7.6 Chemical Admixtures

Chemical admixtures are used to enhance both the fresh and hardened properties of concrete. The department pre-approves several types of chemical admixtures for use. The pre-approved types are listed below.
  • Type A – Water Reducing Admixture: These admixtures reduce the quantity of mixing water required to produce concrete of a given consistency.
  • Type B – Retarding Admixture: These admixtures retard the setting of concrete.
  • Type C – Accelerating Admixture: These admixtures accelerate the setting and early strength development of concrete.
  • Type D – Water-Reducing and Retarding Admixture: These admixtures reduce the quantity of mixing water required to produce concrete of a given consistency and retard the setting of concrete.
  • Type G – High Range Water Reducing Admixture: These admixtures reduce the quantity of mixing water required to produce concrete of a given consistency by 12% or greater.
  • Type F – High Range Water Reducing and Retarding: These admixtures reduce the quantity of mixing water required to produce concrete of a given consistency by 12% or greater and retard the setting of concrete.
  • Air Entraining Admixture: These admixtures are used to stabilize air mixed into the concrete during mixing and create a system of small, closely spaced air voids within the concrete.
The common types of chemical admixtures used in paving concrete are Type A, Type B, Type D, and air entraining admixtures. Since most concrete pavement are placed with a slip form paver, a high degree of workability is not necessary or wanted; therefore, high range water reducers are not used. Some of the recent admixtures are called mid-range water reducers. There is not an official ASTM classification for these admixtures, but they are usually approved as Type A or Type F admixture. These mid-range admixtures provide slightly more water reduction compared to normal range water reducers, reduce stickiness, and improve finishing, pumping, and placing properties.