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, we’re 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. We’ve 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 says “Anyone
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. We’ll 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!
It takes an engineer to build a bridge that barely stands.
That made me chuckle.
So I was thinking about this then other day and hopefully someone can finally provide me with a definite conclusion. He says "generally we like our concrete to.be more viscous, more soupy". If something is viscous doesnt that mean it is thicker? Isnt viscosity the resistance to flow?
The romans had access to high quality limestone and volcanic ash. both of which improved the quality of their concrete.
In addition, the romans did not use rebar in their concrete. Rebar has two effects: Its strength is improved, but it is much more susceptible to water damage. Concrete is porous and the iron in the rebar eventually absorbs the water and rusts. The rusted metal expands and concrete is horrible at dealing with tension, so the outer layers of the concrete slough off, which leads to extensive patching issues.
I really felt like this was one of his most informative videos. Absolutely nerded out watching it.
Lesson learned after that first test, eh?
They had Good Quality limestones nearby...