Upon approval of such maps, plans, and specifications and the filing thereof the board shall, if it determines that the work shall be done by contract, advertise a call for bids upon such construction work by publication in the official county paper and also one trade paper of general circulation in the county, in one issue of each such paper at least once in each week for two consecutive weeks prior to the time set in the call for bids for the opening of bids. All bids shall be submitted under sealed cover before the time set for the opening of bids.