Slaves were used to work on infrastructure projects in the United States. One example I know of is the Stono slave rebellion happened in South Carolina in 1739. Slaves from local plantations were forced to work on their day off on sunday. They were building canals for the local government. Some of these slaves spoke Portuguese which allowed them to organize since they spoke a similar language that the masters wouldn't know. Some had military training being from the Kingdom of the Kongo and had been drilled by Europeans there. They organized themselves and rebelled against the locals stole guns and fought the local milita with some success. The government used slaves to build the White House as well. Slave owners often hired out their slaves to supplement their income during non-harvest or planting season. Now slaves were not directly owned by the government but they were loaned out to local governments. Their labors would be used to fulfill their master tax burden or to pay off liens against the master. In some areas of the South since planters had little cash assets since all their wealth was in their slaves they had to tax them by extracting labor out of the masters slaves. Slaves were even a taxable assets as can be seen on census records and inventories of people's estates. Slaves were even used to help build the White House. And once against slaves that escaped to Union lines early in the war were taken by the government as contraband from the rebelling states. The male slaves were used by some Union commanders to dig fortification and do other menial tasks around their camps. And the female slaves that had escaped were used to do domestic task around the camp as well like cooking etc.

I will say that numerous presidents owned slaves and employed them at the White House, essentially engaged in affairs related to managing the state. The most famous of which was Paul Jennings, who actually published his memoirs which you can read. There were also a number of slaves involved in the building of said White House--and were most recently invoked in a speech by Obama--as well as building the capitol itself. (According to the Pulitzer Prize winning Politfact: "The records reflect 385 payments between 1795 and 1801 for "Negro hire," a euphemism for the yearly rental of slaves. "

So while I cannot say if the government itself directly owned slaves, they did regularly pay for their labor and employ them not just in service of the government but to literally build it.

Well after the civil war the South had vagrant laws or black codes in place. African-Americans if they even did minor crime like trespassing would go to jail. Then sheriffs would lease out their prisoners to companies like mines in the South or chain gangs to work on roads etc for local municipalities. Old habits tie hard lol. Also the 13th amendment has a provision that allowed slavery to still be imposed on someone if they are convicted of a crime so these prisoners were not paid for their labor.

http://www.amazon.com/American-Taxation-Slavery-Robin-Einhorn/dp/0226194884

http://www.nber.org/chapters/c0606.pdf Page 194 deals with the practice of hiring out slaves.