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  • The largest unreinforced concrete dome in world is on the Pantheon.

  • It’s not a modern marvel, but rather an ancient Roman temple built almost two thousand

  • years ago.

  • So, if concrete structures from the western Roman Empire can last for thousands of years,

  • why does modern infrastructure look like this after only a couple of decades?

  • Hey I’m Grady and this is Practical Engineering.

  • In today’s episode, were taking a look at the factors that affect the design life

  • of concrete.

  • This video is sponsored by Brilliant.

  • More on that later.

  • If you haven’t seen the previous videos in this series about concrete, here’s a

  • quick synopsis.

  • Weve talked about how concrete’s made, why it often needs reinforcement, and how

  • that reinforcement can sometimes lead to deterioration.

  • Concrete reinforced with steel bars is the foundation of our modern society.

  • The reinforcement is required to give the concrete strength against tensile stress.

  • We use steel as reinforcement because of its strength, its similar thermal behavior, its

  • availability, and low cost.

  • But steel has an important weakness: it rusts.

  • Not only does this corrosion reduce the strength of the reinforcement itself, but its by-product,

  • iron oxide, expands.

  • This expansion creates stresses in the concrete that lead to cracking, spalling, and eventually

  • the complete loss of serviceability - i.e. failure.

  • In fact, corrosion of embedded steel reinforcement is the most common form of concrete deterioration.

  • But it hasn’t always been that way.

  • The Romans got around this problem in a very clever way: they didn’t put steel in their

  • concrete.

  • Simple enough, right?

  • They harnessed the power of a few clever structural engineering tricks like the arch and the dome

  • to make sure sure that their concrete was always resisting compression and never tension,

  • minimizing the need for reinforcement.

  • One of those clever tricks was just making their structures massive, and I mean that

  • literally, because the simplest way to keep concrete in compression is to put heavy stuff

  • on top of it, for example, more concrete.

  • We use this trick in the modern age as well.

  • Most large concrete dams are gravity or arch structures that rely on their own weight and

  • geometry for stability.

  • In both gravity and arch dams, the shape of the structures are carefully designed to withstand

  • the water pressure using their own weight.

  • You can see how they get larger, the deeper you go.

  • So, even with the tremendous pressure of the water behind the structure, there are no tensile

  • stresses in the concrete, and thus no need for reinforcement.

  • But lack of steel reinforcement isn’t the potential only reason Roman concrete structures

  • have lasted for so long.

  • One of the other commonly-cited suggestions for the supremacy of Roman concrete is its

  • chemistry.

  • Maybe they just had a better recipe for their concrete that somehow got lost over time,

  • and now those of us in the modern era are fated to live with substandard infrastructure.

  • In fact, in 2017, scientists found that indeed the combination of seawater and volcanic ash

  • used in ancient roman concrete structures can create extremely durable minerals that

  • aren’t normally found in modern concrete.

  • But that’s not to say that we can’t make resilient concrete in this modern age.

  • In fact, the science of concrete recipes, also known as mix design, has advanced to

  • levels a Roman engineer could only dream of.

  • One of most basic, but also most important factors in concrete’s chemistry is the ratio

  • of water to cement.

  • I did an experiment in a previous video that showed how concrete’s strength goes down

  • as you add more water.

  • Extra water dilutes the cement paste in the mix and weakens the concrete as it cures.

  • The Romans knew about the importance of this water to cement ratio.

  • In historical manuscripts, Roman architects described their process of mixing concrete

  • to have as little water as possible, then pounding it into place using special tamping

  • tools.

  • Interestingly enough, we have a modern process that closely mimics that of the ancient Romans.

  • Roller Compacted Concrete uses similar ingredients to conventional concrete, but with much less

  • water, creating a very dry mix.

  • Rather than flowing into place like a liquid, RCC is handled using earth moving equipment,

  • then compacted into place using vibratory rollers like pavement.

  • RCC mixes also usually include ash, another similarity to Roman concrete.

  • It’s a very common construction material for large gravity and arch dams because of

  • its high strength and low cost.

  • Again, these are usually unreinforced structures that rely on their weight and geometry for

  • strength.

  • But, not everything can be so massive that it doesn’t experience any tensile stress.

