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everybody, to Wednesday Nite >> Tom Zinnen: Welcome,
I'm Tom Zinnen, and I work here at the University of
Wisconsin-Madison Biotechnology Center and for the UW Extension
Cooperative Extension.
at the Wisconsin Alumni Association and Wisconsin Public
Television, welcome again to Wednesday Nite at the Lab.
We do this every Wednesday
This is a very special night night, 50 times a year.
start of our sixth year, and because this is the
we've been able to flourish this long, and I'm looking
forward to doing this for many
The talk for tonight is appropriate, I think, because it
talks about the durability of a material, the endurance of an
idea, and the foresight to
future. launchTonight's speakernto the
is Steve Cramer.
Academic Affairs at the College He's of Engineering.an for
And he leads the undergraduate
activities there. As a researcher, Cramer studiesd
structural engineering with primary expertise in the
mechanical behavior of wood and wood-based materials, the design
and analysis of wood structures,
construction materials. He's a member of the UW Madison
Teaching Academy. In 2002, he earned the
Chancellor's Distinguished
He received his bachelor's in civil and environmental
engineering from UW Madison in
Colorado state for his masters 1979, and then he headed to
He joined the College of and his PhD.
And you can see the title of his Engineering faculty in 1984.
Centennial Study of Concrete. And, again, I'm very pleased
that he is going to do it tonight because I think it's an
appropriate set of themes for all of us.
They're celebrating a hundred years; we're celebrating
starting our sixth year.
lectern. I'd like to welcome you to the
Steve Cramer: Thank you. [APPLAUSE]
on in the world and all of the So with all of the things going
you're here to learn about things going on in the state,
[LAUGHTER] concrete tonight.
I don't think I could ever have Thatsuch a great audience.ul.
We want to know what's going
[LAUGHTER] on in Walker's head.
take a break from the politics >> Well, now, we're going to
about concrete. tThe other folks in my team arek
Jess Silva, a PhD candidate in
And Jess is actually in the audience towards the back, and
when I get into trouble she's going to bail me out.
And then Jake Effinger is a master's candidate in civil and
environmental engineering, and
well. he waI want to acknowledgeect as
Jim Schmitt, the president of Schmitt Technical Services.
He helped us do the petrographic
Ruipeng Li is a graduate analysis.
is Ramsey Kropp. assistant working with me, as
structures and materials lab. He helped with the tests.
And I could go on for quite a while if I acknowledged everyone
who worked on this project over the past 100 years.
You'll see some of the names
But certainly this isn't a study later, so I won't list them.
we're completing. Now, there's some preliminariest
here before we get started. I'd like to pass out my cement
grains. My cement grains are atomic fire
balls.
please take one. If you like an atomic fire ball,
You can still get the gist of what I'm going to talk about
today without taking one. But I have mine up here, and at
a certain point in my talk I'm
into your mouth. going to ask you to plop that
So I'm going to pass this out and get this going and please
help yourself.
say go, and if you'd like to And at the appropriate time I'll
Now an atomic fire ball, if you've never had one, generates
a little bit of heat. It's kind of a red hot cinnamon
candy.
tonight, that's all right. So if you're not up for that
Now I'm watching. And don[LAUGHTER]em yet.
No one eats them yet. >> What does the word atomic
have reference to?
apparent when you put it into >> Wellyour mouth. become
[LAUGHTER]
And it will create a hot tingly It kisensation.odes.
And are you going to put it in your mouth?
I am. I am.
At the appropriate time.
Now, one of the beauties of Portland cement concrete is it
can be formed to really any shaped desired.
And that's why it's the most
material in the world, but at times the general public doesn't
really understand this concept
[LAUGHTER] very well.
They think it's durable. They think it's hard.
And by the time you leave here, in a short while, I think you'll
find that it's a much more
material than probably what you delicate mimagined.nd complex
So cement is any material that
together. I have to give you some of the
basics here first before we get into the meat of the research.
We generally refer to mortar as Portland cement plus water plus
fine aggregates plus, in this day and age, add mixtures.
They didn't have add mixtures in
And cement paste we'll think of as Portland cement plus water.
So what do you see here?
[INAUDIBLE] What Okay.u see?
I'll be giving you a quiz throughout this presentation.
There's a couple things you should see here.
Many would refer to this as oh, look at they're doing a cement
project.
