Pumping is a frequently used method of efficiently transporting concrete from the delivery vehicles to the formwork. Unfortunately, pumping concrete can affect the air content of a concrete mix. This has caused concern about the possible loss of resistance of the concrete to freezing and thawing. The actual effect of pumping on the air content measured for the concrete is not consistent; in some cases air content has increased, while in others it has decreased. Previous work has shown that pumping causes pressure in the concrete, and this pressure can alter the air-void system. The purpose of this work was to verify previously identified changes in the air-void system, and to examine the effect of pressure on concrete mixes containing various admixture combinations.
The stability of an air-void system with respect to pressurization was found to depend upon both the actual air content and the admixture(s) used in the mix. Higher air contents improved resistance to changes from pressurization, while the use of lignosulfonate-based water reducers decreased the stability. Type F fly ash improved the stability. Air-entraining agent type appeared to also affect stability, but additional testing would be required to establish statistically significant effects based on air-entraining agent type.