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Why was Roman concrete so durable?

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Re: Why was Roman concrete so durable?

#151

Post by LegitShady »

ggm wrote: post #86
ogogmad wrote: post #81

For people who don't know the difference between cement, mortar and concrete: Cement is a substance used for producing mortar and concrete. It's never used on its own. It is a "binding agent". Note that the term "cement" is often not used correctly in everyday speech, with people talking about gluing bricks together with cement, when bricks are actually glued together with mortar. Mortar is the glue between bricks. Y…

Reinforced concrete in part is designed to provide structural strength under tension forces. Concrete is stable in compression but has weak resistance to tension. Pretensioned reinforced concrete permits longer unsupported spans but even vertical concrete can benefit from reinforced bars, to prevent spalling. (I believe). Not all reinforcing steel has to be tensioned to be useful, that's a technique for long spans. R…

>Reinforced steel bars hence rebar.

More like reinforcing bars, rather than reinforced bars. And technically the overwhelming majority of what is sold today are actually deformed reinforcing bars, since the intentional addition of ridges on the outside of the bar helps the concrete adhere as compared to smooth mild steel.

Re: Why was Roman concrete so durable?

#152

Post by saalweachter »

moloch-hai wrote: post #146
mitthrowaway2 wrote: post #135

Earlier quoted context omitted.

Survivor bias comes up every time this topic is mentioned, but it really has less explanatory power than most people think. If you meet a man born in 1932 who is still running marathons and doesn't need glasses, it's probably luck, although he might have some valuable diet tips to share. If you meet a man born in 1732 running the same marathon, it's time to rule out luck, and start collecting blood samples because he…

Right. Romans experimented with the formula for concrete for centuries, and learned a lot about how centuries-old concrete structures deteriorated. They didn't have modern chemistry, but they had observational skills and were serious about their work. It would not be surprising to find that most structures weren't built to last any longer than ours, but that some were meant for the ages. An important difference today…

I wonder if generational reputations and transmission of methods from master to apprentice is sufficient.

"Secundus was an apprentice to Marius who was an apprentice to Darius, who built that awesome aquaduct; Marcus was an apprentice to Felix who who was an apprentice to Marcellus, whose shit bridge fell down after twenty years. We're going to hire Secundus instead of Marcus."

Re: Why was Roman concrete so durable?

#153

Post by LegitShady »

aidenn0 wrote: post #149
jhallenworld wrote: post #142

Earlier quoted context omitted.

There are different types of cement: Portland and CSA (Calcium Sulfoaluminate). https://caltra.com/wp/wp-content/uploads/2016/11/What-is-CSA... Portand is the cheap stuff- like the $6 Quikrete bag at Home Depot. CSA is more expensive, but also available at Home Depot- look for the "Rapid Set" brand (Rapid Set Cement All is like $28 a bag). It's kind of awesome, it's so fast you can sculpt vertically with it. You use…

There are many, many, types of cement. Another common type of cement that people are probably familiar with is asphalt (a.k.a bitumen).

There are many types of even just portland cement, or portland cement with additives. Early strength development, sulfate resistance, air entrainment (trapping little bubbles of air in the concrete so that it cracks less in freeze thaw cycles), different heats of hydration, etc.

Re: Why was Roman concrete so durable?

#154

Post by LegitShady »

masklinn wrote: post #134
oakwhiz wrote: post #132

Earlier quoted context omitted.

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…

> 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.

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.

Re: Why was Roman concrete so durable?

#155

Post by aidenn0 »

ricardobeat wrote: post #127
dreen wrote: post #123

Earlier quoted context omitted.

I don't think the knowledge is gone, we could make that or maybe even better. The point is nobody wants it, most things we build need to be torn down after at most a few decades anyway.

The knowledge is actually gone, like Damascus Steel (what we have today is just a visual reproduction) and Greek Fire. For a more recent example: we already "forgot" how to make the spaceships and rockets that went to the moon. Even if you had the full plans - and apparently we don't, a lot of the materials and processes are not around anymore and would have to be 'reinvented'.

