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Comment by boogoob | Hacker News Reader

Isn't the particularly short lifespan of modern concrete construction mostly a combination of its use in areas with regular freeze-thaw cycles and using a reinforcement material that rusts (and in doing so expands, damaging more concrete and hastening further water inclusion)?

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The rusting and getting wet is cyclical in nature and part of what accelerates that is the formation of cracks in the first place. Part of the maintenance of concrete is to fill any cracks that you can find to prevent it from getting worse. The cracks typically start small, and they might form in places that are hard to get to as well as being too small to see, so if the concrete is self-healing it will prevent some of these small cracks from getting worse on their own which should reduce the effect of expanding water (ice) as well as expanding rust.

The reinforcement material (rebar) already has a very close thermal coefficient of expansion to concrete and this can be thought of as kind of a lucky coincidence.

> as well as expanding rust.

Ideally they’d seal before the rebar is exposed and can rust. As long as the rebar is encased, unless you fucked up dramatically it should be safe.

The rust is essential to bind it to the cement. Without, it adds little strength.

Modern concrete isn't expected or designed to last more than a century. It could have been made to last longer, if they wanted, but that would cost a little more. Making no provision for when it will predictably fall apart is a modern failing. That major construction with concrete is about a century old should worry everyone.

We have a very great deal of infrastructure that will fail on a predictable schedule, with nothing budgeted to replace it all.

Yes although it is true what the grandparent comment said about sealing - that keeping things dry, using coatings or topper materials, dewatering, etc. will all help extend the longevity and reduce maintenance requirements a bit.

On the plus side it makes for some wicked post-apocalyptic ruins. Remove maintenance, add 20 years, and you get the most photogenic wasteland you could ask for.

> The rust is essential to bind it to the cement. Without, it adds little strength.

Do you have a citation for this? As far as I know this is entirely not true.

Rust is an expansionary product - when steel rusts its volume increases something like 10x, which reduces the connection between the concrete and the steel and causes issues like cracking and spalling. Minimizing rust improves the long term performance of rebar.

They leave the rebar out to rust a bit, on purpose, before they pour. Surface rust is good, but more is bad. Like many things in life.

I read your comment as I finished eating my third slice of pie and wondering why I cut myself 3 slices.

3 is the limit. More than 3 is bad.

One would think the limit for pie is closer to 3.14159265359

It is, but it's not easy to cut pi slices of pie without being unfair to others who want to have equal slices of pie. Plus, the effort to be so exact may not be worth the utility. You may hit the true pi limit by random luck too.

Perhaps you’re just mathematically inclined.

You could have cut one big slice instead.

Rust is perfect. We don’t criticize rust on HN.

So the Hoover dam will fail in 10 years?

Fortunately, Hoover Dam is under compressive load.

Only if the Hoover dam was made with standard concrete using standard building methods, I suppose.

Concrete lasts a really long time when it's under pure compression. You only run into issues when you try to build structures that need to survive in tension. That's when you start having to add rebar and lower weight and you run into issues.

> The rust is essential to bind it to the cement

What? This is total nonsense.

> Modern concrete isn't expected or designed to last more than a century. It could have been made to last longer, if they wanted, but that would cost a little more.

Is hacker news a conspiracy theory website now?

> We have a very great deal of infrastructure that will fail on a predictable schedule

Wood rots in a few years under certain conditions and can last hundreds in others. Concrete works similarly.

The 3rd pantheon is currently standing, but it is a huge mix of repairs across centuries. The Romans built many structures that are similar but now are in ruins because the Catholics didn’t maintain the roofs on bathhouses.

It is a conspiracy theory that engineering construction is optimized for cost against requirements? Maybe ask literally any civil engineer how much cost matters.

Steel gives concrete tensile strength. Unfortunately, defects in the epoxy coating become a focal point of oxidation, often pitting that single point more than otherwise uncoated steel. The result is, the pitted area fails quickly and the integrity of the entire member is compromised.

this isn't really possible. For the rebar to take up meaningful tensile stresses, the concrete nearby must have cracked.

Reinforced concrete ALWAYS cracks.

If you want better corrosion resistance, you do other things - lower water ratios, increased concrete cover, sulfate resistant concrete with lower permeability, different reinforcing or epoxy coated reinforcing, etc.

Not always, if rebars are pre-tensioned before concrete solidifies. So the rebar is always under tension. Pretty common in prefab parts, like bridge bars.

pre-stressed concrete is great, and has many advantages over regular reinforced concrete and is more efficient in terms of cross section so cracks less. It still cracks, though, and in different locations like on the ends horizontally instead of at high moment/deflection areas vertically. Pre-stressed does experience fewer shrinkage cracks, which is nice. It's more expensive to build and repair.

