A reinforcement insert includes superposed laps of fiber which are coupled to each other by a coupling fiber which passes through the laps in order to form fiber sections wherein at least certain sections thereof are rectilinear. The fiber sections and laps are immersed in a cured resin. The total density of the fibers that make up the insert ranges from 50% to 60% of the volume of the insert and the fiber sections take up 3%-10% of the volume of the insert. A method for creating a sandwich structure includes at least one such insert.