# A Novel Approach to Quantifying the Failure Modes of Concrete-Epoxy Interface
## 1 Introduction  
Over the years since the last three decades, FRP (Fiber-Reinforced Polymer) has become the material of choice for strengthening and retrofitting aging and deteriorating infrastructures due to its relative ease of installation, high strength-to-weight ratio, resistance to corrosion, and many other advantages over traditional construction materials used for structural repairs like bolts, steel, and concrete, etc. [1, 2, 3, 4]. FRP consists of high-strength continuous fibers embedded in a polymer matrix (resin); the embedded fibers are the reinforcement elements, while the resin serves as binders to the fiber and facilitates the transfer of loads to and within the fiber [5, 6]. The polymer matrices used in the construction industry are categorized into thermostating and thermoplastic matrices. The vinyl esters, epoxies, and polyesters are grouped under thermosetting matrices, while polypropylene, polyvinyl chloride, and polyurethane are thermoplastic matrices. Thermosetting resins are often used in FRPs for retrofitting due to their enhanced mechanical performance and high impregnation and adhesion to fibers [6, 7]. CEI (Concrete Epoxy Interface) is formed during the installation of epoxy-based FRP on retrofitted or aged concrete surfaces; the CEI is the region formed at the concrete and epoxy touching surfaces [8, 9, 10]. In FRP concrete repair systems, the CEI is usually theregion where the debonding and transition between the FRP and concrete occur [8; 9; 10; 11; 12; 13; 14], and the structural performance of the FRP-repaired concrete is dependent on the behavior of the CEI [8; 10; 13; 15; 16; 17]. Over time, under sustained loading and severe environmental conditions, the structural performance of the FRP-repaired concrete weakens, subsequently fails, and debonds [4; 7; 8; 9; 10; 12; 13; 16; 18; 19; 20]. Failure may occur in either or combination of three ways; CC (Cohesive failure in Concrete), CE (Cohesive failure in Epoxy), and IF (Interfacial Failure), representing crack propagated along the concrete substrate, epoxy layer, and concrete epoxy interface respectively [7; 8; 10; 13; 16; 20]. The CEI failure modes are depicted in Figure 1.  
The CC is usually more dominant on the CEI failure surfaces than other failure types, and this is because the concrete substrate is more susceptible to failure than the FRP-epoxy bond and the concrete-epoxy interface [4; 7; 8; 9; 10; 12; 13; 16; 18; 19; 20]. The CE and the IF failure type ratio increases as the adhesive bond between the FRP-epoxy and concrete-epoxy degrades under sustained loading and exposure to high temperature and humidity [4; 7; 8; 9; 10; 12; 13; 16; 18; 19; 20; 21; 22; 23]. Research efforts have been made toward categorizing the failure modes of CEI, yet more research needs to be made to quantify the failure modes on CEI failure surfaces. The relationships of the failure types have been based on researchers' intuition, which is usually a product of visual inspection, as done by [9; 11; 12]. Attempts were made by [8; 10; 13] to eradicate errors influenced by human subjectivity by applying an image-processing technique to determine the ratio of failure modes. However, ref. [8; 10; 13] classified the failure modes based on empirical relationships. To our knowledge, there is no scientific, non-subjective, and reproducible method of quantifying the failure modes of CEI failure surfaces. This study detects and quantifies the failure modes on CEI failure surfaces using HSV (Hue, Saturation, Value), International Commission on Illumination--_Commission Internationale de l'Eclairage_ (CIE) L*a*b*, YCbCr, or RGB (Red, Green, Blue) color space image segmentation-based image processing technique. The RGB color space is the primary color space and is based on color trichromatic theory, which assumes that any color can be matched by mixing the appropriate amount of the primary colors [24]. CIE L*a*b and YCbCr color spaces are categorized under luminance chrominance color space, which expresses a color by its luminance and chromaticity.