## 4 Conclusions  
This study evaluated the influence of spacer surface treatment on the performance of chloride-resistant spacer and concrete composite specimens via the modified non-steady-state migration experiments, as well as the compressive strength. The surface treatment method proposed in this study can be adapted by spacer manufacturers for mass production. A spacer (both with and without surface treatment) 40 mm \(\times\) 40 mm \(\times\) 80 mm in size was placed in the middle of the bottom surface of a 100 mm\({}^{3}\) cubic mould and formed a composite specimen with concrete. The composite specimen and testing method utilised in this study can be easily integrated with the state-of-practice testing of hardened concrete. The main conclusions are as follows:  
* The surface treatment of the spacer had a positive effect on improving the bonding between the spacer and concrete at the interface. The surface treatment technique using retarder to roughen the surface of the spacer can be easily integrated within the current manufacturing process.
* The state-of-practice tests on the hardened concrete can be modified to evaluate the performance of the spacer-concrete composite specimens and provide an estimation of the long-term mechanical and durability properties of the structure at the spacer locations.
* The spacer-concrete composite specimen had a lower compressive strength and a similar resistance to the chloride ion than the control concrete.
* The surface treatment on the spacer improved the compressive strength of the spacer-concrete composite specimen locally, compared to those without surface treatment. The surface-treated spacer exposed a partial aggregate, thereby forming a sufficiently strong interlink at the concrete interface. The failure mode of the composite specimen with surface treatment of the spacer was  
Figure 8: Six hour coulomb electric flux.  
similar to that of the concrete. However, the composite specimen without the surface treatment of the spacer experienced debonding at the interface when a critical load was applied.
* The surface treatment of the spacer improved the resistance to chloride ions of the composite specimen compared to those without surface treatment. The advantage of surface treatment was partially represented by either the diffusion coefficient \(D_{RCM}\) or the column electric flux, as the poor quality at the interface would result in the outlier data that were excluded in the data process.
* The maximum chloride ion penetration depth at the spacer-concrete interface can be used as an additional proxy for the evaluation of the chloride resistance performance of composite specimens. If the maximum penetration depth at the interface is 1.3 times higher than the average penetration depth, the spacer-concrete interface can be identified to have a weak bond and the spacer would require surface treatment to improve the quality of the spacer-concrete interface.  
Conceptualization, Y.Y. and W.G.; supervision, Y.Y. and W.G.; project administration, C.H. and H.Z.; methodology, Y.Y., C.H., J.B.; investigation, C.H., H.Z., Z.L., J.B.; validation, C.H., H.Z., Z.L., J.L., J.B.; data curation, Z.L., J.L., J.B.; resources, H.W.; funding acquisition, Y.Y. and W.G.; visualisation, C.H.; writing--original draft, C.H.; writing--review and editing, Y.Y. and W.G. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the Water Resource Science and Technology Innovation Program of Guangdong Province, China (No. 2017-22).  
The authors thank South China University of Technology for its technical support. The authors thank [PERSON] for helping with the experiments. The authors thank two anonymous reviewers for the constructive comments and suggestions, which helped improve the quality of this paper.  
The authors declare no conflict of interest.