Air Entrainment in Concrete: Benefits, Challenges, and Practical Implications

Key Points

• Research suggests air entrainment improves concrete durability, especially against freeze-thaw cycles, but may reduce strength.
• It seems likely that measuring air content can be affected by mix factors, with potential flaws in testing methods.
• The evidence leans toward pumping reducing air content, requiring adjustments at the discharge point.

Air Entrainment Benefits and Drawbacks

Air entrainment involves adding tiny air bubbles to concrete, enhancing its resistance to freezing and thawing, which is crucial for cold climates. It also makes concrete easier to work with during pouring. However, it can lower the concrete’s strength, with each 1% air increase potentially reducing strength by 3–5%.

Measurement Challenges

Measuring air content, typically at 5–6% for best frost resistance, can be tricky. Factors like cement type, mixing, and temperature can alter results, and standard tests (e.g., ASTM C231) may have errors if not done carefully.

Impact of Pumping

Pumping concrete often reduces air content due to pressure, especially in non-agitated systems. It’s recommended to check and adjust air levels at the final discharge to maintain quality.

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Survey Note: Comprehensive Analysis of Air Entrainment in Concrete

Introduction

Air entrainment in concrete, the process of introducing microscopic air bubbles during mixing, is a critical technique for enhancing durability, particularly in freeze-thaw environments. This survey note explores the pros and cons of air entrainment, examines how air entrainment might be altered during measurement, assesses potential flaws in the measuring process, and investigates the impact of pumping on air content. The analysis is grounded in recent research and industry standards, providing a detailed overview for professionals and enthusiasts alike.

Pros of Air Entrainment in Concrete

Air entrainment offers several significant benefits, making it a preferred choice for certain applications:

• Enhanced Freeze-Thaw Resistance: The introduction of air bubbles, typically 0.01–1 mm in diameter, creates space for water to expand during freezing, preventing cracking and scaling. This is vital for concrete exposed to cyclic freezing and thawing, such as pavements and bridge decks .
• Improved Workability: Air-entrained concrete is more fluid, facilitating easier pouring, leveling, and compaction. This enhances placement efficiency and reduces labor effort, as noted in various industry guidelines .
• Reduced Segregation and Bleeding: The uniform distribution of air bubbles minimizes the separation of aggregates from the cement paste, reducing water bleeding to the surface. This improves the overall homogeneity and quality of the concrete mix .
• Increased Resistance to Scaling: Air entrainment enhances resistance to surface scaling caused by deicing salts or chemicals, extending the service life of concrete in harsh winter conditions .

Cons of Air Entrainment in Concrete

Despite its advantages, air entrainment has notable drawbacks that must be considered:

• Reduced Compressive Strength: The presence of air bubbles reduces the solid material volume, leading to a decrease in compressive strength. Research indicates that each 1% increase in air content can reduce 28-day compressive strength by 3–5%, as detailed in FHWA documentation .
• Risk of Over-Entrainment: Excessive air can weaken the concrete excessively, potentially compromising structural integrity. This requires careful control during mix design to balance durability and strength .
• Measurement and Control Challenges: Maintaining optimal air content is complex due to influences from mixing, transportation, and placement, which can lead to inconsistent quality if not managed properly .

Measurement of Air Entrainment

Air entrainment is quantified by measuring the percentage of air in the concrete mix and analyzing air void characteristics, such as spacing factor and specific surface. Standard methods include:

• Pressure Method (ASTM C231): Measures air content by applying pressure to displace air, commonly used in the field.
• Volumetric Method (ASTM C173): Involves measuring the volume of air by displacement, often used for laboratory testing.

