Abstract: Construction element for forming a reinforced concrete slab (2), consisting of the combination of at least a hardened concrete layer (3), at least a number of reinforcement elements (4) and elements (5) extending at least partially from the concrete layer (3) and defining cavities (6), whereby these elements (5) are designed to be covered with concrete (7) at a later stage characterised in that the above-mentioned elements (5) defining the cavities (6) consist of elements (5) which can be mutually nested as such.