In recent years, the terminology "High-Performance Concrete" has been introduced into the construction industry. This edition of Technical Talk explains high-performance concrete and how it differs from conventional concrete.
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.
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.
The American Concrete Institute (ACI) defines high-performance concrete as concrete meeting special combinations of performance and uniformity requirements that cannot always be achieved routinely when using conventional constituents and normal mixing, placing and curing practices. A commentary to the definition states that a high-performance concrete is one in which certain characteristics are developed for a particular application and environment. Examples of characteristics that may be considered critical for an application are:
* Ease of placement
* Compaction without segregation
* Early age strength
* Long-term mechanical properties
* Permeability
* Density
* Heat of hydration
* Toughness
* Volume stability
* Long life in severe environments
Because many characteristics of high-performance concrete are interrelated, a change in one usually results in changes in one or more of the other characteristics. Consequently, if several characteristics have to be taken into account in producing a concrete for the intended application, each must be clearly specified in the contract documents.
A high-performance concrete is something more than is achieved on a routine basis and involves a specification that often requires the concrete to meet several criteria. For example, on the Lacey V. Murrow floating bridge in Washington State, the concrete was specified to meet compressive strength, shrinkage and permeability requirements. The latter two requirements controlled the mix proportions so that the actual strength was well in excess of the specified strength. This occurred because of the interrelation between the three characteristics. Other recent commercial examples where more than one characteristic was required are given in Table
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.
High-strength concrete A high-strength concrete is always a high-performance concrete, but a high-performance concrete is not always a high-strength concrete. ACI defines a high-strength concrete as concrete that has a specified compressive strength for design of 6,000 psi (41 MPa) or greater. According to a paper(1) by Paul Zia of North Carolina State University, other countries use a higher compressive strength in their definitions of high-strength concrete with 7,000 psi (48 MPa) minimum. Other countries also specify a maximum compressive strength, whereas the ACI definition is open-ended.
The specification of high-strength concrete generally results in a true performance specification in which the performance is specified for the intended application, and the performance can be measured using a well-accepted standard test procedure. The same is not always true for a concrete whose primary requirement is durability.
Durable concrete Specifying a high-strength concrete does not ensure that a durable concrete will be achieved. In addition to requiring a minimum strength, concrete that needs to be durable must have other characteristics specified to ensure durability. In the past, durable concrete was obtained by specifying air content, minimum cement content and maximum water-cement ratio. Today, performance characteristics may include permeability, deicer scaling resistance, freeze-thaw resistance, abrasion resistance or any combination of these characteristics. Given that the required durability characteristics are more difficult to define than strength characteristics, specifications often use a combination of performance and prescriptive requirements, such as permeability and a maximum water-cementitious material ratio to achieve a durable concrete. The end result may be a high-strength concrete, but this only comes as a by-product of requiring a durable concrete.
Concrete materials Most high-performance concretes produced today contain materials in addition to portland cement to help achieve the compressive strength or durability performance. These materials include fly ash, silica fume and ground-granulated blast furnace slag used separately or in combination. At the same time, chemical admixtures such as high-range water-reducers are needed to ensure that the concrete is easy to transport, place and finish. For high-strength concretes, a combination of mineral and chemical admixtures is nearly always essential to ensure achievement of the required strength. Examples of concrete mixes for durable and high-strength concrete are shown in Table 2.
Most high-performance concretes have a high cementitious content and a water-cementitious material ratio of 0.40 or less. However, the proportions of the individual constituents vary depending on local preferences and local materials. Mix proportions developed in one part of the country do not necessarily work in a different location. Many trial batches are usually necessary before a successful mix is developed.
High-performance concretes are also more sensitive to changes in constituent material properties than conventional concretes. Variations in the chemical and physical properties of the cementitious materials and chemical admixtures need to be carefully monitored. Substitutions of alternate materials can result in changes in the performance characteristics that may not be acceptable for high-performance concrete. This means that a greater degree of quality control is required for the successful production of high-performance concrete.
Mix proportions for high-performance concrete (HPC) are influenced by many factors, including specified performance properties, locally available materials, local experience, personal preferences, and cost. With today's technology, there are many products available for use in concrete to enhance its properties. Consequently, there are many alternatives for mix proportions that will result in concrete with the desired properties. Here, Technical Talk briefly addresses selection of mix proportions for high-strength and low permeability concretes.
