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High-performance concrete mix proportions
Jun 1, 2000 12:00 PM, Henry G. Russell
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.
Table 2 lists a selection of concrete mix proportions used to achieve low permeability concretes. It demonstrates that a variety of materials may be used to achieved lower permeability.
Conclusions For HPC, mix proportions must be selected to meet the specified performance criteria at a reasonable cost while using locally available materials. This will require more trial mix batches and testing than are necessary with conventional concretes.
1. ACI Committee 211, "Guide for Selecting Proportions for High-Strength Concrete with Portland Cement And Fly Ash (ACI 211.4R)," American Concrete Institute, Detroit, 1993.)
2. Farny, J. A. and Panarese, W. C., "High-Strength Concrete," PCA Engineering Bulletin EB114.01T, Portland Cement Association, Skokie, IL, 1994, 48 pp.
3. HPC Bridge Views, No. 4, July/August 1999, published by the Federal Highway Administration and National Concrete Bridge Council.
4. ASTM C 1202-97-Standard Test Method for Electrical Indication of Concrete's Ability to Resist Chloride Ion Penetration, American Society for Testing and Materials, Volume 04.02, West Conshohocken, Pa., 1999.)
5. Kosmatka, S.H. and Panarese, W.C., Design and Control of Concrete Mixtures - Thirteenth Edition, PCA Engineering Bulletin EB001.13T, Portland Cement Association, Skokie, IL, 1988, 205 pp.)
6. Moore, J. A., et al., "High-Performance Concrete for Bridge Decks," Concrete International, Vol. 21, No. 2, February, 1999, pp. 58-68.