Henry's Hydro-Flex® Waterstop is an easy to use, preformed, self-adhesive hydrophilic waterstop that provides a watertight seal in cold joints on concrete structures. Designed to swell into unconsolidated concrete surfaces upon contact with water, Hydro-Flex® Waterstop features an advanced blend of butyl rubber, polypropylene elastomers, hydrophilic fillers and plasticizers. For exterior use only.