With today's requirements for high performance concrete (HPC), mix proportions containing cementitious materials in addition to portland cement are used more commonly. According to a survey by the Portland Cement Association published in 2000, at least 60 percent of ready mixed concrete contains other cementitious materials, often referred to as mineral admixtures or supplementary cementitious materials.

The benefits of using these materials, either separately or in various combinations, include higher early strength, higher later age strength, reduced permeability, control of alkali-aggregate reactivity, lower heat of hydration, and reduced costs. Mineral admixtures may be used in addition to the normal amount of portland cement or as a substitute for a portion of the cement depending on the required or specified properties of the concrete. This issue of Technical Talk reviews how HPC can benefit from the three most commonly used materials — fly ash, silica fume, and ground granulated blast-furnace slag.

Fly ash

Fly ash is the fine material that results from the combustion of pulverized coal in a coal-fired power plant. Fly ashes are classified in ASTM C 618 (AASHTO M 295) as either Class F or Class C. Class F fly ash has pozzolanic properties. Class C fly ash has pozzolanic and cementitious properties. Fly ash is used in about 50 percent of ready mixed concrete. The Class C fly ash content of concrete generally ranges from 15 to 40 percent of the total cementitious materials, and Class F fly ash content ranges from 15 to 25 percent.

Fly ash is frequently used in mass concrete as a cement replacement to reduce the heat of hydration, which in turn reduces peak temperatures, temperature gradients, and the likelihood of thermal cracking. Generally, mass concrete only requires a low compressive strength, so development of strength is not a controlling factor in selecting mix proportions.

Fly ash reduces permeability and chloride diffusivity and increases resistivity, making it a beneficial material in concrete that is exposed to chlorides such as bridge decks. Fly ash also binds up the alkalis in the concrete and, thereby, reduces the potential for alkali aggregate reactivity. The addition of fly ash to concrete enhances the strength gain at later ages, making it beneficial when high-strength concrete is specified at ages of 56 or 90 days.

Silica fume

Silica fume, also known as condensed silica fume and microsilica, is a very fine pozzolanic material produced as a by-product in the production of silicon or ferro-silicon alloys. The silica fume content of concrete generally ranges from 5 to 10 percent of the total cementitious materials content. The use of silica fume can be specified using ASTM C 1240 (AASHTO M 307).

For most applications where durability is a concern, the use of silica fume will reduce the permeability of the concrete, thereby slowing the rate of penetration of aggressive chemicals such as deicing salts. The use of silica fume can result in rapid chloride permeability values of less than 500 when tested in accordance with ASTM C 1202 (AASHTO T 277).

The use of silica fume improves the early age strength development of concrete and is particularly beneficial in achieving high release strengths in precast, prestressed concrete beams. Use of silica fume often allows a reduction in the total amount of cementitious materials. At later ages, concretes made with silica fume can achieve compressive strengths in excess of 17,000 psi (117 MPa).

Ground granulated blast-furnace slag

Ground granulated blast-furnace slag (GGBFS), also called slag cement, is made by rapidly quenching molten blast-furnace slag and grinding the resulting material into a fine powder. GGBFS is classified by ASTM C 989 (AASHTO M 302) according to its level of reactivity. Depending on the desired properties, the amount of GGBFS can be as high as 80 percent of the total cementitious materials content.

The use of GGBFS lowers concrete permeability, thereby reducing the rate of chloride ion diffusion. For alkali-silica reaction, GGBFS consumes some of the alkalis produced from the portland cement leaving them unavailable for reaction with the aggregates. Proper proportioning of slag cement can eliminate the need to use low alkali or sulfate-resistant portland cements.

In mass concrete applications, dosage rates of 50 to 80 percent of the total cementitious materials reduce the heat of hydration and the likelihood of thermal cracking. GGBFS can also be used to enhance the strength gain at later ages. Concrete strength is usually optimized when GGBFS replaces 40 to 50 percent of the portland cement.

Combinations

Each of the three mineral admixtures described above can be used individually in combination with portland cement to achieve the desired characteristics of the hardened concrete. The mineral admixtures can also be used in combination with each other and portland cement to achieve the desired results. The “Concrete Mix Proportions” box shows an example of a concrete mix that used all three mineral admixtures and portland cement. Since admixtures — both mineral and chemical — can affect the properties of both the fresh and hardened concrete, they should always be tested in trial mixes prior to the start of production. This will ensure that the desired characteristics are achieved and that no undesirable properties are present.

Acknowledgement

Portions of this article are based on articles in HPC Bridge Views Issue Nos. 16, 19, and 20. The full articles may be viewed and downloaded at www.portcement.org/br/newsletters.asp. HPC Bridge Views is published jointly by the Federal Highway Administration and the National Concrete Bridge Council.

Henry Russell is an engineering consultant based in Glenview, Ill. He is a member of American Concrete Institute, American Society for Testing and Materials, Precast/Prestressed Concrete Institute, and American Segmental Bridge Institute. He is a former chairman of ACI's subcommittee on High-Performance Concrete.

Concrete Mix Proportions

Deck mixes for the 2001-2002 Revive Wacker Drive project in Chicago

Material	Quantities
	per yd3	per m3
Portland Cement(1)	525 lb	311 kg
Fly Ash, Class F	53 lb	31 kg
Silica Fume	27 lb	16 kg
GGBFS	79 lb	47 kg
Fine Aggregate(2)	1140 lb	676 kg
Course Aggregate(3)	1800 lb	1068 kg
Water	254 lb	151 kg
Water Reducer	41 fl oz	1.59 l
HRWR	55-110 fl oz	2.1-4.3 l
Note: Air entrainment as needed; water/cementitious materials ratio, 0.37.
(1) Type I/II
(2) Natural siliceous sand
(3) 3/4-in. (19-mm) maximum size limestone