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A lot of the problems with concrete are actually problems with steel reinforcement. Concrete alone is incredibly long-lived and durable, see the Pantheon in Rom
by fredley 5y ago
A lot of the problems with concrete are actually problems with steel reinforcement. Concrete alone is incredibly long-lived and durable, see the Pantheon in Rome. However steel rusts, expands and weakens within decades, even when you do everything right.
- silicon2401 5y agoWhy don't we just use concrete by itself? Is steel necessary for using concrete the way we do in modern times?
- namirez 5y agoBasically concrete is good only for compressive loads. Under tension it easily cracks. The steel reinforcement adds tensile strength to concrete.
- codeOnMaster 5y agoIn fact, the concrete MUST crack in the tension zone in order for the steel to take effect and provide tensile capacity to the member.
- kube-system 5y agoOnly if you don’t prestress the reinforcement. https://en.m.wikipedia.org/wiki/Prestressed_concrete https://en.m.wikipedia.org/wiki/Prestressed_concrete
- acchow 5y agoHow often are skyscrapers built using prestressed concrete?
- codeOnMaster 5y agoYes, steel is required. In the design of concrete structure, concrete is assumed to have zero (0) tensile strength.
- PeterisP 5y agoIn order to use concrete by itself, you have to limit yourself to specific architectural structures which are very much unlike modern buildings - essentially, you can build quite high ancient-looking structures with everything held up on arches (which take up space and material), but you can't build a simple box-shaped building from concrete without steel rebar.
- mc32 5y agoIt is now possible to use wooden structural components to build modern skyscrapers. Maybe not 100 storeys buildings but 30 storeys is feasible[1]. [1] https://www.bbc.com/future/article/20171026-the-rise-of-skyscrapers-made-of-wood https://www.bbc.com/future/article/20171026-the-rise-of-skys...
- yodelshady 5y agoFor the way we use it, yes. Concrete works well in compression but is horribly weak in tension; a supervisor of mine compared it unfavourably to a ginger nut biscuit. Probably hyperbolic but you get the idea. Big, arch-y structures like the Roman pantheon work, because everything is in compression. But that's a lot of not-economically-useful space.
- mrfusion 5y agoWhy can’t we use carbon fiber or aluminum or anything that won’t rust?
- dTal 5y agoCost would be my guess.
- toss1 5y agoWe actually can - fiberglass rebar has many advantages over steel, among the first being that it does not corrode and expand. It's also much lighter and easier to transport to and on the worksite. It is starting to see widespread use in the construction industry, but far too slowly. A quick search on "fiberglass rebar" will return many manufacturers and articles.
- UI_at_80x24 5y agoTo avoid the problems of rust, epoxy coated steel is often used. It costs more, so in situations where the contract is won by the lowest bidder (unless it is expressly required), you don't get it. So as a result, buildings that could last 200 years start to crumble after 50.
- baybal2 5y agoEpoxy coated rebar was a trap, and is now being banned around the world. Paradoxically, the epoxy coat can make corrosion worse than the bare steel, let alone galvanised.
- ggcdn 5y agoNot sure why you're being downvoted, because you're right, atleast on the west coast of north america. My firm hasn't use epoxied rebar since like 1990, and we removed all references to it on our drawings long ago. The coatings were super fragile, and often damaged while being placed in the field. Then someone had to come along with magic paint and touch up all the cracks. But of coarse they would never find 100% of them...
- jcims 5y agoGreat video on prestressed concrete members - https://www.youtube.com/watch?v=P13Mau2VUWw https://www.youtube.com/watch?v=P13Mau2VUWw (Really good channel overall on civil engineering)
- marcosdumay 5y agoExplaining the asnwers in another way, yes, steel is necessary for both large horizontal openings without pillars and large vertical heights.
- deleted 5y ago[deleted]
- baybal2 5y ago> Concrete alone is incredibly long-lived and durable It can be, but usually isn't. People don't make concrete buildings to last 100 years. Normally, chlorides, and carbonation set the expiration date on a building. In China, 20 year old, if not 10 year old highrises are knocked down. Before I saw that myself, I thought that it's madness.
