Self-compacting concrete needs high fluidity, excellent filling ability, and resistance to segregation. Cellulose ethers play a key role in achieving that stability. But there’s a downside. The wrong cellulose ether — or poor process control — can push the air content above 8%. The result is a serious loss of hardened strength and a surface riddled with pores.

Tracing the source and applying the right defoaming strategy is essential to restoring SCC performance.

Start by Checking the Cellulose Ether Itself

Grade and viscosity
Different cellulose ethers entrain air very differently. High-viscosity, coarse-particle, or untreated HPMC tends to trap more air during mixing, creating large, uneven bubbles. If your air content has suddenly spiked, the first question is whether the cellulose ether grade or batch has changed. Check the dissolution speed and particle morphology.

Dosage level
Higher cellulose ether dosage almost always pulls in more air. When chasing longer open time or better anti-washout performance, it’s easy to push the dosage too high. Revisit whether the dosage has crept beyond a reasonable range — typically no more than 0.05% of binder weight.

Next, Look at Mixing and Batching

Mixing speed and duration
SCC mixing involves high shear, which amplifies the air-entraining effect. Over-mixing or excessively high mixer speed can sharply increase bubble counts. Review the mixer RPM and keep the mixing time within the specified window.

Addition sequence
The order in which cellulose ether and defoamer are added strongly influences bubble control. If the cellulose ether dissolves first and forms a stable foam network, adding defoamer later often fails to knock it all down. The right approach is to add the defoamer together with the mixing water, or adopt a “defoam first, then thicken” sequence.

Defoaming and Prevention Plan

Choose an effective defoamer
For cellulose-ether-related air, polyether-modified silicone defoamers combine fast bubble breaking with lasting foam suppression. Select a grade with rapid spreading ability. The typical dosage is 1–5% of the cellulose ether mass, optimized through air content measurements.

Split addition for better results
If the air content is already too high, split the defoamer into two additions. Add one portion at the start of mixing to suppress bubble formation, and the remainder mid-mix to knock out existing bubbles. This approach improves efficiency while avoiding side effects like oil spots.

Prevention starts at the source
For new formulations, choose low-air-entraining cellulose ethers from the start. Fast-dissolving, low-viscosity products work best. They hydrate quickly and form a gel network before large amounts of air become locked in. Combined with a small amount of defoamer, these can keep air content in a safe 2–3% range.

Verify and adjust
After making changes, always re-test the air content. Cut a hardened sample and examine the pore distribution — the goal is fine, evenly dispersed pores, not large clustered voids. Confirm that strength has returned to the target.

Conclusion

When SCC air content climbs too high, the cause usually lies in the cellulose ether choice, its dosage, or how it interacts with the mixing process. By systematically checking these three areas and applying the right defoaming strategy, you can bring the air content back under control and deliver the dense, high-strength concrete you intended.