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Set-Retarding

Retarding admixtures (retarders) are known to delay hydration of cement without affecting the long-term mechanical properties. They are used in concrete to offset the effect of high temperatures, which decrease setting times, or to avoid complications when unavoidable delays between mixing and placing occur (Mindess and Young 1981). Use of set retarders in concrete pavement construction 1) enables farther hauling, thus eliminating the cost of relocating central mixing plants; 2) allows more time for texturing or plastic grooving of concrete pavements; 3) allows more time for hand finishing around the headers at the start and end of the production day; and 4) helps eliminate cold joints in two-course paving and in the event of equipment breakdown (Amer. Concrete Pavement Assoc. 1975). Retarders can also be used to resist cracking due to form deflection that can occur when horizontal slabs are placed in sections (Mindess and Young 1981). Because of these advantages, set retarders are considered the second most commonly used admixtures in the highway industry, especially in the construction of bridge decks (U.S. Dept. Trans. 1990).

Composition and Mechanism of Retardation. Many water reducers have a retarding tendency. Therefore, some of the ingredients in water reducers, such as lignosulfate acids and hydroxycarboxylic acids, are also a basis for set-retarding admixtures. Other important materials used in producing set retarders are sugars and their derivatives.

Mechanisms of set retardation were studied by many researchers. Several theories have been offered to explain this mechanism. A review of these theories was presented by Young (1972). The role of retarding admixtures can be explained in a simple way: the admixtures form a film around the cement compounds (e.g., by absorption), thereby preventing or slowing the reaction with water. The thickness of this film will dictate how much the rate of hydration is retarded. After a while, this film breaks down, and normal hydration proceeds (Fattohi 1958). However, in some cases when the dosage of admixtures exceeds a certain critical point, hydration of cement compounds will never proceed beyond a certain stage, and the cement paste will never set. Thus, it is important to avoid overdosing a concrete with a retarding admixture.

Other factors influencing the degree of retardation include the w/c, cement content, C3A and alkali contents in cement, the type and dosage of the admixture, and the stage at which the retarder is added to the mix. The effectiveness of retarder is increased if its addition to the fresh concrete is delayed for a few minutes.

Effect on Concrete Properties and Application. In addition to their role in controlling setting time, retarders—like any other admixtures—influence the properties of fresh and hardened concrete. Air entrainment of concrete is affected and fewer air-entraining agents need to be used because some retarders entrain air (see water reducers). Slump loss might increase even when abnormal setting behavior does not occur.

Because of retarding action, the 1-day strength of the concrete is reduced. However, ultimate strength is reported to be improved by using set-controlling admixtures. Rates of drying shrinkage and creep could increase by using retarders, but the ultimate values cannot increase.

One of the most important applications of retarding admixtures is hot-weather concreting, when delays between mixing and placing operation, may result in early stiffening (Fattuhi 1958). Another important application is in prestressed concrete, where retarders prevent the concrete that is in contact with the strand from setting before vibrating operations are completed. Set retarders also allow use of high-temperature curing in prestressed concrete production without affecting the ultimate strength of the concrete.

Recommendations

  1. Verification tests should be performed on liquid admixtures to confirm that the material is the same as that which was approved. The identifying tests include chloride and solids content, ph and infrared spectrometry.

  2. Water reducers and retarders may be used in bridge deck concrete to extend the time of set. This is especially important when the length of placement may result in flexural cracks created by dead load deflections during placement.

  3. Increased attention needs to be placed on curing and protection due to the potential for shrinkage cracks and bleeding when water reducers are used.

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.

American Concrete Pavement Association. Technical Subcommittee on Admixtures. 1975. Instruction and check-list for practical field use and control of chemical admixtures in concrete. Arlington Heights, III.: American Concrete Pavement Association. May.

Fattuhi, N. I. 1958. Influence of air temperature on the setting of concrete containing set retarding admixtures. Cement, Concrete, and Aggregates 7 (1):15-18.

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

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

Young, J. F. 1972. A review of the mechanisms of set-retardation in portland cement pastes containing organic admixtures. Cement and Concrete Research 2:415-33.


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