No. No, this is a concrete project.
They're making a concrete road. The distinction is important
when it comes to engineering. The second part of this is you
might think to yourself: this will be ready to drive on when
it dries. No.
Concrete does not dry, it hardens by hydration.
And we're going to get into some
Now, these lessons here are of the hydratbasic.pects today.
My children had the distinct
time they were five. pleasure of knowing this by the
It's just the way it is when [LAUGHTER]
faculty member. You'll often see these referred
to as cement trucks.
They're concrete ready-mix They're not cement trucks.
They're moving along and mixing the cement with other
constituents down the road to a job site.
Perhaps the most common mischaracterization is this is a
cement sidewalk.
It's a concrete sidewalk. And it's a chemistry experiment.
Really, all of these pictures that I've been showing you
there's a vast amount of
on the highway, on the sidewalk. chemistry going on in the truck,
And if we could perfect the chemistry, probably concrete
would last even longer than it
So this is a cement truck, and does now.
And, of course, cement is the powdery material that's added
with water, fine aggregates,
mixtures. What else do you see there?
[INAUDIBLE] Fruit cake.
Cement is to concrete as flour
So getting the terminology right is to fruit cake.
talking about concrete. isNow, Portland cement is a of
hydraulic cement, and that means
chemical reaction to water. it'Tends not to degrade whenthe
exposed to water. In fact, many projects are done
where you pour the concrete directly into water, and it will
harden under those conditions and function just fine.
If we look at what Portland cement actually consists of, it
consists of tricalcium silicate,
language we call C3S, and that's which in cement chemist's
cement today. It will have dicalcium silicate,
which we call C2S, and around 19%-20% of Portland cement will
be that compound. It will have tricalcium
aluminate, C3A, relatively small
And tetracalcium aluminoferrite, amount.
And you can see the depiction there of some of these compounds
that would be in a cement grain.
much smaller than my model that And cemI've given to you.se, is
What's the significance of
We will come to that. That will come shortly.
So, we don't have time tonight
cement is manufactured. tRaw materials include calciumd
carbonates. Those could be limestones, those
could be seashells. They could be a variety of
different kind of calcium type
oxide. materials: silica, alumina, iron
hydrates and hardens most And we frapidly. the C3S
And that's responsible for the
the early strength of the initial setconcrete.oncrete and
C2S hydrates and hardens a bit
more of a contributor to more slowly and that tends to be
C3A has a high heat release in strength after seven days.
Really doesn't do much to the structural performance of
concrete but it's necessary for the manufacturing, and similarly
with the C4AF, it's really more of a manufacturing issue.
Now there are some other
They're in rather small quantities, but they can have a
significant impact on the outcome of the concrete product.
Okay, so the time has come. If I can get the package open.
There we go.
exothermic reaction here, and Syou have the model, the atomice
fire ball is my model of the
And probably the only moisture cement grain.
mouth. you have available is in your
particle is going to go. So that's where your cement
C3S plus the water produces this And we're going to see that the
compound, and that's notation for calcium silicate hydrate and
calcium hydroxide.
(coughs), excuse me. Similarly but slower,
The C2S plus water is going to [LAUGHTER]
and calcium hydroxide. produce calcium silicate hydrate
attention as well not only to the screen but what's happeningy
in your mouth.
happens. Keep track of the timing of what
is not a well-defined compound, Nso we can't chemically controle
this in a very precise way. The stoichiometry is going to
vary depending on the temperature of hydration, the
ratio of water to cement and so forth.
But it does make up a large percentage of cement paste,
which, of course, is cement plus
Now here's what's going on in my water.
this in our mouth a couple of minutes ago and there was a nice
little sweet taste right off the
Kind of enjoyable. bat.
while I was talking, And then for a few minutes
We were in this stage two, not much happened.
induction period. Right about now, for me, I am
in this acceleration phase. [LAUGHTER]
I am too. >> Yeah, and it's really going
to take off.
Yeah, and it's going to generate quite a bit of heat.
This would typically with concrete occur in
the two and a half hour to
We don't have that kind of time three and a hhere.our timeframe.
So the model works a little bit faster than that.
