Concrete, Flyash, and the Environment - Proceedings

A forum held 8 December 1998 - Sponsored by EHDD Architecture and Pacific Energy Center

Back to Flyash Table of Contents

---

Open Forum Discussion

Panelists:

P.K. Mehta, Ron Middlebrook, Scott Shell, Jon Asselanis, Burt Lockwood

 

Audience: Ron, as a structural engineer do you ever specify the rate of strength gain of concrete? In my experience in Southern California, it's usually a 28 day strength that's required, I've not seen a strength gain of 3, 7, and 28 days.

Ron Middlebrook: The way we do that is we would limit the strength of the cylinders. For a post tensioned slab, before they can stress the slab, they have to meet a minimum strength.

Audience: So the rate of strength gain would be up to the discretion of the contractor. As a structural engineer it doesn't matter if the concrete makes that 3,000 psi in one day or in 28, as long as it meets the 28 day requirement.

Audience: Burt, have you had the opportunity to batch a concrete with a higher percentage of flyash? Like Ron, we usually limit flyash to 20%, and if so what's your experience with that and can you elaborate on it?

Burt Lockwood: No and Yes, most of our concrete, 80-85% of it, has 15% flyash replacement. Up to 25 or 30% we've done a little testing with Master Builders who has a new admixture out. With what we've learned here tonight, I was always under the impression that flyash mixes took longer to set up, so we have used 25-30% using an accelerator type admixture to get those strengths up equal to what a 15% flyash or a 100% portland cement mix would be, and we've had pretty good results. But like I say, we really haven't had the opportunity, even with Cal-Trans to use the 25-30% mix. Regarding the 15% flyash replacement that we do use, when compared to the straight cement mixes that we could use, I would say that our seven day strengths might vary 150-200 psi. The 28 day strengths are the same.

Audience: I wasn't asking so much about the strengths, but just physically in the plant, just batching it. If I say I want 40% and I get a plant to say yes Iill do that for you, is the plant going to come back to me and say "You can't believe what it's going to do to my machinery and so forth". It's the practical aspects that Iim concerned about. Do you see any problems on a commercial scale making this type of concrete?

Burt Lockwood: On the actual batching, No. Most of your batch plants, Kelly Idiart from Central and Al Kaufman from RMC Lonestar are here, most of it's done by computer anyway. You put your percentages in and it batches those percentages automatically. The one problem that I can foresee is with the increase in the use of flyash is not having adequate storage facility. We're very limited in how much cement, and how much flyash we can store at one time. But that could be worked out.

Audience: And then when I ask for 10% rice hull ash what's going to happen?

Burt Lockwood: That's going to have to be batched manually at our plants right now. We would have to break sacks. But if it became just a commodity product that we used every day, then we would have the same facilities that we have to handle flyash.

P.K. Mehta: There is a possibility that rice hull ash may come pre-mixed with flyash.

Audience: The last time a did a large concrete job about four years ago and wanted to use flyash, I called around to all the local suppliers, RMC Lonestar, Walker, and others, and I got a variety of answers at that time "Yes we know flyash", or "we'll charge you extra", or "we don't have it", or "we use it as part of our mix". I don't know how that's changed. But even though I wanted to use it and ask for it, how do I know that I got it?

Burt Lockwood: the way that we verify it is that all of our plants are batched by computer. The computers are checked once a year by Cal-Trans and then they are sealed. Everything that we batch is printed out, every thing that goes into the truck, and that's the only way we have unless you want to come down and look in the silos and see which one has cement and which one has flyash. Why would we say we are going to sell it and then not?

Audience: Do most suppliers have flyash now?

Burt Lockwood: Everyone I know has it.

Audience: Does the flyash cost less than Portland cement?

Burt Lockwood: Yes

Audience: Can one get it in bags?

Jon Asselanis: Yes. I believe a company called Quik-Crete has sack flyash.

