A rigid plastic foam material is being adapted to a number of interesting uses in the concrete construction field. Said to be the lightest of all rigid insulation materials, it combines low thermal conductivity with high resistance to water and water vapor. For perimeter insulation purposes, 1 inch thick boards are installed either horizontally or vertically to a width of 12 to 24 inches, depending on climatic conditions and job requirements. In horizontal applications the foam boards are simply placed on well-tampered fill and the concrete slab is poured directly over them, while in vertical installations the boards are applied to the inside of the foundation walls before the backfill is shoveled in. In highway pavement applications, « by 2 inch strips of foam are used to form transverse contraction joints. The slab is grooved with a joint bar and the strips of plastic foam are fed in, leveled and straightened manually, and then the finishing operations proceed. In the case of dam construction, the plastic insulation planks are being used to prevent sudden temperature fluctuations.