For optimal frost resistance, an air content of 5–6% by volume is targeted, with air bubbles spaced no more than 0.25 mm apart. The spacing factor should be around 0.2 mm, with bubble sizes ranging from 0.1 to 0.3 mm in diameter, resulting in a specific surface of 20–40 mm⁻¹

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Occurrences Where Air Entrainment Might Be Altered or Changed for Measurement

Air entrainment can be altered during various stages, affecting measurement accuracy:

• Mix Constituents: Changes in cement content, fineness, sand grading, and dust in aggregates can influence air content. For instance, increasing cement fineness tends to reduce air content .
• Mixing and Handling: Over-vibration can reduce air content by forming foam, and transportation methods like pumping can cause air loss. Longer mix times or higher mixer speeds (up to 20 rpm) can increase air content, as per FHWA guidelines .
• Temperature and Environment: Variations in temperature and humidity can affect the effectiveness of air-entraining admixtures, leading to inconsistent air levels.
• Admixture Interactions: The use of supplementary cementitious materials (e.g., fly ash, silica fume) or other chemical admixtures can alter air content. For example, fly ash has a strong negative effect (↓↓) on air content .

The following table summarizes the effects of various materials and practices on air content, based on FHWA research:

Material/Practice	Change Effect
Cement: Increase in content	↓↑
Cement: Increase in fineness	↓
Cement: Increase in alkaline content	↑
Supplementary cementitious materials: Fly ash	↓↓
Supplementary cementitious materials: Silica fume	↓↓
Supplementary cementitious materials: Slag with increasing	↓
Aggregates: Increase in maximum size	↓
Aggregates: Sand content	↑
Chemical Admixture: Water reducers	↑
Chemical Admixture: Retarders	↑
Chemical Admixture: Accelerators
Chemical Admixture: High-range water reducers	↑
Batching: W/CM Increase	↑↓
Production: Increase mixer capacity	↑
Production: Mixer speeds to 20 rpm	↑
Production: Longer mix time	↑
Transport and delivery: Transport distance	↓
Transport and delivery: Long hauls	↓
Transport and delivery: Retempering	↑
Placing and finishing: Belt conveyors	↓
Placing and finishing: Pumping	↓↓
Placing and finishing: Prolonged internal vibration	↓
Placing and finishing: Excessive finishing	↓

This table highlights the complexity of maintaining consistent air content, particularly during transportation and placement.

Is the Measuring Process Flawed?

While standard methods like ASTM C231 and C173 are reliable when conducted properly, potential flaws can arise:

• Inconsistent Testing Procedures: Variations in how tests are performed, such as sample preparation or equipment use, can lead to different results.
• Equipment Calibration: Improperly calibrated pressure meters or volumetric devices can introduce errors, affecting accuracy.
• Environmental Factors: Temperature fluctuations and humidity can impact test outcomes, especially for field measurements.
• Mix Variability: The inherent variability in concrete mixes, influenced by raw materials and production practices, can make consistent measurements challenging.

These factors underscore the importance of standardized procedures and regular calibration to ensure measurement accuracy.

Does Pumping Concrete Change the Air Content?

Yes, pumping concrete typically reduces air content due to the pressure exerted during the process, which can force air bubbles out of the mix. This effect is particularly pronounced in non-agitated delivery systems. The FHWA notes pumping as having a strong negative effect (↓↓) on air content

. To mitigate this, it is recommended to:

• Check air content at the final discharge point to assess losses.
• Adjust the dosage of air-entraining admixtures accordingly, especially for mixes with higher workability, which are more prone to air loss during pumping .

This adjustment ensures that the concrete maintains the desired air content for durability, particularly in freeze-thaw environments.

Conclusion

Air entrainment in concrete significantly enhances durability and workability, particularly for freeze-thaw resistance, but it reduces strength and requires careful measurement and control. The measuring process, while generally reliable, can be flawed due to mix variability, environmental factors, and testing inconsistencies. Pumping concrete often reduces air content, necessitating adjustments at the discharge point. Understanding these aspects is essential for effective concrete production and placement, ensuring long-term performance and structural integrity.

Key Citations

• Air Entrainment overview ScienceDirect Topics
• FHWA Chemical Admixtures for Concrete Paving Mixtures
• How to Use Air-Entrained Concrete Concrete Construction Magazine
• Air Entrained Concrete Folsom Ready Mix

Air-Entrained Concrete: Air Content Slider (1% to 7%)