1 (6 ounce) can frozen limeade concentrate
14 fluid ounces water
1 1/2 cups rum
3/4 cup grenadine syrup
1/2 cup lime juice
1 tablespoon aromatic bitters
2 liters club soda
Pour limeade into a large pitcher with a lid; stir in water, rum, grenadine syrup, lime juice, and bitters until well mixed. Cover pitcher and freeze punch until slushy, 8 hours to overnight.
Fill individual serving glasses with ice; Spoon about 3 tablespoons punch-slush mixture over ice. Top with club soda and stir gently.