It shall be the duty of every public body which proposes the making of a contract for any project of public work to determine from the secretary the prevailing minimum wage rates which shall be paid by the contractor to the workmen upon such project. Reference to such prevailing minimum rates shall be published in the notice issued for the purpose of securing bids for such project of public work. Whenever any contract for a project of public work is entered into, the prevailing minimum wages as determined by the secretary shall be incorporated into and made a part of such contract and shall not be altered during the period such contract is in force.