What Causes Air Bubbles on UHPC Component Surfaces? Production Process and Material Control Guide

UHPC components are distinguished by their smooth surface finish and fine texture, but if the production process is not properly controlled, surface bubble defects are likely to appear, affecting both decorative effect and structural durability. Drawing on 28 years of UHPC/GRC/GRG/GRP production experience, Qinglong Group has kept the bubble defect rate below 0.5% through 58 patented technologies and strict quality control, and has summarized the four core influencing factors—material mix proportion, mixing process, casting operation, and curing regime—together with targeted solutions.

1. Material Mix Proportion: The Root Cause of Bubble Formation

Unreasonable material selection and mix proportion are the root cause of bubble formation. The low water-binder ratio of UHPC requires precise aggregate gradation; if the particle size distribution of the fine aggregate (quartz sand) is uneven (e.g., fine powder content ≤15%), voids tend to form and lead to bubbles. Improper admixture selection can also cause problems: an insufficient dosage of superplasticizer (below 1.2% of the total cementitious materials) reduces fluidity and makes bubbles hard to expel, while an excessive dosage may introduce too much air. Using its self-developed composite aggregate gradation (quartz sand of 0.15-1.2mm accounting for 70%) and a custom superplasticizer (dosage 1.5%-2.0%), Qinglong achieved UHPC components with surface bubble diameters ≤2mm and no more than 3 bubbles per square meter in the Shanghai New World project. In addition, uneven dispersion of steel fibers can hinder bubbles from rising. Through fiber pretreatment technology (surface coating with a dispersant), Qinglong ensures that steel fibers are evenly distributed in the mixture, reducing bubble retention.

2. Mixing Process: The Key Stage Where Bubbles Are Introduced

Air entrainment during mixing is the main pathway for bubble formation. Mixing too fast (above 60r/min) creates vortices in the mixture and draws in large amounts of air; insufficient mixing time (less than 5 minutes) results in uneven material mixing, making closed bubbles likely to form inside; an improper feeding sequence (such as pouring powder directly into water) causes the powder to agglomerate, trapping air that is difficult to expel. Qinglong adopts a three-stage mixing process of "low-speed start - medium-speed mixing - high-speed dispersion": initial mixing at 30r/min for 2 minutes to avoid vortices; intermediate mixing at 45r/min for 3 minutes to ensure uniform mixing; and final mixing at 55r/min for 2 minutes to disperse micro bubbles. The feeding sequence strictly follows "aggregate → cement → reactive powder → steel fiber → admixture → water", and water is added by spraying to reduce air entrainment. In the Yangshengtang Pharmaceutical Hangzhou Industrial Park project, this process reduced the bubble defect rate to 0.3%.

3. Casting and Forming: Core Control for Bubble Expulsion

Improper casting operations prevent bubbles from being expelled effectively: casting too fast (above 0.5m/min) creates excessive impact in the mixture, squeezing air between the mold and the material; casting from too great a height (over 1.5m) without flow-guiding devices allows air to be drawn in as the mixture falls; insufficient or excessive vibration also causes problems—insufficient vibration prevents bubbles from rising, while excessive vibration damages the UHPC microstructure and generates new bubbles. Qinglong adopts a "layered casting + high-frequency low-amplitude vibration" process: each layer is cast at a thickness ≤200mm, using 200-300Hz high-frequency vibrators inserted at spacings ≤300mm with a vibration time of 10-15 seconds per point, ensuring that bubbles rise without damaging the material structure. For complex molds (such as hollowed-out and carved components), vacuum-assisted forming technology is used, applying a vacuum of -0.06 to -0.08MPa after casting to extract internal bubbles. The UHPC hollowed-out panels for the Ouargla Hotel in Algeria achieved bubble-free surfaces through this technology.

4. Curing Regime: The Later-Stage Effect on Bubble Appearance

Improper curing causes surface moisture to evaporate too quickly, exposing internal bubbles prematurely or forming drying-shrinkage bubbles. In steam curing, a heating rate that is too fast (above 15℃/h) causes internal moisture to vaporize rapidly, forming expansion bubbles; when the curing environment humidity is below 90%, surface moisture evaporates too quickly and bubble walls cannot heal in time, forming concave bubbles. Qinglong adopts a "constant temperature and humidity + gradient heating" curing scheme: after casting, pre-cure for 4h at 20℃ with humidity ≥95%, then heat to 50-60℃ at a rate ≤10℃/h, maintain constant temperature for 12-18h, and finally cool down at a rate ≤8℃/h, ensuring that bubbles are steadily expelled during hydration. In the Shanghai Astronomy Museum public art project, this curing scheme produced a 3-meter-high UHPC sculpture with no visible surface bubbles and flatness ≤2mm/m.

5. Qinglong's Bubble Control System and Engineering Verification

Qinglong has established a full-chain bubble control system of "raw material inspection - process monitoring - finished product testing": aggregate air content and admixture air-entraining properties are tested when raw materials enter the plant; online air content monitors are used during production to keep mixture air content ≤2% in real time; finished products are scanned by laser to detect surface bubbles, and components exceeding the limits are immediately reworked. Relying on this system, Qinglong achieved a bubble pass rate of 99.7% in producing 3000㎡ of UHPC curtain wall panels for the Shenzhen Yirui Biotechnology Building, and in the Guangxi New Media Center double-curved UHPC component project, the bubble defect rate of complex-shaped components was only 0.4%. As an enterprise that participated in drafting the standard "General Technical Conditions for Non-Load-Bearing Components of Ultra-High Performance Concrete (UHPC)", Qinglong has incorporated bubble control technology into its industry standard proposals, driving overall quality improvement across the industry.

Controlling surface bubbles in UHPC components is a systematic undertaking that requires precise control throughout the entire process of materials, techniques, and curing. Qinglong Group has always upheld the mission of "creating beautiful architecture". With advanced production equipment, mature process technology, and strict quality control, it effectively resolves bubble defect problems and provides customers with UHPC component products featuring smooth surfaces and outstanding performance.