All contracts let for the construction of any work authorized to be done under the provisions of the Act, where the amount thereof is in excess of a small purchase amount, as defined in Section 20-20 of the Illinois Procurement Code, shall be let to the lowest responsible bidder, or bidders, on open, competitive bidding after public advertisement made at least 15 days prior to the opening of bids, in the Illinois Procurement Bulletin, in such manner and at such intervals, as may be prescribed by the Authority except for contracts for the completion of a terminated or defaulted contract. The successful bidders for such work shall enter into contracts furnished and prescribed by the Authority. Such contracts shall contain a provision that such successful bidder shall indemnify and save harmless the State of Illinois for any accidental injuries or damages arising out of his negligence in the performance of such contract, and shall, and in addition, execute and give bonds, payable to the Authority, with a corporate surety authorized to do business under the laws of the State of Illinois, equal to at least 50% of the contract price, one conditioned upon faithful performance of the contract and the other for the payment of all labor furnished and materials supplied in the prosecution of the contracted work.