I've posted so much about asphalt that I've not mentioned enough about sidewalk. Sidewalk and heat too soon is just asking for blisters for too long of a distance, especially on sidewalk with pointed gravel embedded in it. During fall is the PERFECT callus development weather. But, for those who don't want roughened and toughened soles, don't do extended rough sidewalk walks since it's similar to sandpapering the soles with a much rougher grit, but sidewalk can be even rougher than the roughest sandpaper. I think weather in the low to mid 60 degrees F range is just enough to barely start to feel slightly cold on the soles, but is more than enough to make rougher concrete sidewalk scuff the soles that much more. It's THE BEST weather possible for making calluses. Walks don't have to be longer distances too soon, but the soles MUST be scuffed white at the end of a walk. No noticeable whiteness, not enough scuffing. Too much whiteness too soon needs to avoided, but is easily determined by a rug burn feeling when stopping momentarily, so just keep walking but at a slower pace when the scuff burn feeling starts to occur and it will go away. Go ahead and push just past the start of scuff burning towards the end of the barefoot walk, make hotspots but not blisters, because only more frequent scuff burning walks will get the soles used to more friction heat when the rougher sidewalks are also summer hot. If the hotspots become too large, wait a day before resuming the scuffing barefooting on sidewalk. Some soles might become more thicker and leathery, with the calluses barely showing and almost no feeling of roughness. Other soles might become so scuffed that they show as almost being scraped away to the skin during longer walks. The scraping all the way to the skin look is only a temporary condition, as seen when the scuffing stops, but that kind of scuffing is the best for building up nearly permanent calluses, those that don't go away completely unless wearing shoes and socks for months. If the calluses on the heels show as very white small particles, the heel is being callused ideally. If a rougher spot occurs on the edge of the heel, watch it for cracking but that is otherwise normal for heel to toe barefooting where the heel hits the most. DO NOT SMOOTH OR FILE unless there is more than the shallowest cracking, and then ONLY on the cracks. I still don't find any benefit to building up calluses on sidewalk in colder weather below 50 degrees F. Colder weather scuffs more during shorter walks, but it's also starting cold-training the soles. In order to keep my existing heat tolerance to increase it even more next summer, I do not need cold-training. So I say don't just use sidewalk as a cooler surface from asphalt way too burning hot too soon, especially if the sidewalk is more than too hot after a long enough distance so that even sidewalk shade is needed, because that's only training for shorter distance heat and not longer distance scuffing and scraping. Start early, start often. Keep doing roughening, toughening, callus making walks on as rough as tolerable sidewalk during fall. Increase the distance, increase the walking speed. Don't overstride and in fact deliberately reduce the step size from that when otherwise wearing shoes. Get those calluses as rough and as white as possible, but almost always avoid pushing past hotspots into blisters. The warning of blisters from scuffing on sidewalk is different than heat, it will feel like a focused almost sharp poking feeling on the parts of the soles that are starting to get too much scuffing than the rest of the soles. Going though the poking for too long feeling WILL blister. That almost always ends the barefoot walk. Now, if specifically training for extend walking on that long of sidewalk on a regular basis, don't worry about it and get that blister if it occurs, just like gymnasts do with dancers and martial arts students do with their soles, but make sure to plan ahead so that doesn't occur in the middle of the walk. Otherwise, slow down and even end the walk early, since there will be another time for the same walk or even longer. Now, I don't do any pavement barefoot running, in fact I no longer run, but many report bruises or even blisters as part of getting used to it. Since barefoot walking is slower than running, I already the soles absorb a lot more heat. Since barefoot walking scrapes the ground instead of the sole bouncing off the ground when running, I also need to get more used much more friction even with less shock to the soles, and I'm still trying to get more of that, maybe up to a half-marathon distance to and from a destination during an all-barefoot walk. And that's what I have to share about sidewalk from my own experience on the soles. Again, new barefooters might find this useful, but regular inland and even desert area barefooters know all of this already. Their sidewalks can get so hot during the afternoon peak heat that they need sidewalk shade in a shorter distance, and even standing in shade can either require heat relief methods of even sitting down to cool off the soles more than enough before continuing. Too much of that, it's smarter to put on shoes and keep them on. But, it's also worth trying to get used to shorter distances on that much heat if only to withstand that much burning until being able to stop and put on shoes, so it doesn't burn almost on contact, and that would mean the asphalt would blister in just seconds. Getting the roughest and toughest possible calluses serves as an additional friction and heat buffer on the most burn prone parts of the soles, because even scuffing and scraping causes a heat buildup over a longer distance.
I pretty much agree! Fall barefooting on rough sidewalks (other than smooth cement) keeps the calluses conditioned, and the myriad sensations are lovely----cool, warm, cold, warm, with some nice crunchy leaves! Sort of like spring training for baseball!
I found that shuffling your feet across smooth tile floors such as those in schools, stores, etc. helps get/keep your callouses in good shape. I remember in HS taking a test and excused myself to the rest room. A girl who really was into going barefoot ditched her sandals and was dragging her soles just as I did in stores. The next year I sat in back of her in class and saw a nice set o white callouses peering through the black dirt that was on the rest of her soles as she propped them out of her sandals. I was used to seeing her barefoot all over town.