High Performance Concrete

Pierre-Claude Aïtcin

• Publisher: CRC Press
• Year: 1998
• Print ISBN: 9780367865986
• eBook ISBN: 9781135823566
• Topic: Architecture
• Subtopic: Architecture Methods & Materials

A complete review of the fast-developing topic of high performance concrete (HPC) by one of the leading researchers in the field. It covers all aspects of HPC from materials, properties and technology, to construction and testing. The book will be valuable for all concrete technologists and construction engineers wishing to take advantage of the re

CHAPTER 1

Terminology: some personal choices

It may seem strange to begin a book on high-performance concrete with a chapter devoted to terminology. Discussions on terminology are tricky and can be endless, but it must be admitted that often the quality of the information in a technical book is diminished by the lack of consensus on the exact meaning of the terms used.

The author makes no claim for the superiority of the terminology he uses; he wants only to make clear the exact meaning of the terms he employs. The reader is free to disagree with the pertinence and the validity of the proposed terminology but, by accepting it momentarily, he will better understand the concepts and ideas expressed in this book.

There is no clear consensus about the meaning of expressions such as high-strength and high-performance concrete. Even the simple concept of water/cement ratio, that has been the pillar of concrete technology for years, is losing most of its meaning since several supplementary cementitious materials and more or less reactive fillers are replacing some of the cement in modern concretes. It is therefore essential to be more precise about these concepts and expressions in order to eliminate the most frequent source of misunderstanding in science and technology: the existence of several meanings to a simple word, expression or concept.

The acceptance of these definitions is essential to make the most of reading this book. As said by A.M.Neville: ‘The choice of one term over another is purely a personal preference, and does not imply a greater accuracy of definition’ (Neville, personal communication, 1996).

1.1 ABOUT THE TITLE OF THIS BOOK

It was without any hesitation that the author selected High-Performance Concrete as the title of this book, rather than High-Strength Concrete. There is no doubt that this new type of concrete offers more than just a high compressive strength, as discussed in section 1.4. In spite of the fact that up to now high-performance concrete has been used primarily in highstrength applications, it is inevitable that in the very near future high-performance concrete will be mostly specified and used for its durability rather than specifically for its high compressive strength. When the civil engineering community comes to understand this, and changes its perception of high-performance concrete, the construction industry will definitely take a big step forward.

In the present state of the art, it is not possible to make a durable high-performance concrete that does not also have a high compressive strength, but this situation can evolve. Moreover, the author is well aware that the expression ‘high-performance’ concrete is not yet universally accepted by the scientific and engineering communities and that in some countries it has a totally different meaning, but the term is his personal choice and he will use it accordingly.

1.2 WATER/CEMENT, WATER/CEMENTITIOUS MATERIALS OR WATER/BINDER RATIO

It took much longer for the author to select the expression to be used to describe the fundamental and universal concept that is hidden behind such an expression. The water/cement ratio concept has been the pillar of concrete technology for almost a century; it is a simple and convenient concept as long as concrete does not contain any cementitious materials other than Portland cement.

When the very simple but very important concept of water/cement ratio was initially developed, at a time when concrete technology was in its infancy, it was an unquestionable expression; concrete was made using solely Portland cement. This is no longer completely true because, when concrete is made with a modern Portland cement, it can contain a small amount of limestone or silica filler that the cement producer is allowed by many national standards to blend within the Portland cement and still call his product Portland cement. Thus the expression ‘water/ cement ratio’ has already lost some of its basic significance even when concrete is made solely with a modern Portland cement (Barton, 1989; Kasmatka, 1991; Shilstone, 1991). Moreover, with the passage of the years, though to different extents in different parts of the world, the use of so-called ‘supplementary cementitious materials’ or ‘fillers’ has become a more common practice so that many modern concretes now incorporate fly ashes, slags, natural pozzolans, silica fumes, limestone fillers, silica fillers or rice husk ash, etc. These finely divided materials can be part of a so-called ‘blended’ cement or simply be added to the concrete at the batching plant. The expression ‘supplementary cementitious material’ does not unquestionably include all of these materials because, stricto sensu, it is not clear that a limestone or a silica filler qualifies as a cementitious material. As discussed by A.M.Neville, the expression water/cementitious ratio that has been used is absolutely inappropriate because cementitious is an adjective (Neville, 1994). What expression should be used, then, to replace the obsolete expression of water/cement ratio? The author thinks that it is time to propose a new expression that will embrace the expressions ‘supplementary cementitious material’, ‘filler’ and ‘Portland cement’. Of course, such a term covering materials with such a wide variety of properties will have a meaning that is more vague, but it would nevertheless be convenient in order to translate the fundamental concept that was hidden in the original water/cement ratio expression. This fundamental concept was related to the ratio between water and the fine particles that give the concrete its strength, and it is still valid when all of these fine materials are now added to modern concretes.

As the use of blended cement was developed much earlier in Europe than in North America, European authors faced this dilemma much earlier, so that a number of different expressions have been proposed to translate the water/cement ratio expression into a more correct one. Influenced by his French culture, the author has decided to use the English equivalent of the French expression ‘rapport eau/liant’, which may be translated as water/ binder ratio (W/B) instead of the lengthy expression ‘water/cementitious materials ratio’. However, the author recognizes here, too, that the use of one expression over another is purely a matter of personal preference, not accuracy of definition.

