# Repair Composite Adhesion Strength: A Comparison of Testing Methods
## 1 Introduction  
In recent years, the use of multi-component cementitious mixes has expanded significantly in various applications. These composites are extensively used to reinforce or repair existing structures and to build new components. A significant amount of Europe's annual construction budget is allocated to the refurbishment and maintenance of existing structures, and this amount is expected to rise as the number of concrete structures continues to grow. For example, the estimated annual cost for maintaining and repairing bridges in Europe ranges between EUR 4 and 6 billion. Replacing these bridges, however, would be an even more substantial expense, exceeding EUR 400 billion [1, 2]. According to [3], 50% of the repaired structures exhibit a durability of less than 10 years, which is significantly lower than the anticipated 25-year lifespan. There are some factors that can affect the success of a repair system, such as material choice, construction techniques, or a combination of these factors [4, 5, 6, 7]. To achieve monolithic behavior, multi-layered concrete requires sufficient interfacial bond strength to ensure effective load transfer between all concrete components. Structural restoration should not be seen only as a simply technical task of applying a repair mixture to damaged sections. Instead, it is a multifaceted process that encompasses the inspecting and diagnosing of the structures, identification of the root causes of damage, planning the repair work, and selection of suitable materials, systems, and technologies [8]. Although applying repairing mixes shows promise as a method for structural rehabilitation, careful consideration of both early-age performance and long-termdurability is necessary. A strong bond at the interface between the repaired substrate (damaged surface), primer composition (contact layer), and new repair composite is crucial for ensuring effective load transfer and the activation of repair materials in existing structures. The thickness of a repair overlay varies depending on the removed damaged subsurface layer and can reach several centimeters. Interlayer bonding between the materials is crucial for the element's long-term durability, as the contact layer is typically the weakest point [9]. This interface layer between zones can be subjected to various types of stresses throughout the operational life of the repaired construction, including stresses caused by restrained drying shrinkage [10; 11]. Furthermore, material discontinuity, high porosity, and presence of micro-cracks make the interface between concrete components a critical zone within the composite [11]. As mentioned [12], the interface zone can contain hydration products of cement with larger crystals of Ca(OH)\({}_{2}\), resulting in lower bond strength; however, this microstructure has a dual effect. The filling of pores by hydration products in the old concrete substrate creates mechanical interlocking, enhancing bond strength compared to the epoxy-to-concrete interface. Each layer's properties, directly related to its composition, are crucial for the repair system's compatibility. Bond strength at the interface is largely influenced by the properties of the repair material itself. Moisture and hydrated products from the repair medium penetrate the dry substrate, forming a strong shear bond between the old and new materials [13; 14]. This \"adhesive\" mechanism depends on both mechanical anchorage between the new material and old substrate and chemical adhesion forces. Nevertheless, the specific stresses generated at these interfaces and the effectiveness of cement paste as an adhesive have not been fully explored. Adequate adhesion is crucial for maximizing the durability of the repaired system; at the repair joint, it facilitates load transfer and ensures uniform stress distribution [15]. Sufficient adhesion ensures the maximum ability of the repaired system to absorb the workload [16].