Basics of Concrete | Chemical Admixtures
ACI 116R defines the term admixture as "a material other than water, aggregates, hydraulic cement, and fiber reinforcement, used as an ingredient of concrete or mortar, and added to the batch immediately before or during its mixing."
Admixture Storage Tanks and Equipment Shed
Chemical admixtures are materials that are added to the constituents of concrete to either enhance its properties; either in its plastic or hardened state, or to modify some specified and desired chemical reaction within the concrete itself.
While the American Concrete Institute (ACI) defines what an admixture is, ASTM sets the standards that an admixture must meet. The ASTM sections concerning chemical admixtures are:
- ASTM C 260 - Air-Entraining Admixtures
- ASTM C 494 -
Type A - Water-Reducing
Type B - Retarding
Type C - Accelerating
Type D - Water-Reducing & Retarding
Type E - Water-Reducing & Accelerating
Type F - High-Range Water-Reducing (HRWR)
Type G - HRWR & Retarding - ASTM D 98 - Calcium Chloride
- ASTM C 869 - Foaming Agents (for use in making Cellular Concrete)
- ASTM C 1141 - Admixtures for Shotcrete
- ASTM C 1017 - Admixtures for Use in Producing Flowing Concrete
- ASTM C 979 - Pigments For Integrally Colored Concrete
Admixture Dispensing Equipment
Other types of admixtures, which are not listed in ASTM specifications, but are just as valuable to the concrete industry, are:
- Microsilica - ACI 234R
- Corrosion Inhibitors
- Shrinkage Reduction
- Hydration Stabilizers
- Flowable Fill Additives
Most admixtures are batched into the concrete through dispensers mounted at the concrete plant, while others may be hand dosed into the ready-mix truck once the proper concrete consistency has been confirmed. Most admixtures are metered into the mix in fluid ounces per 100 pounds of total cementitions.