In order to defray the costs and expenses of improvements in any improvement district in a city of the first class, the mayor and city council shall have power and authority to levy and collect special taxes and assessments upon the lots and pieces of ground adjacent to or abutting upon the street, avenue, alley, or sidewalk, thus in whole or in part improved or repaired or which may be specially benefited by such improvements. The provisions in this section shall not apply to ordinary repairs of streets or alleys, and the cost of such repairs shall be paid out of the road fund. The mayor and city council are authorized to draw warrants against such fund not to exceed eighty-five percent of the amount levied as soon as levy shall be made by the county board.