When the funds arising from the road district bonds shall have been expended in the construction of roads and the contracts for construction of roads have been completed, then the road engineers, by and with the consent of the board of supervisors, are hereby authorized and empowered to purchase teams, implements and material, employ labor, work said roads and build bridges, and may do all things necessary to be done to work, maintain and improve the roads and build bridges, as herein provided; but all contracts for material and supplies shall be made according to state law governing public purchases, as provided in Sections 31-7-1 et seq., and all labor shall be employed by and under the direction of said engineers, by and with the consent of the board of supervisors. If the road district be composed of only a part of a supervisors district and the principal of the bonds issued does not exceed Fifty Thousand Dollars ($50,000.00), the road engineers, by and with the consent of the board of supervisors, are hereby authorized and empowered, in their discretion, to purchase and hire teams, implements and material and employ labor to build and work said roads and build bridges, and may do all things necessary to be done to construct, work, maintain and improve the roads and build bridges, as herein provided; but all contracts for material and supplies shall be made on and after competitive bids for the same, except as otherwise provided by law, and all labor shall be employed by and under the direction of said engineers, by and with the consent of the board of supervisors. The board of supervisors may use any teams and implements which it owns to assist on the roads of such district; and the like authority may be granted by the board for the maintenance of the roads, as to those not under construction, during the period when the roads or some of them are being constructed.