What is an Air-Entraining Agent?
An air-entraining agent (AEA), also called an air-entraining admixture, is a concrete admixture that creates and stabilizes a system of very small, evenly distributed air bubbles in fresh concrete and mortar. These bubbles stay in the hardened material as tiny voids. Entrained air bubbles are typically between about 10 and 1,000 micrometers in diameter, with most well below 1 millimeter. Entrapped air is different: it consists of larger, irregular voids left by incomplete consolidation, and it does not give the same protection.
The main reason to use an air-entraining agent is to improve the resistance of concrete to repeated freezing and thawing, and to scaling caused by deicing salts. Pavements, bridge decks, parking structures, sidewalks, curbs, and other exterior concrete in cold climates are the most common applications. Without entrained air, water in the capillary pores expands as it freezes and can crack the cement paste over many freeze-thaw cycles.
Air entrainment also changes fresh concrete. The bubbles act like small, flexible bearings between solid particles, which improves workability and cohesion and reduces bleeding and segregation. Because the mix becomes more workable, producers can often lower the mixing water slightly while keeping the same slump. Entrained air does reduce compressive strength, though. A widely cited rule of thumb is a loss of roughly 5% in compressive strength for each 1% of added air, so air content has to be controlled rather than maximized.
Common Chemical Types
Commercial air-entraining agents are surfactants drawn from several chemical families. Each family behaves somewhat differently in the mixer and in the hardened concrete. The table below summarizes the types most often found on the market.
| Type | Typical Source | General Characteristics |
|---|---|---|
| Wood resin salts (e.g., neutralized Vinsol-type resin) | Pine wood resin derivatives | Long industry track record; produces small, stable bubbles; used as a reference in many studies |
| Tall oil and fatty acid salts | By-products of wood pulping and vegetable or animal fats | Good bubble stability; widely used in ready-mix and precast |
| Synthetic surfactants (e.g., alkylbenzene sulfonates, alkyl sulfates) | Petrochemical feedstocks | Air forms quickly and consistent quality is easy to maintain; bubbles may be somewhat larger depending on formulation |
| Protein-based and other specialty surfactants | Hydrolyzed proteins and specialty chemistry | Used in niche applications such as some mortars and lightweight products |
In practice, many commercial products are blends designed to balance how fast the air forms, how stable the bubbles are, and how well the product works with other admixtures such as water reducers and superplasticizers.
Relevant Standards and Test Methods
Several industry standards define requirements and test methods for air-entraining admixtures. In North America these include ASTM C260 for air-entraining admixtures, ASTM C231 (pressure method) and ASTM C173 (volumetric method) for air content in fresh concrete, ASTM C457 for the air-void system in hardened concrete, and ASTM C666 for resistance to rapid freezing and thawing. In Europe, EN 934-2 covers concrete admixtures, including air-entraining types. Buyers should ask suppliers for test reports or certificates that show whether a specific product has been tested against the standard their project requires. Naming a standard does not show that a given product meets it.
How Air-Entraining Agent Works?
Air-entraining agents work at the boundary between air and water. Each surfactant molecule has a hydrophilic (water-attracting) head and a hydrophobic (water-repelling) tail. When the concrete is mixed, the mixing action folds air into the batch. The surfactant molecules collect at the surface of these air pockets, with their tails pointing toward the air and their heads toward the surrounding water. This layer lowers surface tension and creates a stable film around each bubble. The film keeps small bubbles from merging into larger ones or escaping from the mix.
Many surfactant types also interact with calcium ions and cement particles. This interaction helps anchor the bubbles in the paste and further improves stability during transport and placement. The result is a large number of tiny, closely spaced, roughly spherical voids distributed throughout the cement paste.
Protection Against Freeze-Thaw Damage
When water in the capillary pores of saturated concrete freezes, it expands by about 9% in volume. The expanding ice pushes unfrozen water through the pore network, which builds hydraulic pressure, and ice crystals can also grow in place. If this pressure cannot be relieved, it exceeds the tensile strength of the paste and causes microcracking, surface scaling, and eventually loss of material.
