DOES ADDING SUPERPLASTICIZER AFTER BATCHING INCREASE ENTRAINED AIR CONTENT?
This question comes up frequently when we discuss superplasticizer additions in transit with Verifi.
Increasing the slump of a load of concrete usually increases the entrained air content. That is true whether the slump is increased with water or admixture; whether slump is increased at the plant, in transit, or at the jobsite; and whether slump is increased gradually or with a one-time addition.
Likewise, a decrease in slump when concrete is in the mixer is usually associated with a decrease in air content.
This first graph shows an air entrained concrete mix delivered without Verifi. The slump and air content decrease over time. Then, water is added just before discharge to increase slump, which results in an increase in entrained air content.
The next graph shows the same scenario, but polycarboxylate-based superplasticizer is added at the jobsite. Similarly, the air content increases as the slump increases.
This last graph shows a Verifi-managed delivery. The slump and entrained air content remain essentially constant.
In conclusion, maintaining a consistent slump—with water or admixture—helps ensure consistent entrained air content. In addition, minimizing variation in other factors such as water content and mixing also help reduce variation in entrained air content. As always, it is important to test with the materials in question and to select the air entraining admixture best suited for the available materials and expected conditions.