Provided is a method of manufacturing an adaptable pre-cast resin-infused carbon-fiber beam, which method includes the steps of arranging a plurality of elongate carbon-fiber blocks side by side; arranging sheets to enclose the blocks and to extend over opposing faces of adjacent blocks; arranging the sheets to converge as an outwardly projecting elongate bead at a junction between adjacent blocks; and pulling on the elongate bead to inhibit resin flow between blocks during a resin infusion step. Also provided is a pre-cast adaptable carbon-fiber beam manufactured using that method; a method of manufacturing a wind turbine rotor blade; and a wind turbine rotor blade.