What Causes Air Bubbles on UHPC Component Surfaces? Analysis of Bubble Formation Causes and Solutions

Surface air bubbles on UHPC components affect appearance quality and durability; in severe cases they may reduce component density and increase the risk of later cracking. Accurately identifying the causes of bubbles and taking targeted prevention and control measures is key to ensuring UHPC component quality. Drawing on 28 years of production practice, Qinglong analyzes the core causes of bubbles and the solutions.

I. Bubble Problems Caused by Improper Raw Materials and Mix Proportions

Unreasonable raw material performance and mix proportion design are the root causes of bubble formation. Unbalanced aggregate gradation—too much coarse aggregate or insufficient fine aggregate—results in poor fluidity of the UHPC paste, which easily entraps air during mixing and forms bubbles; improper selection of admixtures, such as using air-entraining water reducers or retarders, introduces too many tiny bubbles into the paste that are difficult to expel. Qinglong's tests show that air-entraining water reducers produce over 30% more bubbles than non-air-entraining types; improper water-binder ratio control—a water-binder ratio that is too low (<0.2) makes the paste too viscous and hinders bubbles from rising, while a ratio that is too high easily causes segregation and forms surface bubbles. In addition, impurities or excessive moisture in raw materials also generate extra bubbles during mixing, affecting component surface quality.

II. Bubble Formation Caused by Defects in Production and Molding Processes

Improper operation during production and molding is the main trigger for bubble formation. Unreasonable mixing processes—mixing speeds that are too fast (>60 rpm) or insufficient mixing time—lead to uneven paste mixing and entrapped air that cannot be fully expelled; improper pouring methods, such as high-position free-fall pouring, cause the paste to impact air during its fall and form bubbles, and pouring too quickly leaves air no time to escape; insufficient vibration—failing to use high-frequency vibration equipment or vibrating for too short a time—prevents bubbles in the paste from being effectively expelled. Qinglong uses high-frequency vibration tables (vibration frequency 200-300Hz) on its automated production lines, which significantly reduces bubbles; poor mold surface treatment—mold surfaces without release agent or with unevenly applied release agent—increases friction between the paste and the mold, hinders bubble expulsion, and may even cause bubbles to adhere to the component surface.

III. Surface Bubbles Caused by Curing and Mold Factors

Unreasonable curing processes and mold design can also cause bubble problems. Steam curing with too rapid a temperature rise causes moisture inside the paste to evaporate quickly, and the resulting water vapor cannot be expelled in time, forming bubbles on the component surface; poor mold air permeability—fully sealed molds cannot expel internal air and water vapor, and the complex cavities of specially shaped components are especially prone to localized bubble accumulation; low mold surface smoothness or the presence of micropores causes air and paste to be adsorbed, forming surface bubbles. Qinglong uses polished steel molds, which reduce surface bubbles by more than 40% compared with ordinary molds. In addition, excessive gaps at mold joints allow paste leakage during pouring to draw in air, forming bubbles distributed along the gaps and affecting the appearance integrity of components.

IV. Practical Solutions for Bubble Prevention, Control, and Repair

Taking prevention and control measures across the entire process based on the causes of bubbles can effectively improve UHPC component surface quality. Raw material and mix proportion optimization: use continuously graded aggregates, control admixture types (prioritizing non-air-entraining types), precisely control the water-binder ratio between 0.22-0.25, and dry raw materials before use; production process improvement: adopt low-speed efficient mixing (30-50 rpm), extend mixing time to 3-5 minutes, use low-position layered pouring (pouring height <500mm), combined with high-frequency vibration and vacuum degassing. Qinglong applied vacuum degassing technology in the Shanghai Century Plaza UHPC project, raising the bubble pass rate to 99%; mold and curing optimization: choose high-smoothness air-permeable molds, apply specialized release agent, use stepped temperature increase for steam curing, and properly seal mold joints; bubble repair: minor surface bubbles are filled and polished with specialized repair agent, while severe bubbles require removal of the defective area and re-pouring to ensure components meet quality standards.