How is concrete air content calculated by the gravimetric method?

Air content can be estimated from theoretical air-free density and measured fresh-concrete density using air percentage = (T − D) / T × 100, where T is theoretical density and D is measured density.

Does total air content equal entrained air?

No. A bulk air-content test measures total measurable air in the fresh concrete under the applicable method. It does not by itself establish bubble size distribution or distinguish entrained and entrapped air.

Why does measured concrete density affect air content?

With the same theoretical air-free density, lower measured fresh-concrete density corresponds to greater calculated air volume.

Can air content be measured by pressure rather than density?

Yes. ASTM C231 covers the pressure method for suitable concretes with relatively dense aggregates, while ASTM C173 covers the volumetric method and ASTM C138 covers gravimetric air content.

Can this calculator replace field air testing?

No. It is a calculation aid. Formal air-content testing should follow the applicable ASTM or project procedure and use representative fresh-concrete samples.

Concrete Air Content Calculator

Estimate fresh concrete air content from theoretical air-free density and measured density, then compare the result with your target range. This approach is useful for checking mix data and understanding why density changes can signal a change in air volume.

Enter density measurements

Theoretical air-free densityUse kg/m³ for SI or lb/ft³ for US units.

Measured fresh concrete densityMeasure representative fresh concrete using the applicable method.

Density unit

Target air contentOptional comparison target only.

The gravimetric calculation estimates total air from density. It does not determine air-void spacing, bubble size distribution, or whether the air is specifically entrained versus entrapped.

Enter theoretical and measured density, then calculate air content.

Calculated fresh-concrete air content

Estimated total air

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—Theoretical density

—Measured density

—Density difference

—Target

Air-free density—

Measured fresh density—

Calculated total air—

Target comparison—

—

How the gravimetric air calculation works

ASTM C138 provides a gravimetric approach for determining density, yield and air content of fresh concrete. The simple relationship used here compares measured density with the theoretical density the concrete would have if it contained no air.

Air % = (T − D) ÷ T × 100

Here, T is theoretical air-free density and D is measured fresh-concrete density. Both must use the same unit basis.

Why lower density generally means more calculated air

For an unchanged air-free mixture, air occupies volume without contributing equivalent mass. That means a lower measured fresh density corresponds to a larger calculated air fraction.

In actual testing, however, the theoretical density and measured density must both represent the same mixture and sampling basis. Changes in material moisture, batch proportions or test procedure can also influence the calculation.

Entrained air is not the same as total air

A bulk air-content test gives a total air value under the chosen test method. It does not independently tell you the size, spacing or stability of the air void system in hardened concrete.

For durability-sensitive work, hardened air-void parameters may require a separate microscopic examination or project-specific procedure.

ASTM C231 explains that pressure testing determines air content of fresh concrete and notes important differences between fresh and hardened air systems.

Pressure and volumetric methods

ASTM currently lists C231 for the pressure method, C173 for volumetric air content and C138 for gravimetric density/yield/air content. Each method has its own scope and equipment requirements. The pressure method is intended for concrete made with relatively dense aggregates and requires an aggregate correction factor under the standard.

This calculator is most useful for understanding the density relationship. It is not a substitute for operating the required field apparatus correctly.

Technical context: ASTM C138/C138M and ASTM C231/C231M-26.

Worked example

With a theoretical density of 2,400 kg/m³ and a measured fresh density of 2,280 kg/m³, the density difference is 120 kg/m³.

Air = (2,400 − 2,280) ÷ 2,400 × 100 = 5.0%

If your target is 5%, the arithmetic comparison is exactly on target. That does not prove the field test is valid; sampling and testing procedure still matter.

Common errors

Mixing density units: kg/m³ and lb/ft³ cannot be combined directly.

Using dry material density as theoretical concrete density: the theoretical value must represent the complete concrete mixture on the chosen basis.

Interpreting total air as entrained air: bulk air testing does not provide a complete hardened air-void characterization.

Ignoring aggregate correction factors: pressure testing under ASTM C231 has specific correction requirements.

Limitations

This calculator performs the gravimetric density relationship only. It does not perform C231 pressure measurements, C173 volumetric measurements, aggregate correction testing or hardened-air microscopy. Use the applicable project specification and field test method for formal acceptance.

Frequently Asked Questions

What is the formula for concrete air content?

For the gravimetric relationship, air percentage = (theoretical density − measured density) ÷ theoretical density × 100.

Does this calculate entrained air only?

No. It estimates total measurable air on the chosen basis.

Can I use lb/ft³?

Yes. Select the US density unit and keep both values in lb/ft³.

Why is my measured density lower than theoretical?

Air content is one possible reason, but mixture and measurement issues can also affect the result.

Can this replace ASTM C231?

No. It is a calculation reference and does not perform the pressure test.