Entrained air voids act as relief chambers. Water moving away from the freezing front only needs to travel a short distance before it reaches an empty void, where it can freeze without generating damaging pressure. For this reason, the spacing between bubbles matters more than the total air volume alone. The spacing factor, measured on hardened concrete according to ASTM C457, is commonly recommended to be no more than about 0.20 mm (0.008 in) for good freeze-thaw resistance. A high specific surface of the air voids also indicates many small bubbles, which is the desired condition.
Typical Air Content Targets
Target air content depends mainly on the severity of exposure and on the maximum aggregate size. Smaller aggregates mean more paste in the mix, so a higher total air content is needed to protect it. Industry guidance such as ACI 318 gives target values similar to those in the table below. Project specifications always take precedence.
| Nominal Maximum Aggregate Size | Moderate Exposure (approx. total air, %) | Severe Exposure (approx. total air, %) |
|---|---|---|
| 9.5 mm (3/8 in) | 6.0 | 7.5 |
| 12.5 mm (1/2 in) | 5.5 | 7.0 |
| 19 mm (3/4 in) | 5.0 | 6.0 |
| 25 mm (1 in) | 4.5 | 6.0 |
| 37.5 mm (1-1/2 in) | 4.5 | 5.5 |
Specifications normally allow a tolerance of about ±1.5 percentage points around the target value.
Factors That Influence Air Content
The amount of air an agent produces is not fixed. It depends on the materials, the mix design, and the production process, which is why field testing is essential. The table below lists the main factors and how they usually affect air content.
| Factor | Typical Effect on Entrained Air |
|---|---|
| Higher cement content or finer cement | Tends to reduce air; may require higher dosage |
| Fly ash with high carbon content (high loss on ignition) | Carbon adsorbs the surfactant, which can sharply reduce air and make it vary between batches |
| Silica fume and other very fine materials | Usually increase the dosage required |
| Higher concrete temperature | Reduces air; dosage often needs to rise in hot weather |
| Higher slump or water content (within normal range) | Generally increases air |
| Mixing time and mixer condition | Air builds up with mixing to a point, then can decrease with prolonged mixing |
| Pumping, extended vibration, and long haul times | Can reduce air content between the truck and the placed concrete |
| Water reducers and superplasticizers | May increase or decrease air depending on chemistry; some polycarboxylate products contain defoamers |
How to Choose an Air-Entraining Agent?
Choosing an air-entraining agent means matching the product to the exposure conditions, the local materials, and the production practices of the plant. A product that performs well with one cement and admixture system may behave differently with another. Trial batches with project materials are therefore the most reliable basis for a decision.
Define the Exposure and Performance Requirements
Start with the project specification. It should state the exposure class, the target air content and tolerance, and sometimes a maximum spacing factor or a freeze-thaw durability requirement. Concrete exposed to deicing chemicals and frequent wet freezing needs a stable, fine air-void system. Interior concrete, or concrete that is not exposed to freezing, usually needs no intentional air entrainment. In fact, specifications often limit air content in hard-troweled interior slabs, because entrained air can cause blistering and delamination during finishing.
Check Compatibility with Other Admixtures and Binders
Modern mixes often combine an air-entraining agent with water reducers, superplasticizers, retarders, or accelerators. Polycarboxylate ether (PCE) superplasticizers can entrain large, unstable bubbles of their own. Many PCE formulations therefore include a defoamer, and the air-entraining agent then rebuilds a controlled air system. The two products need to work together, so buyers should evaluate them as a system and ask the supplier about tested combinations. Admixtures should normally be added to the mix separately rather than premixed, unless the supplier states that they can be blended.
Supplementary cementitious materials also affect selection. Plants that use fly ash with variable carbon content should look for an air-entraining agent that is less sensitive to carbon adsorption, and should check the air content of each batch closely. Mixes with slag cement or silica fume may need dosage adjustments that are best established through trials.
