65 ILCS 5/8-9-1
In municipalities of less than 500,000 except as otherwise provided in Articles 4 and 5 any work or other public improvement which is not to be paid for in whole or in part by special assessment or special taxation, when the expense thereof will exceed $25,000, shall be constructed either (1) by a contract let to the lowest responsible bidder after advertising for bids, in the manner prescribed by ordinance, except that any such contract may be entered into by the proper officers without advertising for bids, if authorized by a vote of two-thirds of all the alderpersons or trustees then holding office; or (2) in the following manner, if authorized by a vote of two-thirds of all the alderpersons or trustees then holding office, to-wit: the commissioner of public works or other proper officers to be designated by ordinance, shall superintend and cause to be carried out the construction of the work or other public improvement and shall employ exclusively for the performance of all manual labor thereon, laborers and artisans whom the municipality shall pay by the day or hour; and all material of the value of $25,000 and upward used in the construction of the work or other public improvement, shall be purchased by contract let to the lowest responsible bidder in the manner to be prescribed by ordinance. However, nothing contained in this section shall apply to any contract by a city, village or incorporated town with the federal government or any agency thereof.
In every city which has adopted Division 1 of Article 10, every such laborer or artisan shall be certified by the civil service commission to the commissioner of public works or other proper officers, in accordance with the requirement of that division.
In municipalities of 500,000 or more population the letting of contracts for work or other public improvements of the character described in this section shall be governed by the provisions of Division 10 of this Article 8.
65 ILCS 5/8-9-1