High-Performance Concrete: Understanding the Basics

Portland Cement Concrete Pavements Research

Special report from the High Performance Concrete Committee

by Stephen W. Forster, Ph.D., P.G.
Federal Highway Administration

Introduction
HPC-What is it?
Considerations for HPC
HPC - A Definition and Concluding Remarks
References

Introduction

Over the last decade, the term "high-performance concrete" (HPC) has more and more come into popular use as an ideal we should all strive for. There seems to be universal agreement that production and use of HPC is a worthy goal; however an additional systematic discussion of the concept may be in order to help clarify what it is we all are seeking. In the course of this discussion I hope to expand, or at least reposition the paradigm or "box" each of us has constructed in our own minds to encompass, and often confine HPC.

HPC - What is it?

The concept of HPC has certainly evolved with time. What exactly is "high-performance?" Various parameters have been attached to HPC, with high strength being a popular descriptor. While equating HPC with high strength certainly has some merit, it doesn't present a complete or, in some cases, accurate picture. Other properties of the concrete must also be considered, and may even override the strength issue. A recent article by Aitcin and Neville (1) addressed many of these issues, however even this excellent discussion perhaps limited the bounds of HPC too much in terms of materials and properties. How then should high performance concrete be defined? Three influences must be considered: the structure in which the concrete will be used, including support; the environment in which the structure will be placed; and the type and number of loads to which the structure will be subjected. Let's look at these considerations in more detail, before finalizing our definition of HPC.

Considerations for HPC

Structure

What are we building? A plain (unreinforced) floor slab-on-grade will require vastly different strength properties than a bridge deck; or a bridge structural member; or a slip-formed building or cooling tower. And yet, for each respective application, these greatly different strength ranges can certainly result in a HPC. Other concrete properties which the type of structure may dictate include allowable heat generation during curing; volume stability; creep; crack susceptibility/brittleness; bond to reinforcing; workability; pumpability; and the list goes on and on. Basically, the structural and construction requirements of the structure must be met by the concrete to be used.

Environment

What conditions (exclusive of loading as used here) will the structure be exposed to? Under this factor consideration must be given to climatic conditions, i.e. heating and cooling, wetting and drying, and freezing and thawing, and the requirements they place on the concrete. Further, potential chemical attack must also be considered for those structures in contact with the ground, exposed to chemicals in the air or exposed to chemicals because of the end use. In short, the concrete must be resistant to the environment in which it is placed. As with the structural requirements, the environmental requirements can vary widely: a concrete pavement built on a sulfate rich subgrade and subject to deicing chemicals in a freeze/thaw climatic zone will certainly have different requirements to achieve long term durability than a second story interior floor slab in a climatically controlled high rise building.

Expected Loads

Depending on the breadth of the definitions, either structural or environmental considerations could easily include the loads to which a structure will be subject, and I would not argue with either interpretation. Because of its importance, however, I have chosen to break loading out separately for clarity in discussion. As with the two previous factors, the influence, or impact of load on performance can vary widely. Because I most often deal with pavements in my position, I tend to think of loads in terms of vehicular traffic, such as ADT (Average Daily Traffic) or ESALs (Equivalent Single Axle Loads (of 8165 kg each)). However, for buildings the engineer must consider much different loads, including those from wind or earthquakes. Marine structures present still another set of conditions and requirements. The loads may be compressive, flexural or tensile, or include multiple types. In short, the type, magnitude and number of expected loads must be carefully considered.

Other Factors

Beyond the three basic influences discussed above, additional factors must also be considered. First, no matter how good the potential of our mix design or "lab-crete" is for meeting the three influences, in order to be practical the concrete must be "constructable." That is, while in the plastic state the concrete must be workable, pumpable (as required) and easily consolidated within the confines of any form-work or reinforcing. It must maintain this plasticity for the time period necessary to transport, place and consolidate the concrete. Any desired concrete properties, such as the entrained air void system, must not be adversely affected by transport, placement or consolidation. A wide range of specific materials requirements may be placed on the mix components, depending on the environmental exposure and type of structure in which the concrete is placed, and the concrete must meet these requirements while still remaining "construction-friendly."

Secondly, good practices must be followed during construction. The base or form-work must be well prepared; adequate coverage must be provided for reinforcing; placement techniques must be such as to avoid segregation of the concrete components; consolidation techniques must be adequate to attain target densities, but not so great as to adversely affect the air void system or produce segregation; and finishing and curing techniques must be properly timed and adequate for environmental conditions.

Thirdly, the interaction of the concrete at an early age with the environment and any loads must be considered. Heat generated during hydration must be estimated and combined with the effects of expected environmental conditions. Hot weather concreting requires special considerations, and changes in mix proportions may become necessary to prevent excessive internal temperatures, thermal gradients and thermal stresses. Similar concerns must be addressed for cold weather, or where large swings in ambient temperature are expected in short periods of time. These factors, and others which may influence the maturity of the concrete, must be considered when determining the allowable load for the concrete at any given age. The load may be (for instance) construction traffic, for pavements, or the mass of movable form-work in the case of some structures. Without paying attention to these and other factors during the early life of the concrete, damage may occur which will prevent the concrete from attaining the design properties or design life which were intended.

