the joint during pouring of the concrete holes or knockouts may also be provided preferably at 24 or 18 inch centers in the side of the beam so as to permit reinforcing rods with sleeves to be placed through the beam again for stability or for improvement in the strength of the resulting concrete the plastic material to be used in the molding of the new joints may be any plastic substance that can be injection or extruded to form the desired joint the plastic should of course be one that can be compressed when heated and will expand back when cold preferred examples of these include the thermoplastic polymers and copolymers such as copolymers of butadiene and styrene polymers of ethylene and propylene polyvinyl chloride and copolymers of vinyl chloride with other monomers such as vinyl acetate vinyl butyrate and the like as a preferred embodiment the invention provides expansion joints wherein the center core of the joint beam has been co extruded with a more flexible compressible material such as foamed rubber foamed polystyrene and other suitable material that has been used heretofore for expansion joints under the preferred embodiment then one has a joint prepared by co extrusion wherein the outer shell of the joint is formed by extrusion of the more rigid and strong plastics such as polyvinyl chloride polypropylene and the like and the center core extruded from material such as the rubber polymers.