Wisconsin Public TV University Place Wednesday Nite at the Lab  Steve Cramer, The Centennial Study of Concrete  February 23, 2011 UP557     cc >> Tom Zinnen: Welcome, everybody, to Wednesday Nite at the Lab. I'm Tom Zinnen, and I work here at the University of Wisconsin-Madison Biotechnology Center and for the UW Extension Cooperative Extension. And on behalf of our partners at the Wisconsin Alumni Association and Wisconsin Public Television, welcome again to Wednesday Nite at the Lab. We do this every Wednesday night, 50 times a year. This is a very special night because this is the start of our sixth year, and I'm pretty pleased that we've been able to flourish this long, and I'm looking forward to doing this for many more years to come. 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 launch an exploration into the future. Tonight's speaker is Steve Cramer. He's an Associate Dean for Academic Affairs at the College of Engineering. And he leads the undergraduate and graduate programs in related activities there. As a researcher, Cramer studies structural engineering with primary expertise in the mechanical behavior of wood and wood-based materials, the design and analysis of wood structures, and the performance of concrete construction materials. 

He's a member of the UW Madison Teaching Academy. In 2002, he earned the Chancellor's Distinguished Teaching Award. He received his bachelor's in civil and environmental engineering from UW Madison in 1979, and then he headed to Colorado state for his masters and his PhD. He joined the College of Engineering faculty in 1984. And you can see the title of his talk is going to be the 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. I'd like to welcome you to the lectern. [APPLAUSE] >> Steve Cramer: Thank you. So with all of the things going on in the world and all of the things going on in the state, you're here to learn about concrete tonight. 

[LAUGHTER] That is absolutely wonderful. I don't think I could ever have such a great audience. >> We want to know what's going on in Walker's head. [LAUGHTER] >> Well, now, we're going to take a break from the politics tonight, and we're going to talk about concrete. The other folks in my team are Jess Silva, a PhD candidate in the material science program. 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 he was active in this project as well. I want to acknowledge Jim Schmitt, the president of Schmitt Technical Services. He helped us do the petrographic analysis. Ruipeng Li is a graduate assistant working with me, as is Ramsey Kropp. 

Bill Lang works in our 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 later, so I won't list them. But certainly this isn't a study that we began, it's a study that we're completing. Now, there's some preliminaries here before we get started. I'd like to pass out my cement grains. My cement grains are atomic fire balls. If you like an atomic fire ball, please take one. If you don't, that's fine. 

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 going to ask you to plop that into your mouth. But not now. So I'm going to pass this out and get this going and please help yourself. And at the appropriate time I'll say go, and if you'd like to take one. 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. So if you're not up for that tonight, that's all right. And don't open them yet. Now I'm watching. 

[LAUGHTER] No one eats them yet. >> What does the word atomic have reference to? >> Well, that will become apparent when you put it into your mouth. [LAUGHTER] It kind of explodes. And it will create a hot tingly sensation. >> And are you going to put it in your mouth? >> I am. I am. At the appropriate time. We're not there yet. 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 widely used construction material in the world, but at times the general public doesn't really understand this concept very well. 

[LAUGHTER] They think it's hard. They think it's durable. They think it lasts forever. And by the time you leave here, in a short while, I think you'll find that it's a much more delicate material and complex material than probably what you imagined. So cement is any material that acts to bind other materials 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 1910. And cement paste we'll think of as Portland cement plus water. So what do you see here? What do you see? [INAUDIBLE] Okay. 

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. They're making a cement road. 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 of the hydration aspects today. Now, these lessons here are basic. My children had the distinct pleasure of knowing this by the time they were five. [LAUGHTER] It's just the way it is when your dad's an engineer and a faculty member. You'll often see these referred to as cement trucks. They're not cement trucks. They're concrete ready-mix 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. 

No, it's not. 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 chemistry going on in the truck, on the highway, on the sidewalk. It's all chemistry. And if we could perfect the chemistry, probably concrete would last even longer than it does now. So this is a cement truck, and there's one at a job site. And, of course, cement is the powdery material that's added with water, fine aggregates, coarse aggregates, and add mixtures. What else do you see there? [INAUDIBLE] Fruit cake. Cement is to concrete as flour is to fruit cake. 

