Composite polymer-concrete beams represent new modern structures that can take an advantage of the polymer's practical tensile properties and combine them with the concrete's favourable compressive properties. Drawing on this knowledge, a set of polymer beams acting compositely with a concrete slab was designed and manufactured. The aim of the research was to utilise the low weight and high strength of the polymer I-sections and combine them with the high stiffness of the concrete slab, which forms the upper part of the cross-section. The advantage of fibre-reinforced polymer (FRP) beams is their anisotropy, where the strength of the material is increased by placing the fibres uniformly in one direction, and the composite elements are then stressed in the most reinforced direction. To ensure the interaction between the polymer element and the concrete slab, strip shear connectors of a precisely defined shape were developed and utilised. The designed composite beam simulates a pre-cast component that can be applied in bridge structures for short and medium spans. The pre-cast beams were subjected to four-point bending. Apart from the overall deflections of the structure, the stresses in the cross-section of the composite material and the relative deformations/strains on the surface of the concrete part of the cross-section were monitored during the test. The whole experiment yielded new results in both laboratory and theoretical respects, not only regarding the interaction of materials with distinct characteristics but also the properties of composites per se.