Before any contract shall be let as aforesaid, the county board shall cause to be published three consecutive weeks, in a newspaper printed and of general circulation in the county or, if there is no newspaper printed in the county, in a newspaper of general circulation in the county, an advertisement inviting contractors to compete for such work. Such notice shall state the general character of the work, the number and kind of bridges required to be built, their proposed location as nearly as can be estimated and determined, the time within which and the place where such bids must be presented, and the time and place of opening such bids. No bids shall be considered unless accompanied by a certified check or cash, the amount of which is to be determined by the county board. All such checks shall be made payable to the county clerk of the county, to be forfeited to the county in case the bidder refuses to enter into a contract with the county, if the same is awarded to him.