P.K. Mehta: Rice hull ash is not commercially available at this time.

Audience: A lot of the work that we do in structural engineering has to do with architectural exposed concrete. How would high volume flyash affect the finishability and the control that an architect has towards the finish of the concrete, and what about the color?

Burt Lockwood: Flyash concrete would be a lighter color, because the flyash is lighter.

Jon Madderom [from ISG Resources, a flyash supplier]: Here are some samples of flyash and regular portland cement that Iill pass around, and as you can see, the flyash is lighter in color.

John Asselanis: The finishability is different than you might think based on a micro-silica based concrete which might be very sticky and hard to work with. It's been my experience that high volume flyash mixes are actually a little easier to finish. They don't bleed, but they don't stick on your trowel either. You don't want to overwork it.

Audience: We've heard that high volume flyash might affect the set time, what about the amount of time it takes for finishers to get on the slab?

John Asselanis: It used to be the old adage that you let the bleed water go away before you started doing anything or finishing it. But without the bleedwater, you will have to go by a firmness test, a thumb test. It's got to be a feel for when you can get on it, it's no longer based on sheen.

Audience: I think the key to that question is what is the temperature at the time of placement. We've talked about the prospect of 50% flyash versus 50% cement, and less cost by using super-plasticizers. The big question I have is what would happen to the cost of placement and time of construction if it was 40 degrees out?

Al Kaufman [Technical Services Manager at RMC Lonestar]: I can answer that one, we did some 30% flyash work in some foundations about 4 years ago, about 19,000 yards worth, in weather about like we had today, and we finished it 30 hours later. But then as long as the finisher knew that he had to wait a while to finish it, it wasn't that big of a problem. It was a dramatic effect, but it's not that negative, because as long as the finisher knows what he has to do its not a problem if you educate people. Now if we had just furnished the concrete in 40 degree weather, and we had just sat back and waited, they would have cut my head off. But because I educated them and I told them that you better have some finishers around, but why don't you send them home and have them on call, and probably in 24-30 hours, and it was 30 hours before we finished. It's a problem, but it can be handled. And while Iim talking, Iid like to make some comments. I've been working with high flyash concrete for a long time. I have some data from my lab that came from Mohan Malhotrais original work at CANMET, and I was really intrigued by it, I was super intrigued by it, that was five of six years ago. And one of the questions was does it work in the Bay Area, and yes it works in the bay area, it works very good. But there's a problem. And I think if you guys, structural engineers, could walk away tonight, if I could tell you one thing to do that you could take away with you tonight to take advantage of all this stuff we're doing, would you do it? I can tell you how to do it. You know why you don't do it? Because you're moon struck. The structural engineers are moon struck. How long does it take the moon to make one revolution around the earth? 28 days. How long do we specify our concrete reach strength? 28 days. Why? I can tell you why. In 1919 Duff Abrams wrote a paper, a very famous paper, all those guys know about it. And he said concrete gets about 90% of its strength in about a month. Well, they tried that and the guy in the testing lab loved it because he'd get overtime for breaking cylinders on the weekend. So if you do 7, 28, 56, 91, 182 days you don't have to work on Saturday, and that's why you do 28 days. RMC Lonestar, and Iim the technical services manager at RMC Lonestar, we've done an awful lot of concrete, Iid say probably 100,000 yards of 30% flyash concrete. But we have to do one thing to do it, we can't get moonstruck. We can get double moonstruck or triple moonstruck. Why do you put 28 days? Now Iim a fan of P.K. Mehta's, and I have been for 30 years. He discovers it all and then I go out and try and make money with it. Why would you, let's say your building a high-rise building and its going to take a year to build, and you're going to have a 6i thick mat in the building, and even if you get the building built, the only way you'd ever need to get all of the strength in the footing is if you get a Richter 7 earthquake. Why do you say I've got to have 28 day concrete. Do you know that by switching from 28 to 56 days in your specifications, our mix would probably save 17-18% cement, and we would use more flyash. There's a job in Sacramento that has about 9,000 yards of concrete in the mat foundation, that sat out, and nobody did anything to that mat foundation for 40 or 50 days. And yet I had to get full strength at 28 days, there is no sense in that. If you take one thing away tonight, if you want to do this thing, if you want to save 18% cement and use flyash across the board, then start going to 56 day strengths, and I've got lots of data that will show you what that will do. And if you want to go even farther then go to 90 days and I further submit that you are not going to get this thing done that you want to do, unless you do this. So go to your spec writers and question why do I have to have the strength when I have to have the strength. And if you'll change your spec a little we can give you all this good cement.

