A large number of medium and small span bridges in Switzerland and in other countries are slab bridges, often prestressed, multi-span and supported by columns. Punching in such structures is typically governing at failure. So far, the knowledge of the influence of prestressing on the punching shear strength is limited. Most results published in the litterature have been obtained on slabs prestressed with tendons. As a result the influence of prestressing is investigated globally, because all its effects (axial force, bending moment and deviation forces) have been investigated simultaneously. This paper presents a test campaign currently under way at the Ecole Polytechnique Fédérale de Lausanne. The aim of the tests is to investigate separately and then quantify the various effects of prestressing on the punching shear strength. The paper presents the first part of the test campaign (consideration of a moment), on reinforced concrete slabs. The first results will be presented and discussed on the basis of the critical shear crack theory.