Ultra-high performance concrete (UHPC) is becoming more widely used in bridge construction applications due to its remarkable structural performance, including: high compressive and tensile strength, long-term durability, low permeability, high flowability, and low water-to-cement ratio (all compared to conventional concrete). Therefore, it is a suitable material for connection elements in bridges designed for rapid construction like slab beam superstructures. However, there is limited information on the structural design and long-term service performance of longitudinal UHPC connection details for adjacent slab beam superstructures. A three-dimensional, non-linear, finite element (NFE) model was developed to study the shape of the shear key, load transfer mechanism, and durability of UHPC-to-beam bond. The fracture mechanisms, material properties, and flexural behavior of the joint model were validated by simulating the laboratory tests conducted at the FDOT M.H Ansley Structure Research Center; good agreement was found between numerical and experimental results. The results highlight the importance of using an accurate model to represent the system behavior such that an efficient design can be achieved.