What Causes Bubbles on the Surface of UHPC Components?

The core causes of air bubbles on the surface of UHPC components fall into four major categories: raw material mix ratio, mixing and vibration processes, mold design, and curing. Qinglong applies refined control over every stage, keeping the bubble qualification rate above 99%, with 95% of components showing no visible bubbles (diameter ≥0.5mm) on the surface.

I. Causes Related to Raw Material Mix Ratio

Improper raw material selection and mix ratio are the source of bubble formation. Excessive water-cement ratio: when the water-cement ratio exceeds 0.25, excess water evaporates and forms bubbles. Qinglong strictly controls the water-cement ratio between 0.22 and 0.25 to reduce bubbles at the source. Poor admixture compatibility: using inferior or incompatible superplasticizers leads to excessive air content in the mix (>3%), making bubbles difficult to expel. Qinglong uses polycarboxylate superplasticizers, keeping air content controlled between 1% and 2%. Unreasonable aggregate gradation: discontinuous aggregate particle gradation and low packing density easily create void bubbles. Qinglong uses 0.15-0.6mm continuously graded quartz sand with a packing density above 1.6g/cm³. Uneven fiber dispersion: fiber clumping creates localized voids or traps air on the surface. Through mechanical dispersion devices, Qinglong improves fiber dispersion uniformity by 60%, avoiding clustered bubbles.

II. Causes Related to Mixing and Vibration Processes

Insufficient mixing or inadequate vibration are the main process factors behind bubble formation. Insufficient mixing time: dry materials are not fully wetted, resulting in poor mix uniformity and entrapped air that forms bubbles. Qinglong controls mixing time at 5-8 minutes to ensure raw materials are thoroughly blended. Inadequate vibration compaction: UHPC has high viscosity and requires high-intensity vibration to expel bubbles. Insufficient vibration intensity (frequency <2000 vibrations/min) or too short a duration (<3 minutes) prevents bubbles from rising and escaping. Qinglong uses a high-frequency vibrating table (frequency ≥2500 vibrations/min) supplemented by poker vibrators, with vibration time of 3-5 minutes. Improper vibration method: insufficient insertion depth or excessively fast movement of the poker vibrator results in localized under-vibration. Qinglong inserts vibrators to 2/3 of the component thickness with movement spacing ≤200mm to ensure full coverage.

III. Causes Related to Mold Design and Curing

Defects in the mold and curing stages can cause residual or newly formed bubbles. Poor mold venting: airtight mold surfaces without vent holes, or release agent applied too thick (>0.2mm), prevent bubbles from escaping. Qinglong molds feature micro vent holes (1-2mm diameter, 200-300mm spacing) on the surface, with release agent thickness controlled within 0.1mm. Rough mold surface: poor mold surface flatness (deviation >0.5mm) or oil stains and impurities cause bubbles to adhere. Qinglong mold surface flatness is ≤0.3mm, with thorough cleaning and degreasing before use. Improper curing: early curing temperatures too high (>30℃) or insufficient humidity (<80%) cause surface moisture to evaporate rapidly, forming drying-shrinkage bubbles. Qinglong cures components in a constant temperature and humidity curing room (temperature 20-25℃, humidity ≥90%) for more than 7 days, avoiding rapid surface drying.