What Causes Air Bubbles on the Surface of UHPC Components?

Air bubbles on the surface of UHPC components are a common production defect, with roots traceable to the loss of control in five key areas: raw materials, mix proportion, mixing process, casting and forming, and release agents. Bubbles not only affect appearance but also reduce component compactness, becoming starting points for stress and corrosion. Through 28 years of production process control, Qinglong Group has classified bubble problems into two categories—“entrapped bubbles” and “precipitated bubbles”—and established a full-process prevention and control system from source to finished product. In the production of high-standard UHPC curtain walls for the Yirui Biotechnology Building in Shenzhen, we kept the visible bubble rate below 0.1% through a vacuum-assisted forming process, ensuring exceptional surface quality.

Raw materials and mix proportion are the internal causes of bubble formation

Air-entraining substances in raw materials (such as certain water reducers and admixtures) and improper mix proportion design form the chemical basis for bubble formation. High-range water reducers with poor compatibility with cement, or applied at excessive dosage, tend to increase the air content of the system. An excessively low water-binder ratio (<0.20) makes the paste overly viscous, making it difficult for air entrapped during mixing to escape. Through extensive compatibility testing, the Qinglong laboratory selects low air-entraining admixtures and optimizes the rheological properties of the paste by adjusting the grading of mineral admixtures (such as silica fume and fly ash), controlling the tendency for bubble formation at the source of the mix design.

Mixing and casting processes are the key stages of bubble introduction

Improper mixing is the main cause of entrapped bubbles. Excessively fast or prolonged mixing entraps large amounts of air; an incorrect feeding sequence may also cause fibers or powder to agglomerate, trapping air inside. Qinglong uses planetary mixers with strict control of rotational speed and mixing time, following a standardized process of “dry mixing first, then wet mixing, with fibers added in batches.” During casting, excessive drop height or material flow striking the formwork can also entrap bubbles. We require tremie feeding, controlled casting speed, and the use of high-frequency external vibrators to assist air release.

The influence of molds and release agents cannot be overlooked

The roughness and cleanliness of the mold surface and the type of release agent directly affect bubble adhesion. Rough or dusty mold surfaces adsorb gas and form air cushions. Oil-based release agents may hinder bubble escape. Qinglong strictly requires smooth mold surfaces, uses water-based or specialized concrete release agents, and applies them by atomized spraying to ensure a thin, uniform coating. In the production of double-curved panels for the Hainan Convention and Exhibition Center, we even established dedicated application and cleaning procedures for each individual mold.

The ultimate solution: vacuum dewatering process

For components with extremely high surface quality requirements, Qinglong adopts a composite forming process of “casting vibration + vacuum dewatering.” Once a component is initially formed, a specialized filter cloth is immediately placed over it and a vacuum of -0.08 to -0.1 MPa is applied to forcibly extract most of the capillary pore water and air bubbles. This process significantly improves the compactness of the surface layer and is a key technology for producing mirror-finish or ultra-high-performance UHPC components. Through systematic control, surface bubble problems can be completely prevented and eliminated.