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

---

High Volume Flyash Concrete

Ron Middlebrook Middlebrook + Louie Structural Engineers

I feel like I've gone back to school, and it feels pretty good. Dr. Mehta's talk was very enlightening for me, as I hope it was for many of you. This is really fresh information, only two or three days old I understand. I've learned a bit about rice husk ash in the last few years, and seen some test results on it, and it's a very exciting material. In combination with flyash, the research indicates that it could very well be a big winner.

The organizers of this forum wanted an engineer on the panel, and I told them that I didn't really know that much about flyash, but they put me on it anyway. I can tell you that I have spent quite a bit of time in the last few weeks reading about it. And I probably learned as much tonight as I did in the last few weeks. The experts are going to be talking about the details of the chemistry, and the details of the curing, and all the things that are necessary to make great concrete and to make environmentally friendly concrete. But I decided after all the reading that I've been doing, and the planning meeting that we had a couple of weeks ago, that I wanted to call myself the "optimistic skeptic" on the panel. That's the role that I think most structural engineers will take, and Iim representing them here tonight. This is an important role because it's what you're going to have to face to bring about the changes and the environmental advantages of this technology. A structural engineer is not going to specify this material only on the basis that it helps the environment, or reduces global warming gases, or because it reduces landfill problems, and so on. He's going to specify it because it makes a better product.

While there have been studies for years and years on flyashes and other materials that go into very high quality concrete, most of us don't know a whole lot about it. I'll be the first to acknowledge that, and I expect that I'm fairly typical in that regard. I looked our standard spec and found, as expected, that we limit flyash use to something less than 20%. We spec it, and we give the right spec, and it's the same as we were doing 20 or 25 years ago.

I had an experience with a project in Nashville, Tennessee, a high-rise project for Marriott hotels. All of the Marriott hotels in the range of this project are post-tensioned flat plates. We put the job out to bid, and then we got a call from a contractor who wanted to use flyash. What we knew about flyash at that time was that it retards set, that it gains strength more slowly. So we said "Why in the heck do you want to do that? You're going to want to pour and stress that slab in three days, and get the forms out of there, and move them up to the next floor. You're going to want to get on a one week cycle. Flyash is going to hurt you". But he insisted.

And I'm glad he did. He forced us to take a hard look at what controls when you can strip the forms in a post-tensioned flat plate, it's when you can stress the tendons. You can't stress them until the concrete is strong enough to take the bursting stress at the stressing point. Now ordinarily, without even thinking about it, you can arbitrarily say that you must have 3,000 psi concrete before you can stress. He said "All I want you to tell me is that I can stress at 2,700 psi". So we took it back to the office and made a few calculations and decided we could do that. Well, that was the best concrete job that I've ever seen. There was not a crack in it. And it was probably only 20% flyash, so maybe we could have made it even better, and helped out along the way.

So, in our office we are going to have to do something about our standard spec. But we're going to have to do it in a reasonable manner. I think some of the things that I've learned here tonight are going to help us do that, and that's where the optimistic skeptic comes in.

I'd like to go through just a really general abbreviated list of some of the pro's and con's that I've picked up. Some of these may have been answered, and some may or may not be appropriate. But for those of you who are in the same position that I am with this material, you may have some of these same questions.

Some of the advertised technical benefits of using high volume flyash:

Higher compressive strength over time

More durable concrete

Less permeable concrete

Less shrinkage

Less creep

Lower heat of hydration

Less migration of bleed water to the slab surface

Better pumpability.

Some of the technical drawbacks:

Slower rate of compressive strength gain

It may be more difficult to finish (i.e.: it may delay finishing)

It is one more product to control at the point of batching

And I left purposely third on my list

The environmental & non-technical benefits of high volume flyash:

Flyash costs less than cement

It saves the energy required for making cement

It reduces the emissions of global warming gasses

It usefully employs a waste product

This last category of benefits is very important, but it alone is not something that is going to make engineers like myself go out and specify it. What will lead to more use of flyash is increased education. What Iid really like to see are some local, comprehensive, year long tests that use local materials, local people, local testing, local concrete producers, and local Bay Area materials. We can publish the results as we go along, and let people know how things are coming, what these materials are doing, and what the benefits are. If the benefits are there, you will see it specified. The benefits have to be there and we have to publish these results.

The first two lists I gave indicate that there are probably more benefits than drawbacks to using high volume flyash concrete. The ratio may actually be much greater than the 8 to 3 in these lists. I think it probably is much greater. I was interested in the CANMET tests, which I had access to recently. Those tests, and some of the things that Dr. Mehta showed towards the end of his presentation, indicate that we can in fact get enough early strength to use this material for such things as post-tensioned flat plate.

I'd like to close with some general questions and comments, and maybe these will generate questions from you during the open forum discussion period. These are in no particular order. How does high volume flyash concrete work with fiber mesh? We're seeing more and more use of fiber mesh, especially over steel deck where it acts as the primary reinforcement for the slab. Dr. Mehta may have answered some of this concern, but what elements could tolerate slow initial compressive strength gain? For example, and these are questions: footings, mat foundations, lower story columns, basement walls, slabs on grade, slabs on steel decks, sidewalks, site paving, curbs, equipment bases? This might require slightly different movement of people and machines around the site timing wise so that we don't load pavements too soon for example. But that's food for thought.

What elements need high early strength--three days or so? I've listed post-tensioned slabs, which Iim quite familiar with, any kind of structured decks, slab and beam, waffle slab, joists, and perhaps the upper level columns and walls that are smaller and don't have as much time to cure before they get fully loaded. Can high early strength cement be used effectively with flyash? The CANMET studies, Langley and Leaman seem to indicate quite good early strengths using low water cementitious materials ratios and super-plasticizers. Could the beneficial effects of flyash allow the use of problem Bay Area aggregates? Iid also like to know the costs, Class F flyash versus Class C flyash versus cement. If you save cement how much would you save? What are the costs of other mineral aggregates, blast furnace slag, rice hull ash, silica fume, and super-plasticizer. It would be helpful to know about the costs of these things. What would be the representative bid for delivered costs for ordinary concrete versus 20% flyash concrete versus 50% flyash concrete? The engineers and cost consultants need to know these figures.

Thank you for having me. We will now assemble into a panel and take questions.

Back to Flyash Table of Contents