And if you bear with us here a
subside. little bit longer, this will
peak yet, personally. And then it will have this long
kind of deceleration, if you
generating so much heat. might, or period where it's not
peter out. And it will just kind of
creating calcium silicate hydrate around the particle as
we get down in here.
going to continue, but it's Agoing to become diffusionis
controlled. It's going to be the ability of
water to penetrate the hydration
the cement grain which is going prto control how fast it gainsnd
strength. Back in here, it's just the
availability of water.
cement grain and that cement The water is getting to that
growth. gI think I got the larger onesl
this time which means that they last a lot longer.
[LAUGHTER]
in your mouth? >> How can you talk with it
Well, we're not through yet. [LAUGHTER]
Practice. You've done this before.
I've done this before.
successful in keeping the lecture going while I'm talking,
but that's usually to the delight of my audience.
Do you have water?
So here we see these different >> Nope.
The large red pieces here represent the C3S in a concrete
matrix, or a cement paste matrix
The C2S is the light blue parts maylike right in there.y.
The C3A is the bright green.
oh, yeah there's some right up And the C4AF, which I don't,
And then as it progresses through these different stages,
it's going to produce calcium hydroxide, which is dark blue,
and the calcium silicate
So you can see that the C3S shrinks and instead we get these
hydration products forming. And that's what going to give us
our strength to the concrete. So really the only reason that
we put stone in it is so that we
because the cement is expensive. don't have to use so much cement
quality is the compressive Now, our primary measure of
And compressive strength is measured by creating the
samples, which these gentlemen
that's sample is a six by here ar12-inch cylinder.pically
And there's a very appropriate
them back to the laboratory and you push on them and you crush
them and that's just universally
measure of concrete quality. across the world the primary
compressive strength is the Nwater/cement ratio.e
And that was discovered in 1918.
history of Portland cement a I'm going to talk about the
It was discovered, at least in little bit more.
in 1845. its most relatively modern form,
70 years, they really didn't So for about 50 some years, 60,
And in 1918, Ben Abrams unddiscovered that water, thed.
amount of water to the amount of
Now we only need about .22 cement was very, very important.
to hydrate all the cement. to a quarter pound of water
need about a quarter pound of For one poundwater.ment you only
So what you'll find, though, is they always add much more than
that. And the reason they add much
more than that is to gain workability.
To finish it off, to make it
it into position, they add more look smooth, water.able to move
So that, in essence, it almost
So that would be fine, but once you have more water than you
need, it works against you. It creates a void system where
the water displaces the solid cement material and, thus, you
have a more porous, more
have if you add less water. And if only a quarter pound of
water is needed to hydrate the
water go? Well it dries out eventually, it
migrates, and behind it will
And those voids are doorways for leave voids.
concrete. baChemicals can get in ande
destabilize the matrix that has been created, and you see this
every day when you see concrete spalling or breaking down out in
the real world. So concrete lasts forever,
right?
The processes of degradation Wrong.
abrasion, expansion and could be pcontraction.echanical,
You hear about in the summer, these blowups on the highway
that occur as the concrete
Freeze/thaw attacks, a big deal in Wisconsin.
Water gets in, it freezes, it expands, creates expansive
pressures.
This is a chemistry experiment. You have acids that can break
down the concrete.
sulfates, and you have just simple carbon dioxide that's in
the atmosphere can cause
concrete. cSo these are the enemies ofhe
concrete. If you drive your car into the
garage, salt comes off your car,
concrete garage floor start to you're starting to see your
during the summer is get a good ssealer on that so that salto
can't penetrate and affect the
So it's time for some history chenow.ry.
As I mentioned, 1845 was really the discovery of Portland cement
concrete. And it was called Portland
cement because it looked very similar to a popular building
stone quarried in Portland, England.
So there's your answer to your
It's been produced in the United question.
And today we use five to six States since 1871.
person per year in the United sacks, a sacStates. pounds, per
Now it's time, you kind of have gotten the basics, I've given
you a little bit of the chemistry, it's time to think
about the history here as it
Wisconsin and the state. relates to the University of
Morton O. Withey, and in 1905 he became a faculty member
in the College of Engineering.
the College of Engineering at I don't thithat time.even called
And what's interesting is in
a hundred-year concrete study, 1910 he didn't launch
study. he launched a 10-year concrete
specimens for testing with the He made a whole slough of
the study would wrap up. idea that after about 10 years
He won a number of awards, and Okay?
just a second. this may become apparent why in
Turner Medal, 1932, 1947, respectively, from the American
Concrete Institute which was
He then served as the ACI president in 1943.
