What Causes Air Bubbles on UHPC Component Surfaces? A Full-Process Analysis from Raw Materials to Curing

The formation of surface bubbles in UHPC components is not caused by a single stage, but results from the combined effects of multiple factors including raw material properties, mixing process, molding process, and curing. Bubbles not only affect the decorative effect but may also reduce component density and durability. Qinglong, as a manufacturer with a comprehensive quality control system, has summarized the core causes of bubble formation and their solutions through 28 years of practice, achieving high-quality delivery with bubble-free surfaces in projects in Algeria, Shanghai, and other locations.

Improper raw material proportions lay hidden risks for bubble formation. The cementitious material system, aggregate gradation, and admixture selection of UHPC directly affect bubble conditions: cement with an excessively large specific surface area in cementitious materials easily adsorbs air to form bubbles; discontinuous aggregate gradation with a high void ratio makes it difficult for air to escape during mixing; improper admixture selection, such as water reducers with excessively high air content or failure to pair air-entraining agents with defoamers, leads to uncontrolled bubble quantities. Qinglong uses high-grade cement with low specific surface area, paired with continuously graded quartz sand (void ratio ≤35%), and applies a self-developed compound admixture system (high-efficiency water reducer + defoamer + water-retaining agent), strictly controlling air content within 2%-3% to reduce bubble formation at the source. By contrast, some manufacturers use cheap admixtures and unreasonable aggregate gradation, resulting in surface bubbles exceeding standards and compromising appearance quality.

Imperfect mixing processes cause air entrapment and uneven dispersion. Manual mixing or ordinary mixers with insufficient rotation speed (below 600r/min) cannot thoroughly mix cementitious materials, aggregates, fibers, and admixtures, easily forming agglomerates that trap air into bubbles; mixing time that is too short (below 3 minutes) leaves air insufficiently expelled, while overly long mixing causes the mixture to heat up, triggering water evaporation and bubble formation. Qinglong uses automated twin-shaft mixing equipment with a rotation speed of 800r/min and mixing time controlled at 4-6 minutes. Through the three-stage mixing process of "dry mixing - wet mixing - fiber dispersion", the mixture is ensured to be uniform and fine, with air fully expelled. In the Yangshengtang Pharmaceutical Hangzhou Industrial Park project, this mixing process kept the surface bubble diameter of UHPC exterior wall panels at ≤2mm with ≤3 bubbles per square meter, far better than industry standards.

Improper operation during the molding process causes bubbles to remain on the surface. When UHPC is cast or sprayed, placing the material too fast or from too great a height (over 50cm) causes the mixture to impact the mold and entrap air; insufficient or excessive vibration - the former prevents air from escaping, while the latter causes aggregate segregation and bubble formation; failure to apply release agent on the mold surface, or uneven application, increases the adhesion between the mixture and the mold, making it difficult for bubbles to escape. Qinglong adopts a high-pressure spraying molding process (spraying pressure 0.3-0.5MPa) with uniform material placement, combined with attached vibrators (vibration frequency 200Hz) to ensure air expulsion; a special water-based release agent is applied evenly on the mold surface (thickness 0.1-0.2mm) to reduce bubble retention. The 50mm-thick UHPC openwork panels of the Ouargla Hotel in Algeria, through precise molding operations, feature surfaces with no obvious bubbles, smooth and fine.

Improper temperature and humidity control during curing exacerbates bubble appearance. Excessively high temperature and insufficient humidity in the early curing stage cause rapid evaporation of moisture from the UHPC surface, and the internal air, after expanding, cannot escape, forming surface bubbles; heating up too quickly during steam curing causes internal air to expand sharply, impacting the surface and forming bubbles. Qinglong's intelligent constant-temperature curing kiln adopts a standardized process of gradient heating (5-10°C/h), constant temperature and moisture retention (temperature 90-95°C, humidity ≥95%), and slow cooling, allowing internal air to escape smoothly and avoiding bubble appearance. Through whole-process cause control and technical optimization, Qinglong has effectively resolved the surface bubble problem of its UHPC components, providing customers with quality products that combine aesthetics and durability.