2S0 increased with an increase in the reinforcement ratio. However, the EF values of specimens P0C1S0 and P0C2S0 decreased as the reinforcement ratio increased, indicating weakened textile–matrix bond strength. The textiles did not snap when P0C1S0 and P0C2S0 failed, however slid from the matrix, thereby resulting in debonding failure and low utilization rate of the carbon textiles. In contrast, the utilization rate of carbon textiles increased remarkably when short dispersed steel fibers of 2% volume fraction were inserted into the grids of the textile.

(2) Short steel fibers are able to improve the mechanical properties of the mortar and the entire composite in experiments. Moreover, steel fibers improve the textile–matrix bond strength, which can be attributed to the "shear resistant ability" of steel fibers inserted into the grids of the textile. Increases in tensile strength were clearly observed in all specimens with added steel fibers. An investigation of the fracture surfaces using an optical microscope further revealed that short steel fibers added to TRM cause finer cracks with smaller spacings and widths. Furthermore, within the scope of this test, the improvements in tensile mechanical behavior were highly correlated with the increase in the steel fiber proportion. Compared with the results that were obtained for the reference TRM plates, the tensile strength increased by approximately 100% following the addition of 2% steel fibers by volume.

(3) Increases in first-crack stress and tensile strength were also observed in prestressed TRM specimens. The enhanced first-crack stress was attributed to the extension (caused by pre-compression of the mortar matrix after being released) of stage I, i.e., the uncracked state corresponding to the related mortar matrix. The tensile strength increased as the bond behavior improved, a result of the strengthened interaction effect between the surface of the textile and the matrix activated by prestressing. Therefore, the serviceability limit states of TRM composites can be extended by exerting a prestressing force on the textiles.

(4) Adding steel fibers at 1% volume to prestressed TRM specimens is an effective method of improving the specimens' mechanical performance, dramatically enhancing the bond strength between matrix and textiles. As a result, the failure mode changes from debonding to the complete fracture of the carbon textiles. In this study, the combination of 1% steel fibers and prestress calculated at 15% of the ultimate tensile strength of the two-layer textiles was found to be the optimum configuration, producing the highest first-crack stress and tensile strength and the most reasonable multi-cracking pattern.

**Author Contributions:** F.Z. designed the experiments; H.L. and Y.D. analyzed the data; L.L. performed the experiments; D.Z. and W.P. revised the paper; all the authors reviewed and approved the paper.

**Funding:** This work was supported by the funds from the Natural Science Foundation of Hunan Province (Grant No. 2018JJ2043), the Major Project of Sci-Tech Plan of Changsha City (Grant No. kq1804002, kq1703002), and the Project of Sci-Tech Plan of Changsha City (Grant No. kq1701032).

**Acknowledgments:** The authors gratefully acknowledge Hunan Good Bond Construction Technic Development Co., Ltd (Changsha, China) for supplying the cementitious materials, super-plasticizer, sand, and epoxy resin.

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