government contractor 加钱怎么开口?

Government IT contractor 2+ years on the same hourly rate. Have more responsibility to cover another contractor who left lately, ~50%+ of his workload.  Is it possible to ask for more $$? There is no such a thing as a raise for a contractor. Should I request review the scope of orginal contract? so get more pay for out-of-scope work? how much to be reasonable? ~20% too greedy? Any one has similar experience to share? Thx