Having landed on one or two I can definitely say made from concrete! they usually came in 5-10 metre square sections whatever that is in old money, WW2 runways were qiuck build and the easiest way was with concrete as the raw material, for some strange reason many of the uk hardstands were tarmac.
At a lot of established airfields ie pre war tarmac throughout was used. or I believe London airport now Heathrow was a grass field!
location is very important, i know for sure that Russian uses smaller slabs than the US. Again not sure of the sizes. Maybe you could sneak into a airforce base at night with a measureing tape.[;)] i reamember taking to someone on this forum who worked in france i think that repaired pumas?(the helicoptors) i will see if i can find him and send a pm[;)]
Back in my FOD walking days, I recall the hardstands being about 10’ squares at a couple of bases. I only walked a couple of runways and don’t recall any patterns there. I seem to think that they were poured in long/wide sections.
The only FOD walks I had to do were short ones…but picking brass out of the flight deck tie downs was always a pain in the [censored] and I wasn’t on the “big deck”