No purchase of lumber and timber products in quantities of one or more carloads shall be made except after competitive bids have been tendered to the purchasing officer, or board; nor sooner than the tenth day after proposal forms have been made available to bidders; nor shall any change be made in specifications after publication unless the time for opening bids be extended ten days (or more) from the date of making such change: Provided, however, That in case of emergencies caused by floods or fires which have caused the highways or bridges to become unusable the provisions of this section shall be waived.