We have a pretty good idea of Damascus steel at this point:

- We know that early Damascus examples were made from imported Wootz ingots, which we absolutely can recreated.

- The high quality of the blades were due to naturally occuring impurities of molybdenum, vanadium, and chromium, all of which are used in various modern alloys.

- Forging and heat treating processes varied greatly across historical blades called "Damascus" and we have sequences that create both visually and metallurgically similar results.

Re: Why was Roman concrete so durable?

#156

Post by agumonkey »

mitthrowaway2 wrote: post #135
adolph wrote: post #90

Earlier quoted context omitted.

One method to reduce cement’s carbon footprint (which accounts for up to 8% of total global greenhouse gas emissions), is to improve the longevity of concrete through the incorporation of self-healing functionalities. The resulting extended use life, combined with a reduction in the need for extensive repair, could thus reduce the environmental impact and improve the economic life cycle of modern cementitious constru…

Survivor bias comes up every time this topic is mentioned, but it really has less explanatory power than most people think. If you meet a man born in 1932 who is still running marathons and doesn't need glasses, it's probably luck, although he might have some valuable diet tips to share. If you meet a man born in 1732 running the same marathon, it's time to rule out luck, and start collecting blood samples because he…

meta comment: I'd love to see the stats about all bias and fallacies mentionned in conversations in the last 20 years. Something tells me wikipedia got deep into people's head.

Re: Why was Roman concrete so durable?

#157

Post by LegitShady »

moloch-hai wrote: post #144
masklinn wrote: post #134

Earlier quoted context omitted.

> 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.

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…

> 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.

Re: Why was Roman concrete so durable?

#158

Post by riversflow »

boogoob wrote: post #129

Isn't the particularly short lifespan of modern concrete construction mostly a combination of its use in areas with regular freeze-thaw cycles and using a reinforcement material that rusts (and in doing so expands, damaging more concrete and hastening further water inclusion)?

I'm just going to throw it out there to spread the word; GFRP Rebar (Glass Fiber Reinforced Polymer) is a thing, and you can buy it today at your local hardware store. I really wish I would have known about it on my last project, probably the last time I will buy steel rebar to put in concrete.

I'm not an ME, but my understanding is that it is a comp in terms of structural reinforcement. I'd be really interested to do a deep dive on how these technologies stack up against each other, it seems like a win for construction longevity, it would be pretty cool if the ecological impact of production penciled out as a win too. My understanding is that there are some trade off when being used in large projects.

My cursory research has indicated that this stuff first hit production in the 80's and was used in a bridge in the US in 96. Still haven't figured out why it hasn't taken over.[2] Is it just for a lack of evangelists?

[1]https://www.lowes.com/pd/Owens-Corning/5013333093

[2] https://www.fiberglassrebar.us/gfrp-rebar-from-the-beginning...

Re: Why was Roman concrete so durable?

#159

Post by hinkley »

masklinn wrote: post #100
hinkley wrote: post #92

Earlier quoted context omitted.

The destroyed concrete, yes, but what about the dexpan?

The dexpan is not a huge concrete structure, it's just a cylinder the size of the hole it was poured in.

Oh, I see, just a bore hole and hydraulic concrete. Sort of like the metal shims you see used in pre-industrial stoneworking.

I was picturing pouring it into a void for some reason.

Re: Why was Roman concrete so durable?

#160

Post by dima_vm »

ogogmad wrote: post #81

For people who don't know the difference between cement, mortar and concrete: Cement is a substance used for producing mortar and concrete. It's never used on its own. It is a "binding agent". Note that the term "cement" is often not used correctly in everyday speech, with people talking about gluing bricks together with cement, when bricks are actually glued together with mortar. Mortar is the glue between bricks. Y…

Well, technically, you can use cement instead of mortar (wet, but without sand). As far as I understand, sand is more like a neutral filler, to save money on cement. Various other things can be used to fill as well, like gravel. Sometimes even empty plastic bottles (reduces material and weight) are suitable (just make sure they don't float up). If one got very smooth bricks (e.g. precisely cut aerated concrete), then it might make sense, as there's very tiny amount of mortar (or even glue) is needed anyway.
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