16 cups chopped rhubarb
12 cups water
3 cups white sugar
3/4 cup lemon juice
1 (6 ounce) can frozen orange juice concentrate
2 (2 liter) bottles lemon-lime soda (such as 7-Up®)
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.