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The problem with reinforced concrete (2016)

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Re: The problem with reinforced concrete (2016)

#161

Post by AceyMan »

_jal wrote: post #118
peteradio wrote: post #64

Earlier quoted context omitted.

That doesn't smell right. I've got 15 year old knives that have never once shown signs of rust. A pair of old scissors left in the rain rusted in under a day. That puts the factor to closer to >10000 than 10 or 100...

There are different grades of steel, doped with all sorts of additives, depending on the desired traits. The most corrosion-resistant variant today used in knives would be one of the nitrogen steels, like Nitro-V. https://knifesteelnerds.com/2019/09/23/nitro-v-its-propertie... But they all corrode, eventually. If you want a true corrosion-resistant metal that stays (kinda) sharp, look at one of the cobalt alloys like…

You may like SPY27 if cobalt is what you seek. (=> new steel which just dropped weeks ago.)

/Acey

Re: The problem with reinforced concrete (2016)

#162

Post by ggcdn »

lazide wrote: post #130
GoToRO wrote: post #92

Earlier quoted context omitted.

We only use reinforced concrete. The type of work where simple concrete is enough are very rare.

Simple concrete is enough in almost all cases for what we do - if we use enough of it. Rebar allows dramatically reducing the amount we use in most cases, with this predictable trade off It’s rarely physics, almost always economics.

Plain concrete is simply not allowed by code for many applications. So even if you could theoretically justify the design, you couldn’t get the permit to build it.

Physics, economics, and bureaucratics is a good summary of structural engineering. The last one can’t be ignored.

Re: The problem with reinforced concrete (2016)

#163

Post by ggcdn »

teh_klev wrote: post #58
ajcp wrote: post #25

I'm not sure this article does a good job of highlighting "the problem with reinforced concrete" than it does "the better attributes of material x with y over concrete". Reinforced concrete seems to do exactly what it's intended to do for the designed life of that intent, with some very well known trade-offs coupled with some brilliant strengths. Sure, compared to other materials it might not be as: long-lasting, che…

> Reinforced concrete seems to do exactly what it's intended to do for the designed life of that intent Until there's apparently "no money" to replace the structure after its design life time. Thinking in decades of life span for many of these structures is very short sighted. I think the article mostly gets that across i.e. re-enforced concrete is hard to recycle and their life span is often within that of a human l…

I think many commentators, including yourself, have an erroneous assumption about structural design which is roughly summarized as: the inputs are known with certainty. This is really not the case especially when considering natural hazards like wind and earthquake. What happens to your 500 year lifespan design when we discover new information about earthquakes at the site. This is happening regularly with every building code update. Our design loads change To reflect the latest research on things like seismicity. When you factor in this consideration, it may actually make a lot of sense to only design for shorter lifespans.

Re: The problem with reinforced concrete (2016)

#164

Post by ggcdn »

LegitShady wrote: post #158
ClumsyPilot wrote: post #154

Earlier quoted context omitted.

"Last I checked stainless rebar was ~3x the cost of regular rebar" But it's 5-10% of the total building cost, and if you bump that to 20% the corrosion goes from "always' to "probably never"

if you need to weld the rebar into cages (like you often do) then the cost goes up even more as welding ss is harder. But you're not wrong in general. There will still be corrosion in the concrete eventually but less likely from rebar oxidation.

Welding rebar is almost never used in North American building construction, and is actually forbidden in high seismic zones.

But then again so is stainless steel rebar and carbon fibre rebar and most of these other types of products because they lack ductility

Re: The problem with reinforced concrete (2016)

#165

Post by ggcdn »

gamblor956 wrote: post #83
ConcernedCoder wrote: post #33

Tangentially related. The romans figured out that volcanic ash and salty sea water made cement that actually gets stronger with age instead of breaking-down after 50 years: https://www.sciencemag.org/news/2017/07/why-modern-mortar-cr...

Left out is the reason we don't use the Roman mix today: it takes years for the concrete to set; Roman concrete is weaker for the first decade compared to modern cement mixes. Moreover, back in Roman times, Roman concrete was not as strong back as it is today. As the article you linked points out, "Because both minerals take centuries to strengthen concrete, modern scientists are still working on recreating a modern…

Very good point. Most building projects actually use admixtures to get as rapid strength increase as possible, even if it means less strength increase long term, because the early strength determines when you can strip formwork and start building the next floor. Builders want concrete reaching ~18MPA in like <2 days.

