Move Over Steel, Carbon-Reinforced Concrete Is Here

Reinforced concrete is the miracle material which made possible so many of the twentieth century’s most iconic structures, but here in this century its environmental footprint makes it something of a concern. As part of addressing this problem, a team at TU Dresden in Germany have completed what is believed to be the world’s first building made with carbon-reinforced concrete, in which the steel rebar is replaced with carbon fiber.

New materials are always of interest here at Hackaday, so it’s worth reading further about the nature of the reinforcement. The carbon fiber is woven into a mesh, or as a composite material that mimics existing rebar structures. These two types of reinforcement can be combined in a composite to produce a concrete structure much lighter than traditional steel-reinforced ones. If you page through the architecture critic description, it’s this lightness which has enabled the curving structure of the Dresden building to be so relatively thin.

The carbon saving comes presumably in the lower energy cost from not smelting iron to make steel, as well as the need for less concrete due to the lightness. All we need now is a low-carbon replacement for Portland cement.

Want to know more about concrete reinforcement? We’ve got you covered.

62 thoughts on “Move Over Steel, Carbon-Reinforced Concrete Is Here

    1. In “Ability of recycling on fiber reinforced concrete” (doi:10.1016/j.conbuildmat.2014.01.060  ) the authors conclude that after crushing, the composite filler (steel or polymer fiber) ends up in the mortar fraction and the aggregate can be recycled.
      Overall, urbanite (I love that name) seems to be used almost exclusively as a filler in road construction. With carbon fiber fillers being environmentally rather inert, they can probably just be left in there.
      It’s nice to get the rebar and perhaps the aggregate back (if it can be obtained in a clean fashion), while the rest is down-cycled.

        1. Silicosis is already a known issue with concrete. I’d _imagine_ adding fibers to that poses no additional risk if you’re already following existing safety precautions. I’ve got no professional experience in the area but I’ve dabbled with GFRC and stuff for hobby use.

        2. Carbon fibers themselves will have 4-7 µm diameter and an internal structure that isn’t very conducive to producing high aspect ratio fragments (they like to cleave perpendicular to the fiber, crack propagation along the fiber is not favored).
          That said, one should avoid inhaling swarf from sanding, cutting and milling of CF composites.
          Luckily, carbon fiber isn’t as terrible as abestos, from which there are absurd amounts in buildings all around the world… and you can bet that part of it ends up in the concrete recycling stream.

    2. Concrete recycling does not truly exist, for any type of concrete. It can be crushed and potentially get used as filler in new concrete, but there is no process which would return the cement back for re-use, so you always need to add new binder to it.

      As for other uses, like road and landscape construction, that’s not “recycling” – that’s just landfill by a different name.

      1. Recycle, reduce, reuse are separate categories among themselves, so I’m not sure it’s fair to say that reuse is the same as landfill. (I doubt you can successfully build a very useful or sustainable road over the top of single use plastic bags, diapers, and McDonald’s Happy Meal toys.)

        I went with my kids to a “Discover Engineering” event our local university this weekend, and some civil engineers were there with our local Department of Transportation. I asked them this same question about, “Can concrete be recycled?” Their answer was (in short), it could be (there is a process that has been built that could reclaim cement), but it’s not cost effective/efficient compared to just making new cement and using the remainder as aggregate.

        There’s a lot of limestone in the ground, so probably don’t hold your breath on reclaiming in the near future (even if it could be done.)

        1. Although it is literally landfill. You can’t necessarily build housing on top, but for industrial uses and roads, it’s perfectly fine. There’s a literal mountain of trash nearby that got turned into a ski slope.

        2. >There’s a lot of limestone in the ground

          Limestone alone makes “traditional” cement which sets in contact with CO2 in air, which makes the setting time essentially “forever”. The inner parts of some ancient Roman structures are still soft, which is why they last such a long time: when there’s a crack, the cement seeps out and re-seals it. They accidentally invented self-healing concrete.

          Hydraulic cement that sets by reaction with water, which is the overwhelming majority of cement/concrete mixes, is made by first burning limestone and then reacting it with silicon dioxide, then with calcium, aluminium, iron, etc. oxides. This process drives the CO2 from CaCO3 out and it won’t consume it back when the hydraulic cement sets. This makes cement production a huge CO2 emitter even if the energy to run the process was renewable and emissions free. It also consumes iron, aluminum, etc. that are not recovered.

