OK, so yours are sheep then? Not goats? I can suggest wood shavings, or even sawdust if that is what you can get, because it will soak up the urine. Then I would suggest to find the "wet spots" and remove them daily. Yeah... I know... that's a lot of work, but I do it for my night pen for my goats. Seems to work. It gets a lot of the urine right out of the pen, so it's not there to create ammonia fumes.
Another thing you might consider: a night pen and a day pen. That way, each pen has a chance to dry out for some of the hours of the day or night. You would put them right next to each other, so it would be a matter of opening a gate and herding them through twice a day. Then once they are on the other side, you can clean out as much of the urine as you can. Those ammonia fumes can be really bad for them, so you are right in trying to cut down on them.
ETA: Put the gate *in the corner*. Animals naturally go to a corner to find the way out... if you put a gate in the center of a fenceline your life will be pure misery trying to get animals to go through it on their own. Yes... I know it's harder to put the gate in the corner. However in my experience, life is much happier if you take the time to do it, as it's so easy to herd them through.