(a) Every contract for the construction, alteration, or repair of any public building or public work in the United States growing out of an appropriation heretofore made or hereafter to be made shall contain a provision that in the performance of the work the contractor, subcontractors, material men, or suppliers, shall use only such unmanufactured articles, materials, and supplies as have been mined or produced in the United States, and only such manufactured articles, materials, and supplies as have been manufactured in the United States substantially all from articles, materials, or supplies mined, produced, or manufactured, as the case may be, in the United States except as provided in section 10a of this title: Provided, however, That if the head of the department or independent establishment making the contract shall find that in respect to some particular articles, materials, or supplies it is impracticable to make such requirement or that it would unreasonably increase the cost, an exception shall be noted in the specifications as to that particular article, material, or supply, and a public record made of the findings which justified the exception.

(b) If the head of a department, bureau, agency, or independent establishment which has made any contract containing the provision required by subsection (a) of this section finds that in the performance of such contract there has been a failure to comply with such provisions, he shall make public his findings, including therein the name of the contractor obligated under such contract, and no other contract for the construction, alteration, or repair of any public building or public work in the United States or elsewhere shall be awarded to such contractor, subcontractors, material men, or suppliers with which such contractor is associated or affiliated, within a period of three years after such findings is made public.