So getting the terminology right is an important aspect here of talking about concrete. Now, Portland cement is a hydraulic cement, and that means it's going to harden due to the chemical reaction to water. Tends not to degrade when 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, which in cement chemist's language we call C3S, and that's the largest portion of Portland 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 amount. And tetracalcium aluminoferrite, an even smaller amount. And you can see the depiction there of some of these compounds that would be in a cement grain. And cement grain, of course, is much smaller than my model that I've given to you. 

>> What's the significance of the word Portland? >> We will come to that. That will come shortly. So, we don't have time tonight to go into exactly how Portland cement is manufactured. Raw materials include calcium carbonates. Those could be limestones, those could be seashells. They could be a variety of different kind of calcium type materials: silica, alumina, iron oxide. And we find that the C3S hydrates and hardens most rapidly. And that's responsible for the initial set of the concrete and the early strength of the concrete. C2S hydrates and hardens a bit more slowly and that tends to be more of a contributor to strength after seven days. C3A has a high heat release in its early age. 

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 compounds as well. 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. So we're going to talk about the exothermic reaction here, and you have the model, the atomic fire ball is my model of the cement grain. And probably the only moisture you have available is in your mouth. So that's where your cement particle is going to go. And we're going to see that the C3S plus the water produces this chemical compound plus this compound, and that's notation for calcium silicate hydrate and calcium hydroxide. 

Similarly but slower, (coughs), excuse me. [LAUGHTER] The C2S plus water is going to produce calcium silicate hydrate and calcium hydroxide. Now, it's very important you pay attention as well not only to the screen but what's happening in your mouth. Keep track of the timing of what happens. Now the calcium silicate hydrate is not a well-defined compound, so we can't chemically control 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 water. Now here's what's going on in my mouth, if not yours, we plopped this in our mouth a couple of minutes ago and there was a nice little sweet taste right off the bat. Kind of enjoyable. And then for a few minutes while I was talking, not much happened. 

We were in this stage two, which is sometimes called the 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. >> We need water. >> 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 three and a half hour timeframe. We don't have that kind of time here. So the model works a little bit faster than that. And if you bear with us here a little bit longer, this will subside. 

I don't think I've reached the peak yet, personally. And then it will have this long kind of deceleration, if you might, or period where it's not generating so much heat. And it will just kind of peter out. And what's happening is it's creating calcium silicate hydrate around the particle as we get down in here. And the hydration process is going to continue, but it's going to become diffusion controlled. It's going to be the ability of water to penetrate the hydration products that are forming around the cement grain which is going to control how fast it gains strength. Back in here, it's just the availability of water. The water is getting to that cement grain and that cement grain is exploding with crystal growth. I think I got the larger ones this time which means that they last a lot longer. [LAUGHTER] >> How can you talk with it in your mouth? [LAUGHTER] >> Well, we're not through yet. 

We'll see how long I go. >> Practice. You've done this before. >> I've done this before. Sometimes I'm not very successful in keeping the lecture going while I'm talking, but that's usually to the delight of my audience. >> Do you have water? >> Nope. So here we see these different phases illustrated pictorially. The large red pieces here represent the C3S in a concrete matrix, or a cement paste matrix maybe more appropriately. The C2S is the light blue parts like right in there. The C3A is the bright green. 

And the C4AF, which I don't, oh, yeah there's some right up in here, is the orange. And then as it progresses through these different stages, it's going to produce calcium hydroxide, which is dark blue, and the calcium silicate hydrate, which is the yellow. 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 don't have to use so much cement because the cement is expensive. Now, our primary measure of quality is the compressive strength. And compressive strength is measured by creating the samples, which these gentlemen here are forming, and typically that's sample is a six by 12-inch cylinder. And there's a very appropriate way to make these and you take them back to the laboratory and you push on them and you crush them and that's just universally across the world the primary measure of concrete quality. Now what controls the compressive strength is the water/cement ratio. And that was discovered in 1918. 

I'm going to talk about the history of Portland cement a little bit more. It was discovered, at least in its most relatively modern form, in 1845. So for about 50 some years, 60, 70 years, they really didn't understand quite how it worked. And in 1918, Ben Abrams discovered that water, the amount of water to the amount of cement was very, very important. Now we only need about .22 to a quarter pound of water to hydrate all the cement. For one pound of cement you only need about a quarter pound of water. 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 look smooth, to be able to move it into position, they add more water. So that, in essence, it almost flows into place. 

