Nothing in this act shall prohibit the board from buying materials and contracting all or any part of the labor or purchasing or renting equipment to do the work by day labor under the direction of the county engineer. Prior to beginning the construction or repair of any bridge or culvert work by day labor the approved plans and specifications and the county engineer’s estimates of the cost must be filed in the county clerk’s office, and the said county engineer shall keep an accurate itemized account of all expenditures for labor and materials and work performed and file a signed statement of the same in the office of the county clerk at the end of each month and final statement when the work is completed. The provisions of this section shall not apply to any work the estimated cost of which exceeds fifty thousand dollars ($50,000).