Tangy Barbecue Sandwiches

3 cups chopped celery
1 cup chopped onion
1 cup ketchup
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 boneless chuck roast (3 to 4 pounds), trimmed
14 to 18 large hamburger buns, split

In a slow cooker, combine the first 12 ingredients; mix well. Add roast. Cover and cook on high for 6-7 hours or until tender. Remove roast; cool. Shred meat and return to sauce; heat through. Use a slotted spoon to serve on buns. Yield: 14-18 servings.