Ron Middlebrook: 28 days is a standard, we know what 28 days means, and what it means out beyond 28 days. Anytime someone came to us with a request like that, we would certainly entertain it, but we are not going to go through the building and look at every piece of structure and say well this one only needs 31 days and that one needs 14 days and that one 51 days, we're not going to do that. But we will listen to someone who makes a proposition like that, if we're not overloading something, and if we know that we're going to get the strengths.

Al Kaufman: Let me explain how we can do that. Right now if we design a concrete mix, when all of us who design concrete mixes design a mix, we have to over-design. You're saying you want 28 day strength, that means Burt and I have to design a concrete at 28 days to be at least 15-20% over-designed average at 28 days. If you'll let us instead of being 20% over on strength at 28 days, if you'll say I want the average strength at 28 days to be the strength, and you will go to project acceptance at 56 days, you'll get very nearly the same strength at 28 days. If fact we have a 30% flyash mat mix that we use for heat of hydration, our 4,000 mix, sometimes it averages higher than our regular mix does, but what we do as a philosophy is instead of using 28 day strength with over-design we try to make our mixes average your Fc at 28 days, and at 56 days we use it for the over-design, and then it doesn't affect your design at all, its just a matter of changing your spec.

Ron Middlebrook: For non-critical elements.

P.K. Mehta: Al has raised some very important issues, and you seemed to convince my colleagues that if it's a critical element then at 28 days we go by the average strength required plus 25% over-design. But the problem is that we have standardized our technology so much, that it has become just like a kind of dictatorship. What we need is a diversification in an age of sustainable development. I think that it's an excellent point, for most of the structure it's not critical. So in those cases where these materials give you slightly lower early strength, but they catch up at 56 or 90 days, then I think that structural engineers and architects should work together to use that. Also, a very important issue that was raised about these materials in cold weather. It's very well known that at low temperatures even with portland cement, if you go too close to the freezing point portland cement doesn't hydrate. So what do you do? You use hot water as the mixing water, you use electrical blankets, and those things, even for portland cement which is very very reactive. But the flip side of the coin is that at many times and in many areas you have hot weather, in Texas and Georgia, and you have many areas of the world where most of the time its quite warm. And in those places, the lab tests which we cure at 73 degrees F and a certain relative humidity, and on which you base all of the decisions, they are totally irrelevant to the field practice. So all these issues require, that if you want to make optimal use of materials, and recycle any of the waste, we'll have to deal with these issues. We'll have to loosen up and make our specs and our construction practices less universal and more specific to the particular situation.

Anthony Bernheim [SMWM Architects]: As an architect that works in the area of green building, there are two elements to be concerned with: there is the resource efficiency such as using flyash and other things in our concrete. But the other side is indoor air quality. If I were to analyze this room, we would look at the carpet, the paint, the tiles, and the exposed concrete. The question Iim facing is in a building where we have exposed concrete, and we're adding new things into it, flyash in particular, and there are different kinds of flyash. What potential are there for emissions into the space for volatile organic compounds, or other chemicals that may be harmful to health? I think that Dr. Mehta touched on something very interesting, I think you said that flyash may tend to bond the molecules more tightly in. But I've also heard stories that there may potentially be some problems. I would like to know, other than doing a chamber test, what do we know?