          That’s why concrete recycling would be kind-of a big deal.

          1. The Romans didn’t “accidently” discover it, we have done decades and more of research into how they made it, and there are numerous very carefully described processes involved. Only very recently have modern science figured out what they knew.

    3. I was on a demolition site where the operator had a concrete crusher. Chunks of concrete slab would look like they were melting as they were broken down in the maw of the machine. Rebar was magnetically separated and spit out the side. The operator was selling the crushed concrete to the county to use as road surface at the landfill. Apparently with rigid and well designed machinery the horsepower demands for crushing concrete (or rock) are surprising low.

  1. producing carbon fiber is not what one would call a carbon neutral process. basalt fiber however has been used as rebar for years and is way less energy intensive, and can be recycled. having said that, so can steel.

    both basalt and steel are heavier than CF, obviously, but basalt fiber is a bit better than glass, which also can be used as reinforcement. besides the novelty factor, I kind of fail to see the benefit here tbh

    1. As long as we don’t have to redo it due to rust, I don’t care if it’s 100% carbon neutral (and iirc the reaction when concrete sets releases a boatload of carbon dioxide anyway).

      Especially with seaside infrastructure, the longevity of the product could lead to considerable environmental benefit.

    2. You’re not wrong but I think the idea here is to prepare for when we have a seemingly endless supply of carbon (as a result of direct carbon capture) which may require less energy than producing new steel. In theory it could be done with solar power that would need minimal battery support.

      1. CO2 =/= carbon fiber, the process to make carbon fiber does not use CO2 as a feedstock. Also carbon capture only works when you have a surplus of clean energy, anything less and it makes more than it captures and it only works as a distraction so industry can continue on polluting

  2. Because for some reason many architects follow (amongst others) LeCorbusiers brutalist idea’s that architecture should clash with it’s environment rather than co-exist with it without distracting.

  3. doing concrete that realy lasts is hard
    its chemicaly reactive for a very long time
    if suitable agregate is not availible then then choice
    of re enforcemen is moot,it just crumbles from the salts
    developed internaly

    1. A lot of Roman concrete is still going strong 2000 years later.
      It’s not as economical as modern concrete, though, and that’s really the problem. There’s good concrete, and there’s cheap concrete, and guess which we’ve chosen.

      1. The Romans did a bunch of tricks with their concrete, such as having burned limestone “nuggets” mixed in. Previously it was thought these were just sloppy workmanship, but it turns out they added them on purpose, because leaving inclusions of burned lime in the mix makes it self-healing when the concrete cracks and water seeps in.

      2. Most Roman concrete crumbled to dust millennia ago. What’s left is the stuff that happened to be mixed perfectly, and had the perfect weather to cure properly, the builders weren’t cutting corners that day etc. etc.
        There’s a survivorship bias going on.

        1. The things weren’t built in a day. Any significant building would be made over many years, decades, centuries even, using materials from various sources that became available. The fact that they’re still standing is a testament that they knew what they were doing and kept it consistent.

        2. While that’s true, modern concrete barely lasts 100 years, so those survivors are ~20x older than our best attempts. The basillica dome versus a WWII bunker, for instance.

          1. Those WWII bunkers aren’t really a fair comparison, they were only planned to last for a couple of years. They tended to be built in a hurry, often by unskilled builders.

  4. A bit off topic, I know. We grew sapphire fiber that was then chopped into short pieces, mixed with ceramic slurry and cast into rocket engine parts. That made the parts tough instead of brittle and could take very high temperatures. Our task was to grow the fibers 200 at a time instead of our normal 25 slot dies and crucible. We never did get the economy of scale, just too hard to handle the numbers needed.

    1. The question with any building material, but especially the hard to recycle composite material is rather more how good its material properties are than how environmentally friendly it is during the construction phase. As a building should in pretty much all cases last decades and often century if the material is any good for construction in that region.

      And even if we assume all of them are the right type of material for the job of being an enduring structure if this can do the same job as a 1M thick concrete with bamboo/steel wall at say 10cm thick, and because its lighter the whole building can actually get thinner too so that 10cm thick wall now only needs to be 8cm or something that is a huge saving even if the material costs more to make per ton – you are using so much less of it! Not to mention gaining back huge volumes of building footprint that can be used for added insulation, pipes and wire conduits, maybe part of a more passive HVAC system etc, and so further reducing the cost to run and maintain the building.