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 void-filled solid than you'd have if you add less water. And if only a quarter pound of water is needed to hydrate the cement, then where's the other water go? Well it dries out eventually, it migrates, and behind it will leave voids. And those voids are doorways for bad things to happen to the concrete. Chemicals can get in and 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? Wrong. The processes of degradation could be physical or mechanical, abrasion, expansion and contraction. You hear about in the summer, these blowups on the highway that occur as the concrete expands. Freeze/thaw attacks, a big deal in Wisconsin. Water gets in, it freezes, it expands, creates expansive pressures. 

Chemical attacks too. This is a chemistry experiment. You have acids that can break down the concrete. You have salts, you have sulfates, and you have just simple carbon dioxide that's in the atmosphere can cause carbonation and break down the concrete. So these are the enemies of concrete. If you drive your car into the garage, salt comes off your car, you're starting to see your concrete garage floor start to spall, the thing you could do during the summer is get a good sealer on that so that salt can't penetrate and affect the chemistry. So it's time for some history now. 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 question. 

It's been produced in the United States since 1871. And today we use five to six sacks, a sack is 94 pounds, per person per year in the United States. 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 relates to the University of Wisconsin and the state. The study began with Morton O. Withey, and in 1905 he became a faculty member in the College of Engineering. I don't think it was even called the College of Engineering at that time. And what's interesting is in 1910 he didn't launch a hundred-year concrete study, he launched a 10-year concrete study. He made a whole slough of specimens for testing with the idea that after about 10 years the study would wrap up. Okay? He won a number of awards, and this may become apparent why in just a second. He won the Wason Medal, the Turner Medal, 1932, 1947, respectively, from the American Concrete Institute which was formed in 1904. 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 to a 50-year study. So, you know, I like to talk about the extraordinary vision that he had for a hundred-year study, but quite honestly it wasn't planned to be a hundred-year study in the beginning. But he did make enough specimens such that it could become a hundred-year study and we have to give him credit for that foresight. Now, what's going on at the same time in the United States? In 1902, Thomas Edison introduced the first large scale cement kiln. So prior to 1902, while it was being produced in the United States since 1871, it wasn't really being used in a broad scale. 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. 

In 1909, horse-drawn 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? >> The wheel power, as I understand it. So they pulled, the horses pulled the mixer along, obviously on a much smaller scale than the trucks I showed earlier, and the wheels as they turned had some sort of paddle or so forth to mix the concrete. Cement demand in 1910 when the study began is 20 kilograms 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 time. So in 1916 we begin to pave streets with concrete. And we're probably still using horse-drawn concrete mixers and very manual methods. And that's a big undertaking because a concrete street or road takes a heck of a lot of concrete. 

If any of you have ever done your own sidewalk, it's a lot of work. And then in 1916, the Portland Cement Association is founded. The Portland Cement Association is an association of cement producers. So the cement producing 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 familiar name to you because the engineering library is the Kurt F. Wendt Library. The picture shows him standing in front of the library. In 1910 when the project started, he was four years old at the time. Hardly could he imagine at that age where his career might go and how he might fit into this study. In 1936, he joins the UW. 

In 1941, he joins the one hundred-year concrete project, as I call it. In 1953, he's appointed dean of the College of Engineering. In 1963, he's elected president of the American Society of Engineering Education and '71 he retires. What's going on in the world at that time? In 1930, air entraining concrete is discovered which greatly enhances the freeze/thaw durability. So I won't go into all of the details, but they learned that if you purposely put small bubbles into a concrete mix, it acted like a pressure reservoir for the expanding ice that would get into the concrete. And the durability was greatly enhanced. It was discovered completely by accident, by the way. I'd like to be able to say a purposeful research project discovered it, but it was really an accidental discovery based on a contaminated mixer mixing in a material that generated bubbles unintentionally in the concrete mix. And to everyone's amazement, the concrete lasted a very long time in freeze/thaw. In 1936 the Hoover and Grand Coulee dams were generated. 

