((a)) ** .—** The Secretary, from time to time as the work progresses, may make payments to a State for costs of construction incurred by the State on a project (including payments made pursuant to a long-term concession agreement, such as availability payments). Such payments may also be made for the value of the materials—

((1)) that have been stockpiled in the vicinity of the construction in conformity to plans and specifications for the projects; and

((2)) that are not in the vicinity of the construction if the Secretary determines that because of required fabrication at an off-site location the material cannot be stockpiled in such vicinity.

((b)) ** .—** No payment shall be made under this chapter except for a project covered by a project agreement. After completion of the project in accordance with the project agreement, a State shall be entitled to payment out of the appropriate sums apportioned or allocated to the State of the unpaid balance of the Federal share payable for such project.

((c)) Such payments shall be made to such official or officials or depository as may be designated by the State transportation department and authorized under the laws of the State to receive public funds of the State.