And in 1946, he became the dean of the College of Engineering.
He retired in 1953. He's the gentleman on the left
in the picture. And in 1961, he describes how
the 1910 study had been expanded
So, you know, I like to talk to a 50-year study.
that he had for a hundred-year about the extraordinary vision
wasn't planned to be a study, but quite honestly it
beginning. hundred-year study in the
such that it could become a Bhundred-year study and we haves
to give him credit for that foresight.
Now, what's going on at the same
In 1902, Thomas Edison time in the United States?
cement kiln. introduced the first large scale
being produced in the United So prior to 1902, while it was
really being used in a broad States since 1871, it wasn't
There just wasn't a ready means to produce it in those
quantities. Two years later the Wisconsin
Idea is established and the American Concrete Institute is
founded.
concrete mixers appear. So you think about the ready mix
truck that I showed you earlier in the slide, now we have a
horse-drawn mixer starting to appear.
What did the mixing?
understand it. >> The wheel power, as I
pulled the mixer along, obviously on a much smaller
scale than the trucks I showed
turned had some sort of paddle earlier, and the wheels as they
Cement demand in 1910 when the or so forth to mix the concrete.
per capita worldwide. Now, we're going to look at this
statistic through time. Remember the population of the
world is growing at the same
So in 1916 we begin to pave streets with concrete.
And we're probably still using
very manual methods. horse-drawn concrete mixers and
because a concrete street or And that's a big undertaking
concrete. rIf any of you have ever done
your own sidewalk, it's a lot of
And then in 1916, the Portland work.
The Portland Cement Association Cis an association of cementd.
producers.
infrastructure or industry of the United States had now grown
to where they can organize. So eventually Morton Withey
passes the study on to Kurt F. Wendt.
Kurt F. Wendt might be a
the engineering library is the Kurt F. Wendt Library.
The picture shows him standing
In 1910 when the project in front of the library.
at the time. started, he was four years old
age where his career might go Hand how he might fit into this
study.
In 1941, he joins the one hundred-year concrete
project, as I call it.
the College of Engineering. In 1963, he's elected president
of the American Society of
retires. Engineering Education and '71 he
that time? What's going on in the world at
is discovered which greatly In 1enhances the freeze/thawrete
durability. So I won't go into all of the
details, but they learned that
bubbles into a concrete mix, it acted like a pressure reservoir
for the expanding ice that would
And the durability was greatly get into the concrete.
It was discovered completely by accident, by the way.
I'd like to be able to say a
discovered it, but it was really purposeful research project
a contaminated mixer mixing in a an accidental discovery based on
unintentionally in the concrete material that mix.rated bubbles
And to everyone's amazement, the
in freeze/thaw. concrIn 1936 the Hoover and time
Grand Coulee dams were generated.
And as those dams were built
I'm in the deacceleration phase, controlling the heat, now
It's getting a lot better. I don't know about you.
Yep, there you go. [INAUDIBLE]
dams were built, and these large So the Hoover and Grand Coulee
you experienced a few minutes damago was one of the designhat
challenges that they faced. Also in the 1930s and later,
changes were made to the cement grinding to make it finer and
with a higher C3S content.
cement demand is six times per And now by this time in 1940
what it was simply 30 years ago caat 180 kilograms per person.o
So the study was eventually passed to George Washa.
In 1910, he was one year old
In 1930, he joined the college as the project commenced.
graduate school. as an instructor while in
time frames he's almost a Icontemporary of Kurt F Wendt.se
In 1941, he joins the hundred year concrete project.
Receives a number of awards from
In 1942, joins the college as an the American Concrete Institute.
Along in there he becomes the chair of the Department of
Engineering Mechanics.
And then I come along. And in 1978, he retired.
And that was when the 50-year In results were published.old.
I came to the College of
in 1975, and lo and behold in 1977 I have a class with George
Washa where he teaches me concrete mix design.
I go off to graduate school and to go on to have my career, and
somewhat by chance, maybe by
come back to Madison and I join the faculty in the College of
Engineering.
another professor by the name of In 1987, George Washa and
why don't you work with us, Jeffwe're going to test someay,
long-term concrete. And thus I become involved.