Evaluate Product Form and Handling
Most air-entraining agents are supplied as liquids, which makes accurate automated dispensing easy, and they are typically dosed in small amounts relative to cement mass. Because dosages are small, the solids concentration of the product directly affects how precise the dosing is and how much product the plant must store and handle. Powder forms exist for dry-mix mortars and premixed bagged products. Buyers should also consider shelf life, sensitivity to freezing during storage, whether the product needs agitation before use, and packaging options such as drums, IBCs, or bulk delivery.
Request Technical Documentation and Support
A credible supplier should provide a technical data sheet with the recommended dosage range and physical properties, a safety data sheet, and batch quality-control data such as density, solids content, and pH. If the project requires conformity with a specific standard such as ASTM C260 or EN 934-2, ask for the corresponding test report or certificate for the exact product being supplied. Technical support for trial batching and for troubleshooting in the field is also valuable, especially when local materials or seasonal temperatures change.
| Selection Criterion | What to Ask or Verify |
|---|---|
| Exposure requirements | Target air content, tolerance, and any spacing factor or freeze-thaw test requirement |
| Chemistry | Resin, fatty acid, synthetic, or blended type, and the bubble stability expected with local materials |
| Admixture compatibility | Tested combinations with the water reducer or superplasticizer in use |
| Binder sensitivity | Performance with fly ash, slag, or silica fume used at the plant |
| Dosage and form | Recommended range, solids content, and liquid or powder form |
| Documentation | Technical data sheet, safety data sheet, batch QC data, and any relevant test reports |
| Supply and support | Packaging, lead time, storage requirements, and access to technical service |
Confirm Performance Through Trial Mixes
Before full production, run trial batches with the actual cement, aggregates, supplementary materials, and other admixtures. Measure fresh air content, slump, unit weight, and temperature, and check how air content changes over the expected haul time. For critical projects, test hardened specimens for compressive strength and air-void parameters. These trials establish a dosage curve that operators can adjust for temperature and material changes during production.
FAQ
What is the difference between entrained air and entrapped air?
Entrained air is intentionally created by an air-entraining agent. It consists of small, spherical, evenly distributed bubbles that improve freeze-thaw resistance. Entrapped air is made up of larger, irregular voids caused by incomplete consolidation. Entrapped air provides little durability benefit and can lower strength.
Does an air-entraining agent reduce concrete strength?
Yes, to some degree. As a general guide, each 1% increase in air content reduces compressive strength by about 5%. Some of this loss can be offset because air entrainment improves workability, which allows a lower water-cement ratio. Mix designs for air-entrained concrete take this effect into account.
Can an air-entraining agent be used together with a superplasticizer?
Yes, and the combination is common in high-performance and self-consolidating concrete. The two products must be compatible, because some superplasticizers affect bubble size and stability. Use products evaluated together, add them separately, and confirm the air-void system through trial batches.
How is air content measured on site?
For normal-weight concrete, the pressure method (ASTM C231) is the most common field test. The volumetric method (ASTM C173) is used for lightweight or porous aggregates. The air-void system of hardened concrete, including the spacing factor, is evaluated microscopically according to ASTM C457.
Is an air-entraining agent needed for all concrete?
No. Air entrainment is mainly needed for concrete exposed to freezing and thawing while wet, or to deicing salts. Air entrainment is also sometimes used to improve the workability of lean mixes and mortars. For interior slabs that will receive a hard-troweled finish, specifications often restrict entrained air to avoid finishing problems.
Why does air content vary from batch to batch?
Air content is sensitive to concrete temperature, mixing time, slump, cement properties, fly ash carbon content, and interactions with other admixtures. Changes in any of these factors can shift air content. Regular testing and dosage adjustment at the plant keep air within the specified tolerance.
Can air-entraining agents be used in mortar?
Yes. In masonry mortar and plaster, air entrainment improves workability and cohesion and can improve freeze-thaw resistance. Excessive air can reduce bond strength, however, so air content should stay within the limits set by the applicable mortar specification.