"I always have this in my freezer for when guests pop by. Easy to use in a punch, or just take out what you need by the glass. You can also add vodka, gin or rum; add 1 to 2 cups per bowl of punch and subtract an equal amount of the soda."
Mix rhubarb with water in a large pot and bring to a boil; lower heat to medium-low and simmer until rhubarb becomes mushy, 10 to 15 minutes. Strain juice into a large freezer-proof container. Discard pulp.
Stir sugar, lemon juice, and orange juice concentrate into the rhubarb juice, dissolving sugar, and place in freezer overnight. Stir mixture the next day, then stir every 2 to 3 hours until slushy. If left in freezer unstirred, the punch base will freeze hard; to thaw, microwave, 1 cup at a time, until slushy, 30 seconds to 1 minute.
Mix slushy punch base with lemon-lime soda in a large punch bowl until combined. To serve in individual glasses, mix half punch base with half lemon-lime soda in a large glass.