A sleeve is a liner, the outside of which is coated with a resin, which is inserted into a damaged pipe in the trenchless rehabilitation process referred to as relining or pipe relining.
The sleeve may be made of fiber reinforced plastic or polyester felt coated with polyvinyl chloride (PVC). The sleeve is pulled into the damaged pipe (called a "host pipe") so that it isolates the damaged area and is expanded until its outer, resin-coated surface contacts the walls of the host pipe.
Trenchlesspedia Explains Sleeve
The first step in installing a sleeve is to take the host pipe out of service. The sleeve is installed into the host pipe after the host pipe's interior walls are cleaned and sandblasted to ensure good contact between the sleeve and the host pipe's inner surface.
When the resin cures and bonds the sleeve against the inner surface of the host pipe, the annular space between the sleeve and the host pipe is grouted to prevent leaks between sleeve and pipe.