## 5 Conclusions  
In this study, a bio-coating material which is effective for enhancing resistance to sulfate ingress was developed using the NPM and an accelerated sulfate resistance test was performed to evaluate the applicability of the proposed technique. The repair material with bio-coating from NPM had excellent mechanical and durability characteristics. After the accelerated sulfate resistance test, the concrete surface under the coating material was analyzed by various methods and the derived results are outlined as follows:  
* As a result of applying a negative pressure method (NPM) that forcibly removes the internal air through a pressure of 10-20 tor, it was possible to obtain the immobilization rate of _Riodobacter capusulatus_ 2.2 to 3.3 times that of the simple soaking method (SSM). The variation in strength was not significantly different between the two methods after 4 weeks of immersion in a 5% sulfuric acid solution.
* As a result of evaluating the decreasing rate in strength and mass after sulfuric acid immersion for the normal repair specimen, SSM specimen, and NPM specimen, resistance to sulfate was highest in the coating repair by NPM. In addition, the NPM  
Figure 14: Thermal analysis of coated concrete samples immersed in 5% sulfuric acid solution for 28 days.  
specimen showed more than 3.0 times the bacterial count compared to the SSM specimen. The slime thickness of bio-coating was evaluated at 90 \(\upmu\)m from the SEM analysis and enhanced the resistance to sulfate ingress. The sulfate diffusion coefficient from NPM showed a lower result from SSM by 15%, which was consistent with the test results of the strength and mass variation after sulfate acid immersion.
* The XRD analysis result confirmed that the effect of the sulfate ions which penetrated the bio-coating repair did not significantly affect the properties of the base concrete. In particular, the peak intensity of gypsum (acid-deteriorated hydrates) from XRD decreased in the bio-coating slime treated with NPM, which showed improved resistance to sulfate ingress. The TGA analysis in NPM also showed the mass reduction curve closest to that of the virgin concrete that is not affected by sulfate attack, which means that the repair material by the proposed NPM is highly effective to blocking the intrusion of sulfate ions and preventing the base concrete from sulfate ingress.
* The newly developed technique for NPM was effective to protect sulfate ion penetration; however, future research is still needed on quality control in the mass production of bio-coating material and the automated repairing method for coating thickness control.  
K.-H.Y.: conceptualization, formal analysis, writing--review and editing; S.-J.K.: formal analysis, methodology, supervision, writing--review and editing; H.-S.Y.: investigation, formal analysis, visualization, writing--original draft. All authors have read and agreed to the published version of the manuscript.  
This research was supported by a grant (21 SCIP-C158976-02) from the Construction Technology Research Program funded by the Ministry of Land, Infrastructure and Transport of the Korean government.  
Not applicable.  
The data presented in this study are available on request from the corresponding author.  
The authors declare no conflict of interest.