In a large saucepan, bring rhubarb and 4 cups of water to a boil. Reduce heat; simmer, uncovered, for 5-8 minutes or until the rhubarb is tender. Mash the rhubarb and strain. Reserve juice and discard pulp. Add sugar, concentrates and remaining water to juice. Transfer to a freezer container and freeze.
Remove from the freezer 30-45 minutes before serving, scraping the surface as it thaws. Place equal amounts of slush mixture and club soda in each serving glass. Stir together and serve immediately.