With today’s concrete mixtures becoming more complex, the possibility for cement-admixture incompatibility problems is  increasing.  Cement-admixture incompatibility is simply when the concrete experiences the opposite performance that was expected when a particular cementitious-admixture system is used.  The occurrence of such problems is ever more evident based on the recent creation of ASTM Sub-committee C09/C01.48, which deals with establishing test methods designed to predict and diagnose material incompatibility issues.

The heart of most incompatibility issues concerns how chemical admixtures can alter the balance of sulfate availability (from the inter-ground gypsum) and aluminate reactivity from the C3A and C4AF phases in the cement.

Remarkably, problems such as poor slump increase and rapid slump loss that can occurr with certain cementitious material-admixture combinations, can be essentially eliminated by delaying the addition of admixture sometimes as little as 30 to 60 seconds after the cement and admixture have mixed together. This delay allows the normal sulfate-aluminate reactivity to occur, thus minimizing unexpected performances.

With the Fall season underway in the northern hemisphere, concrete producers and contractors readily understand that the setting rate and strength development of cementitious mixtures will be delayed as concrete and ambient temperatures drop. These delays in set and strength gain can be offset with the use of set accelerating chemical admixtures. The performance requirements of such admixtures are clearly stipulated in various regional nation standards such ASTM C494, where two types of set accelerating admixtures are desribed, Type C and Type E. Compositionally, set accelerators fall into two classes: chloride or calcium chloride-based, and non-chloride. More importantly, it is very important to understand corrosivity of an admixture over the dosage range recommended by the manufucturer. My next posting will provide further guidance on the use of set accelerating admixtures.

Water Reducer performance can significantly be influenced by the physocal and chemical properties of all the concrete mix components, as well as the time of addition, – that is, with the mix water, or delayed after cement and water have made contact. Becuase of all these possible influences on water reducer performance, testing the admixture under a wide range of controlled condtions is important both for achieving maximum performance as well as anticipating unexpected concrete behavior.

Every cubic yard or cubic meter of concrete, batched with ordinary portland cement, can readily benefit from the use of a conventional ASTM C494 certified water reducing admixture (WRA). The benefit can be realized in one of three basic ways relative to a reference mix without any admixture:

1. All the mixture proportions of the reference mix can be kept constant. Upon adding the water reducing admixture and increase in slump or workability can be realized. The amount of slump increase will be a function of the type (or chemistry) and dosage rate of the water reducing admixture.
2. With the addition of a WRA, the water content of the reference mix can be reduced (accompanied by an increase in fine and coarse aggregate). The resulting mixture will have increased strength and reduced permeability versus the reference mix.
3. Both the water and cement contents can be reduced proportionately, maintaining a constant w/c ratio. The resulting mixture will have approximately the same workability and strength at a lower production cost.

All these benefits are made possible by the action of water reducing chemicals on the inter-particle forces between hydrating cement grains. To learn more, an excellent course on chemical admixtures for concrete is available from Dr. Ara A. Jeknavorian.