None of the amounts paid to the counties shall be used for the construction or improvement of any highway or street if the contract for such construction or improvement specifies the use of any patented or proprietary paving material, unless the contract has been awarded to the lowest responsible bidder therefor after alternate bids have been called for and opportunity afforded for bids to be submitted for nonpatented or nonproprietary paving material in competition with an equal thickness and like design of such patented or proprietary paving material. This section shall not be deemed nor construed to prohibit the use of any patented or proprietary paving material in the maintenance of any highway or street when such highway or street has been constructed of such material and, in the opinion of the body, board or officer ordering such maintenance, it would be impractical to use a different paving material for such maintenance.