ASTM C231 Standard Test Method for Air Content of Freshly Mixed Concrete by the Pressure Method

Get Certified ASTM C231 Testing for Mixed Concrete Materials

Professional ASTM C231 testing provides quantitative information about the air content of freshly mixed concrete. Technicians can use the results to determine whether the concrete contains the required amount of entrained and entrapped air for the applicable mixture or project requirements. Accurate air-content measurement is important because concrete air content can change during transportation, pumping, placement, and consolidation. Proper testing at the right time provides a picture of the condition of fresh concrete when tested. Skilled technicians test with a calibrated pressure meter, a representative sample, proper consolidation, and proper procedures. This data can be used for quality control, acceptance, mixture confirmation, and monitoring.

• Material Type: Freshly mixed concrete and mortar containing relatively dense aggregates.
• Property Measured: Air content of fresh concrete, excluding air contained within aggregate particles.
• Method: Pressure-based measurement of the change in volume of the concrete.
• Aggregate Correction: Requires determination of the aggregate correction factor for applicable mixtures.
• Applications: Field and quality-control measurement of fresh concrete air content.
• Limitations: The method does not apply to concrete made with lightweight aggregates, air-cooled blast-furnace slag, or highly porous aggregates.
• Nonplastic Concrete: It is also not applicable to nonplastic concrete commonly used for products such as pipe and concrete masonry units.
• Units: ASTM C231 recognizes SI and inch-pound units separately as standard; the two systems should not be combined for conformance purposes.

What are the uses of ASTM C231 Testing?

ASTM C231 testing is typically employed to track the air content of freshly mixed concrete in the process of production and placement. It is a measurement used to verify the concrete delivered to the job site has the desired air-content properties prior to placement or per project specifications. In concrete that relies on entrained air for protection against freezing and thawing, air content is of special significance. The test can also detect changes in the consistency of mixtures or loss of air during delivery, pumping, mixing, or handling. Common uses include:

• Quality Control: Monitoring air content during concrete production.
• Acceptance Testing: Providing air-content measurements for applicable project specifications.
• Mixture Verification: Confirming the air-content characteristics of a designed concrete mixture.
• Field Testing: Measuring fresh concrete at the construction site.
• Production Monitoring: Identifying changes in air content between batches.
• Placement Control: Checking concrete before or during placement.
• Freeze-Thaw Concrete Evaluation: Supporting control of air content where entrained-air requirements apply.
• Troubleshooting: Investigating unexpected changes in fresh-concrete air content.

Which Materials Can Be Tested According to ASTM C231?

ASTM C231 is intended for freshly mixed concrete and mortar made with relatively dense aggregates. The aggregate correction factor must be capable of being determined satisfactorily for the mixture being tested. Materials and mixtures that may be evaluated include:

• Normal-Weight Concrete: Fresh concrete containing relatively dense aggregates.
• Cement Mortar: Applicable mortar mixtures containing suitable dense aggregates.
• Air-Entrained Concrete: Fresh concrete in which controlled air content is part of the mixture requirements.
• Structural Concrete: Fresh structural concrete where air-content verification is required.
• Quality-Control Batches: Production concrete sampled for routine air-content monitoring.
• Field Concrete: Concrete sampled during delivery, placement, or construction operations.

Why is ASTM C231 Testing Important?

Air content is an important fresh-concrete property because the amount and stability of air voids can influence concrete performance. Measuring air content during production and placement helps identify whether the fresh mixture is within the applicable requirements. ASTM C231 provides a standardized pressure method that can be used to obtain repeatable measurements when the apparatus, aggregate correction factor, sample, and testing procedure are properly controlled. Do not automatically treat the measured air content of fresh concrete as the air content of hardened concrete. ASTM notes that consolidation, air-bubble stability, environmental exposure, and the stage at which technicians test the fresh concrete can cause the air content of hardened concrete to increase or decrease.

ASTM C231 Testing Process and Data Collection

Technicians first obtain a representative sample of freshly mixed concrete and identify the mixture, batch, sampling location, and relevant testing information. The pressure meter is checked for cleanliness, sealing condition, and calibration status before the concrete is placed in the measuring bowl. The concrete is placed and consolidated according to the applicable procedure. Technicians strike off the surface, clean the sealing surfaces, assemble the pressure apparatus, and establish the required initial condition. Pressure is then applied, and the meter reading is observed to determine the apparent air content. Where applicable, the aggregate correction factor is applied to account for air contained within aggregate particles. The corrected air-content result is then recorded together with sample identification, test conditions, apparatus information, and other required observations.

Common Challenges and Troubleshooting

A common way in which the results might vary is that the concrete sample is not representative. The air content may vary through handling, transportation, pumping, and placing, and the technician should determine the sampling point and conduct the sample at the proper stage. Other consolidations can have an impact on the measurement in an improper way. Unwanted voids in the sample may occur if consolidation is insufficient, and variation between tests may occur if handling is inconsistent. The consolidation procedure must be followed in the same manner by the technicians. Air leakage around the pressure meter is another potential problem. Sealing surfaces that are dirty or damaged, components that are not properly fitted or leaky valves can affect the ability to retain pressure and result in poor accuracy. The meter should be checked and serviced prior to testing.

• Analysts interpret the result by considering the measured pressure response, the applicable aggregate correction factor, sample condition, mixture characteristics, and the air-content requirement established by the project specification or concrete mixture design. Results should be evaluated against the applicable acceptance or quality-control criteria rather than against a universal ASTM C231 limit.
• Air content is reported as a percentage by volume. The report should identify whether the result is an apparent or corrected value as applicable and should document the aggregate correction used in the calculation.
• Technicians record the concrete sample identification, batch or delivery information, sampling location, test time, meter identification, calibration information, aggregate correction factor, observed air-content reading, and corrected result. Analysts document calculations and any relevant observations or deviations.
• The final report should include the specimen/sample identification, concrete mixture information, test method, pressure-meter information, aggregate correction factor, measured and corrected air content, units, testing conditions, and applicable specification or project requirement. Consistent sampling, consolidation, apparatus calibration, pressure control, correction-factor determination, and data recording support comparable results.

Link to ASTM C231