P.K. Mehta: If we remember the slides, there were three types of tests. Tests A & B were leach tests, and C was a tank test. The leach test is a standard EPA test, you have a piece of concrete containing flyash and some toxic materials, and then you pulverize that material, and then try to leach it out. But this is not the way that concrete behaves, in concrete you have a piece of concrete and then you try to leach out whatever is bound inside. Can it come out or not? And that is the tank test, test C, and very little can be leached out. Professor Schiessl's tests, A & B, are not realistic tests. They are just being specified because they are valid for other things, but not for concrete.

Scott Shell: At the end of the booklet we handed out there is an article by the EPA that I found quite convincing about indoor air quality issues.

[The EPA in Cement and Concrete Containing Flyash; Guideline for Federal Procurement January 28, 1983 concludes that "little, if any, flyash exhibits characteristics defined a hazardous in the Federal regulations." There appear to be two primary health concerns that have been raised regarding flyash. One is the trace amounts of heavy metals which Dr. Mehta addressed above, and the EPA article agrees leaching of these should not be a concern. The second issue is radon emissions. The EPA article states that flyash may have a slightly higher radium content that the cement it replaces, but its emanation fraction is much lower than most materials due to its glassy structure. Consequently, "the use of flyash as a partial cement replacement is likely to reduce the radon gas contribution of the final concrete product".]

Anthony Bernheim: What Iid like to see is to put a piece of concrete with a lot of flyash into a chamber test, and run the air through and on the other end analyze which of 2,500 chemicals come out, that's how many we can detect now. What comes out of it over a period of time?

SS: I think that's a great idea. We've mentioned that we are planning on doing a testing and demonstration program. Several people have volunteered to help fund that. This is one part of the testing program that we might ask you, Anthony, to help direct.

Drew Stelman [Gateway Associates Architects]: I was wondering if any of the panel has had any discussions with building code officials about high volume flyash use, and what we might expect on that front.

Scott Shell: Bruce, do you want to answer that one?

Bruce King [Structural Engineer, author of Strawbale: Structural Engineering, and articles on Flyash published by ICBO's Building Standards Magazine]: Scott and I have had some private conversations about this issue, and in a way Iim qualified to talk about it because I do a lot of strawbale and rammed earth, and stuff that's way out there, the kind of things that really freak out building officials. But for the most part what I have found is that if we have a rational argument, they will usually listen. Just like the very first sentence of the code says, there nothing in the rest of this code intended to preclude any other system or material. The code is written to be inclusive, but we forget that and we don't design that way. But if you present a rational argument, and if you've done your homework and you present the data, generally you won't have a problem with the building department. With something like strippable, there isn't much data so that's a lot harder to do. But what we're talking about here tonight, there are reams of data, all the backup in the world that you need is already at hand. So even though what we're talking about may exceed some code restrictions, it's been my experience having done it already, that it's not that hard to do. You have to have the engineer on board, and arguing with you and all of that, but the data is all there, there is more than enough evidence.

[The 1994 Uniform Building Code and the 2000 International Building Code - Final Draft do not appear to limit flyash use. The exception is for concrete exposed to deicing chemicals, in which flyash is limited to 25%, or 50% flyash + pozzolans + slag + silica fume (see UBC Table 19-A-3 and IBC Table 1904.2.3). Since building structures, and even most civil work in California, are not subject to deicing chemicals, this should not be an impediment to high volume flyash use. On the other hand, the 1994 California Building Code 1903A.6.6 limits flyash to 15% replacement in all locations. This means that schools, hospitals, and other projects that are under the jurisdiction of the CBC will have to make their case to the building official per the rationale outlined by Bruce King above. We hope these CBC code sections will be revised to to encourage these improved mix designs.]

Audience: The impacts of cement manufacturing is energy use and the CO2 emissions. Recently I read a little blurb that someone was talking about the fact that concrete can absorb CO2. Does anyone know anything about that?

