Upon the completion of any such work or improvement, the City Engineer shall make a report in writing whether or not the same has been done according to the terms of the contract, and upon acceptance and approval of such report by the City Council, a warrant shall be drawn upon the City Treasurer for the amount due such contractor. During the progress of any such work the Council may require of the Engineer to make a report of the amount of work that has been completed and whether the same has been done according to the contract, and upon the acceptance and approval of such report by the Council it may order such contractor to be paid not to exceed 85% of the proportionate share of the contract price due for such work so completed, provided that at least 15% of the contract price shall always be reserved for payment upon the final acceptance of the work by the Council.
('72 Code, § 3.20(k))