Influence of glass-fiber epoxy coating on bond behavior of FRCM systems. Experimental campaign and analytical interpretation

Fiber-reinforced cementitious matrix (FRCM) composite systems show excellent potential for the reinforcement of existing structures. However, modeling these materials remains challenging, and test methods for mechanical characterization are still debated. The current study aims to enhance understanding of the mechanical behavior of FRCM systems, with a particular focus on observing how the epoxy coating of the fiber and the mechanical and physical characteristics of composite components affect bond behavior. Six FRCM systems were implemented by combining the same glass-fiber textile, dry or epoxy-coated, and three different matrices based on lime, gypsum, and cement binders, which were fully characterized through petrographic and mechanical tests. Then, the six composite systems were tested through single-lap shear test, direct tensile test, and textile pull-out test. The experimental results highlighted the beneficial effect of the epoxy resin coating that greatly improves the fiber-matrix adhesion capacity in terms of stress, although resulting in brittle failure of the textile. The differences between dry and epoxy-coated fibers were observed through a digital microscope as well. Finally, two analytical models were developed to simulate the distinct boundary conditions of the two mortar layers in single-lap shear tests. The first model assumes rigid mortar and incorporates two tri-linear distinct cohesive material laws (CMLs): one for the internal interface and a weaker one for the external interface. The different CMLs should not be intended as two distinct material laws but as model assumptions representing the system more closely than the symmetrical layout model. Results highlight the significant role of mortar granulometry and aggregate skeleton in fracture energy distribution across linear-elastic and softening phases. Epoxy coatings improve tensile strength, elastic modulus, and interlayer collaboration but shift failure behavior from pseudo-ductile to brittle. The second model restores the deformation of the upper mortar layer, allowing for the consideration of different interface behaviors without the need for two distinct CMLs. This approach provides a refined understanding of the fiber-matrix bond.