What would happen if a Renaissance concrete engineer and a leading-edge Ready-Mix pro got together at a 50's style breakfast diner in Omaha, Nebraska to compare notes? Who would learn from whom? Who would order biscuits and gravy? Who would order the pineapple plate? Most important, who would get stuck with the bill?
We took our cameras and tape recorder in to find out the answers to these and other pressing questions. Let's listen in...
A breakfast conversation between L&M Technical Director, Phil Smith and Mark Deetz, vice-president of sales for Lyman-Richey Corporation, Omaha's largest ready-mix concrete company. Mark and Phil had breakfast recently when the subject of concrete came up. Mark opened the subject.
MARK: Phil, how did you get into concrete?
PHIL: I was introduced to concrete in the mid-1950's as a teenager. I found myself on the heavy side of a wheelbarrow being gently encouraged by the job site foreman to quickly pour the mud into the footings. At this time, it occurred to me that concrete must have an academic dimension. I think you'll agree with me that today's concrete is a blending of an ancient art and modern day technology. Most of the "technology" aspect has occurred in the last 50 years.
The concrete of my youth was not self-compacting. As a matter of fact, it was not self-anything; we had to do it all. The most commonly used plasticizer had a chemical name of H2O. As for curing, at best you did one or two days of water curing and you were out of there. Many of the things that we are putting onto and into concrete now did not exist 50 years ago. The structures that we're building using today's concrete would have been science fiction in my youth. We can certainly say to the present generation of concrete people, "Your concrete is not your grandfather's concrete."
What I've observed is that while most things in concrete have not changed, certain things that have changed now appear to work together to produce a better end-product. Aggregates, for example, have not changed. However, the way we grade and proportion them in the mix has improved. Water has not changed, but we have found ways to use it more efficiently. And Portland cement has been continuously improved over the years, even though it is still chemically about the same as it was in my youth.
The two things that have changed the most are the additives that we are now putting into the concrete mix and our knowledge about how all of the components of a good concrete mix design work together.
MARK: Phil, you're exactly right. Not too may years ago, concrete aggregates were proportioned without much regard to cement efficiency. Additional cement was added to concrete mixtures not only to overcome aggregate gradation deficiencies, but also to address workability issues.
Today, much has been written and practiced in the area of aggregate gradation and cement optimization. For example, in 2002, most industrial floor slab specifications require increased proportions of significantly larger coarse aggregates, reduced cement content, along with lower compressive strengths. These requirements address the issues of shrinkage, warping and curling that are so prevalent in high-strength concrete mixes.
The high range and mid-range water reducing admixtures being used today offer to specifiers, producers and contractors many more options than in the past. By using these admixtures in combination, today's concrete is able to meet lower water-cement ratio demands while providing excellent finishability and consistent set times. In addition, we've seen the recent introduction of "self-consolidating concrete." This appears to be the next level of chemically-enhanced concrete. I find this subject interesting because self-consolidating concrete is much more than just a new admixture; self-consolidating concrete takes in consideration all components of the concrete mix design. (See related article in this issue of Concrete News, Page 8.)
PHIL: Good point, Mark. Speaking of admixtures, I think that the use of air entrainment in concrete was a watershed moment in the history of concrete. While air has significantly improved concrete's durability in exterior conditions, it has also created problems in handling, placement and finishing. As a matter of fact, many concrete authorities have now disallowed the hard troweling of concrete with an air content over 3% simply because of the increased potential for delamination.
MARK: Floor delamination continues to be a problem with industrial floor slabs. But air content of the concrete is just one factor. There are issues with vapor barriers, cool sub-grades, and over-aggressive finishing operations that prematurely close or "seal" the surface before entrapped air and water vapor can escape. Fast-paced construction schedules contribute to this problem, as well.
PHIL: I remember when we had a real problem in the mid-70's with air-entrained fly ash modified concrete. We could produce the air required at the beginning of the mix cycle, but five minutes later the air was gone. It disappeared. We eventually discovered that the high carbon content in the fly ash was absorbing the air-entraining agent. Those companies that found themselves in the admixture industry did their part by producing an air bubble that had greater stability, and the power industry did its part by producing a fly ash with lower carbon content. It was a very successful collaborative effort.
MARK: Phil, I understand that in the eastern portion of the United States the carbon content was an issue with Class F fly ash. Here in the Midwest, however, carbon content wasn't a big issue with our Class C fly ash. The bigger challenge for us was working together with our contractor customers to understand the changes in the working and setting characteristics of Class C fly ash modified concrete mixes. This has been a definite learning experience.
PHIL: These hot summer days bring back memories of hot weather concreting in the early 60's. I think the sun was closer to the earth then. Those were the days before evaporation control agents like my company's E-CON were available. All we had were chemical retarding admixtures. While they helped us get the concrete placed, they did not offer any protection from rapid evaporation. The best we could hope for was a friendly cloud to come over and give us some shade. The only real protection from plastic shrinkage cracking was fogging with a fine spray of water, but this was rarely done. The other solution was to place concrete at night. With only these options at hand, most hot weather concrete was doomed to crack.
I have found that many people think that by applying an evaporation control agent to the surface of concrete we are just adding water to the concrete surface. That's not true. What is happening is that we are applying a chemical to the surface of the concrete. This chemical does not replace the escaping water. Instead, it chemically "grabs" the water molecule and holds it at the surface of the concrete slab.
MARK: Probably one of the most commonly misunderstood aspects of concreting practices is knowing which factors most affect fresh concrete evaporation rates. Most people think that high temperature is the only big problem. Industry experience, however, has shown that low humidity and high winds also have a big effect on the rate of evaporation. Plastic shrinkage cracking and other finishing problems are more likely to occur as evaporation rates increase due to any combination of these factors.