And as those dams were built controlling the heat, now I'm in the deacceleration phase, I don't know about you. It's getting a lot better. [INAUDIBLE] Yep, there you go. So the Hoover and Grand Coulee dams were built, and these large dams controlling that heat that you experienced a few minutes ago was one of the design 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. And now by this time in 1940 cement demand is six times per capita in the world compared to what it was simply 30 years ago at 180 kilograms per person. So the study was eventually passed to George Washa. In 1910, he was one year old as the project commenced. In 1930, he joined the college as an instructor while in graduate school. If you're keeping track of these time frames he's almost a contemporary of Kurt F Wendt. 

In 1941, he joins the hundred year concrete project. Receives a number of awards from the American Concrete Institute. In 1942, joins the college as an assistant professor. Along in there he becomes the chair of the Department of Engineering Mechanics. And in 1978, he retired. And then I come along. In 1961, I was four years old. And that was when the 50-year results were published. I came to the College of Engineering as an undergraduate 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 chance, maybe not, in 1984 I come back to Madison and I join the faculty in the College of Engineering. 

In 1987, George Washa and another professor by the name of Jeff Seaman come to me and say, why don't you work with us, we're going to test some long-term concrete. And thus I become involved. In 2003 cement demand is now 320 kilograms per capita, worldwide. So, again, you need to think not only about this demand of cement that's increasing but the population of the world is increasing too. And then in 2006 I joined the Dean's Office in academic affairs. And that's a picture of yours truly and Jim Ottlane, who was a technician in the college, setting up to do one of the 50-year tests in 1987. Right, that's me just yesterday. No, not quite. [LAUGHTER] So now let's talk a little bit about the hundred-year test program specifically. There were three series that were developed over the years. 

In 1910, series A was made. The test concluded as we conducted them here in 2010. There's also a 1923 series B, and we still have cylinders to do that and those tests will conclude in 2023. Let's see, that's 11 more years, I don't know if that's me or not. 1937 was series C. Those were just 50-year tests. And those tests concluded in 1987, and that's what I alluded to a minute ago. The tests have included, in our most recent go around, petrographic examination, cement chemistry, compressive tensile strength. There was in some earlier work looking at the expansion and shrinkage of concrete and freeze/thaw durability. The series A tests broke into two kinds of specimens. 

Those that were submerged in water. So there was these specimens sitting in water for essentially a hundred years. And then there were some specimens "in the cellar." "In the cellar" is in the 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 air is I know that basement in Engineering Hall gets pretty damp in the summertime. So there's probably a higher humidity and higher moisture in that environment than you might see just outside. 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 chemistry, and they measured this at the time they made the specimens, the C3S, the alite remember that generates that early strength. That was about 29% in 1910. It was Atlas Cement. Medusa Cement from 1937 had 53%. And then if you look at a modern cement, it's like 60%. So remember I mentioned that in the '30s on, they started to make the cement finer and put more C3S in it. And indeed we see that here. And, conversely, the C2S, and an impure form of that would be belite, had 44%, and you can see now it's decreased down to 12%. Also, as I indicated, the fineness has changed. In 1910 that cement only about 60% of it passed the number 325 sieve. 

The number 325 sieve has a .045-millimeter screen opening. So that's pretty small. And then you can see that today that number has increased to pretty much 95%. Almost all of the cement will go through a 325 sieve. Jess did the petrographic 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 that the wet stored concrete was harder and less fragile than the concrete that was "in the cellar." And the overall matrix contained a lot more binder than concretes today. I had mentioned earlier that CO2, just the CO2 in the atmosphere can chemically degrade concrete, and the way that that works is the CO2 on the surface will attack the calcium hydroxide first, combining with the CO2, and it will generate calcium carbonate plus water. And you all know calcium carbonate is pretty chalky, pretty crumbly, there's not much strength to that structure. 

So that's not a good thing. Once 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. So that, again, will create calcium carbonate, and of course this is the main strength component of the concrete, so as that calcium silicate hydrate gets degraded and is replaced by calcium carbonate, you see a degrade then in the strength. She found that there were poorly graded materials. There wasn't much size diversity in the sand. You have to remember concrete's a brand new material in 1910 when they made these. So many of the rules and refinements that exist today, they didn't know then. That's part of why they were 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 carbonated to the point that carbonation products were replacing the concrete products. 

In the water-cured specimens, the cement particles in general were very large, some unhydrated cement compounds. 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 belite there were hydration rings, and some of those were even visible with a stereo microscope. So, again, the C stands for cellar and the W stands for water. And different observations could be made here, but based on these observations she could see that those in the cellar were going to be weaker than those cured in the water. 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 higher water/cement ratio are going to be more porous and the CO2 is going to be able to penetrate them more easily and the carbonation is going to occur. There's not as much CSH, calcium silicate hydrate, there to begin with. 

