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I had some interesting thoughts while walking my dog this morning.

Let's assume there has been some catastrophic event that leaves the world habitable, but kills off a large amount of people (99% of the world population) and breaks down governments and global supply chains. All that are left are small groups of survivors that band together and might be the only people for dozens of miles.

One of these groups happens upon a subdivision and starts ripping apart houses looking for useful things (let's say they've set up in one of these houses or in a nearby area). My expectation would be that most of the contents of the house would be the first to be pillaged (food, appliances, furniture, etc), followed by the materials that made up the house (drywall, wiring, wood beams that make up the supports and walls), but I can't figure out if the concrete that makes up the foundation and driveways would be useful in any way.

Is there any good use that can be made of concrete that would be worth breaking it up and hauling it out?

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    $\begingroup$ I can't submit an authoritative answer on the subject, but there are two key ways concrete recycling are done. The most obvious is using busted up concrete as aggregate solids so less liquid cement is needed to form new concrete. The other is to take pulverized concrete, smash it into dust and heat the bejesus out of it so it actually becomes concrete mix again. More or less. $\endgroup$
    – user8827
    Jul 18, 2017 at 0:26
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    $\begingroup$ @SeanBoddy Spot on. Recycling concrete into cement is currently not worth doing, but it is smashed up and used as aggregate. In a world with sacks of cement in every abandoned DiY store existing concrete would have value as aggregate. The answers below all miss the fact that this is already done, and would continue to be! $\endgroup$
    – Grimm The Opiner
    Jul 18, 2017 at 8:08
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    $\begingroup$ The most useful thing to rip out of a house is the house. A house is designed to be shelter, which is something humans always need. The best house is an intact one. Anything you rip out of it will only diminish its usefulness in that regard. $\endgroup$
    – Harper - Reinstate Monica
    Jul 18, 2017 at 15:27
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    $\begingroup$ You'll need to kill off a lot more than 99% of the population if "small bands of survivors" are to be "the only people for dozens of miles", except in regions that are already sparsely populated. A 24-mile radius circle is 1,800 square miles, so if your "small band" is 18 people, that's one person per 100 square miles. That's less than 0.01% of the current global population density (looking at land only, excluding Antarctica) of 142 people per square mile, so you need to kill more than 99.99% of the current population. $\endgroup$
    – Mike Scott
    Jul 18, 2017 at 15:38
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    $\begingroup$ If 99% of the population is wiped out, we would have concrete (and a lot of other stuff) in large abundances. So unless you make it something that can be consumed... $\endgroup$
    – NoDataDumpNoContribution
    Jul 19, 2017 at 8:02

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Not really.

In a (horribly reductive) nutshell concrete is essential to modern engineering because it allows us to create an arbitrary shaped rock, on demand, anywhere in the world. Once it's cured it's effectively a weird shaped rock. While rocks are useful for construction there are easier rocks to quarry especially if your post apocalyptic survivors don't have access to the tools necessary to break up concrete. Modern cement will also slowly degrade from exposure to water, making it a poor choice as a structural stone.

Like any rock it could be in the way of something, like a field or a foundation. If your survivors are settling an area eventually some of the existing concrete may be in the way of agrarian activities and will need to be moved, probably the minimal distance possible because moving rocks is hard work.

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  • $\begingroup$ I figured as much.....there goes my plan for the survivors to descend upon a suburb and render it back to nature by taking literally everything. $\endgroup$
    – Marshall Tigerus
    Jul 17, 2017 at 20:18
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    $\begingroup$ People do break down concrete in order to get access to the soil underneath for gardening. I have a book on urban gardening that suggests that people should break up concrete back yards to do exactly that. You might have your survivors break up the concrete in order to clear land for farming. Rather than hauling it away, they could do what the colonial New Englanders did when clearing rocks out of their gardens: pile it up to make rock walls between fields. $\endgroup$
    – workerjoe
    Jul 17, 2017 at 20:23
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    $\begingroup$ @MarshallTigerus Nature is a vague and arbitrary concept. Modern paved roads are carefully constructed out of layers of sand and gravel that have been shipped on site. Even if you removed the asphalt and concrete it would be pretty obvious where the roads used to be. $\endgroup$
    – sphennings
    Jul 17, 2017 at 20:35
  • $\begingroup$ I am not sure there always be some "easier rocks to quarry" $\endgroup$
    – Alexander
    Jul 17, 2017 at 20:39
  • $\begingroup$ @Alexander In a horribly reductive nutshell there will always be easier rocks than concrete rubble. I'm sure there are some edge cases, or field expedient solutions, where concrete will be useful. In general however any other rock is going to as good or better. $\endgroup$
    – sphennings
    Jul 17, 2017 at 20:45
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Not the concrete, but the rebar would be useful long after societies collapse.

