Vacuum dewatering gives the floor contractor control over an important variable- the time required for the concrete to set up firm enough to finish. Without vacuum this may vary from half an hour to more than three, depending on amount of mix water, temperature, humidity, and wind. With vacuum, the contractor can count on a definite time, usually less than half an hour. When the concrete is placed, it is vibrated with immersion vibrators if the slab thickness is more than about four inches. It is then leveled and vibrated with a double beam vibrating screed, which works either on the form edges or on elevated rails above the concrete. The concrete surface is then immediately covered with the suction mat or plate and vacuum dewatering is begun. The duration of the treatment should be about 2 and one-half to 3 minutes per inch of slab thickness. After dewatering has been finished the mat is moved to a fresh surface. It is already possible to walk on the concrete if necessary and the dewatered surface is immediately floated with a power trowel fitted with a planing disc. The surface of the vacuum-treated concrete is generally finished in two stages by machine because a vacuum treated surface is generally too hard for manual finishing. For the first stage a power trowel is fitted with either a planing disc or extra large float shoes. If a smoother surface is specified, a second stage must be undertaken: power troweling.