The present invention relates to a liquid (meth)acrylic syrup for impregnating a fibrous substrate. The present invention relates in particular to a viscous liquid syrup mainly containing methacrylic or acrylic components. The invention also relates to a process for manufacturing such a syrup. The invention also relates to a process for impregnating a fibrous substrate or long fibers with said viscous liquid syrup. The invention also relates to a fibrous substrate preimpregnated with said syrup, which is useful for manufacturing mechanical or structured parts or products.The present invention also relates 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 applies to the preparation of mechanical parts or structural elements made of fire-resistant thermoplastic composite material.