The commissioners shall pay all contractors every thirty (30) days upon estimates made by the engineer in charge of the work for the county, or upon estimates of the assistant engineer for the county, reserving not in excess of ten percent (10%) of the amount of work done at that time as may be evidenced by estimates of the engineer or the engineer's assistants; but the engineers shall be sure at all times that their estimates are not for more than the amount of work actually done, and the commissioners shall at all times reserve an ample amount or percentage to protect the interest of the county.