## Materials and Methods
### Preparation of Spacers and Concrete Composite Specim  
The composite specimen containing the spacer and concrete was designed to test the collective performance of the two components. [PERSON] et al. [2] have designed a cylindrical composite specimen of 100 mm in diameter and 25 or 50 mm in height, having the spacer in the middle of the bottom surface of the specimen (i.e., the lower edge of the mould). This type of composite specimen has been tested for the mass transport properties and to evaluate the microstructure of the spacer-concrete interface. [PERSON] [19] drilled cores of 100 mm in diameter containing spacer from the column of 1.2 m in length and 0.6 m in diameter, which were exposed to the marine environment. These samples were used to evaluate the influence of spacer on chloride diffusion and reinforcement corrosion in concrete. [PERSON] et al. [12] considered a cylindrical sample 100 mm in diameter and 50 mm in height that contained half-spacer and half-concrete for bond strength and mass transport testing. These types of cylindrical composite specimens have a similar diameter to the cylinder specimen used for the testing of hardened concrete in British standard BS EN 12390-1:2012 [25] (i.e., \(d\) mm diameters and \(2d\) height, where \(d\) could be 100 mm), although the height of the composite specimen is considerably different from that of the concrete specimen.  
It would be convenient for both the manufacturers and contractors if the spacer-concrete composite specimen conforms to a standardised shape and dimension as those specified for the hardened concrete testing. First, it will facilitate the quality control process, as the performance of a composite specimen with a spacer (i.e., compression strength and chloride resistance) can be directly compared with the control concrete specimen without a spacer, which helps examine whether the spacer and concrete as a whole meet the requirements of the structural designers and clients. Moreover, it takes advantage of the existing standards and apparatus for the testing of hardened concrete, although certain adaptations to the current practice and training for technicians and workers are needed.  
This study proposes a spacer-concrete composite specimen based on the conventional cubic concrete specimen, as illustrated in Figure 3. The 40 mm \(\times\) 40 mm \(\times\) 80 mm spacers with and without surface treatment were placed in the middle of the bottom surface of the 100 mm\({}^{3}\) cubic steel mould and the chloride-resistant HPC Mix C was cast around the spacer. The spacer-concrete composite specimen was compacted in two layers and each layer was vibrated by a vibrating table until no significant release of air bubbles was observed. The spacers were manually held in place during the vibration process and the check showed little relative movement between the spacer and mould. It is worth noting that the vibrating table may not be available on the construction site for the preparation of composite specimens. Alternatively, compacting by hand with a compacting rob or bar is recommended and great care is needed to ensure that the spacer is in place. The composite specimen was cured in a fog room at a temperature of 20 \({}^{\circ}\)C \(\pm\) 2 \({}^{\circ}\)C and a relative humidity of \(\geq\)95% for 28 days.  
The composite specimens are labelled in the following order: spacer type-surface-treatment-concrete. For example, the composite specimen comprising a small-size coarse aggregate concrete spacer (SS) with surface treatment (T) and chloride-resistant HPC (C) is referred to as the SS-T-C sample.  
Figure 2: Examples of spacer surface with and without surface treatment: (**a**) the spacer without surface treatment; (**b**) the chloride-resistance small-size coarse aggregate concrete spacer (SS) with surface treatment; and (**c**) the chloride-resistant mortar spacer (MS) with surface treatment.