When this expression is used the following definition is implicit: the term ‘binder’ as used in this book represents any finely divided material that is used in a concrete mixture having about the same fineness as or finer than Portland cement. However, the use of ‘water/binder ratio’ does not mean that the author does not recommend the simultaneous use or calculation of the water/cement (W/C) ratio, because, as will be seen, both ratios are very important from a technological point of view. As most of the supplementary cementitious materials and fillers used in conjunction with Portland cement in high-performance concrete are much less reactive than Portland cement, during setting and early hardening the actual value of the water/cement ratio is, of course, very important, because the early strength and impermeability of the hardening concrete are almost entirely a function of the bonds created by the early hydration of the Portland cement part of the binder. In the case of blended cements, it is not always easy to calculate the exact water/cement ratio because the exact amount of cement contained in the blended cement is not always known, some national standards specifying only a range of potential compositions and not a precise composition.

1.3 NORMAL STRENGTH CONCRETE/ORDINARY CONCRETE/USUAL CONCRETE

The author does not like the expression ‘normal strength concrete’ used to describe the usual concretes that are used presently by the construction industry. This expression implies that concretes other than these are abnormal. Very often it is these usual concretes that are abnormal because they are used in environmental conditions under which they are unable to fulfil, on a long-term basis, their structural function. As this type of concrete will continue to be used in the future since there are many applications in which a high water/binder ratio concrete, having a not-so- high compressive strength, is perfectly adequate and economical, the author has had to select another expression to describe them. In spite of the fact that, with present technological standards, the expression ‘lowstrength concrete’ would have been more appropriate, the author has finally decided to use the expression ‘usual concrete’ rather than ordinary concrete or normal strength concrete. The expression ‘ordinary concrete’ was rejected because it might imply that other concretes could be extraordinary, while the expression ‘unusual concrete’ still applies well in most parts of the world to designate high-performance concrete. Ten years from now, when high-performance concrete will not be as unusual, the expression ‘usual concrete’ will not be appropriate to designate lowstrength concrete, but this is not very important, because this book will be obsolete long before the concrete industry uses high-performance concrete on a routine basis.

1.4 HIGH-STRENGTH OR HIGH-PERFORMANCE CONCRETE

In the 1970s, when the compressive strength of the concrete used in the columns of some high-rise buildings was higher than that of the usual concretes used in construction, there is no doubt that it was legitimate to call these new concretes ‘high-strength’ concretes. They were used only because their strength was higher than that of the usual concretes generally specified at that time. In fact, by present standards, they were only improved usual concretes. They were made using the same technology as that used to make usual concrete except that the materials used to make them were carefully selected and controlled (Freedman, 1971; Perenchio, 1973; Blick, Petersen and Winter, 1974).

However, when superplasticizers began to be used to decrease the water/cement or water/binder ratios rather than being exclusively used as fluidifiers for usual concretes, it was found that concretes with a very low water/cement or water/binder ratio also had other improved characteristics, such as higher flowability, higher elastic modulus, higher flexural strength, lower permeability, improved abrasion resistance and better durability. Thus the expression ‘high-strength concrete’ no longer adequately described the overall improvement in the properties of this new family of concretes (Malier, 1992). Therefore the expression ‘high-performance concrete’ became more and more widely used. However, the acceptance of this expression is not yet general; for example, the name of ACI Committee 363 is still the High-Strength Concrete committee and not High-Performance Concrete committee.

Most of the detractors of the expression ‘high-performance concrete’ criticize this expression because it is too vague. What is the ‘performance’ of a concrete? How can it be measured? When using the expression ‘high-strength concrete’ there is no misunderstanding possible, except that the limit at which a concrete is no longer a usual concrete and becomes a high-strength one is not the same for everybody. This kind of endless discussion can be overwhelming (or can generate a new one!) if concrete is considered in terms of its water/binder ratio. For the author, a high-performance concrete is essentially a concrete having a low water/binder ratio. But how low? A value of about 0.40 is suggested as the boundary between usual concretes and high-performance concretes.

This 0.40 value, which might be perceived as being totally arbitrary, is based on the fact that it is very difficult, if not impossible, to make a workable and placeable concrete with most ordinary Portland cements that are presently found on the market, without the use of a superplasticizer if the water/binder ratio is lower than 0.40. Moreover, this value is close to the theoretical value suggested by Powers to ensure full hydration of Portland cement (Powers, 1968) and, as will be seen later in this book, it seems that this value denotes concretes that are starting to present autogenous shrinkage.

If this definition is accepted, it is evident that a 0.38 water/binder ratio concrete is not very much stronger and will not exhibit much better performance than a 0.42 one. But as soon as the water/binder ratio deviates significantly from the 0.40 value, usual concretes and high-performance concretes have not only quite different compressive strengths but also quite different microstructures (quite different shrinkage behaviour) and quite different overall performances.

