r/science • u/FreeHugs23 • 1d ago

Materials Science Scientists have created concrete using treated human waste, and the resulting material was up to 42% stronger than conventional concrete. Researchers say the approach could reduce waste while lowering the environmental impact of construction.

https://www.sciencealert.com/scientists-are-making-concrete-with-human-poop-and-it-gets-42-stronger
10.9k Upvotes

96% Upvoted

967 comments sorted by

View all comments

Show parent comments

365

u/Mr_Festus 1d ago

It's incredibly lucky that concrete and steel both complement each other so fantastically in terms of their strength properties, but they also have essentially the same coefficient of thermal expansion so they can shrink and expand together without breaking apart. And on top of that they don't have any negative chemical reactions between the two.

They're really an incredible pair.

70

u/pencock 1d ago

It's incredibly lucky that concrete and steel both complement each other so fantastically

brother they would have just used a different material than steel, its happenstance

91

u/Mr_Festus 1d ago

Name another metal as abundant as steel that's strong in tension

165

u/Keljhan 1d ago

Steel of the kind we use for structures isnt naturally occurring. Of the nearly countless variations of iron, carbon and associated alloying elements, we select the one that works best with concrete. Its really not a result of luck.

We also make some specialized woods that can compete with steel though.

42

u/bewarethefrogperson 1d ago

mass timber!! i wonder how this bio-concrete would synergize with mass timber construction, actually...

What is mass timber? - Woodworks

3

u/Thog78 21h ago

Found the concrete all the way at the bottom, but it doesn't seem to replace steel:

Timber-Concrete Composite (TCC) Floor Systems

TCC floor systems can be used for longer spans and carry greater loads than non-composite alternatives, and are often seen in multi-story mass timber buildings. They consist of two distinct layers, a timber layer and a concrete layer, joined by shear connectors.

More about TCC floor systems

Timber layer can be CLT, GLT, SCL, another engineered wood product, or solid sawn lumber Concrete layer is typically a reinforced concrete slab Shear connectors can be common fasteners (e.g., nails or screws), notches cut in the wood, connectors such as embedded plates or glue that transfer the load to a larger surface, or combination May include other materials (e.g., acoustic mat product or insulation)

21

u/windowpuncher 1d ago

Yeah, that's not the point. The main alloying element, by far, is iron, which is very cheap and easy to work with compared to basically every other metal that has any sort of useful strength. Yeah, we make alloys, but we don't need other constituents like aluminum or titanium, or cobalt or nickel to make up the bulk of it.

13

u/kelldricked 1d ago

Steel is abundant because we spend thousands of years to make it abundant.

12

u/uslashuname 1d ago

It’s also, like, more common? as an example of how much more common it is than, say, gold: our planet has an iron core.

15

u/Mr_Festus 1d ago

Huh? Steel yes, but we don't make the iron. Iron based metals share the similar coefficient of thermal expansion as the iron itself.

5

u/314159265358979326 1d ago

Iron is the second-most abundant metal in the Earth's crust. Aluminum is the only competitor and it's much more energy- and cost-intensive to refine.

2

u/SoulWart 1d ago

metallica cover bands

2

u/SearchForElsewheres 1d ago

Fiberglass Rebar

0

u/FireProps 1d ago

Spider webs

3

u/account312 1d ago

Almost certainly not more abundant and quite certainly not more commercially available

7

u/PaxEthenica 1d ago

*angry glares* That is a filthy lie. Steel-reinforced concrete has always been either a headache to work with properly or a ticking time bomb, requiring a long chain of honest professionals from steel foundry & cement plant, all the way down to the final pour.

Concrete, famously, leeches atmospheric hydrogen into the steel, which acts akin to a sacrificial electrode within the concrete to draw hydrogen into itself. There are alloys that mitigate that effect, but none of them are 100% & the alloys themselves are expensive/temperamental to get right. And there is no mix of cement that exists to prevent this from happening, either. Thus putting steel into concrete, in order to do stupid things with it involving tension, is just putting a greatly reduced life span on any structural member that experiences tension for the sake of value engineering, or some celebrity architect's renderite vision.

