Reinforcer, composite material comprising such a reinforcer, method of using such a reinforcer

A reinforcer, comprising:

• a shrinkable layer, comprising shrinkable yarns causing the shrinkable layer to be able to shrink irreversibly in a direction of shrinkage, under the effect of a heat-shrink heat treatment, from an initial state to a shrunk state, by irreversible shrinkage of the shrinkable yarns from the initial state to the shrunk state under the effect of the heat-shrink heat treatment, the shrinkable yarns extending in the direction of shrinkage,
• a first corrugatable layer, which comprises a flat flexible grid work of yarns added against the shrinkable layer, the grid work of yarns of the first corrugatable layer comprising first yarns and second yarns, the first yarns extending along the direction of shrinkage, in alignment with the shrinkable yarns of the shrinkable layer, the second yarns extending transversally relative to the first yarns, the first yarns being assembled with the second yarns without weaving, in that the first yarns are connected to the second yarns by gluing or welding, wherein the grid work of yarns of the first corrugatable layer is connected to the shrinkable layer by connection lines spaced apart and extending transversely with respect to the direction of shrinkage and to the shrinkable yarns of the shrinkable layer, the first corrugatable layer exhibiting a shrinkage that is substantially zero or that is smaller than that of the shrinkable layer along the direction of shrinkage under the effect of the heat-shrink heat treatment, so that:
• when the shrinkable layer is in the initial state, portions of the first corrugatable layer, each defined between two consecutive connection lines, are folded down against the shrinkable layer, so that the corrugatable layer extends flat along the shrinkable layer, and
• when the shrinkable layer is in the shrunk state, the portions of the first corrugatable layer are curved.