5 ms·
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 co
by oakwhiz 4y ago
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.
- masklinn 4y ago> 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.
- moloch-hai 4y agoThe 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.
- saalweachter 4y agoOn 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.
- oakwhiz 4y agoYes 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.
- LegitShady 4y ago> 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.
- moloch-hai 4y agoThey 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.
- zoklet-enjoyer 4y agoI read your comment as I finished eating my third slice of pie and wondering why I cut myself 3 slices.
- PakG1 4y ago3 is the limit. More than 3 is bad.
- aborsu 4y agoOne would think the limit for pie is closer to 3.14159265359
- PakG1 4y agoIt 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.
- imron 4y agoPerhaps you’re just mathematically inclined.
- chrisdhoover 4y agoSo the Hoover dam will fail in 10 years?
- moloch-hai 4y agoFortunately, Hoover Dam is under compressive load.
- anamexis 4y agoOnly if the Hoover dam was made with standard concrete using standard building methods, I suppose.
- deleted 4y ago[deleted]
- adgjlsfhk1 4y agoConcrete 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.
- jcampbell1 4y ago> 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.
- moloch-hai 4y agoIt is a conspiracy theory that engineering construction is optimized for cost against requirements? Maybe ask literally any civil engineer how much cost matters.
- Slava_Propanei 4y ago[dead]
- anonuser123456 4y agoSteel 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.
- LegitShady 4y agothis 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.
- dima_vm 4y agoNot always, if rebars are pre-tensioned before concrete solidifies. So the rebar is always under tension. Pretty common in prefab parts, like bridge bars.
- LegitShady 4y agopre-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.
- sporkland 4y agoFor 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?
- walty 4y agoYou'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=PLTZM4MrZKfW90PdaBFt70BLTbz1bTF6Mn https://www.youtube.com/watch?v=UOHURuAf5iY&list=PLTZM4MrZKf...
- PicassoCTs 4y agoAnother method is to connect all the rebar and add a https://en.wikipedia.org/wiki/Galvanic_anode https://en.wikipedia.org/wiki/Galvanic_anode
- tuatoru 4y ago> 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) https://en.wikipedia.org/wiki/Passivation_(chemistry)