Therefore, if it is the water/binder ratio that is to be used to differentiate high-performance concrete from usual concrete, why have I not selected as a title for this book Low Water/Binder Ratio Concrete instead of High-Performance Concrete? In spite of the fact that this title would be more correct from a scientific point of view, it would be much less appealing and it would probably be rejected by the publishers from marketing considerations.

REFERENCES

Barton, R.B. (1989) Water-cement ratio is passé. Concrete International, 11(11), November, 75–8.

Blick, R.L., Petersen, C.F. and Winter, M.E. (1974) Proportioning and Controlling High-Strength Concrete, ACI SP-46, pp. 141–63.

Freedman, S. (1971) High-Strength Concrete, IS1 76-OIT, Portland Cement Association, Skokie, IL, 19 pp.

Kasmatka, S.H. (1991) In defense of the water-cement ratio. Concrete International, 13(9), September, 65–9.

Malier, Y. (1992) Introduction, in High-Performance Concrete—From Material to Structure (ed. Y.Malier), E and FN Spon, London, pp. xiii–xxiv.

Neville, A. (1994) Cementitious materials—a different viewpoint. Concrete International, 16(7), July, 32–3.

Perenchio, W.F. (1973) An Evaluation of Some Factors Involved in Producing Very High- Strength Concrete, Portland Cement Association Research and Development Bulletin RD 104, Skokie, IL, 7 pp.

Powers, T.C. (1968) The Properties of Fresh Concrete, John Wiley and Sons, Inc., New York, 664 pp.

Shilstone, J.M. (1991) The water-cement ratio—which one and where do we go? Concrete International, 13(9), September, 64–9.

CHAPTER 2

Introduction

The purpose of this introduction is to provide an overview of the different chapters that make up this book. It can be considered as a kind of executive summary for readers who want to have a general idea of the content of the book, for readers who are not interested in all the chapters, or for readers who do not like the order of the chapters. It follows a chapter on terminology, in which the exact meaning of certain terms and expressions has been clearly defined, because it is the author’s experience that very often a lack of understanding is simply the result of the use of terms or expressions that have different meanings for different individuals in different countries. These choices do not pretend to be the perfect ones, because in matters of terminology the pros and cons of a definition can be discussed endlessly, but the precise sense of the terms used in this book must be accepted in order to have a comprehensive reading.

In Chapter 3, a historical perspective of the development of high-performance concrete over the past 25 years is presented. It shows how, slowly, high-strength concrete was introduced to the high-rise building market in the Chicago area in the late 1960s and early 1970s, and how it then spread throughout the world and came increasingly to be called high-performance concrete. In 1997, high-performance concrete is used in most parts of the world. Many, in fact too many, workshops, symposia and conferences are held almost every year on the subject; similarly, many papers, too many papers, are published in technical journals, so that readers are submerged by a flood of information. What is needed right now is more papers synthesizing what is really known, what is not well enough known and what should be known, rather than detailed academic papers having very little impact on the science and technology of high-performance concrete. It is one of the ambitious objectives of this book to present a comprehensive synthesis of what I presently know, or at least what I think I know, about high-performance concrete.

In Chapter 4, the rationale for high-performance concrete is developed, followed by a presentation, in some detail, of 11 case studies. Why should an owner, why should a designer, why should a concrete producer become interested in high-performance concrete? What are the technical and economic advantages of specifying a high-performance concrete? There is no unique answer. The 11 case studies selected show that high-performance concrete was selected to build both outstanding structures and some modest ones because it was found, for various reasons, that the use of high-performance concrete could achieve the most economical and durable structure under the circumstances. The cases selected do not pretend to cover all past, present and possible future uses of high-performance concrete. However, they suffice to explain the main advantages and characteristics of high-performance concrete that have led to its increasing use as a construction material. The discussion is given from the point of view of the owner, the designer, the contractor and the concrete producer. The conclusion of this chapter discusses some of the environmental advantages of using high-performance concrete. It demonstrates how future development of the use of high-performance concrete could result in a better use and conservation of our shrinking natural resources and decrease the environmental damage associated with any production of concrete.

Chapter 5 develops the principles of high-performance concrete. First, there is a review of some fundamental principles that govern the compressive strength and the tensile strength of materials. Then it is noted how the strength of high-performance concrete can be increased at each of the following levels: the hydrated cement paste, the transition zone and the aggregates. There is a discussion of the conflicting demands concerning the amount of mixing water required to make a workable concrete. This conflict can be solved throu...

Table of contents

• Cover Page
• Title Page
• Copyright Page
• Foreword
• Foreword
• Preface
• Acknowledgements
• CHAPTER 1
• CHAPTER 2
• CHAPTER 3
• CHAPTER 4
• CHAPTER 5
• CHAPTER 6
• CHAPTER 7
• CHAPTER 8
• CHAPTER 9
• CHAPTER 10
• CHAPTER 11
• CHAPTER 12
• CHAPTER 13
• CHAPTER 14
• CHAPTER 15
• CHAPTER 16
• CHAPTER 17
• CHAPTER 18
• CHAPTER 19
• CHAPTER 20
• Afterword
• Suggested reading