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Comment by mikehotel | Hacker News Reader

Would you consider insulated concrete forms (ICF) addressing the water intrusion concerns, while retaining the other benefits of reinforced concrete?

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Maybe you know more about this than I do. Does the insulation last, say, 200 years?

ICF requires rebar to be dropped in the forms before pouring so... maybe not much better than standard reinforced concrete re: water intrusion.

Water could reach the rebar from the ground, but I would think waterproof membranes and proper drainage should allow it to remain mostly dry. The foam panels should also reduce the amount of air that gets to the concrete and the rebar. Of course, you want concrete to be wet for curing, but how would water and oxygen reach it after it drys out? Do you think the oxidization process will not stop after curing?

concrete is porous. IF you wrapped your concrete in a waterproof membrane, the first tear or piercing in the membrane would then become an entryway for water and there would be no way for the water to leave because of the rest of the membrane. Proper drainage is a better solution.

It's like the bridges that used epoxy coated rebar for chloride corrosion resistance and then found out that any tiny nick in the epoxy coating then causes that rebar to somehow corrode FASTER than uncoated rebar.

Does it require steel rebar or could you use glass rebar? I think most homes (that are "boxes") have concrete in compression, and not tension. That means the structural considerations for rebar are a lot lower. I think, though, as always, the lifetime of a structure is always dependent on its roof. I don't think I've seen any "multicentury" solution for roofs. Even a lead roof is only definitely good for about a century before it needs maintenance; although, it's useful for twice that long. I've got a metal roof on my house, and it came with a 100 year warranty. (The UV eventually gets to the paint, which will take out the roof.)

Copper roofs maybe? Not sure what maintenance they have to do for the roofs of Prague, but their green color is due to a copper patina.

Unfortunately you need to spend some time understanding loading on structures to understand what reinforcing is doing. Reinforcing is strictly a tension reinforcement - if the loading in compression is too great for concrete the solution is not to reinforce the concrete, it's to use a different material like steel. Concrete in pure compression is better off with no rebar at all.

Just because things fall under gravity doesn't mean they only experience compression. A simple moment frame would create tension in vertical members with only a downward load on the crossbeam creating moment.

It also would exclude the ability to do crack control in concrete with reinforcing, which would greatly lower its lifespan.

I probably don't know more than you, but I am interested in learning more about it. The insulation most often used is EPS (expanded polystyrene), which many studies are trying to find ways to accelerate decomposition from an ecology perspective. Sheltered from UV and solvents, it should last more than 200 years, with some marketing materials giving it an indefinite lifespan. We need more history on ICF, since it is a relatively new building material/technique.

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