I see that one of the convicted soldiers from Abu Ghraib has served 90 days at hard labor at Fort Bragg, the specifics to be determined by some superior officer there. I’m wondering what those guys are actually doing during their work day when they’re at hard labor. Do they still break up boulders with sledge hammers? What’s that like these days? What type of punishment could this man have been subjected to for his crime?
A skim of this 2004 Army law article seems to indicate it can be anything from extra latrine duty, to pouring cement, to literal breaking of rocks, and that to constitute punishment, it’s done in addition to the soldier’s regular duties, with no allowance for the extra work.
Four hours of cleaning latrines or scraping gum off of sidewalks may actually feel like “hard labor”…if you’re doing it at the end of an already hard 16 hour day. 
When I was in the Marines, a lot of Marines would be sentenced to 30/60/90 days of CCU (Correctional Custody Unit), which was basically like the brig but with hard labor. We would see them out there all of the time, exactly as you would think. They would have sledgehammers and in unison (while singing some kind of cadence) would be breaking huge rocks. Life was HELL for them, and most actually came back a better Marine.
From what I understand, hard labor is just that. Back breaking work.