The county commissioners of the Counties of Bucks, Lancaster, Juniata, Venango, Adams, Erie, Mercer, Westmoreland, Montgomery, Clarion and Schuylkill, before making any contract for the erection of any new building or buildings, bridge or bridges, or for the alteration or addition thereto, shall, by public advertisement, printed in not less than two weekly newspapers of the county, if so many be published therein, where the contract is to be awarded, for not less than four weeks, one of which shall be published in the county town, invite sealed proposals for the same, according to specifications, which shall be written or printed in a book to be kept by the commissioners for that purpose, and kept open for inspection of all persons for at least four weeks before the time appointed by said advertisement for the opening of said sealed proposals, and which, at the time fixed, shall be publicly opened and the contract awarded to the lowest bidder or bidders, who shall, within ten days thereafter, give security to said commissioners for the faithful performance of said contract, according to said specifications: provided, that no contract shall be awarded by county commissioners until the same, with the sureties for its faithful performance, shall be approved by one of the judges of the court of common pleas of the proper county.