If you want a source of steel and you have already recycled all the cars and large appliances, foundation rebar would be quite a plentiful source of steel for your re-emerging blacksmith industry.

Also being imbedded in concrete would offer protection from the elements making the steel last longer against the environment, so after all the cars rust out, you could still move into an uninhabited area and break up a foundation for some steel. These are the iron mines of the future.

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    $\begingroup$ rebar is also ideally shaped for fences, cages, furniture legs and supports, piercing weapons or traps (spear tips, arrows, and the like), and a variety of other applications. You could break it out of the concrete and use it without having to smelt it into a useable shape. $\endgroup$
    – Logan Kitchen
    Jul 17, 2017 at 22:47
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    $\begingroup$ If would suggest that if they've reached the point of having to extract the rebar from concrete this is a survival group who aren't going to be surviving much longer. It's very high energy cost for not a lot of reward. $\endgroup$
    – Separatrix
    Jul 18, 2017 at 9:17
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    $\begingroup$ Rebar in driveways? Some foundation types do have some reinforcement, but that's the minority, and usually mesh rather than bar, but house driveways are almost exclusively bulk concrete. $\endgroup$
    – Pete Kirkham
    Jul 18, 2017 at 9:25
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    $\begingroup$ There is no rebar in driveways. $\endgroup$
    – Fattie
    Jul 18, 2017 at 12:10
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    $\begingroup$ Eh? Rebar rusts, even when it's inside concrete -- indeed, that's the primary limit on concrete structures' lifespans (which, unmaintained, is considerably shorter than one might hope). $\endgroup$
    – Charles Duffy
    Jul 18, 2017 at 23:24
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Maybe.

Cement is sometimes a conveniently shaped rock. A drive way is a thin wide rock. Foundations are often round with some metal attachment points embedded in them. If labor is not an issue digging it up and moving it by hand might be worth it for some things.

Rarely is cement more than a foot thick, and generally can be demolished with hand tools. If your people have no experience quarrying and there are no obvious convenient sites nearby they might take cement as low hanging fruit.

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  • $\begingroup$ Note that Cement is not the same as Concrete. And breaking up a even a non-reinforced concrete foundation with hand-tools is a remarkably difficult job. $\endgroup$
    – RBarryYoung
    Jul 18, 2017 at 20:51
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Having done some concrete work over the years, I can tell you that it's amazingly​ labor intensive even with air hammers and gas powered saws.

Things would have to be pretty bleak, post apocalypse, to make the amount of energy expended worth it...

Foundations will typically be around 6 to 8 inches thick and be reinforced with rebar. Cutting or hammering through takes a lot of time and fuel even with the best tools. Basically any resources you could get by dismantling a foundation you could get much more easily by raiding a construction supply manufacturer or warehouse.

Big box stores will be useful alternative in the beginning, but I would look further up the supply chain. Concrete and asphalt are heavy and shipping is obviously time sensitive, because asphalt cools and mixed concrete sets, so most cities will have their own local manufacturer. These manufacturers will usually have extraordinarily large stockpiles of both raw materials and ready to use product.

Rather than dumping time, fuel, and effort into breaking, moving or otherwise recycling concrete slabs, you could put in a tiny fraction of the required resources into finding and transporting materials from the manufacturer just like we did it in the good old days before the apocalypse.

