Ultra-high-performance concrete (UHPC) is a cementitious material with a substantially higher mechanical capacity and durability than conventional concrete. The enhanced compressive strength (over 120 MPa) and high tensile capacity of UHPC make it unique compared with other types of concrete. However, one of the most challenging aspects of UHPC is its high early-age autogenous shrinkage, owing to its low water-to-binder ratio and minimal to no bleeding of the specimens. Currently, while some standards and guidelines for UHPC exist, comprehensive design codes are still developing, particularly for specialty applications such as UHPC in shear keys and bridge construction, where more specific details are required. This study investigates the early-age shrinkage of UHPC shear keys constrained by precast high-strength concrete (HSC) beams. Factors influencing shrinkage, such as surface preparation, reinforcement, and curing environment, were investigated. Furthermore, the HSC and UHPC bond characteristics were studied by performing slant shear, bi-shear, and direct tension tests. The same geometric shear key design for the beam was applied to analyze the performance of the UHPC shear key with two surface preparations under a negative bending moment. The experiment's results showed that rough surface preparation showed significantly higher bond strength and lower shrinkage of the UHPC shear key. Further, they resulted in higher adhesion between two surfaces, which was studied through the microanalysis of a failed cross-section of the bi-shear specimens.