Blog

Concrete leaves the mixer with the correct air content.

But after it passes through the pump, the measured air content is lower.

Does this mean the air-entraining agent is not working?

Not necessarily.

Pumping changes the pressure and movement experienced by fresh concrete. The stability of the air bubbles, concrete mixture, pipeline configuration, and testing procedure can all influence the final result.

In air-entrained concrete, millions of small air bubbles are intentionally introduced to improve freeze-thaw durability and workability.

During pumping, the concrete experiences pressure, shear, changes in velocity, and sudden pressure release at discharge.

These conditions can change the measured air content.

ACI notes that some air loss during pumping air-entrained concrete can occur, particularly depending on pump pressure, pipeline configuration, and concrete properties.

A small change does not automatically mean the entire air-void system has failed. However, a large or inconsistent drop should be investigated.

Different air-entraining agents can produce air bubbles with different stability. An AEA should not only create enough air during mixing—it should also maintain a stable bubble system through:

• transportation;
• agitation;
• pumping;
• placement;
• vibration.

SidleyChem’s air-entraining agent for concrete range includes different chemistries designed to create stable air-void systems and improve concrete workability and durability.

Air stability is also affected by the other materials in the concrete.

Important factors include:

• cement type and content;
• PCE dosage;
• supplementary cementitious materials;
• fine aggregate content;
• temperature;
• slump;
• mixing time.

A change in cement, sand, or polycarboxylate superplasticizer may change air content even when the AEA dosage remains unchanged.

For this reason, investigate any raw-material changes before assuming the AEA itself is defective.

Not all pumping conditions are the same. Air loss may become more noticeable when:

• pumping pressure is high;
• the pipeline has long vertical sections;
• concrete experiences significant free fall;
• discharge conditions change suddenly;
• the mix is highly flowable.

ACI recommends controlling pumping pressure and avoiding excessive free fall where possible when placing air-entrained concrete. If one pump produces stable results while another produces significant air loss, equipment and pipeline layout should be included in the investigation.

Another common problem is comparing two results that were not measured under the same conditions.

For example:

• Sample A is taken directly from the truck.
• Sample B is taken after pumping.
• Different operators perform the tests.
• The samples are tested at different times.

ASTM specifically notes that the stage at which fresh concrete is sampled—including before or after pumping—can influence measured air content testing results.

Therefore, record the sampling location and test procedure every time.

A drop in concrete air content after pumping does not automatically mean that more air-entraining agent is required.

The real cause may involve:

• air-bubble stability;
• pump pressure;
• pipeline configuration;
• cement or PCE compatibility;
• concrete temperature;
• sampling and testing conditions.

The best approach is to compare air content before and after pumping under controlled conditions and identify exactly where the change occurs.

For air-entrained concrete, the goal is not simply high initial air content—it is a stable air-void system that survives transportation, pumping, and placement.