Mixture of badly maintained concrete/tarmac, and lots of high speed gravel. To put that into context a guy I know who had a D2 a few years ago had shredded a set of normally quite hard wearing Michelin XPCs in 8000 miles...
I don't know for sure about the construction I am just going on what the tyres "feel like" when off the rim, the AT2s seem to have a very tough sidewall and are b*ggers to change! Also if you have two tyres side by side, not mounted on a rim, and give them the "shove test" (push down on the centre of the tread with your hand and see how much the carcass deforms) the AT2s seem some of the strongest in this test and at least as strong as the BFGs. It is not very scientific but is usually proven right in the performance of the tyres after a year or two i.e. the ones that do badly are usually not up to much for puncture resistance, tyre wear etc etc.
I am running the AT2s on my Ranger (1500kg unladen wt) at about 21psi and they seem happy at that, though not recommended for motorway speeds!