And while you absolutely have pretensioned girders on bridges commonly, the bridge decks generally aren't, although they're almost always completely in compression and the steel is there for shrinkage cracks etc.

For someone totally outside this space I had assumed concrete is rigid and brittle and rebar reinforcement allows for it to bend and flex without breaking. But your comment makes it feel like it keeps the concrete in tension with compression forces to make it sturdier. Can you ELI5 what the rebar does to help?

You're going to love this playlist from Practical Engineering [1]. Grady explains all of this much better than I could. The second video explains reinforcement and the fifth explains pre-stressing.

[1] https://www.youtube.com/watch?v=UOHURuAf5iY&list=PLTZM4MrZKf...

you have it backwards. The reinforcement handles tensile stress, keeping more of the concrete section in compression, where its strong. In concrete where the reinforcing isn't tensioned, the concrete below the reinforcing will crack as it is very weak in tension. In concrete that is pretensioned, the tension in the rebar is used to apply compression on the concrete so that it might never get into a tensile load and crack, which also reduces deflection under load.

The rebar handles tensile loads, the concrete handles compressive loads. You put the rebar where tension will exist so that it can handle that. Some amount of cracking on non-pretensioned concrete is normal and unavoidable.

Another method is to connect all the rebar and add a https://en.wikipedia.org/wiki/Galvanic_anode

> expanding rust

The alkaline environment inside concrete "passivates" the reinforcing bar, greatly reducing the rate of oxidation and changing its form.

The problem exists only in concrete that is badly made with poorly chosen aggregate, and in corrosive environments.

1. https://en.wikipedia.org/wiki/Passivation_(chemistry)

I'm just going to throw it out there to spread the word; GFRP Rebar (Glass Fiber Reinforced Polymer) is a thing, and you can buy it today at your local hardware store. I really wish I would have known about it on my last project, probably the last time I will buy steel rebar to put in concrete.

I'm not an ME, but my understanding is that it is a comp in terms of structural reinforcement. I'd be really interested to do a deep dive on how these technologies stack up against each other, it seems like a win for construction longevity, it would be pretty cool if the ecological impact of production penciled out as a win too. My understanding is that there are some trade off when being used in large projects.

My cursory research has indicated that this stuff first hit production in the 80's and was used in a bridge in the US in 96. Still haven't figured out why it hasn't taken over.[2] Is it just for a lack of evangelists?

[1]https://www.lowes.com/pd/Owens-Corning/5013333093

[2] https://www.fiberglassrebar.us/gfrp-rebar-from-the-beginning...

Any comparison in tensile strength?

The main reason is stiffness—off the top of my head, it’s roughly a quarter as stiff as typical steel, so it needs significant engineering and often much more reinforcement to keep the concrete within allowable deformation limits. Basalt and carbon fiber are more promising replacements, but are significantly more expensive.

I recommend Tyler Ley's youtube channel[0] entirely devoted to concrete. He has an episode[1] addressing GFRP that might interest you.

[0] https://www.youtube.com/@TylerLey

[1] https://www.youtube.com/watch?v=thUZImUTZn0

Mike Holmes, a home reno personality in Canada, had been touting its benefits in about the mid-2000s for home foundation re-pours.

And being driven over by semi trucks full of heavy cargo 24/7.

> Isn't the particularly short lifespan of modern concrete construction mostly a combination of its use in areas with regular freeze-thaw cycles and using a reinforcement material that rusts

No. It's a consequence of designing to requirements of minimum cost and a 50-year lifespan. It'd be easy to make concrete with much longer lifespan, but it costs more.

It's a nice thought that all these depressive brutalistic/modernist builduings will all soon crumbly by the same mechanism that spawned them!

Most brutalist buildings are already 60+ ish years old and doing great (actual brutalism, rather than “concrete buildings I don’t like,” was mostly a style of the 1950s and 60s in the west, though it lasted quite a bit longer in Eastern Europe). Even the Boston government services center, everyone’s most hated brutalist building in the US is already past 50.

for many people, myself included, actual brutalism is "concrete buildings I don’t like"

Eh, there’s no accounting for taste, but there have always been shitty buildings in every vernacular. We tend to not see the worst buildings in older styles because they’ve long since been torn down.

“Any idiot can build a bridge that stands, but it takes an engineer to build a bridge that barely stands.” - Unknown

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