A concrete mixture is provided which is homogeneous and free of lumps and comprises 5-15 parts by weight of sand, 2-3 parts by weight of cement and 1 part by weight of water. Said mixture is charged into a mold in a molding machine in a charging operation having a duration of about 30-120 seconds. Said mold is vibrated in said molding machine in a vibrating operation from the beginning of said charging operation until after said charging operation, whereby said mixture is compacted and air is expelled from said mold and a molding is formed therein. At least part of said mold is removed from said molding when said vibrating operation has been completed and before said molding has completely set. Said molding is stored outside said molding machine until said molding has completely set.