*3.3. Effect of Prestress*

Figure 18 presents the stress–strain curves of the prestressed TRM specimens. The results, including a summary of the tensile strength, ultimate strain, first-crack stress, crack number, crack spacing, and EF values are listed in Table 4. The first-crack stress and tensile strength of the prestressed TRM specimens are depicted in Figure 19. According to the results shown in Figure 18a, pre-tension to 10% and 20% of the ultimate tensile capacity of the one-layer textile increased the specimens' first-crack stress. Although both the first-crack stress and tensile strength improved with increases in the prestress level, the improvements are not obvious (Figure 19). For example, the first-crack stress and ultimate tensile strength of P20C1S0 only increased by 25.7% and 30.5%, respectively, compared with those in P0C1S0. Figure 18b shows that adding 1% steel fibers to the prestressed TRM specimens improves their mechanical properties. The first-crack stress, ultimate tensile strength, and strain capacity of P20C1S1 increased by 51.5%, 114%, and 57.4%, respectively, compared with those in the reference specimen P0C1S0.

According to the results presented in Figures 18c and 19, TRM specimens pre-tensioned to 15% of the ultimate tensile capacity of the two-layer textiles (P15C2S0, P15C2S1) exhibit higher first-crack stress and tensile strength than the control TRM specimen (P0C2S0). Compared with P0C2S0, the first-crack stress of P15C2S0 and P15C2S1 increased by 44.8% and 102%, and the tensile strength increased by 51.2% and 124%, respectively. The average crack number and crack spacing of P15C2S1 increased from 9.8 and 11.92 mm to 11.5 and 9.18 mm in comparison with P0C2S0 (Figure 10 and Table 4). It can be concluded that adding 1% steel fibers by volume to prestressed TRM specimens (P15C2S1) is an effective method of improving the specimens' mechanical performance.

The effect of prestress can be explained by referring to Figure 20. Point *O* is the origin of the tension force *N* with respect to displacement Δ*l*. The curve includes three distinctive stages, i.e., elastic, multiple cracking, and post-cracking. In this case, *Ncr* indicates the critical tensile load at which the mortar matrix first cracks, corresponding to the moment that the tensile load is mostly transferred to the textiles, whereas *Nu* is the ultimate tensile load, beyond which the specimen loses its load capacity. Whether the slope of the post-cracking stage is steeper or shallower than that in stage I depends on the stiffness of the reinforcing textiles. Once the textiles in the TRM specimen are pre-tensioned, the origin *O* shifts to point *O*, permitting the previous short uncracked stage to extend to an ideal duration [26]. With respect to this particular curve, a prestressing force is exerted on the textiles at point *O* and released at point *Np*. Thus, an initial compressive stress on the concrete matrix was achieved, leading to an increase in the first-crack stress [35]. Moreover, after releasing the prestress on the mortar matrix, the bond strength between the textile and matrix is considerably improved, so the ultimate tensile strength of the TRM specimens also increases (Figures 18 and 19). The development process after releasing the prestress in TRM specimens runs along the same path as the un-prestressed specimens until the final failure. As a result, exerting a prestressing force on the textiles extends the serviceability limit states of TRM and produces more reliable workability.

**Figure 18.** Stress–strain curves of the prestressed TRM specimens: (**a**) reinforced with one-layer textile and without steel fibers, (**b**) reinforced with one-layer textile and 1.0% steel fibers, and (**c**) reinforced with two-layer textiles.

**Figure 19.** First-crack stress and tensile strength of the prestressed TRM specimens.

**Figure 20.** Influence of prestress on TRM.

Although prestressing the textiles improves the cooperative bearing ability between the textile and the matrix to a certain extent, specimens P10C1S0, P20C1S0, and P15C2S0 exhibited debonding failure. The failure modes of P10C1S1, P20C1S1, and P15C2S1 changed from debonding to the complete fracture of carbon textiles; thus, adding 1% steel fibers by volume to prestressed TRM specimens could significantly improve the textile–matrix bond properties. The fracture morphologies of prestressed TRM specimens are shown in Figures 21 and 22.

**Figure 21.** Fracture morphology of prestressed TRM specimens with one-layer textile.


**Figure 22.** Fracture morphology of prestressed TRM specimens with two-layer textiles.
