Fresh concrete contains a continuous system of water filled channels. The application of a vacuum to the surface will result in a large quantity of water being extracted from the close, say 100mm depth, surface. The process was invented and patented in 1935 by K P Bilner.
The technique is generally used on horizontal surfaces, the vacuum being applied through porous mats in turn placed on fine filter pads, which minimise the removal of cement fines. The vacuum is generally applied for 3 to 5 minutes for each 25mm depth of slab. The mats are up to 5 m square. The process allows for power-finishing to be started earlier. The resulting surface is denser and the w/c ratio reduced with resulting lower permeability and greater durability.
The effectiveness of the process reduces with time and is generally only applied for up to 30 minutes. Vacuum dewatered concrete stiffens rapidly. Limits to the mat size and the logistics of moving the apparatus about a site has probably responsible for its lack of use in the UK, especially with the tendency of the flooring industry towards large area pours.