The concrete from which water is extracted by a vacuum process before the hardening takes place is known as vacuum concrete. The type of concrete having relatively high slump in order to achieve consolidation is usually useful in case of thin slabs and walls. This is achieved by withdrawing considerable amount of water and air near the surface by use of vacuum mats connected to a vacuum pump.
A vacuum mat consists of a plywood backing with a vacuum chamber formed out of expanded metal and faced with a fine wire gauge and fabric covering. Each mat is fitted with a valve controlled outlet for connection to a vacuum pump.
Advantages of Vacuum Concrete
Vacuum processing of concrete results in
Increase of final strength of concrete by about 25%
Sufficient decrease in permeability of concrete
High density concrete
Increase of about 20% bond strength of concrete
Appreciable reduction of time for final finishing
Early removal of wall forms
Surfaces entirely free from pitting
Some water extracted may leave behind small voids or channels because of the vacuum operation, but this can be taken care of by vibrating concrete at the start of application of vacuum. Vacuum process has been found to be satisfactory up to 20 cm depth of concrete.