## 1 Introduction
The results from a series of experiments on assorted spacers have shown that spacers increase the mass transport, which is due to a porous spacer-concrete interface that spans the cover where preferential transport occurs. By investigating the reinforced concrete columns exposed in the tidal zone for more than 30 years, the study of [PERSON] [19] also observed that the presence of plastic spacer increased the chloride penetration along with the spacer-concrete interface. Therefore, it is of  
Figure 1: Schematic plot of the possible penetration paths of the aggressive agent through (**1**) the spacer, and (**2**) the interface between the spacer and concrete, and (**3**) the concrete. great engineering importance to improve the interface between the spacer and concrete so that the overall structure reaches the expected design performance, particularly the durability and mechanical properties. The quality of the spacer-concrete interface could be influenced by its rough surface texture. [PERSON] et al. [12] have introduced a range of surface textures to semi-circular cementitious spacers that were later cast into the concrete to form the cylindrical specimens. Their tests on composite specimens have shown that spacer surface texture positively contributes to its interlocking with concrete, leading to a significant improvement in the bond strength of the spacer-concrete interface. However, the effect of surface textures on mass transport properties has remained unclear. These codes of practice and research conclusions have significant implications on the construction practice, such as the surface treatment of spacers on construction sites and the spacer-concrete performance testing during the construction processes. Following the current spacer specifications and suggestions, the spacer should be compatible with the surrounding concrete in terms of mechanical property and durability. Moreover, the spacer should be manufactured in a factory or prepared on the construction site in such a way that it has an adequately rough surface to increase the bonding with concrete at the spacer-concrete interface. Furthermore, the state-of-practice testing of the hardened concrete (i.e., taking a sample of the fresh concrete on site and testing the corresponding hardened concrete for compressive strength and other engineering properties) can be adapted to evaluate the performance of a spacer-concrete composite specimen (i.e., composed of the fresh concrete and the spacer that is used on site) through compressive strength testing as well as other durability tests. By testing the quality of the spacer and the spacer-concrete composite specimen during the construction process, the structural design can be conducted properly (e.g., with sufficient cover and the correct position of reinforcement bar). Moreover, it also provides a long-term estimation of construction performance, particularly at the spacer location, to avoid or mitigate the undesirable corrosion observed in the field investigation. This study investigated the influence of surface treatment on the performance of a chloride-resistant spacer-concrete composite specimen. In the following section, the methodology utilised in this study is described. In particular, the preparation of the spacer-concrete composite specimen and the modified testing method for the resistance to the chloride ion of the composite specimen were introduced. The results of the compressive strength and the chloride-resistance properties of the composite specimen were discussed. Recommendations on the manufacturing of spacers with a rough surface in a factory and on the testing of the spacer-concrete composite specimen were provided based on the results of this study.