Expansion Joints Filler Cork Expansion Joint Filler is preformed from clean, granulated cork particles, bonded together with an insoluble phenolic resin and molded under heat and pressure. The resultant material is strong, light in weight, flexible, acid-resistant and waterproof. Cork will not extrude when compressed up to 50 percent of its original thickness, and regains up to 95 percent as joint expansion takes place. Expansion joints fillers principally accompany the movement of the materials. They are used to suppress the effects of thermal variations in concrete structures. Expansi