## 3 RR-FRC Under Chloride Ingress
### Corrosion Initiation and Corrosion Propagation in RR-FRC
#### 3.1.3 Other Parameters Influencing Corrosion Initiation and Propagation  
Deterioration of tunnel lining segments in chloride-rich environments was studied by [PERSON] et al. [74], focusing on subsea shield tunnels of the Xiamen metro line 2 (Figure 5). The experimental program was divided in three stages: (1) loading with a 250 mm eccentric force of 130 kN; (2) application of direct current for 20 days to accelerate the corrosion of steel bar reinforcement; and (3) loading until failure. The authors found that the increase in \(V_{\text{f}}\) delayed the depassivation of steel bars, as indicated by measurements of corrosion potential (\(E_{\text{corr}}\)). Corrosion was first detected in RC elements after 17 h, followed by RR-SFRC (0.5%) after 28 h, and finally, in RR-SFRC (1.0%) after 35 h.  
[PERSON] et al. [76] used electrochemical polarisation (galvanostatic pulse) to investigate the corrosion of rebar in a chloride-rich environment and freeze-thaw cycles (100 cycles with temperature variation between \(-\)18 \({}^{\circ}\)C and +18 \({}^{\circ}\)C). Specimens were partially submerged in NaCl solution, which allowed the authors to differentiate between a corrosion process where chlorides penetrated directly from the liquid solution (submerged part) and a corrosion process where chlorides reached reinforcement by capillary action and salt fog (floating part). In the submerged parts of the specimens, measurements of corrosion current density (\(i_{\text{corr}}\)) showed the highest corrosion rate in the case of RC specimens (3.42 \(\mu\)A/cm\({}^{2}\)). Relative to RC, the values of \(i_{\text{corr}}\) were observed to decrease in RR-SFRC as the fibre quantity increased. The same trend was observed in the case of the floating parts. In another study, [PERSON] [77] used a similar methodology as in [76] but without exposing specimens to freeze-thaw cycles. For the submerged parts of RC specimens, average values of \(E_{\text{corr}}\) and \(i_{\text{corr}}\) of \(-\)234 mV and 4.61 \(\mu\)A/cm\({}^{2}\) were measured, respectively, with similar values observed in RR-PPFRC, indicating a moderate value of corrosion activity. In contrast, low corrosion activity was observed in the floating parts. [PERSON] et al. [87] produced a report for the Florida Department of Transportation investigating the durability of RC and RR-FRC elements (and their counterparts without rebar). Two different concrete mixes were considered: concrete class II with a \(w/c\) ratio of 0.44 and concrete class V with a \(w/c\) ratio of 0.37. Pre-cracking was performed in 3 PBT. Electrochemical measurements on RC and RR-FRC were performed in accordance with ASTM G 109 [90]. The corrosion of steel bars in RR-FRC elements was tested under the cyclical wetting and drying conditions. Uncracked PC and uncracked and pre-cracked FRC elements were subjected to environmental exposure conditions characteristic of Florida for 27 months. Three different environmental exposure conditions were considered: exposure to lime water, exposure to salt water (5% CI) and exposure to swamp water (pH 4.5). In the case of exposure to salt water, after 21 months, the concentration of chlorides was increased from 5% to 7%. For exposure to lime and salt water, both immersion and wetting and drying cycles were considered. Exposure to swamp water was considered only in an immersed state. After the end of exposure, surface degradation due to the corrosion process was evident in all concrete types. Measurements of \(E_{\text{corr}}\) were taken for 617 days for all RC and RR-FRC beams. During this period, no corrosion initiation was detected in any specimens made from concrete class V. The shortest corrosion initiation time was reported for RR-SFRC (137 days), followed by RC (190 days), RR-PPFRC (315 days) and RR-PVAFRC (609 days). The study concluded that the best performance under the considered  
Figure 5: Loading setup performed by [PERSON] et al. [75]. environmental and exposure conditions was observed in PPFRC elements, followed by SFRC and then PVAFRC elements.