HPC - A Definition and Concluding Remarks

In the foregoing discussion, the various factors that must be addressed in the design, construction and application of HPC have been considered. Based on this information, let's now revisit the definition of high performance concrete which was touched on at the outset of the article. Incorporating the information provided, it may now be said that:

"High Performance Concrete is a concrete: made with appropriate materials combined according to a selected mix design; properly mixed, transported, placed, consolidated and cured so that the resulting concrete will give excellent performance in the structure in which it is placed, in the environment to which it is exposed and with the loads to which it will be subject for its design life."

I'm sure this will not be the final word on HPC, since there are still possible areas for refinement/improvement in this definition. For instance, what is "excellent" performance? It is probably not distress-free, since in a practical sense some level of distress would be acceptable at some magnitude of impact on performance. Maintenance-free is a possible alternative, although repair-free or rehabilitation-free may be more appropriate since maintenance may, in some cases, imply preventative activities. Other refinements to this new "paradigm" are certainly welcome. It should be noted that no special ingredients are listed as necessary for HPC; their use is determined by the factors and considerations discussed. Figure 1. shows these main categories of factors and considerations to be addressed in order to obtain HPC in place.

Finally, it is interesting that the definition given sounds very much like the concrete we have always tried to produce, and in the final analysis HPC may perhaps be another term for what all of us involved in the concrete industry have targeted for years, and continue to strive for on every project: quality concrete structures that perform well throughout their design life. If the term high performance concrete helps to concentrate our efforts in this regard, however, it will certainly have served its purpose.

Fig. 1. Factors to be considered for high-performance concrete.

Factors to be considered for high-performance concrete

References

1. Aitcin, P.-C., and Neville, A., "High-Performance Concrete Demystified," Concrete International, V. 15, No. 1, pp.21-26.

Biography - Dr. Stephen W. Forster, ACI member
Dr. Stephen W. Forster is a research geologist in the Pavements Division of the Federal Highway Administration. He has worked in the Office of Research of FHWA since 1975 on a range of materials and materials related issues, including aggregates and Portland Cement concrete. He is manager of FHWA's research program on cement and concrete.

Contact Dr. Cheryl Richter at cheryl.richter@fhwa.dot.gov for additional information.