Scott Shell: We talked about that before hand, and I don't have a definitive answer. But if you think that the energy use is responsible for about 60% of the CO2 emissions, and you're left with 40% due to the chemical reaction, which could possibly be reversed. And if you think what percentage of that concrete is close to the surface and exposed where it could absorb CO2, I think it would be quite a small percentage.

[In the EPA's Inventory of U.S. Greenhouse Gas Emissions & Sinks: 1990-1996, p. 3-5, they write "As cement reacts with water, alkaline substances ... are formed ... and these compounds my react with CO2 in the atmosphere ... this reaction only occurs in the outer 0.2 inches of surface area and the amount of CO2 thought to be absorbed is minimal". See also Environmental Building News Volume 4, Number 5, page 5].

Darryl DeBoer [Architect]: The problem they had in Biosphere II is the exposed concrete kept giving off CO2 for the year that people were living in it, and that caused the potential for failure of the whole operation [editor's note: it was lack of oxygen due to soil microbes that was the primary problem. In trying to understand this problem, scientists discovered that the concrete actually absorbed CO2]. I also had a question to push the envelop a little further. In Texas, Pliny Fisk has been working with mixes that are just pozzolans with no portland cement. He claims to be having great success with that. Are there other kinds of Pozzolans that you can blend together to act as cement that will allow it to completely replace the portland cement?

P.K. Mehta: Pozzolans are mostly aluminum silicate materials. Without additional calcium, they cannot form any cementitious products which can glue together with the gravel aggregate. Now there are other waste materials like blast furnace slag which is not a pozzolan, which is not by itself a strong cement, but which in combination with portland cement can develop very good strength. There are some flyashes that are very high in calcium that are similar to slag, and you can mix high calcium class C flyash with class F flyash, so you get a source of calcium from the class C flyash. This can give you reasonable strengths, not very high strengths. But for certain purposes like making bricks and blocks it's fine. Otherwise, you need a good powerful activator and a calcium source.

Audience: The gentleman from RMC Lonestar said that high volume flyash can expect to save about 17% of cement usage. What kind of cost savings could we expect?

Al Kaufman: Put one out and we'll bid it and see. One of the problems that has plagued the industry is getting the cost savings back from flyash. I used to sell pozzolans. I started pozzolans in Northern California about 20 years ago and I worked with the industry trying to get them in, and I had this awful time trying to get the cost reflected back. Now you have to understand the mentality of the Ready-Mix industry, and then you'll understand why Iim answering you the way that I am. We bid against each other, and we'll lower your prices to what we can most economically compete with. So if you'll allow us the opportunity to compete, then we'll use our cleverness and you'll find those costs built in. But as long as you're limiting it to the specifications that we have, and tying our hands, then there's no way that the cleverer of us can lower our cost. But I can tell you, Greg Allen does our mix design for RMC Lonestar, if we have a 56 day strength specification for concrete, do we use a lower cement content Greg? Yes. Our cement salesmen price our concrete by and large based on the cement content. So yes it does reflect right straight back. If you go to 56 day strength you'll find the cost savings coming right back to you.

Audience: But I don't think our purpose here tonight is to find cost savings, I really think the savings are nickels and dimes. Our purpose here tonight is about making a better product and reducing energy use and environmental impacts. Yes, there's probably a slight cost savings, but as Burt said, a few years ago flyash and cement were very close to the same price. The savings may be relatively small, but the value in the product is the key.

Audience: Can I ask why that is? A material that you are going to throw away cost the same amount as something that you're going to put into a kiln and heat up to 2,700 degrees?

Jon Madderom: I can tell you exactly why. Cement comes from Davenport, from Bonetti [short shipping distances], what's the freight on cement from those locations? Flyash comes from Arizona, Wyoming, Colorado, etc.

George Loisos: I've heard of cement being shipped from Korea and competing economically with cement manufactured 100 miles away.

