Slabs are fine as a floor - better than mud - but remember hens love to scratch around, so top them with bark chips. You can replace these and hose down the slabs to keep things clean.
I would also look at roofing the run as that makes a difference to how well the flooring litter keeps. If the run catches the sun, you can use Onduline or something similar to provide shade, which is important.
If the hens are confined to a small area, it is also worth adding extra amusements. A couple of logs to perch on are popular, hang up greens and chuck in an apple for them to peck at.
Finally, don't overstock. A few happy chucks will give you more eggs than a load of cramped, stressed ones