Products and Research Projects Products Publications Concrete Labs	Portland Cement Concrete Pavements Research High-Performance Concrete: Understanding the Basics Special report from the High Performance Concrete Committee by Stephen W. Forster, Ph.D., P.G. Federal Highway Administration Introduction HPC-What is it? Considerations for HPC HPC - A Definition and Concluding Remarks References Introduction Over the last decade, the term "high-performance concrete" (HPC) has more and more come into popular use as an ideal we should all strive for. There seems to be universal agreement that production and use of HPC is a worthy goal; however an additional systematic discussion of the concept may be in order to help clarify what it is we all are seeking. In the course of this discussion I hope to expand, or at least reposition the paradigm or "box" each of us has constructed in our own minds to encompass, and often confine HPC. HPC - What is it? The concept of HPC has certainly evolved with time. What exactly is "high-performance?" Various parameters have been attached to HPC, with high strength being a popular descriptor. While equating HPC with high strength certainly has some merit, it doesn't present a complete or, in some cases, accurate picture. Other properties of the concrete must also be considered, and may even override the strength issue. A recent article by Aitcin and Neville (1) addressed many of these issues, however even this excellent discussion perhaps limited the bounds of HPC too much in terms of materials and properties. How then should high performance concrete be defined? Three influences must be considered: the structure in which the concrete will be used, including support; the environment in which the structure will be placed; and the type and number of loads to which the structure will be subjected. Let's look at these considerations in more detail, before finalizing our definition of HPC. Considerations for HPC Structure What are we building? A plain (unreinforced) floor slab-on-grade will require vastly different strength properties than a bridge deck; or a bridge structural member; or a slip-formed building or cooling tower. And yet, for each respective application, these greatly different strength ranges can certainly result in a HPC. Other concrete properties which the type of structure may dictate include allowable heat generation during curing; volume stability; creep; crack susceptibility/brittleness; bond to reinforcing; workability; pumpability; and the list goes on and on. Basically, the structural and construction requirements of the structure must be met by the concrete to be used. Environment What conditions (exclusive of loading as used here) will the structure be exposed to? Under this factor consideration must be given to climatic conditions, i.e. heating and cooling, wetting and drying, and freezing and thawing, and the requirements they place on the concrete. Further, potential chemical attack must also be considered for those structures in contact with the ground, exposed to chemicals in the air or exposed to chemicals because of the end use. In short, the concrete must be resistant to the environment in which it is placed. As with the structural requirements, the environmental requirements can vary widely: a concrete pavement built on a sulfate rich subgrade and subject to deicing chemicals in a freeze/thaw climatic zone will certainly have different requirements to achieve long term durability than a second story interior floor slab in a climatically controlled high rise building. Expected Loads Depending on the breadth of the definitions, either structural or environmental considerations could easily include the loads to which a structure will be subject, and I would not argue with either interpretation. Because of its importance, however, I have chosen to break loading out separately for clarity in discussion. As with the two previous factors, the influence, or impact of load on performance can vary widely. Because I most often deal with pavements in my position, I tend to think of loads in terms of vehicular traffic, such as ADT (Average Daily Traffic) or ESALs (Equivalent Single Axle Loads (of 8165 kg each)). However, for buildings the engineer must consider much different loads, including those from wind or earthquakes. Marine structures present still another set of conditions and requirements. The loads may be compressive, flexural or tensile, or include multiple types. In short, the type, magnitude and number of expected loads must be carefully considered. Other Factors Beyond the three basic influences discussed above, additional factors must also be considered. First, no matter how good the potential of our mix design or "lab-crete" is for meeting the three influences, in order to be practical the concrete must be "constructable." That is, while in the plastic state the concrete must be workable, pumpable (as required) and easily consolidated within the confines of any form-work or reinforcing. It must maintain this plasticity for the time period necessary to transport, place and consolidate the concrete. Any desired concrete properties, such as the entrained air void system, must not be adversely affected by transport, placement or consolidation. A wide range of specific materials requirements may be placed on the mix components, depending on the environmental exposure and type of structure in which the concrete is placed, and the concrete must meet these requirements while still remaining "construction-friendly." Secondly, good practices must be followed during construction. The base or form-work must be well prepared; adequate coverage must be provided for reinforcing; placement techniques must be such as to avoid segregation of the concrete components; consolidation techniques must be adequate to attain target densities, but not so great as to adversely affect the air void system or produce segregation; and finishing and curing techniques must be properly timed and adequate for environmental conditions. Thirdly, the interaction of the concrete at an early age with the environment and any loads must be considered. Heat generated during hydration must be estimated and combined with the effects of expected environmental conditions. Hot weather concreting requires special considerations, and changes in mix proportions may become necessary to prevent excessive internal temperatures, thermal gradients and thermal stresses. Similar concerns must be addressed for cold weather, or where large swings in ambient temperature are expected in short periods of time. These factors, and others which may influence the maturity of the concrete, must be considered when determining the allowable load for the concrete at any given age. The load may be (for instance) construction traffic, for pavements, or the mass of movable form-work in the case of some structures. Without paying attention to these and other factors during the early life of the concrete, damage may occur which will prevent the concrete from attaining the design properties or design life which were intended. HPC - A Definition and Concluding Remarks In the foregoing discussion, the various factors that must be addressed in the design, construction and application of HPC have been considered. Based on this information, let's now revisit the definition of high performance concrete which was touched on at the outset of the article. Incorporating the information provided, it may now be said that: "High Performance Concrete is a concrete: made with appropriate materials combined according to a selected mix design; properly mixed, transported, placed, consolidated and cured so that the resulting concrete will give excellent performance in the structure in which it is placed, in the environment to which it is exposed and with the loads to which it will be subject for its design life." I'm sure this will not be the final word on HPC, since there are still possible areas for refinement/improvement in this definition. For instance, what is "excellent" performance? It is probably not distress-free, since in a practical sense some level of distress would be acceptable at some magnitude of impact on performance. Maintenance-free is a possible alternative, although repair-free or rehabilitation-free may be more appropriate since maintenance may, in some cases, imply preventative activities. Other refinements to this new "paradigm" are certainly welcome. It should be noted that no special ingredients are listed as necessary for HPC; their use is determined by the factors and considerations discussed. Figure 1. shows these main categories of factors and considerations to be addressed in order to obtain HPC in place. Finally, it is interesting that the definition given sounds very much like the concrete we have always tried to produce, and in the final analysis HPC may perhaps be another term for what all of us involved in the concrete industry have targeted for years, and continue to strive for on every project: quality concrete structures that perform well throughout their design life. If the term high performance concrete helps to concentrate our efforts in this regard, however, it will certainly have served its purpose. Fig. 1. Factors to be considered for high-performance concrete. References 1. Aitcin, P.-C., and Neville, A., "High-Performance Concrete Demystified," Concrete International, V. 15, No. 1, pp.21-26. Biography - Dr. Stephen W. Forster, ACI member Dr. Stephen W. Forster is a research geologist in the Pavements Division of the Federal Highway Administration. He has worked in the Office of Research of FHWA since 1975 on a range of materials and materials related issues, including aggregates and Portland Cement concrete. He is manager of FHWA's research program on cement and concrete. Contact Dr. Cheryl Richter at cheryl.richter@fhwa.dot.gov for additional information.	Events View all Upcoming Pavements Events More Information Pavement Design & Performance Modeling Related Links Ultra-Thin Whitetopping (UTW) Contact Cheryl Richter Turner Fairbank 202-493-3070 E-mail Cheryl
	This page last modified on 06/09/06

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