After formwork is removed, air bubbles, bugholes, or small surface voids may appear on concrete. The first question is often: Was the concrete insufficiently vibrated?
Not necessarily.
Not all air bubbles in concrete are harmful. What really matters is their size, distribution, location, and cause.
01 | Some Air Bubbles Are Actually Beneficial
Tiny air bubbles introduced by air-entraining agents are generally small and evenly distributed.
In freeze-thaw environments, these bubbles can provide small pressure-relief spaces for ice expansion and help improve the concrete's freeze-thaw resistance.
So, fewer air bubbles do not always mean better concrete.
Beneficial microbubbles do not need to be completely removed.
02 | Large Air Bubbles Are the Ones That Need Attention
Large and concentrated air bubbles may affect concrete compactness and surface appearance.
They may be related to:
Admixture system and dosage
Mixing speed and mixing time
Air introduced during mixing
Placement and vibration procedures
After formwork removal, they may appear as noticeable bugholes, surface voids, or localized defects.
03 | Another Common Problem: Trapped Air
Air is more likely to become trapped around formwork corners, congested reinforcement, beam-column joints, and changes in section geometry.
If vibration is insufficient, the concrete may not fully fill these areas. The trapped air can remain inside and eventually form larger voids or poorly compacted zones.
Therefore, surface bubbles should not automatically be attributed to insufficient vibration.
04 | How Can You Tell If Air Bubbles Are Harmful?
Pay attention to three factors:
Size:
Small and uniformly distributed bubbles are not necessarily a problem, while larger voids require further evaluation.
Distribution:
Uniformly distributed bubbles and concentrated voids may have different causes.
Location:
If defects are concentrated around congested reinforcement or formwork corners, the concrete compactness should be carefully checked.
If internal voids are suspected, additional methods such as sounding or ultrasonic testing can be used for further evaluation.
05 | How Can Harmful Air Voids Be Reduced?
During mixing:
Properly control mixing speed, mixing time, and admixture dosage.
During placement:
Use appropriate layer-by-layer placement and avoid excessive accumulation in one area.
During vibration:
Pay particular attention to corners, congested reinforcement areas, and changes in section geometry to achieve proper compaction.
Formwork:
Improve air-escape conditions according to structural and construction requirements to reduce trapped air.
Final Thoughts
Air bubbles in concrete should not simply be judged as “the fewer, the better.”
Uniform, fine entrained air can provide beneficial effects, while large bubbles and trapped air around formwork and congested reinforcement are the conditions that require greater attention.
In simple terms:
Keep the beneficial microbubbles, and remove the air that needs to be released.
Only by considering the size, distribution, location, and cause of air bubbles can concrete surface and internal defects be properly evaluated and addressed.
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