Why Using an Air-Entrainer to Increase Workability Is Not a Great Idea for Deep Foundations-Paper

Paper

Pdf Summary

The paper argues that entraining air to improve concrete workability is unreliable for deep foundations because air bubbles compress under pressure and can dissolve into the surrounding water. Their contribution to flow therefore depends on pressure, shear rate, bubble size, and concrete viscosity; workability measured at atmospheric pressure may not represent behavior underground.<br /><br />Full-scale pumping tests compared similar self-consolidating concrete mixtures with and without air-entraining admixture. In 125 mm pipes, the air-entrained mixture required less pressure to flow. In 100 mm pipes, where pressure was higher, this advantage largely disappeared. The results suggest that at about 5 bar (500 kPa), air no longer meaningfully reduces flow resistance.<br /><br />The authors relate this pressure to foundation placement: hydrostatic pressure reaches roughly 5 bar beneath a 20 m column of concrete, or a 50 m column of support fluid. Thus, air added to increase workability may lose its benefit at depths common in deep foundations. The concrete could become stiffer or more difficult to place than atmospheric-pressure quality-control tests indicate.<br /><br />The study concludes that air entrainment should not be relied on as a workability-enhancing strategy for deep foundations. Its findings are based on pumping tests and a simplified hydrostatic-pressure estimate; the authors note that air-void changes were not confirmed through hardened-concrete analysis.

Keywords

air entrainment

concrete workability

deep foundations

hydrostatic pressure

self-consolidating concrete

pumping tests

bubble compression

air-entraining admixture

flow resistance

pressure effects