      You could use the least eco freindly to create material imaginable and still have an ‘eco building’ if its got a design that will last in that area and be low energy cost in use and maintenance. Yes the upfront cost is obviously going to be high, but eventually it has more than paid for that and still going strong while the bulk of the timber eco houses have cost a fortune in maintenance and are now needing to be replaced or extensively repaired again… Not saying wood or your suggested bamboo-reinforced concrete doesn’t have merits for an ecofriendly focused build though, just that the more costly but enduring and durable materials can as well!

      1. Modern building codes and efficiency regulations do not permit you to build a “century building” anymore. Arguably, modern comfort standards don’t either.

        The main problem is all the plastic barriers and insulation you have to use to prevent air leaks, because the plastic degrades in a few decades and crumbles away.

        And finally, safety margins don’t allow you to reduce material thickness arbitrarily. Suppose you endure chemical weathering, chipping from temperature changes, etc. at a certain rate. Well, an 8 cm slab is going to wear out proportionally faster than a 10 cm slab because there’s less material to remove.

        1. I see nothing in building regulations (here anyway) that really make it difficult to build enduring and comfortable, it just costs more to build that way and makes the building developers less profit, so they don’t. (and replacing the odd window seal, running new wiring as required etc isn’t at odds with that IMO)

          When you can probably sell a new building that won’t last much over 20 years without maintenance for only a fraction less than the one that will outlive your grandkid’s grandkid and in all that time only want minor cosmetic repair and change of use tinkering… Especially when the modern style for so many buildings seems to be a rendered or clad finish so the new owner can’t actually see how you put it together to know. Also in many places globally there is a huge demand for housing – when you need a roof over your head in a market full of competitors the prices skyrocket and being choosy on the lifespan and maintenance requirements of building is a position its hard to have – as long as it is not just a thin cardboard box that is a problem for the you of so many decades in the future…

          And if the material is ‘better’ for that role then you can shrink its thickness and still have just as much safety margin. Maybe even more safety margin – In more than a few cases buildings have used massively overkill amount of a material because it doesn’t look right or safe against the old buildings style to use the new (or newly affordable) materials and techniques near their limits, not because it isn’t still massively overkill. And assuming the weathering effect is identical if one version is able to carry the loads with less thickness then it doesn’t need as much extra thickness for the safety margin anyway – that extra 1cm of thickness added is perhaps worth 1.5cm of the other stuff…

          1. You also have to look at urban development. The cosy sub-urban neighborhood is going to get steamrolled over in 30-40 years as the city expands, so there’s no point in building the proverbial brick shithouse.

          2. > then you can shrink its thickness and still have just as much safety margin.

            These things don’t go hand-in-hand.

            For example, in cars, UHS steel is used to make the box beams out of thinner sheet, but the speed at which it rusts has nothing to do with the case. If it rusts, it will rust at the same rate, so where you previously had 2-3 millmeters of steel you now have 1 millimeters and it will go through 2-3 times as fast.

            With concrete, you can have water and frost cycles erode the material off the top just the same whether it’s rebar or carbon fiber reinforced. When you use 8 versus 10 cm wall thickness, well you just reduced the lifespan of the wall by 20%

          3. >steamrolled over in 30-40 years as the city expands

            You say that Dude, but across the UK for instance a huge supply of the houses are over 100 years old and still going. Cities don’t always have to expand in footprint either, at some point it makes no sense to let them get bigger either as the centre can’t take the increase in surrounding population and would be impossibly expensive and time-consuming to fix. Plus even if they are expanding they are much much more likely to expand over the oldest, poorer neighbourhood that even if it was build to last is already going to be old enough it wasn’t a waste to build it properly (or the greenbelt, but that really shouldn’t be allowed too easily).

            Also once you get to the buildings that are actually in ‘the city’ odds are they won’t go anywhere until they are unfit for use, and even then are more likey to be refurbish at huge expense ans its still cheaper than trying to deal with brining it down first. Building the high rise that I assume you mean by ‘the city’ costs too much, and demolishing it first for no reason just to put a slightly newer bigger one up costs even more too much! So building those to last and be maintained is just common sense.

            My point on safety margins is even if you assume weathering is identical (which it probably won’t be) is that 1mm of the better material is worth structurally the same as 1.5mm, 2mm of the weaker stuff – so you most likely have massively more material to loose before it actually matters, even if you made the part thinner! As people won’t accept a part made near the better materials actual limits when they are expecting the chunkiness of the old – it looks wrong, and if you are building with endurance in mind you can still make it massively thinner and have the same thickness of excess to be lost to weathering if you wish. So you certainly don’t have to loose lifespan.

