Abstract: An expansion joint for sealing the channel between two sections of roadway has a plug and a fluid-polymer poured into the joint and allowed to set in situ in the channel. Enclosed within the fluid is a core of a mastic material which is more plastic and less elastic than the fluid-polymer. The elasticity of the fluid-polymer allows the joint to accommodate changes in the size of the joint due to e.g. thermal expansion and contraction. Plastic flow of the core permits changes of shape of the joint to be absorbed without causing excessive stressing of the sides of the channel to which the fluid-polymer adheres.