The Coupling Effect of Viscosity Modifying Agents and Printing Process on the Air-Void Structure Formation of 3D Printed Air-Entrained Concrete (2026-01)¶

3D printed concrete (3DPC) holds significant potential for applications in extreme environments. Air-entraining agents (AEAs) can enhance the frost resistance of concrete in cold regions by introducing uniformly distributed small air-voids. However, in 3DPC, the use of viscosity-modifying agents (VMAs) and the unique printing process may adversely affect the characteristics of entrained air-voids. This study employs X-ray computed tomography (X-CT) to quantitatively characterize the evolution of air-void structures in AEA-modified 3DPC containing different VMAs across three critical manufacturing stages: before-printing, in-printing-nozzle and after-printing. The results reveal that the shear action of the screw rod effectively refines the air-void size distribution while increasing overall air-void volume fraction. Hydroxypropyl methyl cellulose (HPMC) effectively stabilizes small bubbles (<500 μm) during before-printing stage, increasing the air-voids count via a protective film that prevents rupture. Conversely, attapulgite leaves small bubbles vulnerable to break. However, during in-printing-nozzle and after-printing stage, attapulgite better protects large bubbles (>1000 μm), maintaining their shape against shear and elongation forces with a stable shell, while HPMC offers little protection, leading to more significant elongated bubble shapes. This study provides an experimental basis for regulating air-void structure in 3D printed air-entrained concrete from the perspective of materials selection and printing processes control.