Tangy Barbecue Sandwiches

3 cups chopped celery
1 cup chopped onion
1 cup catsup
1 cup barbecue sauce
1 cup water
2 tablespoons vinegar
2 tablespoons Worcestershire sauce
2 tablespoons brown sugar
1 teaspoon chili powder
1 teaspoon salt
1/2 teaspoon pepper
1/2 teaspoon garlic powder
1 (3 to 4 pound) boneless chuck
roast, trimmed
14 to 18 hamburger buns, split
In a crockpot, combine the first 12 ingredients; mix well. Add roast. Cover and cook on HIGH for 6 to 7 hours or until tender.
Remove roast; cool. Shred meat and return to sauce; heat through. Use a slotted spoon to serve on buns.