## 3 Results and Discussion  
### Compressive Strength  
The compressive strength results are presented in Figure 4. As shown, the compressive strengths of composite specimens were generally lower than that of the control chloride-resistant HPC (C),  
Figure 3: Spacer and concrete composite cross-sections (Unit: mm).  
suggesting that a partial replacement of the concrete by the spacer would reduce the compressive strength of the specimen. Moreover, the compressive strength of the composite specimen with surface treatment (T) was approximately 5% higher than those without surface treatment (N). This is expected as the surface treatment is intended to improve the bonding between the spacer and the concrete. Additionally, the compressive strength of the composite specimen containing the chloride-resistant mortar spacers (MS) was generally higher than those containing the small-size coarse aggregate concrete spacers (SS).  
The theoretical and experimental failure modes of the spacer-concrete composite specimens under the compressive load are illustrated in Figure 5.  
As shown in Figure 5a, the spacer and concrete composite specimen (with a spacer 80 mm in height) was expected to fail along the red line due to the hoop effect, as an analogy to the concrete specimen. However, the composite specimen without the surface treatment of the spacer had an unsatisfactory failure mode as shown in Figure 5b. The applied compressive load crushed the specimen along the smooth spacer-concrete interface, suggesting that the bond between the spacer without surface treatment and concrete was weak. On the contrary, the composite specimen with surface  
Figure 4: Compressive strengths of the specimens. The control chloride-resistant high-performance concrete was denoted by C. The chloride-resistant small-size coarse aggregate concrete spacers and the chloride-resistant mortar spacers were denoted by SS and MS, respectively. The composite specimen with and without surface treatment were denoted by T and N, respectively.  
Figure 5: Failure modes of the spacer and concrete composite specimens after the compressive strength testing: (**a**) the theoretical failure mode of the spacer and concrete composite specimen, where the red line indicates the expected cracking pattern; (**b**) the composite specimen without surface treatment (N); and (**c**) the composite specimen with surface treatment (T).  
treatment of the spacer presented a failure mode that was consistent with the theoretical failure mode (Figure 5c), suggesting that the bond between the spacer and concrete was sufficient. These results indicate that the surface treatment that allows the aggregate to be partially exposed can form a sufficient bond at the spacer-concrete interface, which locally improves the mechanical property of the composite specimen.  
These results have important engineering implications in terms of the strength of the concrete structure. The overall resistance strength of the structure at the spacer location may be compromised due to the existence of the spacer. Moreover, when the critical load is applied at the spacer location, the concrete may crack along the smooth spacer-concrete interface, which can be avoided or mitigated by introducing the surface treatment of the spacer.