Air Change in Hydraulic Concrete Due to Pumping

The air content of concrete plays an important role in the durability of concrete. Excess air may reduce the strength of the concrete, while inadequate air will result in premature deterioration of the concrete due to scaling caused by freeze–thaw cycles and deicing chemicals. Pumping of hydraulic concrete has been a common practice for placing concrete. Although the air content before pumping is relatively easy to control, the air change during pumping has not been well understood. The purpose of this study was to investigate the change in air content of hydraulic concrete caused by pumping. Air content and other properties of concrete before and after pumping were measured at 36 construction projects in two phases. Analysis on data from 31 projects in Phase I indicated that most concrete experiences air loss because of pumping. Many factors could be behind this air loss. Analysis on data from five control projects in Phase II indicated that sampling methods had a moderate effect on the air change. Errors in measurement were minimal for certified testers and calibrated pots. The air change of concrete was significantly affected by the configuration of the boom, in addition to the air content before pumping. Attachments to the boom played key roles in reducing the air loss. Two mathematical models were developed for the prediction of air loss.