What Causes Air Bubbles on the Surface of UHPC Components?

Surface bubbles on UHPC components are a common defect affecting both aesthetics and performance, arising from multiple factors including materials, processes, and equipment. Based on 28 years of production practice, Qinglong Group categorizes the causes of bubbles into four major types: air contained in raw materials, air introduced during mixing, air entrapped during casting, and air released during curing, and has reduced surface porosity to below 1% through systematic control.

Air contained in raw materials is the main source of bubbles. Insufficient fineness of cementitious materials and excessively strong air-entraining properties of water reducers both lead to increased bubbles. Through strict control of raw material quality, Qinglong Group keeps the cement specific surface area within 350-400 m²/kg and the air content of water reducers at ≤3%. At the Nanning production base, air content tests are conducted on every batch of incoming raw materials to ensure material consistency. Mix proportion optimization is also critical: the water-binder ratio is controlled at 0.20-0.22, and defoamers (silicone-based, dosage 0.1%-0.3%) are added to effectively reduce bubble formation.

The mixing process directly affects the number of bubbles. Excessively fast mixing speed or prolonged mixing time increases entrapped air. Qinglong Group adopts a three-stage low speed-high speed-low speed mixing process, with high-speed mixing controlled at 60-90 seconds to avoid excessive shear-induced air entrainment. Vacuum mixing is an effective means of eliminating bubbles: mixing under a vacuum of -0.08 MPa for 2-3 minutes can remove more than 95% of bubbles with a diameter over 0.5 mm. In the Hainan International Convention and Exhibition Center project, this process reduced panel surface porosity to 0.5%.

Casting and curing also require fine control. Excessive casting drop easily entraps air, so Qinglong Group requires a casting drop of ≤0.5 m and adopts layered casting with each layer ≤200 mm thick. Vibration process parameters are optimized: frequency 50-60 Hz, amplitude 0.5-0.8 mm, and duration 20-30 seconds. During curing, film covering is applied to prevent surface moisture evaporation that forms shrinkage bubbles. Through these systematic measures, UHPC components can achieve a mirror-like smooth finish.