# A Novel Approach to Quantifying the Failure Modes of Concrete-Epoxy Interface
## 4 Conclusions  
This study has presented a novel approach based on image processing to quantify the failure modes (cohesive failure in epoxy (CC), cohesive failure in epoxy (CE), and interfacial failure (IF)) on concrete-epoxy interface failure surfaces. This study evaluated the quantity of the failure modes of twenty-eight (28) CEI failure surfaces specimens from the 3 PB experiment conducted by [13] using four significant steps: image acquisition, image pre-processing, epoxy area selection, and failure mode detection and quantification. Based on fracture surface analysis, the following conclusions can be drawn:  
1. The epoxy area and the evaluated failure modes vary across the color spaces. RMSEs (Root Means Square Errors) of 0.19, 0.10, 0.23, and 0.26 was obtained when the CEs evaluated by the manual approach were compared with the CEs evaluated by the HVS, CIE L*a*b*, YCbCr, or RGB color space-based image segmentation approach, respectively.
2. The CIE L*a*b* is the ideal color space for the novel approach proposed in this study, as it produced the most accurate model of the epoxy areas on the failure side and the closest evaluation of the failure mode (CE) to those made by the manual approach.
3. Analysis of the evaluated failure modes shows that the failure modes have some correlations with the bond performance. The agreement of the correlations with established literature further implies the suitability of the novel approach for quantifying the failure modes of the concrete-epoxy failure surfaces.
4. The epoxy matrix and the concrete substrate are the least and most susceptible to failure, respectively. The performance of the interfacial bond between the concrete and the epoxy weakens as environmental loads and sustained loading duration increase.
5. The novel approach proposed in this study provides a scientific, non-subjective, and reproducible technique to quantify the failure mode on concrete-epoxy interface fracture surfaces in externally bonded FRP-repaired concrete, provided that both concrete and epoxy have distinctive colors. The novel approach proposed in this study is perfect for forensic engineering as it can give valuable insight into the probable causes of failure.  
Figure 15: Percentage of IF failure modes (determined using CIE L*a*b* color-space) of 3 PB and fracture energy against Sustained loading duration (a) 7-days and (b) 90 days cured specimen.  
**Author Contributions:** Conceptualization, [PERSON] ([PERSON]); Supervision, [PERSON] ([PERSON]) and I.Y.; Formal analysis and Writing--original draft, A.S.I.; Data curation, Y.J. ([PERSON]), Project administration, I.Y.; Software, D.A.; Writing--review & editing, [PERSON] ([PERSON]), [PERSON], [PERSON] ([PERSON]) and I.Y. All authors have read and agreed to the published version of the manuscript.  
This research was supported by the BK21 FOUR (Fostering Outstanding Universities for Research) funded by the Ministry of Education (MOE, Republic of Korea) and National Research Foundation of Korea (NRF) (NO. 5199990214511).  
**Data Availability Statement:** Not applicable.  
The authors would like to thank Department of Construction and Disaster Prevention Engineering of Kyungpook National University for their support throughout the research process.  
The authors are unaware of any conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.