The application on existing Reinforced Concrete (RC) slabs of cast-on-site Ultra-High Performance Fiber Reinforced cement-based Composite (UHPFRC) layers is an efficient reinforcement technique, currently spreading. The thin layer of UHPFRC, with or without steel rebars, serves as a tensile reinforcement for the RC slab, creating a composite element. This thesis combines material and structural engineering to study the behavior and resistance of two-way spanning composite slabs, with a certain focus on punching shear resistance. When analysing the behavior of composite elements, the effect of fiber orientation on the in-plane tensile response of the UHPFRC layer needs to be accounted for. Theoretical tools are derived herein to analyze the complementarity of fiber orientation in perpendicular directions and determine the average effect of fiber orientation on fiber efficiency at pull-out. A comprehensive material testing campaign on a strain-hardening UHPFRC is carried out on specimens with various thicknesses and casting processes. The most likely fiber orientation in a layer of UHPFRC for the casting method considered herein is finally estimated with the results of the theoretical and experimental work and a representative tensile response is scaled accordingly. An experimental campaign is carried out on six composite slabs without transverse reinforcement. The parameters of the tests include the thickness of the UHPFRC layer and the amount of reinforcement in it. The punching shear resistance of all composite slabs is higher than the resistance of the reference RC slab. The layer of UHPFRC increases the rigidity of the slab and provides added shear resistance to the cracked RC section by out-of-plane bending accompanied by limited or inexistent Near Interface Cracking (NIC) in the concrete section prior to failure. By doing so, it allows more deformation to take place in the RC section before punching shear failure. This results in rotations at maximum resistance close to what is observed for the reference RC slab. An analytical model is then developed to predict the global bending behavior of the composite slab and the punching shear resistance. A multilinear moment-curvature relation for composite sections is used to calculate the force-rotation curve of a slab. The intersection between this curve and a deformation based composite failure criterion predicts the punching shear resistance. This criterion combines the concrete and the UHPFRC layer contributions. The latter resists to punching shear by out-of-plane bending over a limited length. This mechanism induces tensile stresses perpendicularly to the interface with the concrete. The contribution of the UHPFRC layer to the punching shear resistance thus depends on the tensile strength of concrete. In the final section of the work, a description of the parameters influencing the shear resistance of composite elements is done. With the tools developed in this work, the effects of fiber orientation in a layer can be mastered and the analytical models allow to simply verify the resistance of a composite section.