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England’s concrete crisis could extend to hospitals and courts, experts say
(www.theguardian.com)
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this post was submitted on 02 Sep 2023
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How the hell have we messed up concrete? The Romans had concrete that is still with us today.
The difference between a Victorian bridge and a modern one, is that the Victorian one was built to definitely stay up, while the modern one is built to just stand up.
Because just is all you need, when you can calculate so much of the design, and know the service life.
What's happened here, is a lot of the buildings were built with a service life that was within the bounds of aerated concrete. The buildings were supposed to be replaced by now, but budget constraints have meant that they've been pushed beyond their service life.
Or in analogous terms: You have to stay in a house for a week, you buy some disposable plates and cutlery.
2 months later, you're still there, and all the plastic forks have broken.
According to other articles that I linked to in this thread, the problem was only discovered in the 1990s, that the stuff had a relatively short lifetime.
Couple that with the bidding process for infrastructure contracts, and anything built in the last 40-50 years by government contract is likely to be falling apart before too long.
I've found nothing is more permanent than "temporary".
It's economically and environmentally bad to make things to be temporary and disposable. Stupid short term thinking.
This is true, but it's also more expensive, which means the owners now don't want to spend the extra 25% to make sure their building lasts 500 years instead of 50.
Roman concrete was more durable than most modern concrete, but was much, much weaker. It also relied on volcanic ash, which isn't as readily available as the ingredients for portland cement. Being able to have larger freestanding spans and lower construction costs due to reinforcement is usually worth a much shorter design lifetime.
The reinforced aerated autoclaved concrete was clearly a mistake, trying to make a concrete foam to reduce weight meant that more water could get to the rebar and cause corrosion much more quickly than in normal reinforced concrete.
To be fair, the Romans also had a lot of concrete that is not with us today, there's a bit of survivorship bias going on here.
Very true. But still, seams like we have been doing it long enough we should know what lasts.
We have definitely built concrete structures that have lasted a lot longer than 30 years. This was a very particular form of concrete construction that was apparently only a few decades old when the issues were discovered.
I mean, building one form of concrete structure doesn't give us a complete understanding of every possible new variant invented and their tradeoffs.
Sure, but Roman concrete was also actually really good due to the ingredients used. They had self-healing concrete millennia before we came up with the idea.
A fair critique is the Romans built their shit to last and didn't have advanced computers to calculate loads to just ~10% of failure, like we do now. We'll use cheaper, local materials if it's good enough and make sure the building stands for maybe a century. The Romans shipped ash and concrete ingredients halfway across Europe to make sure they were using the good stuff.
Do you think they built everything that way? Cause they certainly didn't. Hence the bias.
No, they also used a lot of wood.
But doesn't change the fact the concrete is good concrete. Better much of ours.
One thing to note regarding the self-healing concrete. They came across that formula by complete accident. All they knew was adding volcanic ash resulted in longer lasting concrete but wouldn't have known about the lime clasts that would mix with water and refill cracks.