I’ve used pallets, lumber, RR ties, landscape timbers, etc… and I hate them all. Every one of them will sink, freeze to the ground, and rot/fall apart. I’ve learned if air can get under the stacks, so can critters… critters lift up the earth packing mud into the wood and also undermine the stacks, causing them to fall. And if air and critters can get under the stacks so can weeds of all sorts… which just attract more critters. I’ve also stacked directly on the ground… and that’s OK for the short-term, but over the course of two or three years you get the same problems along with wet, rotted, mud-caked firewood frozen to the ground on the bottom.
I’ve gone to just using a moisture barrier such as old rubber belting, rubber roofing… currently I’m using old vinyl siding that came off my brothers house, and that seems to work best of anything. I cut the moisture barrier a few inches wider than the wood so I can just run the grass cuttin’ machine along it… no trimming, weed-whacking, etc. No ground-digging critters, no weeds, no falling stacks; no wet, rotted, mud-caked firewood frozen to the ground. Ice and snow won’t stick to vinyl and it won’t freeze to the ground… any water attempting to puddle in low spots just runs off/out at the joints.
Getting air under the stacks is over-rated… besides, most of the time critters just block it off with rooted-up dirt and weeds anyway. I suppose if ya’ stack several rows deep there is some advantage to air under the stacks… maybe. But if ya’ stack in single rows, like I do, there ain’t any need whatsoever. Stacking on concrete is nice… but if you live where there’s lots of ice and snow you’ll have the bottom three rows or so solidly frozen down in winter (ice clings to concrete with an iron-like grip). The concrete block idea ain’t too bad other than it raises the stacks, reducing the amount of wood that can be placed in that area… and they can tend to sink, tip and freeze down. Really, anything non-flexible will sink, lift, tip and even break when the frost comes out in the spring... a flexible moisture barrier will "give" where needed and then settle back in after the first good thunderstorm.