In 2003 cement demand is now
worldwide. 320 kilograms per capita,
only about this demand of cement So, again, you need to think not
population of the world is that's increasing but the
And then in 2006 I joined the Dean's Office in academic
affairs.
truly and Jim Ottlane, who was And that's a picture of yours
setting up to do one of the a t50-year tests in 1987.ge,
Right, that's me just yesterday.
[LAUGHTER] No, not quite.
about the hundred-year test So nprogram specifically.e bit
There were three series that
In 1910, series A was made. werThe test concluded as wers.
conducted them here in 2010.
and we still have cylinders to There's also a 1923 series B,
conclude in 2023. Let's see, that's 11 more years,
I don't know if that's me or
1937 was series C. not.
And those tests concluded in Those were just 50-year tests.
to a minute ago. 1987, and that's what I alluded
most recent go around, The tests have included, in our
chemistry, compressive tensile petrographic examination, cement
There was in some earlier work strength.
shrinkage of concrete and lofreeze/thaw durability.and
The series A tests broke into two kinds of specimens.
Those that were submerged in
So there was these specimens water.
a hundred years. sitting in water for essentially
specimens "in the cellar." And then there were some
basement of Engineering Hall. Now, there were probably other
locations of that cellar over time but they were in the
basement. Now, the reason I call it "in
the cellar" rather than just out
in Engineering Hall gets pretty in damp in the summertime.ment
So there's probably a higher
that environment than you might humiditsee just outside.ture in
There were two water/cement ratios, .63 and .90.
Those are high by today's standards.
We're doing some concrete experiments for the Wisconsin
Department of Transportation. We typically work with a water
to cement ratio of .4, .42, somewhere in there.
If we look at the cement
this at the time they made the chemistry, and they measured
remember that generates that specimens, the C3S, the alite
That was about 29% in 1910. It was Atlas Cement.
Medusa Cement from 1937 had 53%.
cement, it's like 60%. And then if you look at a modern
the '30s on, they started to So remember I mentioned that in
more C3S in it. mAnd indeed we see that here.
And, conversely, the C2S, and an
belite, had 44%, and you can see impure form of that would be
Also, as I indicated, nowthe fineness has changed.2%.
In 1910 that cement only about
325 sieve. 60% of it passed the number
.045-millimeter screen opening. TSo that's pretty small. a
And then you can see that today that number has increased to
pretty much 95%.
through a 325 sieve. Almost all of the cement will go
analysis. She took thin sections for
optical mineralogy. She looked at polished surfaces
to look at the structure, and she looked at fractured
surfaces. And, you know, some of the
general findings she had was
was harder and less fragile that than the concretencrete
that was "in the cellar." And the overall matrix
contained a lot more binder than
I had mentioned earlier that concretes today.
atmosphere can chemically degrade concrete, and the way
that that works is the CO2 on the surface will attack the
calcium hydroxide first,
will generate calcium carbonate combiningplus water.O2, and it
And you all know calcium
pretty crumbly, there's not much carbonate is pretty chalky,
So that's not a good thing. sOnce the CO2 consumes the.
calcium hydroxide and if it's still continuing to penetrate
into the concrete, next it will attack the calcium silicate
hydrate.
calcium carbonate, and of course So that, again, will create
component of the concrete, so as this is the main strength
gets degraded and is replaced by that calcium silicate hydrate
degrade then in the strength. She found that there were poorly
graded materials. There wasn't much size diversity
in the sand.
a brand new material in 1910 You have to remember concrete's
So many of the rules and when they made these.
they didn't know then. refinements that exist today,
doing the experiment. They wanted to learn more about
how these properties changed in near term and in long term.
This specimen, C340, C stands for cellar, that's why the using
the term, was completely
carbonation products were replacing the concrete products.
In the water-cured specimens, the cement particles in general
were very large, some unhydrated
Very interesting. After a hundred years she's
still finding some unhydrated cement compounds in the
concrete. One would think at some point
that process would end. But the particles are larger
than they are today so it takes a little bit longer to react.
She found in the alite and
rings, and some of those were beven visible with a stereo
microscope. So, again, the C stands for
cellar and the W stands for water.
And different observations could
observations she could see that bthose in the cellar were goinge
to be weaker than those cured in
And as I mentioned, there's two water/cement ratios occurring
here as well, .63 and .9, and from the microstructure she
could see that the ones with a
going to be more porous and the hiCO2 is going to be able toe
penetrate them more easily and
occur. the carbonation is going to
silicate hydrate, there to begin There's not as much CSH, calcium
So she also did some scanning electron microscope analysis.
