The use of superplasticizers (high range water reducer) has become a quite common practice. This class of water reducers were originally developed in Japan and Germany in the early 1960s; they were introduced in the United States in the mid-1970s.
Superplasticizers are linear polymers containing sulfonic acid groups attached to the polymer backbone at regular intervals (Verbeck 1968). Most of the commercial formulations belong to one of four families:
The sulfonic acid groups are responsible for neutralizing the surface charges on the cement particles and causing dispersion, thus releasing the water tied up in the cement particle agglomerations and thereafter reducing the viscosity of the paste and concrete (Mindess and Young 1981).
ASTM C 494 was modified to include high-range water-reducing admixtures in the edition published in July 1980. The admixtures were designated Type F water-reducing, high range admixtures and Type G water-reducing, high-range, and retarding admixtures (Mielenz 1984).
Effect of Superplasticizers on Concrete Properties. The main purpose of using superplasticizers is to produce flowing concrete with very high slump in the range of 7-9 inches (175-225 mm) to be used in heavily reinforced structures and in placements where adequate consolidation by vibration cannot be readily achieved. The other major application is the production of high-strength concrete at w/c's ranging from 0.3 to 0.4 (Ramachandran and Malhotra 1984).
The ability of superplasticizers to increase the slump of concrete depends on such factors as the type, dosage, and time of addition of superplasticizer; w/c; and the nature or amount of cement. It has been found that for most types of cement, superplasticizer improves the workability of concrete. For example, incorporation of 1.5% SMF to a concrete containing Type I, II and V cements increases the initial slump of 3 inches (76 mm) to 8.7, 8.5, and 9 inches (222, 216, and 229 mm), respectively.
The capability of superplasticizers to reduce water requirements 12-25% without affecting the workability leads to production of high-strength concrete and lower permeability. Compressive strengths greater than 14,000 psi (96.5 MPa) at 28 days have been attained (Admixtures and ground slag 1990). Use of superplasticizers in air-entrained concrete can produce coarser-than-normal air-void systems. The maximum recommended spacing factor for air-entrained concrete to resist freezing and thawing is 0.008 inch (0.2 mm). In superplasticized concrete, spacing factors in many cases exceed this limit (Malhotra 1989; Philleo 1986). Even though the spacing factor is relatively high, the durability factors are above 90 after 300 freeze-thaw cycles for the same cases (Malhotra 1989). A study conducted by Siebel (1987) indicated that high workability concrete containing superplasticizer can be made with a high freeze-thaw resistance, but air content must be increased relative to concrete without superplasticizer. This study also showed that the type of superplasticizer has nearly no influence on the air-void system.
One problem associated with using a high range water reducer in concrete is slump loss. In a study of the behavior of fresh concrete containing conventional water reducers and high range water reducer, Whiting and Dziedzic (1989) found that slump loss with time is very rapid in spite of the fact that second-generation high range water reducer are claimed not to suffer as much from the slump loss phenomenon as the first-generation conventional water reducers do. However, slump loss of flowing concrete was found to be less severe, especially for newly developed admixtures based on copolymeric formulations.
The slump loss problem can be overcome by adding the admixture to the concrete just before the concrete is placed. However, there are disadvantages to such a procedure. The dosage control, for example, might not be adequate, and it requires ancillary equipment such as truck-mounted admixture tanks and dispensers. Adding admixtures at the batch plant, beside dosage control improvement, reduces wear of truck mixers and reduces the tendency to add water onsite (Wallace 1985). New admixtures now being marketed can be added at the batch plant and can hold the slump above 8 inches (204 mm) for more than 2 hours.
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.
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Mindess, S., and J. F. Young. 1981. Concrete. Englewood Cliffs, N.J.: Prentice-Hall, Inc.
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Siebel, E. 1989. Air-void characteristics and freezing and thawing resistance of superplasticized air-entrained concrete. In ACI SP-119: Superplasticizers and other chemical admixtures in concrete, ed. V. M. Malhotra, 297-319. Detroit: American Concrete Institute.
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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.
Wallace, M. 1985. Flowing concrete produced at the batch plant. Concrete Construction 30 (4):337-43
Whiting, D., and W. Dziedzic. 1989. Behavior of cement-reduced and flowing fresh concretes containing conventional water-reducing and second generation high-range water-reducing admixtures. Cement, Concrete, and Aggregates 11 (1):30-39.