## 3 Results and Discussion
### Resistance to Chloride Penetration  
The chloride ion diffusion coefficients are presented in Figure 6. As shown, the 28 day \(D_{RCM}\) values of composite specimens, except for the MS-T-C specimen, were generally higher than that of the control concrete specimen. The 56 day \(D_{RCM}\) values of the composite specimens were slightly smaller than the control specimen except for the SS-T-C specimen. The 28 day \(D_{RCM}\) values of the composite specimen with the surface treatment of the spacer were systematically smaller than those without surface treatment. Similar observations hold for the 56 day \(D_{RCM}\) values for the composite specimens with mortar spacer. The composite specimens with mortar spacer (with or without surface treatment) had generally lower \(D_{RCM}\) values than those with the small-size aggregate spacer. These results suggest that the composite specimens with a spacer had a similar capacity to resist the chloride ion as the control concrete specimens over time. The lower 56 day D\({}_{RCM}\) values for the composite specimens compared to the control concrete may be because the cementitious spacer was prepared in advance and the total curing time of the spacer (i.e., the curing of the spacer and the curing of the composite specimen) was slightly longer, which will be further investigated in a future study. Moreover, the surface treatment could improve the resistance to chloride penetration at the spacer location. It is worth noting that the calculation of the diffusion coefficient, \(D_{RCM}\), was based on the average value of the penetration depth of the chloride ion at the split surface. To visualise the penetration depth, the 0.1 mol/L silver nitrate solution was sprayed on to the freshly split section. It was observed that a higher-than-average penetration depth occurred at the spacer-concrete interface for the composite specimen without surface treatment, as presented in Figure 7a, which was considered to be a significant defect in the specimen and was ignored in the calculation of the diffusion coefficient. As presented in Figure 7a,c, regarding the composite specimen without the surface treatment of the spacer, the white silver chloride precipitation on the split surface presented a peak along the spacer-concrete interface, although the penetration depth in the spacer area was similar to that in the concrete area. In terms of  
Figure 6: Chloride ion diffusion coefficient \(D_{RCM}\). the composite specimen with the surface treatment of the spacer (Figure 7b,d), the penetration front was nearly horizontal and no peculiar peak occurred at the spacer-concrete interface. These results suggest that surface treatment improves the quality of the spacer-concrete interface by forming a sufficient interlink with the concrete. Further analysis (e.g., microstructural analysis, scanning electron microscopy (SEM) or micro X-ray fluorescence (\(\upmu\)-XRF) elemental mappings) will be taken in the future study to visualise the chloride ingress. The 6 h coulomb electric flux is presented in Figure 8. The 28 day coulomb electric flux of the composite specimen was lower than that of the control concrete, and the 56 day coulomb electric flux was similar between the composite specimen and the control concrete. The surface treatment of the spacer contributed positively to the resistance to chloride ion of the composite specimen with a mortar spacer while it negatively contributed to that with the small-size aggregate spacer. These results suggest that the benefit of surface treatment of the spacer is unclear. It is worth noting that the aforementioned issue of the spacer-concrete interface (Figure 7) of the composite specimen without the surface treatment of the spacer also affected the testing of the coulomb electric flux. Since the chloride ion could deeply penetrate the composite specimen at the spacer-concrete interface, it also results in an excessively large electric flux which is considered as an outlier sample. Several engineering implications can be drawn from these results.