In civil engineering, construction projects often involve assembling various components at different stages, creating interfaces where load transfer must be considered. This load transfer is governed by dowel action (reinforcement), mechanical interlock, adhesive bonding, and friction. Without reinforcement, most concrete interfaces are brittle and weak. However, interfaces don't have to be brittle and weak. Natural hard materials like bone, tooth, and seashells have tough interfaces due to geometric interlock and friction, which could be used to enhance concrete connections' performance, making them more ductile.

This study focuses on applying a natural interface design to a concrete-to-concrete connection under bending conditions. The design is inspired by jigsaw-like contours made from arcs of circles with radii R1R1​ and R2R2​, blended tangentially at specific angles θ1θ1​ and θ2θ2​. This interlock shows two stable positions during pullout, making the design bistable. The goal is to replicate this interlocking connection using Ultra-High Performance Fiber-Reinforced Concrete (UHPFRC), known for its higher tensile strength and strain-hardening behavior compared to traditional concrete.

A literature review covered three areas: the jigsaw-like interlock's bistable behavior in various materials, the behavior of interlocks under three-point bending, and the composition and properties of UHPFRC. Following the review, a numerical study was conducted, simulating a three-point bending test on the interface. The model was used to design and optimize the interface profile, resulting in three designs connecting two UHPFRC specimens with five interlocks each. A parametric study investigated the effects of various parameters, including E-modulus, material strength, friction coefficient, and plastic strain at peak resistance.

The fracture behavior under bending revealed several phases: initial linear loading with geometric hardening, plastic deformation of the lowest tab, and eventual failure of the middle tabs at peak load. The smoothest interlocking design, with minimal surface deviation, showed the most ductile response. This design was achieved with low θ1θ1​ and θ2θ2​ and radii close to a quarter of the tab width. A more ductile behavior was obtained using materials with low elastic modulus, higher tensile strength, lower friction coefficient, or more plastic strain, although this reduced strength.

Two additional designs were investigated to improve ductility: one with a single-circle interlock and one with a gap. The single-circle design showed more ductility due to fewer contact points, while the gap design slightly improved ductility at the cost of some strength. Experimental research on scaled designs revealed discrepancies with numerical predictions, likely due to poor fiber distribution and fabrication issues. Adjusting the numerical model's friction coefficient and increasing experimental sizes could resolve these discrepancies.

Ultimately, a very smooth interlocking design was optimal for UHPFRC connections, achieved with specific angles and radii. For further research, a bent interlocking tab design is recommended to address off-center forces and improve ductility without significantly decreasing strength. Additionally, enhancing the analytical model by revising assumptions could improve its accuracy, particularly under higher displacements. This includes incorporating bending moments in the tabs, adding a plastic hardening phase, and accounting for non-rigid movements.