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The advantages to using mineral admixtures added at the batch plant (Popoff 1991; Massazza 1987).

Mineral admixture replacement levels can be modified on a day-to-day and job-to-job basis to suit project specifications and needs.
Cost can be decreased substantially while performance is increased (taking into consideration the fact that the price of blended cement is at least 10% higher than that of Type I/II cement [U.S. Dept. Int. 1989]).
GGBFS can be ground to its optimum fineness.
Concrete producers can provide specialty concretes in the concrete product markets.

At the same time, several precautions must be considered when mineral admixtures are added at the batch plant.

Separate silos are required to store the different hydraulic materials (cements, pozzolans, slags). This might slightly increase the initial capital cost of the plant.
There is a need to monitor variability in the properties of the cementitious materials, often enough to enable operators to adjust mixtures or obtain alternate materials if problems arise.
Possibilities of cross-contamination or batching errors are increased as the number of materials that must be stocked and controlled is increased.

Modified Portland Cement (Expansive Cement)

Expansive cement, as well as expansive components, is a cement containing hydraulic calcium silicates (such as those characteristic of portland cement) that, upon being mixed with water, forms a paste, that during the early hydrating period occurring after setting, increases in volume significantly more than does portland cement paste. Expansive cement is used to compensate for volume decrease due to shrinkage and to induce tensile stress in reinforcement.

Expansive cement concrete used to minimize cracking caused by drying shrinkage in concrete slabs, pavements, and structures is termed shrinkage-compensating concrete.

Self-stressing concrete is another expansive cement concrete in which the expansion, if restrained, will induce a compressive stress high enough to result in a significant residual compression in the concrete after drying shrinkage has occurred.

Types of Expansive Cements. Three kinds of expansive cement are defined in ASTM C 845.

Type K: Contains anhydrous calcium aluminate
Type M: Contains calcium aluminate and calcium sulfate
Type S: Contains tricalcium aluminate and calcium sulfate

Only Type K is used in any significant amount in the United States.

Concrete placed in an environment where it begins to dry and lose moisture will begin to shrink. The amount of drying shrinkage that occurs in concrete depends on the characteristics of the materials, mixture proportions, and placing methods. When pavements or other structural members are restrained by subgrade friction, reinforcement, or other portions of the structure, drying shrinkage will induce tensile stresses. These drying shrinkage stresses usually exceed the concrete tensile strengths, causing cracking. The advantage of using expansive cements is to induce stresses large enough to compensate for drying shrinkage stresses and minimize cracking (ACI Comm. 223 1983; Hoff et al. 1977).

Physical and mechanical properties of shrinkage compensating concrete are similar to those of portland cement concrete (PCC). Tensile, flexural, and compressive strengths are comparable to those in PCC. Air-entraining admixtures are as effective with shrinkage-compensating concrete as with portland cement in improving freeze-thaw durability.

Some water-reducing admixtures may be incompatible with expansive cement. Type A water-reducing admixture, for example, may increase the slump loss of shrinkage- compensating concrete (Call 1979). Fly ash and other pozzolans may affect expansion and may also influence strength development and other physical properties.

Structural design considerations and mix proportioning and construction procedures are available in ACI 223-83 (ACI Comm. 223 1983). This report contains several examples of using expansive cements in pavements.

In Japan, admixtures containing expansive compounds are used instead of expansive cements. Tsuji and Miyake (1988) described using expansive admixtures in building chemically prestressed precast concrete box culverts. Bending characteristics of chemically prestressed concrete box culverts were identical to those of reinforced concrete units of greater thickness (Tsuji and Miyake 1988). Expansive compounds are also available in the United States. They can be added to the mix in a way similar to how fly ash is added to concrete mixes.

References

Sections of this document were obtained from the Synthesis of Current and Projected Concrete Highway Technology, David Whiting, . . . et al, SHRP-C-345, Strategic Highway Research Program, National Research Council.

ACI Committee 223. 1983. Standard practice for the use of shrinkage-compensating ACI 223-83. Detroit: American Concrete Institute.

ACI Committee 225R. 1985. Guide to the selection and use of hydraulic cements. AC225R-85. Detroit: American Concrete Institute.

Bogue, R. H. 1955. The chemistry of portland cement. 2d ed. New York: Reinhold Publishing Corp.

Call, B. M. 1979. Slump loss with type "K" shrinkage compensating cement, concrete, and admixtures. Concrete International: Design and Construction, January: 44-47.

Energetics, Incorporated. 1988. The U.S. cement industry: An energy perspective. Final report. Columbia, Md.: Energetics, Incorporated.

