## 1 Introduction  
Spacers are important components in reinforced concrete structures to provide adequate cover between the steel reinforcement and formwork or the nearest concrete face. Although the small spacers seem inconsequential compared to the overall reinforced concrete structure, they are indeed vital to ensure the correct placement of reinforcement to the design requirements and to produce durable reinforced concrete [1, 2]. Spacers of various shapes and sizes are utilised in the construction industry and they can be generally classified into three categories based on the materials: cementitious spacer, plastic spacer, and steel chair. As the steel chair is welded to the reinforcement cages, it is recommended [1] that the steel chair should not be used off faces exposed to aggressive agents (e.g., chloride ion). The plastic spacers were found to be susceptible to degradation by ultra-violetradiation [1] and among the three types of spacers, to have the largest increase in the transport of aggressive agents when exposed to the marine environment [3]. The cementitious spacer, if not carefully designed (i.e., the mix design) or manufactured, could also result in early corrosion at the spacer location as observed in the field investigation [4; 5]. Thus, for the construction of reinforced concrete structure subjected to extreme weathering and chemical corrosion in offshore areas, the cementitious spacers were widely used because of the desired engineering properties of cement, which can be designed to have comparable strength, durability, and porosity to the surrounding cement-based chloride-resistant high-performance concrete (HPC). The engineering concern of permanently leaving considerable amounts of spacers along the reinforcement in a structure is that the presence of spacers potentially facilitates the ingress of aggressive agents [2; 6] through paths such as the spacer itself, or its interface with concrete, in addition to the concrete near the spacer, as illustrated in Figure 1. The quality (e.g., microstructure, porosity) of the spacer and the surrounding concrete (i.e., the possible penetration path 1 and 3 in Figure 1) can be improved by carefully calibrated variables, such as water-to-binder ratio (W/B), the type and amount of mineral admixtures as well as the maximum size of aggregates. Extensive researches [7; 8; 9; 10] have been conducted in this regard and conclusions have been applied in the construction industry to build durable costal structure e.g., [11]. However, although the spacer-concrete interface has been well recognised as a weak link with regard to ingress in practice, only a few laboratory investigations [2; 12] are available to explain the phenomena and to improve the interface. Furthermore, limited guidance on the evaluation of the properties of the spacer and its collective performance with concrete, particularly at the spacer-concrete interface, is available for site engineers and contractors. The state-of-practice codes and standards [13; 14; 15; 16; 17] provide general specifications on the spacers and their placement. It is specified that the spacer should be durable and it should not lead to the corrosion of reinforcement or the spalling of concrete cover. Moreover, the spacer should be fixed to reinforcement bars at a spacing not exceeding 50 times the diameter of the rebar or 1000 mm and the layout of the spacer can be in staggered rows for parallel bars. However, the code-of-practice neither offers guidance on how to reduce the negative effects of spacers nor provides a recommendation on the performance testing of the spacer itself or the spacer-concrete specimen. The recent laboratory findings have advanced the knowledge of the production and application of the spacers. The study conducted by [PERSON] et al. [2; 18] has systematically evaluated the effects of commercial spacers on the mass transport properties and the microstructure between the spacer and concrete.