All contracts for the construction of sidewalks as hereinbefore provided, when the expense thereon shall exceed $500, shall be let to the lowest bidder, in the following manner: Notice shall be given by said City Engineer, by advertisement in some newspaper of general circulation in the city, that bids will be received for the construction of such sidewalks in accordance with the ordinance therefor. Such notice shall state the time of opening bids, not more than ten or less than five days thereafter. All proposals of bids offered shall be accompanied by cash, or a check payable to the order of the city, and certified to by a responsible bank for an amount which shall not be less than 10% of the aggregate of the proposal and contracts shall be approved by the Council.
('71 Code, § 9-3-10)