What Causes Surface Air Voids in Concrete? 3 Key On-Site Practices to Control Them
Created on 09.22
After formwork removal, you may sometimes find the concrete surface covered with many small pinholes, almost like tiny needle marks or sesame seeds.
These are commonly known as surface air voids.
They are usually shallow and do not directly indicate a structural problem, but they can affect the appearance of concrete and may increase surface permeability if they are extensive.
To control surface air voids effectively, it is important to look beyond vibration alone. Three factors should be considered together:
concrete rheology, vibration, and formwork condition.
1. Where Do Surface Air Voids Come From?
During concrete mixing, transportation and placement, a certain amount of air is inevitably introduced into the concrete, forming air bubbles.
Under normal conditions, these bubbles should gradually escape during placement and vibration.
Some bubbles rise toward the top surface, while others move toward the formwork and travel upward along the formwork surface.
When these bubbles cannot escape effectively, they remain trapped near the concrete surface.
After the formwork is removed, the trapped bubbles leave behind the small holes we see as surface air voids.
2. Why Don't the Air Bubbles Escape? Three Common Causes
1. The Concrete Is Too Viscous
When concrete is too viscous, air bubbles have difficulty moving upward or migrating through the mixture.
Think of bubbles trapped in honey: the more viscous the medium, the harder it is for the bubbles to move.
Excessive viscosity can be related to factors such as:
- Poor aggregate grading;
- Excessive powder content;
- A very low water-to-binder ratio;
- Poor compatibility between admixtures and cementitious materials;
- A mix design that places too much emphasis on high powder content or strength.
Therefore, lower water-to-binder ratio or higher powder content does not automatically mean better concrete performance.
The goal is to maintain appropriate flowability and rheology while meeting strength, durability and construction requirements.
This gives entrapped air a better chance to migrate and escape.
2. Insufficient or Uneven Vibration
Vibration is one of the key methods for helping entrapped air escape.
If the spacing between vibrator insertion points is too large, areas of insufficient vibration may occur.
For example, when insertion points are spaced too far apart, some areas may not receive enough vibration energy.
Vibration time also matters. If the vibrator is removed too quickly, air bubbles may not have enough time to move toward the surface.
In practice, vibration should be carried out according to the concrete condition, vibrator type and site conditions.
A basic principle is:
Insert quickly, withdraw slowly, and vibrate evenly.
When the surface begins to show mortar paste and visible air bubbles have significantly reduced or largely stopped emerging, the area has generally received sufficient vibration.
For layered placement, the layer thickness should also be controlled so that the vibrator can effectively reach the current layer and provide adequate integration with the previous layer.
3. The Bubbles Reach the Formwork but Cannot Escape
This factor is often overlooked.
When air bubbles reach the formwork surface, they should ideally move upward along the formwork and eventually escape.
However, problems such as:
- Rust;
- Residual mortar;
- Dust;
- Oil contamination;
- Uneven release-agent application;
- Excessive release agent accumulation;
can affect the concrete-formwork interface and make it more difficult for bubbles to move along the surface.
Therefore, surface air voids do not always mean insufficient vibration.
In some cases, vibration may already be adequate, but poor formwork cleanliness, an unsuitable release agent, or uneven application can still contribute to surface air voids.
3. Three Key Practices That Matter Most On Site
Practice 1: Optimize Concrete Flowability and Viscosity
Start with the mix design.
Aggregate grading, powder content, water-to-binder ratio and the selection of compatible water-reducing admixtures can all affect the rheology of fresh concrete.
The goal is not simply to make concrete "more fluid."
The goal is to achieve appropriate flowability and viscosity while maintaining the required strength, durability and construction performance.
Practice 2: Vibrate Properly and Consistently
Insertion points should be arranged according to the effective working range of the vibrator.
In general:
- Keep insertion points evenly distributed;
- Avoid excessive spacing;
- Insert quickly and withdraw slowly;
- Avoid under-vibration;
- Avoid over-vibration;
- Control the thickness of each concrete layer.
The exact vibration parameters should be adjusted according to the vibrator, slump, member dimensions and site conditions.
Practice 3: Keep the Formwork Clean and Use the Release Agent Properly
Formwork condition has a direct influence on the final concrete surface.
Before placing concrete, thoroughly remove:
- Rust;
- Hardened mortar;
- Dust;
- Oil;
- Other contaminants.
The release agent should be compatible with the formwork and concrete system.
It should also be applied thinly and evenly, avoiding excessive accumulation in local areas.
Water-based release agents can offer practical advantages in some applications, but the final choice should be based on the formwork type, concrete system and site trials.
4. Surface Air Voids Are Rarely Solved by One Measure Alone
Surface air voids are often the result of several factors working together.
Effective control requires attention to the complete process:
Mix Design → Rheology → Vibration → Formwork → Release Agent
If the concrete is too viscous, air bubbles have difficulty moving.
If vibration is insufficient, the bubbles cannot escape.
If the formwork interface is not properly prepared, bubbles that reach the formwork may still remain trapped.
So, if you want a cleaner and more uniform concrete surface after formwork removal, focus on three key areas:
the mix, the vibration, and the formwork.
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ARICH provides building additives and supports material selection and application solutions for different construction and dry-mix systems.
If you are working on challenges related to workability, flowability, water retention, adhesion, defoaming or formulation stability, contact us to discuss suitable materials and formulation solutions for your application.
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