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I don't really understand the chemistry of concrete, but one of the main ingredients is cement. For Portland cement, Clinker is the key part. It takes huge amounts of energy to make Clinker. When you combine the Belite in Clinker with water and time, you get Portlandite, which is a rare, but natural occurring, mineral. With a little bit of handwavium, maybe Portlandite, or the alkaline solution it forms when dissolved in water, could be something that is critical for survival.

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  • $\begingroup$ pretty nifty !! $\endgroup$
    – Fattie
    Jul 18, 2017 at 12:11
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I'd not think hauling it away would be useful (easier to just dig up some already loose gravel if you need small stones to pave a path or something like that). but why not leave it in place and use it as a foundation for your own buildings?

Concrete tiles used for terraces and parking lots are a different matter. You can just lift those out and use them again elsewhere.

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Ever hear of a tip-up building?

It has walls made of concrete that are tipped up to vertical, like raising a barn. Well chosen sizes of concrete slab could be dug up and tipped up into walls of a structure. The ability to move large pieces and the height of the structure would be very dependent on the strength of the concrete, obviously, which is to say its reinforcing if any.

I surely wouldn't want a roof made of found concrete. So you'll have to come up with something for that.

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Maybe if the land is poisoned by something in the air during the near-extinction event, only the soil formerly under concrete driveway could be clean enough to plant crops etc. Perhaps rain has pushed the contamination deeper than topsoil? Tearing up a drive would be easier than having to dig up buildings to get fresh soil.

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Instead of using concrete as a material, could you use the concrete slab just as it is?

I've been to Venezuela pre-Chavez/Maduro, and one thing that stuck out to me at the coffee farms was the use of large slabs of concrete for drying coffee beans. On a sunny day, the surface will become dry very quickly, and it will retain heat at night, making it a great surface for this application. With a little scavenged fencing to keep the animals off, it would be great for drying fruits and meat.

Next, what applications would require a hard surface that plants will not grow on? If you have an intact basement, damming up any sides/windows would make for a nice cistern, if you live in an area containing houses with basements. If you have any sort of chisel, you might also write something on the concrete.

Finally, for the "stone" application, you would most likely be looking for some sort of concrete drill (just the bit - the rest would be manual) to drill breakpoints, after which you would crack pieces off with a hammer. This is the lowest-energy way to break up concrete, but per the other comments this would not be effective on foundations, so much as on patios or other, thinner pads.

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Concrete slabs might be useful as they're essentially big flat rocks that you just have to pick up and carry, rather than quarry first, then pick up and carry. How big a slab will be useful would depend on how much weight the survivors have the equipment to move around.

Things that are embedded in the concrete might be worth the effort to tear it apart for, once other sources have been exhausted. (But probably not before then, concrete is tough to break up.)

Once the survivors have enough energy at their command, the concrete can be "recycled" by heating it until the water is driven out of the chemistry and it reverts to pebbles, sand, and cement. This does take fairly high temperatures, but it could be done with a solar furnace for a relatively small investment in reflectors. Of course, concrete is still heavy, so just being able to remix and recast it doesn't help much if they don't have the manpower to haul it to where they need it.

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I have to say that concrete could be used as protection

This first scenarios would be, there are some structures left to use as shelters.

They could use building or a house capable of protecting them, maybe with a roof or so, and check if they could live there for as long as they could because having four walls is a good protection against the weather and wild animals. They could also use it while gathering energy to partake other tedious tasks, maybe get a location where resources are close.

the second scenario would be, there are no buildings or houses standing, just the concrete and the driveway.

I'd still think that they should survey an area where there would be lots of concrete and other resources, then eventually, prepare a settlement with whatever materials they could gather and using the concrete as crude walls. It may not stave most wild animals but It is still better than nothing.

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A small group is going to have a hard time recycling concrete into something useful, unless they can get their hands on industrial demolition equipment (excavator with huge drill bit, rock crusher). Or lots of explosives.

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No. By the time the concrete is potentially useful, they will not be survival groups, but new civilizations. At which point, they will probably be a bigger nuisance than benefit.

As a driveway, they are large rocks that are basically impossible to deal with due to their weight. As smaller rocks, they are more easily obtained elsewhere.

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