So she also did some scanning electron microscope analysis. She found a very well-bonded matrix in some cases, but then there'd be these little dolomite particles in others kind of indicating that there was an excessive amount of fine particles in some of the aggregates that were used. And you can see cracking that's occurring here because that aggregate's not as clean and hard as we'd like it to be. And as she went into some of the voids, the ettringite was present. Ettringite is a secondary hydration product that can occur. And actually if there's, ettringite will only form when there's water and moisture present. So if you're out and there's water getting in your concrete, likely some ettringite is forming. And if it fills all the voids, it will begin to expand and break your concrete up. Cement with a reaction ring. A large belite particle. 

So there's unhydrated cement that she's finding in here. And there's some ettringite. The ettringite is going to attract water. It can actually form a gel which becomes expansive. You can see a crack forming there. Here's some calcium hydroxide particles surrounding aggregates. Calcium hydroxide tends to fill in some of the nooks and crannies that the calcium silicate hydrate doesn't form in. And calcium hydroxide is fine, there's nothing wrong it, it's strong, but it's less stable. There are more things that can attack it and break it down. So, no surprise, as she looked at carbonation depth in the wet curing, she found carbonation only on the surface only. 

But when she went to the dry curing specimens, it was carbonated all the way through. This is what a compressive test looked like. And I had indicated earlier that typically compressive specimens are six inches in diameter by 12 inches long. Back in 1910 they were six by 18, so they tended to be a little bit longer. They tended to be very, very chalky. As you broke them apart there was just this dust coming out. And, of course, that was the calcium carbonate forming based on the carbonation. Okay, finally we get to some results here. And here we have age on a log scale on the bottom and compressive strength on the Y axis here. And here's their first test after about a month. 

And the specimen that's been moist-cured after about a month is just slightly stronger than the specimen that's been put in the cellar. And that's not surprising because there's plenty of water in the concrete at that time. As long as it's not dramatically dried out, there's enough water there for both specimens to proceed and gain strength. And they do that all during pretty much that first year. And then something happens in here. This is in 1911. I don't know what happened 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 specimens tend to kind of parallel each other. So whatever was going on was happening to both. Then we get into this period of one to 10 years. 

We're seeing some strength gain. The cellar ones have kind of flattened out. And then very interesting at the 10-year mark things take off again, and they're actually parallel to each other but now the distance is starting to come apart. And then we see that the cellar ones have dropped off in strength after a hundred years, and, no surprise, they're completely carbonated. The CO2 has now, after a hundred years, taken its toll. This concrete is in decline. Kind of the opposite situation is occurring in the water. Of course, they're submerged. So CO2 is not really getting to them, and even after, from 50 to 100 years, and it's may be not obvious here in the log scale, but it's still going up in 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. This is the water/cement ratio of .63. We see a similar result with the water/cement ratio of .9. But, of course, the strengths are lower. And they're lower again because there's less calcium silicate hydrate there, there's more 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 strength is in decline due to carbonation and yet it's slight, and perhaps barely perceptible 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 specimens that were made in the C series, 1987, the 1937 series. First of all, they had a lower water/cement ratio. So they start out at a higher strength. 

These were actually left outside, not in a cellar, not in water, but outside. You can see that they peaked out at 10 years, and then just kind of flatten out after that. Part of that, again, is perhaps the fineness of the cement. They shot up in strength much more quickly, and then they didn't have as much C2S or belite to carry it on longer and longer. And at some point this was a much finer cement. The reaction perhaps ceased completely. So to summarize, concrete's an amazing material. Truly an amazing material. I hope as you walk out of here today when you look at, not the cement sidewalk, the concrete sidewalk, you'll see it in a new way. Because of its longevity, studying it and testing it is always a challenge. 

But what's really unique in this case is it not only traces the history of what was this new material at the time in the United States, it actually traces some of the history of the University of Wisconsin Madison. Even after a hundred years, the concrete continues to gain strength, but what you can see is that when we take it out of the water and put it out in the air, CO2, simple old CO2, tends to take its toll and essentially render it structureless or with no structure or strength. Thank you. [APPLAUSE]