The present invention relates to a liquid (meth)acrylic syrup for impregnating a fibrous substrate; a viscous liquid syrup mainly containing methacrylic or acrylic components, and preferably hollow beads; to a process for manufacturing such a syrup; to a process for impregnating a fibrous substrate or long fibers with said viscous liquid syrup; to a fibrous substrate preimpregnated with said syrup, which is useful for manufacturing mechanical or structured parts or products; to a manufacturing process for manufacturing mechanical or structured parts or articles; and to three-dimensional mechanical or structured parts obtained by this process. The invention makes it possible to significantly reduce the exothermic peak during the polymerization of the syrup, to reduce the residual content of monomer at the end of polymerization, and to obtain parts made of composite material that have few or no defects.