The board of county commissioners may, in constructing, surfacing, repairing or maintaining the county roads, let contracts for all or any part of such work, or said board may buy the materials and contract all or any part of the labor, or may purchase or rent machinery and other equipment, and employ labor, under the direction of the county engineer: Provided, That before beginning to construct, surface or repair any road by day labor, the approved plans and specifications and an estimate of the cost must be filed in the office of the county clerk. On all day-labor work the county engineer shall keep an accurate itemized account of the expenditures for labor, materials and work performed and file a sworn statement of the same in the office of the county clerk at the end of each month, and a final statement when the work is completed.