What Causes Air Bubbles on the Surface of UHPC Components?

Air bubbles appearing on the surface of UHPC components are a common quality defect in the production process, mainly stemming from improper control in three areas: "material properties, mold condition, and production process." These bubbles not only affect appearance; if located on the surface layer or in stress concentration zones, they may also become microscopic defects that compromise durability. Through systematic cause analysis and process control, Qinglong Group has reduced the occurrence rate of such defects to an extremely low level.

The intrinsic characteristics of the material are an internal cause of bubble formation. UHPC uses an extremely low water-binder ratio (typically 0.18-0.22) and high-range water reducers, giving its mix a very high viscosity in which entrapped air bubbles can hardly rise and escape. Meanwhile, to achieve high strength and self-compaction, UHPC typically contains no air-entraining agents, which means it lacks stable micro-bubbles to refine the pore structure, making large bubbles accidentally entrained even harder to eliminate. If raw materials (especially silica fume and cement) are damp or contain impurities, gases may also be introduced during mixing.

Mold condition and release agent application are the key external causes. An unclean mold surface with moisture or oil stains, or a mold temperature that is too low (causing water vapor to condense on the surface in contact with the mix), can both form bubbles at the interface. An improperly selected release agent, or one applied unevenly or too thickly, hinders close contact between the mix and the mold, forming "sticky-film bubbles." Qinglong strictly requires molds to be cleaned and dried before use, selects specialized release agents with good compatibility with UHPC, and applies them thinly and evenly by spraying.

Casting and densification techniques are the core of control. Improper casting methods (such as pouring directly from a great height) entrap large amounts of air. Insufficient vibration leaves bubbles unable to escape; over-vibration, however, may cause fibers to float, paste-aggregate segregation, and may turn bottom bubbles up to the surface. Depending on the component shape, Qinglong adopts layered casting with reasonably arranged pouring points, combined with high-frequency form-attached or poker vibrators, and has introduced "vacuum-assisted molding" technology. After casting and before initial setting, a brief vacuum is applied to the mix inside the mold, using negative pressure to draw out internal bubbles—an extremely effective method for eliminating bubbles inside UHPC. Through this combined set of measures, Qinglong ensures the smoothness and denseness of UHPC component surfaces.