All city improvements of whatever kind or character, including the erection of all public buildings made or to be erected at the expense of any constitutional charter city having more than seventy-five thousand and less than eighty thousand inhabitants, except where otherwise provided herein, shall be let by contract to the lowest and best bidder, and shall be prescribed by ordinance; provided, that nothing in this section shall be so construed as to prevent the repair, by day’s work, of boulevards, streets, sewers, culverts, buildings or other city property, so far as may be necessary for the preservation thereof; and provided that nothing in this section shall be construed to prevent the city from specifying by ordinance the kind and character of materials and the kind and character of labor to be used by contractors making city improvements and the maximum hours that the labor shall work thereon and the minimum wages that shall be paid the labor.