Whenever contracts are to be let for road improvements, it shall be the duty of the county board to cause to be prepared and filed with the county clerk an estimate of the nature of the work and the cost thereof. After such estimate has been filed, bids for such contracts shall be advertised by publication of a notice thereof once a week for three consecutive weeks in a legal newspaper of the county prior to the date set for receiving bids. Bids shall be let to the lowest responsible bidder. The board shall have the discretionary power to reject any and all bids for sufficient cause. If all bids are rejected, the county board shall have the power to negotiate any contract for road improvements, but the county board shall adhere to all specifications that were required for the initial bids on contracts. The board shall have the discretionary power to authorize the Department of Transportation to take and let bids on behalf of the county at the offices of the department in Lincoln, Nebraska. When the bid is accepted the bidder shall enter into a sufficient bond for the use and benefit of the county, precinct, or township, for the faithful performance of the contract, and for the payment of all laborers employed in the performance of the work, and for the payment of all damages which the county, precinct, or township may sustain by reason of any failure to perform the work in the manner stipulated. It shall be the duty of the county to determine whether or not the work is performed in keeping with such contract before paying for the same.