I have no problem understanding how the freeze-thaw cycle causes potholes, nor is it a mystery how salt reduces the freeze point of ice on roads resulting in safer travel.
What is a mystery to me is how the addition of salt to roads increases the freeze thaw CYCLE that causes terrible roads. According to every source found, mainly DOT’s from Tennessee to North Dakota, there is no explanation as to how or why. Everyone simply states it’s “because”.
There is no doubt in my mind that salt is the culprit here and that the freeze-thaw cycle is likely the answer but I’d like to see a reference as to how… preferably with charts or graphs to delineate.
Water that is under 32 degrees F will freeze, and not thaw, until the temperature finally rises back above 32.
What the article says is that salted water has a freezing point of 15 F. So, if the temperature stays below 32F, but varies back and forth between, say, 25F in the daytime, and 10F at night (which it certainly does, here in Chicago, for example, for days on end during the wintertime), the salty water freezes every time it drops below 15, and thaws again every time it goes above 15. If the water weren’t salty, in those temperatures, it’d stay frozen.
Your cite is inherently flawed (as above) however, all things considered: you’re assigning arbitrary numbers here that don’t necessarily apply to all conditions.
I’m looking for an explination applicable to all conditions
Are you thinking that, somehow, despite what paving professionals have to say, that there must be some other, additional, impossible-to-find-online explanation, that explains why road salt makes the freeze-thaw cycle worse, under every possible temperature and weather condition?
I don’t think it would be a universal thing: It would depend on how the temperature varies in any given location. It would then be an empirical fact, not a fundamental one, that in the locations served by those road authorities, salt makes things worse.
What I see in that cite isn’t that road salt makes a particular freeze-thaw cycle worse, it’s that it makes freeze-thaw cycles happen more often. Which, in the net, means that it makes the fact that the freeze-thaw cycle exists worse for roads, and potholes, over the course of time.
Wild-ass supposition: when salty water freezes, does it thaw under pressure more easily than more pure water? That would increase the number of cycles a lot
But if this is the reason, the implication is that temperature is more likely to fluctuate above and below 15 than it is to fluctuate above and below 32.
Intuitively (for me), I would’ve thought the answer was something more along the lines of water with salt expanding more than water alone.
In fact, if we were talking about concrete, the salt would cause the rebar to rust, expand and create potholes.
I think the missing point is that salted water has different freezing/melting points depended on its concentration of salt.
Salt on the road creates a range of salt concentrations from some areas that have no salt in the water, to fully saturated. These include areas immediately adjacent to each other. That means the range of freezing/melting points goes from 0 C down to -21.1 C.
Any and every temperature fluctuations within that range will result in freeze thaw cycles of some of the water, which then seeps into other spaces … Not only the freeze thaw at going above and below 0 C.
To throw at the more physics minded here: those are freeze points at one atmospheric pressure. How much up of an impact might be had by the increases of pressure of large vehicles driving over these areas of cracks filled with heterogeneous concentrations of salted water and ice at temperatures that vary in those ranges? Does that meaningfully impact the freeze thaw cycling too?
I think there may be a survivor-bias-esque situation here. In a place where there’s enough snow or ice on the streets to need salting, then the climate is likely cold enough that water in would-be potholes would have stayed frozen solid if it weren’t for the salt. So in a climate like Chicago, Minneapolis, Buffalo, etc., the addition of salt causes more freeze/thaw cycles than if they just left the roads untreated and they stayed frozen for weeks or months at a time. Normally there’d be a handful of freeze/thaws in the autumn, a long frozen winter, and then freeze/thaw in the spring. That’s not the most damaging thing for streets, and the winter deep freeze is not as cold as it used to be, but with a lot of salt then the streets are going to be cycling a lot more often.
In the sunbelt, even though it gets below freezing plenty, they don’t need to salt the streets because they rarely get snow or ice, and if they do it usually melts by itself pretty quick. In some middle climate zones however, like Cincinnati, Washington DC, or St. Louis, there’s already a lot of natural freeze/thaw cycles because for most of the winter it’s typically above freezing during the day and below freezing at night. Rain is just as likely as snow, so there’s less need for salting in the first place, and salt may in fact reduce the number of freeze/thaw cycles by keeping the salted streets above the freezing point throughout the colder nights. On the other hand, at least here in Cincinnati, they will do everything they can to not plow snow and instead just salt salt salt, so the streets are still pretty crap. Point being, there is no “spring thaw” here, snow rarely lasts for more than a week. The foot of snow we got three weeks ago is more than half gone, but we’ve had a long string of very cold days, and enough warm days with some light rain mixed in to solidify what’s left into chunky ice. I suspect salt is still a net-negative here, even if you ignore the damage it does to rebar in concrete, but I’d say farther north is where the salt really hurts the pavement.