In the United States, dewatering of concrete by vacuum was patented in 1935 by K. P. Billner and all subsequent development has been based on his extensive ideas. Vacuum dewatering got a new lease on life mainly after two or three Scandinavian firms succeeded in simplifying the equipment enough to make it practical for almost any builder. Dewatering can produce impressive strength gains. In Sweden, the method is now used for 40 to 50 percent of all floors. The use in wall, column and other construction is rare. The aim in vacuum dewatering is to remove as much as possible of the portion of the mix water introduced for workability. The amount generally is two to four times as much as the amount required for hydration of the cement. Water not needed for hydration but left in the concrete after placing and finishing is a drawback in that it leads to lower strength, a decreased rate of strength gain, increased shrinkage and creep, and more susceptibility to frost damage. The Billner method seals off a part of the concrete surface with a rigid plate or a flexible mat below which vacuum is created in the concrete slab. Atmospheric pressure will then act on the plate or mat with a considerable force, about 1,600 to 2,000 pounds per square foot at the vacuum generally used. The concrete is compressed and part of the surplus water is squeezed out and sucked away from the concrete surface by the vacuum pump. The water squeezed out of the concrete passes through the filter cloth into the space immediately below the top cover, moves toward the suction inlet and is sucked away to the vacuum pump. The solid particles are pressed against each other with a pressure corresponding to that caused by the mat while the water and air are subject to vacuum.