As defined by the American Concrete Institute, an admixture is a material other than water, aggregates, hydraulic cement, and fiber reinforcement used as an ingredient of a cementitious mixture, which is added to the batch before or during mixing to modify its freshly mixed, setting, or hardened properties. The following discussion reviews how materials generally known as chemical admixtures are used in high performance concrete (HPC).

Air-entraining admixtures

Air-entraining admixtures are used in HPC primarily to increase the resistance of the concrete to freeze/thaw damage when exposed to water and deicing chemicals. They may also be used to increase workability and facilitate handling and finishing. Air-entraining admixtures should conform to ASTM C 260 (AASHTO M 154). Because the presence of air reduces concrete compressive strength, air entrainment is generally not used in producing high-strength concrete (HSC). For most applications of HSC, air-entrainment to provide freeze/thaw resistance is not required.

Chemical admixtures

ASTM Specification C 494 (AASHTO M 194) lists the following seven types of chemical admixtures:

Type A	- Water-reducing
Type B	- Retarding
Type C	- Accelerating
Type D	- Water-reducing and retarding
Type E	- Water-reducing and accelerating
Type F	- Water-reducing, high-range
Type G	- Water-reducing, high-range and retarding

Water-reducing admixtures and high-range, water-reducing admixtures in HPC minimize the quantity of water necessary to produce a concrete with the required workability. They are often used in HPC specified for durability and are almost always used in high-strength concrete. Retarding admixtures are used to delay the setting, i.e., decrease the setting rate, of concrete. They can be beneficial in offsetting the accelerating effects of hot weather or when a long transit time is required. Accelerating admixtures decrease setting time and increase early strength development. They are sometimes used in cold weather placements and are useful in precast concrete fabrication to facilitate form removal and release of prestressing. The same effect, however, can be achieved naturally in HSC because of the higher cementitious materials content.

Flowing concrete

Flowing concrete is defined in ASTM C 1017 as concrete characterized by a slump greater than 7¼ in. (190 mm) while maintaining a cohesive nature. It is produced by using a plasticizing admixture or a plasticizing and retarding admixture. It is especially suitable for applications with congested reinforcement where conventional consolidation methods can not be used. The admixtures used to provide flowing concrete are generally identical to high-range, water-reducing admixtures Types F and G.

Corrosion inhibitors

Corrosion inhibitors are liquid admixtures used in concrete to raise the chloride threshold level at which corrosion starts and to reduce the rate of corrosion after it begins. They are used primarily in producing structures that are exposed to chloride salts, such as bridge decks, parking garages, and marine facilities. Various levels of corrosion protection can be achieved depending on the quantity of inhibitor used in the concrete.

Shrinkage-reducing admixtures

Shrinkage-reducing admixtures are used in applications where it is desirable or necessary to reduce the shrinkage of concrete. Drying shrinkage reductions up to 50 percent have been achieved in laboratory tests. It has also been reported that curling of slabs is reduced by means of shrinkage-reducing admixtures.

Antiwashout admixtures

For casting under water, antiwashout admixtures may be used to increase the cohesiveness of concrete, thereby reducing the loss of cement and increasing resistance to segregation. A mixture's resistance to mass loss during underwater placement may be measured by the U.S. Army Corps of Engineers Method CRD-C61 entitled “Test Method for Determining the Resistance of Freshly Mixed Concrete to Wash Out in Water.”

Self-compacting concrete

Self-compacting concrete (SCC) or self-consolidating concrete is a fluid, cohesive concrete mix that is able to flow and consolidate under its own weight. SCC is produced using a high-range, water-reducing admixture in combination with a stabilizer. Since the concrete is very fluid, a traditional slump test is not appropriate. Accordingly, slump flow — the diameter of the puddle of SCC produced when a standard slump cone is lifted — is a more useful measurement. The concrete, of course, has to flow without segregation. The use of SCC is beneficial in precast fabrication due to its ease of placement, minimal labor requirements, and reduced noise levels. In addition, SCC can result in a good surface finish.

High performance concrete

High performance concrete is characterized by properties developed for a particular application and environment. Careful selection of all constituent materials and optimization of mixture proportions are essential. Such a mix invariably requires the use of one or more admixtures and trial batches to ensure that the specified and desired characteristics are achieved with the proposed mix proportions. When multiple admixtures are used, compatibility of the admixtures with each other and with other constituent materials must be ensured. On-site trials as well as laboratory trial batches are indispensable.

---

References

ACI 212.3-Chemical Admixtures for Concrete, ACI 212.4-Guide for Use of High-Range Water-Reducing Admixtures (Superplasticizers) in Concrete, EB 001 Design and Control of Concrete Mixtures, Fourteenth Edition (Portland Cement Association)

---

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.