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Comment by fredley | Hacker News Reader

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.

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> 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.

Building with concrete to knock down in 10-20 years is actual madness.

There'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?

On the other hand, building with an eye to increase density in a couple of decades looks like smart planning for growth.

Not 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.

Part of that is because they have to other wise the building would collapse on its own.

A big issue people don't talk about is that a lot of the concrete China makes is subpar and not up to spec by US or EU standards - they don't use the right (aka more expensive) grade of sand, so the concrete doesn't bind as strongly as it should. The sand used to make concrete has to have rough edges, something about the surface area, which ironically means that desert sands aren't very useful - they've been rounded by wind erosion and rubbing against other sand grains.

It’s not surface area so much as the faceted sides of crushed sand lock together when compressed instead of sliding past each other

What I was wondering some time ago...couldn't you sieve desert sand for larger grains and then crush them?

> In China, 20 year old, if not 10 year old highrises are knocked down.

Isn’t it part of their 12% year on year growth strategy? Whatever it takes to make these numbers happen? Which is also why China builds cities designed for millions of people that remain ghost towns once they’re done: they’re not really built to be lived in but to boost the economy activity via construction costs.

To some part, it must be. I don't believe that even in communists China there must be somebody who don't realise how bonkers it is.

But economic gain is there too. The surreal real estate bubble makes knocking down even 10 year old buildings profitable.

Here’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-concre...

By using basalt or other composite rebars the concrete cancer problem is solved.

What are basalt rebar? Never heard of these.

It'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.

The 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.

Basalt 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

More detailed explanation of tensile strength implications in structures: https://www.youtube.com/watch?v=thUZImUTZn0

Nice! Thank you for a few hours of entertainment.

LOL!

No problem.

Tyler's definitely a nerd, but he's one of those magnificently brilliant nerds that you can't help watching for hours at a time.

> The expense is an upfront cost. Using basalt rebar we could be building 100 or 150 year bridges instead of 50 year bridges.

But is that something we're trying to build? As the saying goes, it takes an engineer to design a building that's barely standing.

We know how to build stuff which lasts for centuries, there's plenty of those lying around in the old world.

https://basalt-rebar.com/

https://www.youtube.com/watch?v=mRWdHpopETI

Basalt is a pretty amazing material. This is just one use. Basalt is also used to make insulation, low skid tiles, and corrosion resistant pipe liners.

Thanks for the links! I love building material science.

Why don't we just use concrete by itself? Is steel necessary for using concrete the way we do in modern times?

Basically concrete is good only for compressive loads. Under tension it easily cracks. The steel reinforcement adds tensile strength to concrete.

In fact, the concrete MUST crack in the tension zone in order for the steel to take effect and provide tensile capacity to the member.

Only if you don’t prestress the reinforcement.

https://en.m.wikipedia.org/wiki/Prestressed_concrete

How often are skyscrapers built using prestressed concrete?

Yes, steel is required. In the design of concrete structure, concrete is assumed to have zero (0) tensile strength.

In 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.

It 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-skys...

For 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.

Why can’t we use carbon fiber or aluminum or anything that won’t rust?

Cost would be my guess.

We 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.

To 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.

Epoxy 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.

Not 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...

He's getting downvoted because more modern versions of epoxy coated rebar are still widely used in new construction. It wasn't the silver bullet for salted roadways it was sold as but it has its place.

which regions and industry do you see them used? In west coast buildings, its uncommon.

> Why can’t we use carbon fiber or aluminum or anything that won’t rust?

We can, we do, just steel is super duper cheap, and workable.

Ironically, it's China now who leads the world in GFRC (glass fibre reinforced concrete) construction to get those miniscule cost savings on steel (despite it being world's biggest steel producer.)

There is also basalt fibre reinforced concrete that is supposed to be superior to glass fibre, and can come in workable varieties (heat it up with a torch, and bend.)

One of the big reasons is ductility. We want structures to show signs of distress before failing catastrophically. With ductile reinforcing (steel), the deformation at which failure occurs is easily 5 times larger than the deformation where yielding occurs. Large cracks will open up, it hopefully gets noticed, and there is a chance to intervene.

With GF or CF reinforcing, failure is sudden and catastrophic.

> Why can’t we use...

You can. Nobody’s stopping you.

Perhaps you meant “they” instead of “we”.

Pet peeve.

I’ve found when you use the proper pronoun — ‘they’ in this case, unless you make concrete — it focuses your thinking.

That is, if you want to know why ‘they’ do something, you’ll naturally ask ‘them’. But, when you use ‘we’, you’re more likely to just imagine your own solution.

Nothing personal.

‘We’ is the bane of my existence...

In order for this post to be somewhat relevant, I’ll add this... I see corroding rebar as a feature and not a bug; it assists in the natural degradation of concrete and it gives me some comfort to know that the distant future will have no signs of the concrete monstrosities that litter out landscape. Thanks, rebar.

"Why can’t we use carbon fiber or aluminum or anything that won’t rust?"

You can. Stainless steel rebar is a thing and would, largely, solve the problem of spalling due to rusting steel reinforcements.

As you can imagine, stainless steel rebar adds a significant cost.

Great video on prestressed concrete members - https://www.youtube.com/watch?v=P13Mau2VUWw

(Really good channel overall on civil engineering)

Explaining the asnwers in another way, yes, steel is necessary for both large horizontal openings without pillars and large vertical heights.

However, in earthquake country, we don't build with unreinforced concrete for the obvious reasons.

> 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.

This 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.

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