The paper presents an experimental study regarding the interface strength and stiffness of composite concrete elements with an unreinforced multiplanar interface. The tested element was made of a precast rib-shaped beam with a width of 200 mm, a height of 120 mm, and a 45 mm layer of concrete overlay above the rib. The research investigated the performance of rib-shaped precast elements with three different surface treatments. The study was performed on a direct shear test stand. Comparative pull-off adhesion tests were conducted for each surface type. Laser sensors were used to measure displacement and then compared with the results of the digital image correlation (DIC) method. The slip value before failure ranged from 0.02 mm to 0.2 mm, depending on the measurement location. The study highlighted the importance of test stand configuration and the selection of measurement points, particularly concerning determining interface slips. The parameters examined included interface stiffness, cracking force, residual force, and mutual displacements of the interface planes in three orthogonal directions. Notably, despite the lack of reinforcement at the interface, a significant residual force was recorded following the cracking initiation.