Hoff, G. C. 1985. Use of steel fiber reinforced concrete in bridge decks and pavements. In Steel fiber concrete seminar (June): Proceedings, ed. S. P. Shah and A. Skarendahl, 67-108. Elsevier Applied Science Publishers.

Hoff, G. C., L. N. Godwin, K. L. Saucier, A. D. Buck, T. B. Husbands, and K. Mather. 1977. Identification of candidate zero maintenance paving materials. 2 vols. Report no. FHWA-RD-77-110 (May). Vicksburg, Miss.: U.S. Army Engineer Waterways Experiment Station.

Kudlapur, P., A. Hanaor, P. N. Balaguru, and E. G. Nawy. 1987. Repair of bridge deck structures in cold weather. Report no. SNJ-DDT4-25156 (December). The State University of New Jersey, College of Engineering, Dept. of Civil Engineering.

Lee, D. Y. 1973. Review of aggregate blending techniques. Highway Research Record, no. 441 111-98

Massazza, F. 1987. The role of the additions to cement in the concrete durability. n Cemento 84 (October-December):359-82.

McCarter, W. J., and S. Gravin. 1989. Admixture in cement: A study of dosage rates on early hydration. Materials and Structures 22:112-120.

Mehta, P. K. 1986. Concrete. Structure, properties, and materials. Englewood Cliffs, N.J.: Prentice-Hall, Inc.

Meyer, L. M., and W. F. Perenchio. 1979. Theory of concrete slump loss as related to use of chemical admixtures. Concrete International. Design and Construction 1 (1):36-43.

Mielenz, R. 1984. History of chemical admixtures for concrete. Concrete International: Design and Construction 6 (4):40-54 (April).

Mindess, S., and J. F. Young. 1981. Concrete. Englewood Cliffs, N.J.: Prentice-Hall, Inc.

National Material Advisory Board. 1987. Concrete durability: A multi-billion dollar opportunity. NMAB-437. Washington: National Academy Press.

Polivka, M., and A. Klein. 1960. Effect of water-reducing admixtures and set-retarding admixtures as influences by cement composition. In Symposium on effect of water reducing admixtures and set-retarding admixtures on properties of concrete. STP-266, 124-39. Philadelphia: American Society for Testing Materials

Pomeroy, D. 1989. Concrete durability: From basic research to practical reality. ACI special publication. Concrete durability SP- 100: 111-31.

Popoff, N. J. 1991. Blended cements. In Concrete construction. A vision for the nineties. Concrete technology seminar MSU-CTS no. 5 (February), eds. P. Soroushian and S. Ravanbakhsh, 2.1-2.16. East Lansing: Michigan State University.

Powers, T. C., L. E. Copeland, J. C. Hayes, and H. M. Mann. 1954. Permeability of portland cement paste. ACl Journal Proceedings 51 (3):285-98.

Previte, R. 1977. Concrete slump loss. ACI Journal Proceedings 74 (8):361-67.

Ramachandran, V. S., and R. F. Feldman. 1984. Cement science. In Concrete admixtures handbook: Properties, science, and technology, ed. V. Ramachandran, 1-54. Park Ridge, N.J.: Noyes Publications.

Ruettgers, A., E. N. Vidal, and S. P. Wing. 1935. An investigation of the permeability of mass concrete with particular reference to Boulder Dam. ACI Journal Proceedings 31:382-416.

Standard specification for portland cement (AASHTO M 85-89). 1986. AASHTO standard specification for transportation materials. Part I, Specifications. 14th ed.

Standard specification for portland cement (ASTM C 150-86). 1990 annual book of ASTM standards 4.02:89 - 93.

Taylor, W. F. W., ed. 1964. The chemistry of cements. 2 volumes. London: Academic Press.

Tsuji, Y., and N. Miyake. 1988. Chemically prestressed precast concrete box culverts. Concrete International: Design and Construction 10 (5):76-82 (May).

U.S. Department of the Interior. Bureau of Mines. 1989. Cement mineral yearbook. Washington: GPO.

U.S. Department of Transportation. Federal Highway Administration. 1990. Portland cement concrete materials manual. Report no. FHWA-Ed-89-006 (August). Washington: FHWA.

Verbeck, G. J. 1968. Field and laboratory studies of the sulfate resistance of concrete. In Performance of concrete resistance of concrete to sulfate and other environmental conditions: Thorvaldson symposium, 113-24. Toronto: University of Toronto Press.

Whiting, D. 1981. Evaluation of super-water reducers for highway application. FHWA/RD 80/132 (March). Washington: FHWA.

Whiting, D. 1988. Permeability of selected concretes. ACI special publication. Permeability of concrete SP-108: 195-222.

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