Indiana Code Title 36. Local Government § 36-10-4-26

Sec. 26. (a) If the board orders acquisition and construction and has advertised for bids for the construction after final adoption of the resolution, it shall require each bidder to deposit with the bidder's bid a certified check for an amount not less than two and one-half percent (2 1/2 %) of the engineer's estimate of the cost of the improvement to insure the execution of the contract for which the bid is made.

(b) The contract must state that payments for all work under the contract shall be made only from the special fund derived from the proceeds of bonds and special assessments.  A contract may not be let for a higher amount than the estimated cost of the work, and the board may let parts of the proposed work under different contracts.  The board may, at any time before the execution of a contract for the work, rescind any acts or orders in relation to the proposed work or take the supplementary proceedings that the board considers necessary.

(c) The validity of a contract may not be subsequently questioned by any person except in an action to enjoin the performance of the contract instituted within fifteen (15) days after the execution of the contract.  After that fifteen (15) day period, all proceedings and orders of the board preliminary to the contract are valid, conclusive, and binding upon all persons and are not subject to attack.  The amount of the benefits resulting to all property in the city and the special tax shall be levied only for the balance.