            Also water and frost cycles shouldn’t erode a properly designed and finished concrete structure anyway – if you make the concrete right and don’t design in some lovely spots for water to fill and then destroy with expansion as it freezes (or have the rusting reinforcements blowing it apart from the inside to then make it easier) it won’t matter at all – lots of stupendously old concrete construction out there that isn’t being eroded by such things in a way that matters even on the longer timescales. For it to really be an issue mistakes have to be made somewhere at the very start of the buildings life or some damage must have been done and not fixed.

  5. Quote: “All we need now is a low-carbon replacement for Portland cement.”

    And we should name that stuff “wood”, because that would avoid confusion with existing solutions.

    1. Most graffiti looks better than a bare concrete wall, even just a wall full of tags can brighten up a grim brutalist building.

      I swear architects should be forced to show “aged” renders of their gleaming concrete creations showing the cracks, stains, dirt and water marks because that’s how the damn thing is bound to look in a few years.

  6. I believe it’s not a “move over” but rather “meet an ambitious newbie”. As always, there are a lot of things that should be learned/solved until it becomes a “high rank” one.
    The structures are not going to be “much lighter”, since the steel part contributes only about 5% to the whole.

  7. Always trying to reinvent asbestos. I would wager the dust from this product is 10x more dangerous than concrete alone, this is a solution looking for a problem, that will create a much larger problem later.

    1. Carbon fiber is the opposite of brittle. You can bend a fishing rod in half and it will regain its shape. Yes, if its ultimate tensile strength is exceeded, it will break, as will every material. The difference with carbon fiber (and most composite materials) is that they have little to no plastic region before failure, i.e. no permanent deformation that is often used as an indicator that design loads have been exceeded. When a steel girder is used as a beam, the design consideration is to keep it in its elastic region. Any deflection under load is thus recovered when the load is removed. If the load is excessive however, the steel deforms plastically, and your beam will be permanently bent. That plastic region is large for steel, it is likely that the beam becomes unfit for purpose well before its ultimate tensile strength is reached.

      However much you might be tempted to think of this as a good thing, consider another viewpoint:

      The yield strength of steel (where it stops being elastic, and enters its plastic region) is ~350MPa (50,800psi), where its tensile strength is 420MPa (60,900psi). 16% of its ultimate tensile strength is effectively unusable because no one wants to drive on bent bridges. Meanwhile, carbon fiber has no listed yield strength because it never yields – 100% of its ultimate tensile strength is usable. That ultimate tensile strength is about 3.5GPa (500,000psi), over 8 times that of steel, and 10 times that of steel’s useful (yield) strength.

      In the real world, financial liabilities due to failure have the potential to be many times the extra cost of carbon fiber rebar. Not spending $45M to repair a 40 year old parking garage suffering from rust-induced concrete “cancer” would have been plenty of motivation to spend an extra $2M on its original $28M cost, had carbon been available then. I’d hope that some civil engineering researchers are keeping tabs on the economics, because from my layman viewpoint it appears to be a slam dunk for just about any infrastructure project, even before considering public and traffic disruption.

      1. Steel is not one material. It can be alloyed, heat-treated and work-hardened to behave exactly like carbon fiber – but then it will also fail catastrophically like carbon fiber instead of just going out of shape from an excess loading.

        You really don’t want a material with the yield strength and the ultimate strength being the same, because the energy you load up in the elastic deformation is released when it breaks, and carbon fiber structures can store a whole bunch of energy to the point that they can “explode” when they fail. Steel will consume the elastic energy into the plastic deformation.

        1. And another thing is the Young’s modulus of the materials. Steel is at 220 GPa give or take. Carbon fiber reinforced plastic is 30 – 50 GPa. That’s very close to plain concrete at 15-30 GPa. That’s right: concrete is actually very bendy – it just doesn’t take a lot of tension before it cracks.

          Why is this a problem? Because the Young’s modulus is like the spring constant of the material. The rebar that binds the concrete together should stretch less than the concrete so it would actually carry the tensile load. Otherwise it will just stretch along and do nothing.

