In a large mixing bowl, combine the graham cracker crumbs and sugar. Then stir in butter. (Mixing in this order will prevent sugar clumps in the crust.)
Set aside 2 cups of the graham cracker mixture for the topping.
Press the remaining 20 cups of crumb mixture into a greased 9-inch x 13-inch baking dish.
Cover and freeze for at least 10 minutes.
In a large mixing bowl, mix the ice cream and orange juice concentrate until evenly combined and smooth. Pour the ice cream mixture over the crust and spread it into an even layer.
Freeze for 10 minutes or until partially firm.
Sprinkle with reserved graham cracker crumb mixture over the ice cream and gently press down. Cover and freeze for at least 4 hours or until set.
Remove from the freezer 15 minutes before serving. Cut into 12 slices. Garnish each slice with a dollop of whipped cream, a mandarin orange, and a sprig of mint if desired.
Notes
You'll need about 21 graham crackers to make 3 cups of crumbs. Place them into a food processor and whirl until you have crumbs, or place them into a large food storage bag, seal it, and pound with a rolling pin until all you have is small crumbs.