- RosanaAnaDana 5y agoBuilding with concrete to knock down in 10-20 years is actual madness.
- magicsmoke 5y agoThere's a kind of twisted logic to it. If your construction techniques are constantly improving and the high volume of construction incentivizes you to develop more cost efficient construction technologies, then planned obsolescence of buildings is like planned obsolescence of phones. Build soviet block apartments, knock down in 20 years, replace with prefab building compartments with wiring and plumbing built in at the factory. Then export the prefab technology worldwide to take advantage of the infrastructure boom in other developing countries. Isn't build fast, break fast, and learn fast the whole silicon valley ethos?
- brandmeyer 5y agoOn the other hand, building with an eye to increase density in a couple of decades looks like smart planning for growth.
- baybal2 5y agoNot so. Most of "knockdown 10 year old building, and rebuild" are done for very expensive, luxury developments with much lower density. Think of 2-4 mansion apartments per floor. Nothing else would've made economic sense. In comparison, the densest tower I seen in China had whooping 24 apartments per floor.
- hguant 5y ago
- tastyfreeze 5y agoBy using basalt or other composite rebars the concrete cancer problem is solved.
- RosanaAnaDana 5y agoWhat are basalt rebar? Never heard of these.
- masklinn 5y agoIt's what it sounds like: rebar made from basalt (basalt fibers specifically): melt the basalt, extrude it, mix it with polymers, shape into a REinforcing Bar. Problem's it's way more expensive than steel. It's also a pretty young material so I don't know how much safety and longevity information we have. Steel's well understood.
- tastyfreeze 5y agoThe expense is an upfront cost. Using basalt rebar we could be building 100 or 150 year bridges instead of 50 year bridges. We know basalt concrete structures wont fail from the reinforcement rusting away. We just haven't been using basalt rebar long enough to know the long term failure modes.
- bilbo0s 5y agoBasalt rebar does not have the tensile strength of steel rebar. During extreme stress, steel rebar allots time for escape, increasing the survivability of an event. The behavior of basalt is significantly different. Just different materials, with different performance profiles, that you can use to beneficial effect in different environments. But you have to know and understand the implications of the different performance profiles. I'm not going to do a whole engineering lecture here, but there is a really good concrete nerd who can outline this, and so much more, for you on his youtube channel if you're interested. He's a good engineer and I've been impressed with the accuracy of the material he presents as well as the accessibility with which it is presented. Tyler's channel: https://www.youtube.com/channel/UCrvfiHNDS_QI-FgKQSmTITQ https://www.youtube.com/channel/UCrvfiHNDS_QI-FgKQSmTITQ More detailed explanation of tensile strength implications in structures: https://www.youtube.com/watch?v=thUZImUTZn0 https://www.youtube.com/watch?v=thUZImUTZn0
- angst_ridden 5y agoHowever, in earthquake country, we don't build with unreinforced concrete for the obvious reasons.
- masklinn 5y ago> Concrete alone is incredibly long-lived and durable, see the Pantheon in Rome. That requires massively overbuilding the concrete structure, and many of the things we mold concrete into simply would not be feasible in unreinforced concrete, it's only strong in compression. Plus the Pantheon and friends are good examples of survivor bias, for the on Pantheon there are hundreds of insulae which didn't survive.
- jbay808 5y agoThis is a total misunderstanding of survivorship bias. It would be like encountering a 1,000 year old man and attributing his age to merely survivor bias rather than investigating whether it has something to do with the magical beans he ate.
- mmmBacon 5y agoHere’s a good explanation on the differences modern concrete and Roman concrete. It turns out they are pretty different materials. https://www.sciencealert.com/why-2-000-year-old-roman-concrete-is-so-much-better-than-what-we-produce-today https://www.sciencealert.com/why-2-000-year-old-roman-concre...