          The other point is that carbon fiber composites are 4-5 times bendier than steel: they just stretch and flop around a whole lot more – or – for the same amount of deformation, steel takes on 4-5 times as much load. Holding a carbon fiber rod in your hand, you get the illusion that the material is stiff because you’re fooled by the light weight of it.

  8. This is massive as concrete typically fails in one of two ways: 1. Cracking due to thermal expansion. 2. Cracking due to the expansion of the reinforcement as it rusts. The first can be mostly avoided through good design, using expansion gaps to allow for concrete to grow and shrink without tearing itself apart. Carbon reinforcement could solve the second.

    Even without any change in the carbon economics of its manufacture versus steel, the advantage of vastly increasing the lifespan of infrastructure such as bridges can not be underestimated. If we can double it, we effectively halve the carbon cost of infrastructures.

    This has the potential to increase the lifespan well beyond that though. Romans used a huge amount of concrete to achieve the strength required in their construction projects because they didn’t have reinforcement. The domed roof of the Pantheon is 43.3m (142ft) in diameter. The concrete at the base of the dome is 6.4m (21ft) thick, and at the top near the oculus 1.2m (3.9ft). It’s the largest non-reinforced concrete structure on the planet, and 2000 years old. The initial carbon cost may have been massive compared to modern reinforced concrete construction, but it probably wouldn’t have lasted much beyond 100 years if it had contained steel.

    It is believed that one of the major contributing factors leading to the 2021 tragic collapse of Champlain Towers South condominiums in Surfside, Florida was concrete failure due to reinforcement corrosion. Water from the pool deck seeped into the concrete foundations. That building was merely 40 years old. There are dozens of other factors still being investigated, but none so far reported as bigger. If that is borne out in the final report, it wouldn’t surprise me to see carbon take over in big condo projects pretty quickly as I am sure that insurance companies will be happy to remove that financial risk. The CTS condo association had a $15M remediation plan to pool deck issues noted in 2018, but hadn’t started them. That is an average of $110,000 per condo unit so financial risk will surely factor into condo buyers’ calculations in the future too.

    On a frequent basis I drive past bridge supports with visible rusting rebar. The bridge in question is part of a parking garage entrance ramp for my local transit station that opened in 1983. By the mid ’90s water damage required fixing, and in 2015 urgent structural fixes were needed. It’s currently undergoing a $45M multi-year renovation where they’re replacing a lot of the concrete.

    A non-rusting concrete reinforcement has so much potential to both save money and reduce the ecological impact of concrete construction. We’d be negligent if we didn’t insist on it for future infrastructure projects.

  9. Hardly the first carbon-fiber reinforced concrete building. The quantum computing lab at waterloo (and I imagine many other scientific facilities worldwide) had it a decade ago. For them it was because they were worried about electromagnetic interference.

  10. It would be very interesting to look at mechanical properties of carbon fiber reinforced concrete in terms of higher temperatures. One of the big advantages of steel reinforced concrete is its good fireproof characteristics.
    Steel and cement production have approximately the same carbon footprint yet concrete is utilized almost uniquely in construction whereas it’s true for only half of the steel produced other half in others industries. Therefore cement production is 2 times more carbon heavy than steel.

  11. What about Glass Fibre Reinforced Concrete (GRC) ? It is proven technology and relaively cheap in contrast to everything carbon-fibre related. They have even built ocean ship’s hull of this material. TiO2 treated glass fibres are rock stable in the environment of concrete and people even managed to handle this techology in the stagnant East-European EU countries, see here:çade-cladding/glass-fiber-reinforced-concrete.html

  12. I’m really doubtful about CF reinforced concrete as becoming mainstream.
    Yes it sounds cool, but basalt fibers seems to make more sense. 2/3 the tensile yield of CF, 1/3 of the price.
    It’s also claimed to be more environmentally-friendly than CF of glass fiber, but I didn’t see any studies to fact check that claim.

    Speaking of studies, the one linked only compared 3 bridge designs, one of each technologies.
    Given that only part of the design considerations are listed (they did not say if expected lifetime, or if weather conditions were comparable), and how close those 3 were together in term of environmental impact, the study really doesn’t prove that CF reinforced concrete is globally better (or worse) for the environment.

  13. Skipped all the comments, I know this is in Germany and I’m not sure about the wind loads or anything. But surely since carbon is much lighter than steel, therefore the structure would actually be weaker? Something has to give?

    Great idea – was curious is anyone had a clue about the actual structural integrity of a structure built like this.

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