In every county where the board of chosen freeholders shall appoint a keeper or warden as provided by sections 30:8-19 and 30:8-20 of this title, such keeper or warden shall be the master of the workhouse in such county, if there be one, and such workhouse, or so much of it as shall be so declared by such board, shall be part of the common jail of such county. Such keeper or warden shall put and keep at such work as they are able to perform, at such place or places in the county as may be designated by such board, all persons who may be by law required to work during their continuance in his custody.