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.
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.
> 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.
> 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.
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.
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.
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.
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.
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.
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.