Indiana Code Title 8. Utilities and Transportation § 8-6-2.1-18

Sec. 18. Any portion or portions of the work of improvement under the resolution which is allotted to the city shall be done by contract or contracts, and all contracts shall be let under statutes governing the letting of contracts by the city.  In event of the execution of a contract for the work, the validity of the contract may not subsequently be questioned by any person, except in a suit to enjoin the performance of the contract instituted within ten (10) days after its execution.  All proceedings and orders of the board preliminary and prior to the contract, and the contract, are considered valid, conclusive and binding upon all persons and are not subject to attack for any cause after the ten (10) day period after its execution has expired.