Effects of Rheological Performance, Antifoaming Admixture, and Mixing Procedure on Air Bubbles and Strength of UHPC

This paper investigates the effects of rheological performance, antifoaming admixtures (AFA 1 and AFA 2), and mixing procedure on air bubbles and the mechanical strength of ultra-high-performance concrete (UHPC). Thirty-eight mixtures were prepared by varying the water-to-binder ratio from 0.21 to 0.27, the dosage of superplasticizer (SP) from 2.5% to 4.0%, cationic polyacrylamide from 0‰ to 3.0‰, AFAs from 0.5‰ to 4.0‰, and mixing duration from 4 to 12 min at two mixing speeds. The air content in a fresh mixture and air bubble parameters in hardened samples were measured using a pressure gauge method and digital image analysis, respectively. Compressive and flexural strengths were tested for all the mixtures after 7 and 28 days of standard curing. Results show that a lower yield stress and/or plastic viscosity and higher incorporation of AFAs reduce the content of air bubbles in hardened UHPC. Higher mixing speed is more beneficial for reducing the content and average diameter of entrapped air bubbles. The 28-day compressive strength linearly increases with the decreasing content and average diameter of air bubbles. Air bubbles with diameters larger than 1 mm have worse effects on strength of UHPC than the smaller bubbles. Therefore, mechanical strength of UHPC can be improved by controlling the content and size of air bubbles through regulating rheological properties, incorporating anti-foaming admixture and optimizing mixing procedure.