High-strength concrete High-strength concrete is defined by the American Concrete Institute (ACI) as concrete with a specified compressive strength of 6,000 psi (41 MPa) or greater. Although concretes with compressive strengths greater than 6,000 psi (41 MPa) can be produced using only cement as the binding material, it is likely that these concretes will also contain a mineral admixture such as fly ash, silica fume, or ground granulated blast furnace slag (GGBFS). For mix proportions of high-strength concrete containing cement and fly ash, the reader is referred to ACI 211.4R(1) entitled "Guide for Selecting Proportions for High-Strength Concrete with Portland Cement and Fly Ash." At the present time, similar guides are in preparation for high-strength concretes containing silica fume or GGBFS. However, many of the guidelines that apply to concrete containing fly ash also apply to concrete containing silica fume or GGBFS. Some of these are summarized below:
Testing age. Concrete tested at an age of 56 or 90 days generally has a higher compressive strength than concrete tested at 28 days. This is more noticeable with concrete containing fly ash and less noticeable with concrete containing silica fume. The use of a later age makes it easier and more economical to achieve the higher strengths. Proportions of cementitious materials are usually selected to produce the desired strength at the selected test age.
Water-cementitious materials ratio. According to ACI 211.4R,(1) many researchers have concluded that the most important variable in achieving high-strength concrete is the water-cement ratio. However, most high-strength concretes contain binding materials other than cement. Consequently, the water-cementitious materials ratio must be considered instead of the water-cement ratio where the cementitious materials include cement, fly ash, silica fume, and GGBFS as appropriate. In general, as the water-cementitious materials ratio decreases, the concrete compressive strength increases.
Portland cement. Proper selection of the type and source of cement is one of the most important steps in the production of high-strength concrete. Variation in the chemical composition and physical properties of the cement affect the concrete compressive strength more than variations in any other single material. There is also an optimum cement content beyond which little or no additional increase in strength is achieved by increasing the cement content.(1) To achieve higher strengths, it is necessary to include other materials such as fly ash, silica fume, GGBFS, or combinations of these materials
Coarse aggregate. For each concrete strength level, there is an optimum size for the coarse aggregate that will yield the greatest compressive strength per unit mass of cement. In general, a smaller size aggregate will result in a higher compressive strength concrete. On the other hand, the use of the largest possible coarse aggregate size is important in increasing the modulus of elasticity or reducing creep and shrinkage
Fine aggregate. According to ACI 211.4R,(1) fine aggregates with a fineness modulus in the range of 2.5 to 3.2 are preferable for high-strength concrete. Concretes with a fineness modulus less than 2.5 may be sticky and result in poor workability and high water requirement.
Chemical admixtures. Water-reducers or high-range water-reducers are essential in high-strength concrete to ensure adequate workability while achieving a low water-cementitious materials ratio. Retarding admixtures may also be used. The optimum dosage of an admixture or combination of admixtures should be determined by trial mixtures using varying amounts of each additive. It is also important to be sure that admixtures are compatible when used in combination.
Sample mixes. Table 1 lists a selection of concrete mix proportions for commercially available high-strength concretes from various sources. These data show a range of materials and quantities that can be used to produce high-strength concrete. Hence, there is no standard mix to produce a high-strength concrete. Trial mixes are needed to obtain the optimum use of each locally available constituent material.
Low permeability concrete Whereas guidelines are available for mix proportions of high-strength concrete, the same is not true for low permeability concretes. Most specifications address the requirements for low permeability by specifying a maximum water-cementitious materials ratio of 0.40 and a maximum permeability per ASTM C 1202.(4)
According to the Portland Cement Association publication EB001,(5) fly ash and GGBFS generally reduce the permeability of concrete even when the cement content is relatively low, and silica fume is especially effective in this regard. Tests show that the permeability of concrete decreases as the quantity of hydrated cementitious materials increases and the water-cementitious materials ratio decreases. Values of permeability less than 2,000 coulombs may be achieved with concretes containing fly ash, silica fume, or GGBFS. Values of permeability less than 1,000 coulombs may require the use of silica fume.
