65 ILCS 5/9-2-136

From time to time, as the work under any contract for such an improvement progresses, upon certificates by the board of local improvements, or by some officer designated by the board for that purpose, payments may be made either in money, vouchers, or bonds, as provided in this Division 2, to apply upon the contract price, reserving, however, a sufficient amount upon each of the payments to properly secure, in the judgment of the board, the faithful performance of the contract. This reserve shall be paid over at such time and on such conditions as the board shall fix, after the specified work has been completed or accepted.