To strengthen a reinforced concrete (RC) slab with deficient shear resistance, it is proposed to add to the top of the slab a 25-50 mm thick layer of ultra-high performance fibre reinforced cement-based composite (UHPFRC) with small diameter rebars (R-UHPFRC). This creates a monolithic composite element where the new UHPFRC layer acts as an external tensile reinforcement. The present work focuses on the parameters influencing the shear resistance of R-UHPFRC-RC composite elements and how the layer influences the shear transfer mechanism involved in the RC section. The two analytical models available to respectively predict the shear and punching shear resistance of a composite section are presented. The test results database on composite elements subjected to shear are used to verify these models. To show how the layer of UHPFRC influences shear resistance, the results of a parametric study are also given.