Jon Madderom: You're talking about a different type of transportation. If you move anything by water, its a lot cheaper than trucking it across town. You can bring a pozzolan in by ship from Sampan into the west coast for $19. Transportation is the issue when it comes to flyash.

George Looses: We'll can we get flyash from China? [nervous laughter]

Mike Sera [Phoenix Flyash in San Diego]: I have to back up a little bit, I think we're making this way too complicated. This gentleman behind me is concerned about the vapors in the air. Remember what flyash does, it makes the exact same glue that portland cement does, under a microscope you cannot tell a difference, its the exact same glue. In this room, I bet you that either this carpet, the paint, the screen, or this plastic has flyash in it for the cenospheres. Which is another byproduct of burning coal. It's around us all the time, it builds a better concrete, for the structural engineer it reduces alkali-silica reaction, it reduces sulfate attack, it makes a higher ultimate strength, it lowers the penetration of chemical attacks. Those are the sound reasons for engineering. The cost is going to shift. The portland cement association is now embracing flyash. The cement companies are buying the marketing rights to these coal burning power plants, and now they are realizing that it's no longer a byproduct, but in fact it is good sound engineering required in our specifications. So you're going to see in the near future, I would say by 2005, that flyash will be equal to or higher than the cost of cement. There are more and more limiting factors on the air pollution requirement for coal burning power plants, so these sources of quality ash are going to diminish. But the benefits are still going to be there, so you're going to see a shift there. In six years you're going to be saying "that little short guy told us this was going to happen". All the benefits of using flyash will drive the price up.

Audience: Iid like to ask a question about the potential effects of flyash on the moisture emissions from the concrete slab.

Scott Shell: I think this is a potentially huge benefit. I would bet that just about every architect in this room has had a project on which the flooring could not be installed at the proper time. This has been a huge headache. The client wants to move-in in a few days, everything is done and the flooring is ready to go down, and then the slab does not pass the calcium chloride moisture emissions tests. All manufacturers of resilient flooring, VCT, and many other floor coverings will not warrant the floor if the slab is releasing more that 3-5 pounds of water per 1,000 s.f. per 24 hours. And most of our slabs have not been meeting this criteria. We've been forced to put in our standard spec a floor sealer on all our jobs, and it's expensive. On the Tenderloin school which was just complete, it cost $100,000 extra and the entire school is less that 60,000 s.f. The sealer can cost as much as the flooring, not to mention the resource inefficiency. We're almost ready to quit using resilient flooring because with the cost of the floor sealer, the price doubles, and we could afford tile or wood floors. There is an article at the back of the handout that makes a strong case that these high moisture emissions are due to the porosity of the concrete, and that the way to solve the problem is to get high quality, dense, impermeable concrete. And this is exactly what these high volume flyashes do. I don't have any test data yet to back it up, but I think this is a promising technology for a very difficult problem, and that it deserves immediate testing.

Audience: Do the Ready-mix plants take any special precautions when handling flyash for health reasons?

Burt Lockwood: No. For us it's in the silos, so we don't ever really come in contact with it. If you were to break sacks, you may want to wear a dust mask because you aren't supposed to breath fine silica particles, but this is true of lots of fine powdered materials.

Scott Shell: As mentioned earlier, we are planning a testing and demonstration program, and we would like a few structural engineers to be on our oversight committee to direct and review the testing. We would like you input to decide what tests it is that you need, and what field work will give you the data and the confidence to revise your standard spec. Anyone that would be interested please come up and see me afterwards. We will be distributing proceedings from this forum in a few weeks to everyone that signed in at the front desk, so make sure your name is on the list. If you need any further information let me know, and we'll try and get it for you. Thank you all very much for coming, and spec more flyash!

Back to Flyash Table of Contents

Notes:

Metric Conversion Factors: 1 psi = 0.00689 MPa.

To convert from kg/m3 to pounds / CY divide by 0.5933