Re: The problem with reinforced concrete (2016)

#166

Post by djmips »

scythe wrote: post #68

Fiber-reinforced cement composites are an interesting alternative: https://onlinelibrary.wiley.com/doi/pdf/10.1002/suco.2017001... Some German researchers seem to think that carbon fiber-reinforced polymer "rebar" could be more durable than steel bars with similar cost and lower weight: https://www.aboutcivil.org/carbon-reinforced-concrete-buildi... Unfortunately, the terminology is not well standardized, and when yo…

Lightweight video. https://www.youtube.com/watch?v=nB9ViglDMmg

Re: The problem with reinforced concrete (2016)

#167

Post by WalterBright »

hdctambien wrote: post #134
culturestate wrote: post #125

Earlier quoted context omitted.

There's nothing wrong with having long eaves if that's your preference; I'm just pointing out that there are a lot of factors that go in to building a long-lasting structure, and the answer to most questions around best practices is "it depends." If you have a two-storey house in a wet area that gets a lot of storm activity coming from the northeast, for example, and you have an exposed northeast-facing wall, the eav…

> might be good advice in Seattle, but if I told a local builder to worry about frost mitigation where I live now (Singapore) they'd probably question my sanity Texas felt the same way until February!

> Texas felt the same way until February!

And I have a friend who lives in Texas. The pipes in the outer walls froze and burst, the ones in the inner walls did not.

Re: The problem with reinforced concrete (2016)

#168

Post by WalterBright »

culturestate wrote: post #125
WalterBright wrote: post #116

Earlier quoted context omitted.

I live in Rain City (Seattle). Quite a lot of homes are built with ridiculously tiny eaves. I'd never buy one of those. My house has eaves that stick out about 2 feet. It added nothing significant to the cost, but boy what a difference it makes. The exterior walls almost never get wet. The windows and their frames stay dry and free of rot. No mildew. Haven't even needed to repaint. There are a lot of things one can d…

There's nothing wrong with having long eaves if that's your preference; I'm just pointing out that there are a lot of factors that go in to building a long-lasting structure, and the answer to most questions around best practices is "it depends." If you have a two-storey house in a wet area that gets a lot of storm activity coming from the northeast, for example, and you have an exposed northeast-facing wall, the eav…

> it might be good advice in Seattle

Jeez, of course one pays attention to the local climate. I don't worry about tornadoes in Seattle, but would if in the midwest.

Re: The problem with reinforced concrete (2016)

#169

Post by rsa4046 »

brutusborn wrote: post #3

This should be an economics piece, not an environmental piece. The author states that "one of iron’s unalterable properties is that it rusts" yet further on acknowledges the existence of stainless steel. There's nothing wrong with reinforced concrete, but the incentives to produce long lasting buildings are not there. The cheapest bidder will generally win and their building will last the "design life" of the buildin…

"This means that concrete structures, for all their stone-like superficial qualities, are actually made of the skeletons of sea creatures ground up with rock. It takes millions upon millions of years for these sea creatures to live, die and form into limestone. This timescale contrasts starkly with the life spans of contemporary buildings."

This description is fairly accurate. The CaCO3 (used as a source of calcium in the cement component of concrete) is completely decarbonated in a 1450°C kiln in the process of cement manufacture, combined with silica (from shale) +/- SO4 (from gypsum) and sintered to form an anhydrous calcium silicate (clinker: e.g. tricalcium silicate, Ca3SiO5, ‘alite’), then powdered (e.g. ordinary Portland cement, OPC). The skeletal limestone is long gone — and the above decarbonation step is the reason cement manufacturing process is a significant GHG source (in addition to fuel consumption by the kiln itself).

Mixing water with the powdered clinker generates a very rapid, exothermic, partial dissolution of the primary silicate. The rapid release of silica results in nucleation and growth of calcium silicate hydrate (CSH) plus Ca(OH)2. CSH binds the remaining unreacted solid mass together, giving cement its durability and strength.

Re: The problem with reinforced concrete (2016)

#170

Post by rsa4046 »

rsa4046 wrote: post #169
brutusborn wrote: post #3

This should be an economics piece, not an environmental piece. The author states that "one of iron’s unalterable properties is that it rusts" yet further on acknowledges the existence of stainless steel. There's nothing wrong with reinforced concrete, but the incentives to produce long lasting buildings are not there. The cheapest bidder will generally win and their building will last the "design life" of the buildin…

"This means that concrete structures, for all their stone-like superficial qualities, are actually made of the skeletons of sea creatures ground up with rock. It takes millions upon millions of years for these sea creatures to live, die and form into limestone. This timescale contrasts starkly with the life spans of contemporary buildings." This description is fairly accurate. The CaCO3 (used as a source of calcium i…

Sorry, typo: fairly inaccurate
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