• 2 thin reinforced concrete walls, linked and held apart by steel spacing rods. • maximum dimensions: 3.8 m x 12.30 m • wall thicknesses: from 16 to 40 cm • inner wall thicknesses: from 4.5 to 7.5 cm • weight: from 280 to 350 kg/m². • quality of concrete: fixed at C 40/50 minimum