What Causes Air Bubbles on the Surface of UHPC Components?

Surface air bubbles in UHPC components are a common quality defect, with their root causes attributable to the combined effects of four factors: mix proportion, mixing process, pouring method, and release agent use. Drawing on 28 years of production practice and laboratory research, Qinglong Group has summarized the main causes of bubble formation: excessive air entrained by high-range water reducers (air content above 6%), gas entrapment caused by mixing speed that is too fast or mixing time that is too short, turbulence-induced air entrapment from excessive pouring drop height (>1m), and gas barrier layers formed by uneven or excessive release agent application. During the initial production of UHPC roof panels for the Hainan International Convention and Exhibition Center, we saw the surface bubble rate rise from 3% to 8% because the mixer speed was set too high (raised from 120rpm to 150rpm). By optimizing the mixing curve (reducing speed to 100rpm and extending wet mixing time by 1 minute) and adding a defoamer (dosage 0.1%), we successfully brought the bubble rate below 1%. Bubble control is the fundamental checkpoint for UHPC surface quality.

Mix Proportion and Admixtures Are the Source of Bubbles

Polycarboxylate water reducers have a certain air-entraining property; when overused (dosage >1.5%) or poorly compatible with the cement, they easily produce fine bubbles (pore diameter 0.1-0.5mm). Through testing, Qinglong Group selects water reducers matched to the cement and keeps air content within the ideal range of 4-6%. Meanwhile, high-quality defoamers (such as silicone-based ones) are added to effectively break up large bubbles.

Mixing Process Parameters Directly Affect Air Content

With a mixing speed that is too high (>150rpm) or a time that is too short (<3 minutes), the slurry cannot fully release air. We adopt a three-stage 'slow-fast-slow' mixing process: low-speed dry mixing (60rpm×1min) → high-speed wet mixing (100rpm×3min) → low-speed defoaming (60rpm×1min). The vacuum mixing equipment at our Nanning production base (vacuum degree -0.08MPa) can further reduce air content to below 2%.

Pouring and Vibration Operations Are Key Control Points

The pouring drop height should be kept within 0.5m, with drop chutes or chutes used to reduce impact. Vibration requires 'quick insertion, slow withdrawal,' avoiding over-vibration (>20s per point) that causes bubbles to rise and gather. For complex molds, vent holes are provided to assist air release. Qinglong Group workers are professionally trained and certified for their posts.

Release Agents and Mold Condition Are Often Overlooked

An overly thick oil-based release agent hinders air release; we prefer water-based release agents, sprayed evenly (thickness ≤20μm). Insufficient mold surface smoothness (Ra >3.2μm) also tends to trap air, so molds are regularly polished and maintained.

Bubble prevention requires systematic control. Through standardized operations and full-process monitoring, Qinglong Group ensures UHPC components with mirror-smooth surfaces.