Changing weather conditions can become very costly to the concrete contractor, too. The start of a nice day can lead a contractor into a false sense of security. Many days which seem to be "ideal" for placing and finishing concrete can quickly become days of increased temperature, wind velocity, and decreasing humidity. Again, any combination of these factors can result in rapid moisture loss from the surface of the concrete slab.
Our contractors have discovered the benefits of evaporation retardants, such as L&M's E-CON, in both residential and commercial applications to combat this real problem. We have found that E-CON is a type of cost-effective insurance that no concrete finisher should be without. I really can't say enough about the benefits of that product to our customers. We're sold on it.
PHIL: Well, thanks for the endorsement, Mark. You know, sometimes one thing has to happen before something else can occur. For example, we had the steam engine long before the airplane. But, it was only after the development of the gasoline engine that the airplane was possible. While I have not heard of a coal-fired, steam engine-powered airplane, I have seen a concrete canoe.
Still, the issue of sequencing holds true for the concrete industry. For many years we have used fly ash in concrete to enhance its strength. Fly ash has a particle size somewhat the same as that of cement. We could add the fly ash to the concrete mix and not drastically change the water demands. The active compound in the fly ash is the silica. It is silica that reacts with the calcium hydroxide to form siliceous hydrate, and it is the formation of siliceous hydrate that contributes to a beneficial increase in the strength of the concrete.
In recent years silica fume, a material that is very similar to fly ash, was found useful. Silica fume, like fly ash, is an industrial by-product which was discarded until a use was found in the concrete industry. A couple of things to remember about silica fume: first, it's about 90% to 95% silica, which is much higher than that of fly ash. The other characteristic about silica fume is its fineness. It is about a hundred times finer than Portland cement.
Because of these two factors, silica fume is very chemically active and, as a result, its presence in the concrete mix produces very high-strength concrete. Its fineness, however, is also a major problem. The general rule of thumb is that for every pound of silica fume added to a concrete mix an additional pound of water is required to maintain the desired slump (consistency). Typically, silica fume is used at the rate of 7% to 15% by weight of cement, which would require an additional 5 to 10 gallons of water. This much additional water would destroy any concrete mix. So, something else must be put into the concrete mix that will significantly reduce the water demands while maintaining a workable slump. Luckily, we have superplasticizers, something already on the shelf.
The ability of superplasticizers to significantly reduce water demands while maintaining slump is well known in the concrete industry. By combining a by-product of the silicon alloys industry (silica fume), a superplasticizer, and calcium hydroxide (free lime), a useless and unwanted by-product of cement hydration, a new concept in concrete is realized: very high strength concrete. Using these materials, concrete mixes in the range of 20,000 psi and higher have been produced. In the 1960's, 5000 psi concrete was considered high strength.
MARK: Phil, as a ready-mix producer, we have had the opportunity to produce silica-fume concrete, and let me tell you this product definitely deserves some attention. First of all, without the use of a superplasticizer the slump would be less than zero, or what we call a negative slump. Therefore, use of superplasticizers is absolutely mandatory in order to achieve a placeable silica fume concrete mix. We have to pay special attention to mix temperature, delivery and placement time, and weather conditions. Finally, it's been our experience that silica fume modified concrete mixes require the use of an evaporation retardant and water curing.
Let me change the subject for a minute, Phil, and ask you about fibers. We've noticed that the plastic fibers have become shorter and finer, which seem to have resulted in less slump loss, better workability and more efficient prevention of shrinkage cracking. What's happening here?
PHIL: Mark, it's interesting that our conversation is moving from silica fume, a relatively recent development, to fibers in concrete. Often we look to new science for our next innovation, but sometimes it comes from our distant past. This is true of fiber-reinforced concrete. I like to refer to it as an ancient concept and a new technology.
Several thousand years ago in Egypt, straw was added to a mud mix to give sun-dried brick additional strength and to prevent cracking. Today, we're adding plastic fibers to fresh concrete to prevent cracking in the early stages of hydration. You might say we're using a 2000-year-old solution for a 2000-year-old problem, but we have made some product improvements since those days.
We have found that finer fibers have, overall, more total surface area per pound than coarse fibers. The finer fiber therefore provides more surface area to which the cement paste can bond and thereby results in more efficient crack prevention. We have also discovered that the strength of the fiber is less important than how well the fiber is dispersed in the concrete. So, well dispersed, finer fibers are the way to go.
MARK: You know, Phil, even with the many recent accomplishments and technological advancements in the concrete industry, the greatest challenge for the ready-mix producer still remains the control and predictability of air content. More of our quality control resources are spent on the control of air than any other ingredient in the concrete mix.
PHIL: I know what you mean. When I was in ready-mix concrete, it was my number one problem, also. While we have come a long way in the development of air entraining agents we still are fighting the same old enemy of a stable air void system.
But this topic will have to wait for another day. My coffee's getting cold and I need to get to the office. So let's sum the last few minutes up like this: We must remember that even though we've come a long way as an industry, we're really just getting started. Al Jolson, said it best in the first line of The Jazz Singer, the first major film with synchronized sound: "Folks, you ain't heard nothing yet!"
Now, Mark, I know what you're thinking, but I did not hear this first hand. My grandmother told me about it -- many years later!
Mark Deetz is Vice President of Sales for Lyman-Richey Corporation in Omaha, Nebraska and has been with the corporation for 20 years. He is responsible for the Sales and Customer Service of concrete, aggregate and related products. Mark can be reached at (402)556-3600 or e-Mail: mark.deetz.@lymanrichey.com.
Mark is past president of the Nebraska ACI Chapter, and currently is an associate member of the ACI International Committee C-610, Concrete Field Testing Technician Certification.
© 2002 L&M Construction Chemicals, Inc. | ConcreteNews Fall 2002.