  • Modern structures like highway overpasses and skyscrapers would be impossible without

  • reinforced concrete.

  • So, generally we like our concrete to be more viscous or soupy.

  • It’s easier to work with.

  • It flows through pumps and into the complex formwork and around the reinforcement so much

  • more easily.

  • So, one way we get around this water content problem in the modern age is through chemical

  • admixtures, special substances that can be added to a concrete mix to affect its properties.

  • Water reducing admixtures, sometimes called superplasticizers, decrease the viscosity

  • of the concrete mix.

  • This allows concrete to remain workable with much lower water content, avoiding dilution

  • of the cement so that the concrete can cure much stronger.

  • I mixed up three batches of concrete to demonstrate how this works.

  • In this first one, I’m using the recommended amount of water for a standard mix.

  • Notice how the concrete flows nicely into the mold without the need for much agitation

  • or compaction.

  • After a week of curing, I put the sample under the hydraulic press to see how much pressure

  • it can withstand before breaking.

  • This is a fairly standard test for concrete strength, but I’m not running a testing

  • lab in my garage so take these numbers with a grain of salt.

  • The sample breaks at around 2000 psi or 14 MPa, a relatively average compressive strength

  • for 7-day-old concrete.

  • For the next batch, I added a lot less water.

  • You can see that this mix is much less workable.

  • It doesn’t flow at all.

  • It takes a lot of work to compact it into the mold.

  • However, after a week of curing, the sample is much stronger than the first mix.

  • It didn’t break until I had almost maxed out my press at 3000 psi or 21 MPa.

  • For this final batch, I used the exact same amount of water as the previous mix.

  • You can see that it doesn’t flow at all.

  • It would be impossible to use this in any complicated formwork or around reinforcement.

  • But watch what happens when I add the superplasticizer.

  • Just a tiny amount of this powder is all it takes, and all of a sudden, the concrete flows

  • easily in my hand.

  • In many cases, you can get a workable concrete mix with 25% less water using chemical admixtures.

  • But most importantly, under the press, this sample held just as much force as batch 2

  • despite being just as viscous as batch 1.

  • The miracle of modern chemistry has given us a wide variety of admixtures like superplasticizers

  • to improve the characteristics of concrete beyond a Roman engineer’s wildest dreams.

  • So why does it seem that our concrete doesn’t last nearly as long as it should.

  • It’s a complicated question, but one answer is economics.

  • There’s a famous quote that saysAnyone can design a bridge that stands.

  • It takes an engineer to build one that barely stands.”

  • Just like the sculptors job is to chip away all the parts of the marble that don’t look

  • like the subject, a structural engineer’s job is to take away all the extraneous parts

  • of a structure that aren’t necessary to meet the design requirements.

  • And, lifespan is just one of the many criteria engineers must consider when designing concrete

  • structures.

  • Most infrastructure is paid for by taxes, and the cost of building to Roman standards

  • is rarely impossible, but often beyond what the public would consider reasonable.

  • But, as we discussed, the technology of concrete continues to advance.

  • Maybe today’s concrete will outlast that of the Romans.

  • Well have to wait 2000 years before we know for sure.

  • Thank you for watching, and let me know what you think!

  • Thanks to Brilliant for sponsoring this video.

  • In my career as an civil engineer, I’m constantly on the lookout for new ways to do my job better,

  • and often that means learning new skills.

  • Recently, I’ve been using Brilliant to brush up on my understanding of probability.

  • Civil engineers work on projects that can last many years, so for me, being able to

  • anticipate risks and estimate their probability has helped me get ahead at work.

  • Brilliant starts you with the fundamentals and provides interesting exercises and puzzles

  • to help you master each concept at your own pace.

  • I find that I learn best when I can apply the skills immediately, so I love the interactive

  • problems you can work in each lesson.

  • To support this channel, go to brilliant.org/practicalengineering and sign up for free.

  • The first 200 people will get 20% off the annual Premium subscription.

  • Again, thank you for watching, and let me know what you think!

The largest unreinforced concrete dome in world is on the Pantheon.

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罗马混凝土更好吗?(Was Roman Concrete Better?)

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    joey joey 發佈於 2021 年 06 月 11 日
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