The concrete section might have been a feed area for cattle. Hard to say though...
And I agree with Ole about cleaning and spraying. You can wear a respirator mask and sand them down too, so nothing hides in the cracks. Or maybe disassemble, knock out the walls between every pair of boxes, then reassemble. A little extra work, but you have the wood all cut and ready to reassemble. You can replace the nest-floor boards if they are really nasty and clean the rest. If you knock out the walls (with or without disassembling) between two nests, you'll have a long box with two openings, which your hens should love. Many like to nest together, and they can always run for it if a bully shows up at one entrance window.