The board of county commissioners may make partial payments upon written estimate of their engineer, on any contract for bridge or culvert work under this act as the work progresses, except that at no time shall more than 95% of the estimate of the materials at such time furnished and of the work at such time done be paid in advance of the full and satisfactory completion of the work. Final payment shall not be made on any of such contracts until the county engineer has inspected the structure and certified in writing that the work has been done properly and completed according to contract, plans and specifications, and the labor and materials have been paid for, and the county engineer’s certificate to that effect has been filed in the office of the county clerk. The acceptance of any bridge or the approval thereof shall not relieve the contractor or sureties of such contractor from any liability for any failure to fully perform the contract or under any provisions of such contractor’s bond.