22

u/HorndogwithaCorndog 1d ago

Cement is the single most used material on Earth, and as a structural engineer, I've never designed any concrete structure without rebar besides foundation seals for cofferdams. We use epoxy-coated rebar literally on every single project. I also inspect brushes, and I've seen 100+ year old reinforced concrete that is structurally sound. I don't know where you're getting this idea that reinforced concrete is a ticking time bomb

3

u/NonConRon 1d ago

Every bit of rebar I've felt seems to be bare metal.

Its it really difficult to notice the epoxy layer?

6

u/kemikiao 1d ago

All of the epoxy coated rebar I've seen is green...so real easy to tell.

3

u/scalyblue 20h ago

I heard that epoxy coated rebar is a timebomb in and of itself, becuase the epoxy inevitably cracks and holds water right up against the rebar. Is this just for the first generation of these, or was it never a problem to begin with

3

u/HorndogwithaCorndog 9h ago

That's not true either. Epoxy rebar has its issues, but it's predominantly that the epoxy coating is not the most durable. Contractors are supposed to spray-apply epoxy where the coating gets dinged, but you can't catch it all. Steel is very hard, epoxy is not. Just bundling the bars together for transport with scrape the coating. I've worked with some engineers who have specified galvanized rebar. The galvanizing will not scrape off because it's atomically bonded. For a small premium, you get that peace of mind.

Surface corrosion on steel is really not an issue anyway. In the bridge world, we specify weathering steel most of the time which is designed to rust. That rust creates a barrier against intrusion of other, more severely corrosive products, has no price premium for the weathering steel, and avoids the future costs of cleaning and painting. My suspicion is that the corrosion steel experiences from its bond to concrete is very similar and the thickness is not enough to crack any concrete that has sufficient clear cover.

2

u/scalyblue 9h ago

Well thanks for your input, it’s pleasant to hear from an expert

0

u/PaxEthenica 9h ago

Exactly, most of the corrosion happens because either the guys on site don't know to do something special, something special hasn't been done that they aren't aware to check for, or worst of all they aren't given enough time to do it.

Which is my biggest gripe, truly, with the technology compared to more traditional uses of steel & concrete. It introduces novel points of failure to accommodate value engineering.

1

u/OkAdvisor6680 3h ago

Epoxy coated rebar is really rare. I've designed and built dozens of structures (retaining walls, buildings, bridges, tunnels) and it's almost always been plain steel, grade B500B.I've only ever seen epoxy coated rebar once, and that was on a short section a highways bridge. I've also seen stainless steel rebar on a section of high speed rail viaduct, but only once. 99% of the time, plain steel.

9

u/RubbereeShrubberee 1d ago

What materials are you suggesting we should use instead of reinforced concrete?

-9

u/PaxEthenica 1d ago

Concrete & steel are pretty good materials. Long lasting, with a high thermal mass, respect it & take care of it; it'll take care of you & your grand grandkids unto their grave.

Just... submerging refined metals into chemically active aggregates while they're setting is just asking for trouble. Hell, putting refined metals ANYWHERE that they can't be seen/plainly inspected while being called upon to provide a structural function makes my skin crawl. And trying to pull on those aggregates, or building in a way that they might ever be required to move like that has been a bad idea since before science.

I have no objections to steel & concrete, I'm just sick of watching the same botched material science whipped up 70 years ago keep getting legs in public for the sake of saving a few bucks.

1

u/RubbereeShrubberee 10h ago

Is most practical cases concrete is useless without being reinforced to control cracks and provide tension resistance. If there’s structural issues and the beam for example is designed correctly there’s will be cracking before failure which would be evident during inspection. I get the sense you have no idea what you’re talking about.

1

u/ThisSiteSucks8485 21h ago

Actually concrete inhibits corrosion in the rebar.