She found a very well-bonded
there'd be these little dolomite matrix in some cases, but then
indicating that there was an particles in others kind of
particles in some of the excessive amount of fine
And you can see cracking that's occurring here because that
aggregate's not as clean and
And as she went into some of the hard as we'd like it to be.
present. voids, the ettringite was
hydration product that can Ettringite is a secondary
And actually if there's, occur.
there's water and moisture ettringite will only form when
So if you're out and there's present.
likely some ettringite is water gettinforming.r concrete,
And if it fills all the voids,
break your concrete up. Cement with a reaction ring.
A large belite particle.
that she's finding in here. And there's some ettringite.
The ettringite is going to
It can actually form a gel which attract water.
You can see a crack forming becomethere.nsive.
Here's some calcium hydroxide particles surrounding
aggregates. Calcium hydroxide tends to fill
in some of the nooks and
silicate hydrate doesn't form crannies thin.the calcium
And calcium hydroxide is fine,
strong, but it's less stable. There are more things that can
attack it and break it down.
at carbonation depth in the wet curing, she found carbonationd
only on the surface only. But when she went to the dry
curing specimens, it was
This is what a compressive test carbonatedlooked like. through.
And I had indicated earlier that
are six inches in diameter typically compressive specimens
Back in 1910 they were six by 18, so they tended to be a
little bit longer.
chalky. As you broke them apart there
was just this dust coming out.
calcium carbonate forming based And,on the carbonation.s the
Okay, finally we get to some
And here we have age on a log scale on the bottom
and compressive strength
And here's their first test after about a month.
And the specimen that's been
is just slightly stronger than moist-cured after about a month
the cellar. theAnd that's not surprising in
because there's plenty of water in the concrete at that time.
As long as it's not dramatically
there for both specimens to driproceed and gain strength.er
And they do that all during pretty much that first year.
And then something happens in here.
This is in 1911.
there, but you can see that the strength actually went down and
then it starts to come back up. What's really interesting is the
water-cured and the cellar-cured
parallel each other. specimens tend to kind of
happening to both. So whatever was going on was
one to 10 years. Then we get into this period of
The cellar ones have kind of We're seeing some strength gain.
And then very interesting at the flattened out.
again, and they're actually parallel to each other but now
the distance is starting to come
And then we see that the cellar ones have dropped off in
strength after a hundred years, and, no surprise, they're
completely carbonated.
years, taken its toll. The CO2 has now, after a hundred
Kind of the opposite situation This concrete is in decline.
Of course, they're submerged. So CO2 is not really getting to
them, and even after, from 50 to
obvious here in the log scale, 100 years, and it's may be not
strength. It's still hydrating, it's still
gaining strength. That's really pretty amazing.
The material is continuing to develop even after a hundred
years.
of .63. We see a similar result with the
water/cement ratio of .9. But, of course, the strengths
are lower.
there's less calcium silicate Anhydrate there, there's morese
water voids existing. And, again, we see the same
trend at a hundred years, between 50 and a hundred, we can
now see that the compressive
carbonation and yet it's slight, strength is in decline due to
here, but at a hundred years, from 50 to a hundred years we're
still gaining strength in the submerged specimens.
Just looking quickly at some
C series, 1987, the 1937 series. specimens that were made in the
water/cement ratio. So they start out at a higherr
strength. These were actually left
outside, not in a cellar, not in
You can see that they peaked out water, but outside.
of flatten out after that. at 10 years, and then just kind
the fineness of the cement. PThey shot up in strength muchs
more quickly, and then they didn't have as much C2S or
belite to carry it on longer and
And at some point this was a longer.
The reaction perhaps ceased muccompletely.ent.
So to summarize, concrete's an amazing material.
Truly an amazing material.
today when you look at, not the cement sidewalk, the concretee
sidewalk, you'll see it in a new
Because of its longevity, studying it and testing it
is always a challenge.
case is it not only traces the But what's really unique in this
material at the time in the hUnited States, it actuallyw
traces some of the history of the University of Wisconsin
Madison.
the concrete continues to gain Even after a hundred years,
is that when we take it out of strength, but what you can see
air, CO2, simple old CO2, tends to take its toll and essentially
render it structureless or
Thank you. with no structure or strength.
[APPLAUSE]