Couple of people have already said this but use PVC to rebuild the casing after you remove all of the rotted wood. Part of the problem there is it's too close to the ground. Another problem is concrete wicks moisture. Wood should never touch concrete. PVC with a polyurethane caulk will solve all of these problems

You can also build up one by six PVC. Stack it to make 2x6 stock. You can glue it together with plumbers glue or polyurethane adhesive.