Regarding air-entraining agents in concrete

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Regarding "air-entraining agents" in concrete

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I. Commonly Used Air Entraining Agents

Commonly used air-entraining agents can be divided into four main categories based on their chemical composition and performance characteristics, each suitable for different concrete applications. The recommended basic selection logic is: prioritize rosin-based agents (for general applications), alkylbenzene sulfonates for pumping/thin-wall concrete, fatty alcohols for high-strength/decorative applications, and silicone-based agents for corrosive environments.

1. Rosin resins (most widely used and cost-effective): rosin thermal polymers, rosin soaps (products of the reaction between rosin and alkali).

Features: Small and uniform air bubbles, good stability, significant improvement in freeze-thaw resistance and impermeability, low dosage (0.005%-0.02%), suitable for ordinary concrete and freeze-thaw resistant concrete (such as road surface and bridge).

2. Alkylbenzene sulfonates (high surface activity, also helps reduce water content): Sodium dodecylbenzene sulfonate, sodium dodecyl sulfate (commonly known as "the main ingredient in laundry detergent", but industrial grade is used in engineering).

Features: It has a slight water-reducing effect while entraining air, and its workability is significantly improved. However, the stability of the air bubbles is slightly poor, and the dosage needs to be controlled to avoid excessive air content. It is suitable for pumped concrete and thin-walled components.

3. Fatty alcohols (fine bubbles, suitable for high-end scenarios): fatty alcohol polyoxyethylene ether, fatty alcohol sodium sulfate.

Features: Smaller and more uniformly distributed air bubbles, resulting in a smoother concrete surface and superior durability. Suitable for high-strength concrete and decorative concrete (such as fair-faced concrete). Price is slightly higher.

4. Other types (adapted for special scenarios)

(1) Lignosulfonates: have a weak air-entraining effect and are mostly used as auxiliary components of water-reducing agents, taking into account both air entrainment and retarding, and are suitable for large-volume concrete.

(2) Protein-based (animal glue, gelatin): It has good bubble stability, but poor alkali resistance and is easy to deteriorate. It is only used in special waterproof concrete.

(3) Silicone: It has extremely strong bubble sealing properties, outstanding impermeability and corrosion resistance, and is suitable for concrete in corrosive environments such as coastal areas and saline-alkali land.

II. Functions of Air-Entraining Agents in Concrete

Adding air-entraining agents to concrete optimizes performance in three key dimensions: workability, durability, and strength compatibility, by introducing uniform, tiny, closed air bubbles. This makes it particularly suitable for complex environments and construction scenarios. Specific details are as follows:

1. Improve workability: Make concrete "easy to mix, easy to pour, and non-stick to the formwork".

(1) Air bubbles act as a "lubricant", filling the gaps between aggregates, reducing particle friction, reducing the amount of water required for mixing (water can be reduced by 5%-10% under the same fluidity), and avoiding segregation and bleeding (water rises and causes surface sand).

(2) Improved plasticity and pumpability: The viscosity is more moderate, and it can easily fill the gaps between steel bars and corners of formwork during pouring. It is especially suitable for thin-walled components and densely reinforced structures, reducing the difficulty of vibration and avoiding the "honeycomb surface" defect.

(3) Stable slump: Reduces fluidity loss during transportation and waiting, and maintains good working condition even at high temperature and long distance pumping, thus reducing construction risks.

2. Enhanced durability: A "protective shield" against harsh environments.

(1) Resistance to freeze-thaw damage (core function): air bubbles provide a "buffer space" for frost heave - when the water inside the concrete freezes at low temperatures, the volume expands (about 9%). Sealed air bubbles can absorb the expansion stress, avoid internal cracking and surface peeling. The number of freeze-thaw cycles can be increased from 50 times for ordinary concrete to more than 200 times, making it suitable for cold regions and outdoor exposed components (such as roads, bridges, and dams).

(2) Waterproof and seepage-proof: The evenly distributed air bubbles form a "maze-like" barrier, which prolongs the penetration path of water and harmful ions (such as chloride ions), reduces the risk of water seepage, and protects the steel bars from corrosion. It is especially suitable for underground engineering and hydraulic structures (such as pools and tunnels).

(3) Resistance to sulfate attack: Reduces the intrusion of sulfate solution and avoids the formation of expansive products (such as ettringite) inside the concrete, which can lead to cracking. It is suitable for corrosive environments such as coastal areas and saline-alkali land.

(4) Improve carbonation resistance: slow down the carbon dioxide penetration rate, delay the strength reduction and alkali-aggregate reaction caused by cement stone carbonization, and extend the service life of the structure.

3. Optimize volumetric stability: reduce shrinkage cracking

(1) Air bubbles can offset the drying shrinkage and plastic shrinkage stress during the hardening process of concrete, reduce the probability of early cracking, and are especially suitable for large-volume concrete (such as raft foundation) and large-area pouring (such as floor slabs and road surfaces) to avoid cracks caused by uneven shrinkage.

(2) Relieve temperature stress: The heat insulation effect of air bubbles can reduce the temperature difference between the inside and the surface of concrete, reduce temperature difference cracks, and improve the overall structure.

4. Strength adaptability: Sacrificing a small amount of strength for core performance can be compensated for through proportioning.

(1) Air-entraining agents will slightly increase the porosity of concrete. If the mix proportion remains unchanged, the strength will decrease slightly (for every 1% air content introduced, the compressive strength will decrease by about 3%-5%). However, this loss can be compensated by "reducing water" or "increasing the amount of cementitious materials". In actual engineering, the strength of air-entrained concrete can be designed as C15-C50 according to the requirements, which fully meets the requirements of conventional structures.

(2) Strength advantage in special scenarios: After freeze-thaw cycles, the strength of ordinary concrete will decrease significantly (up to 30% or more), while air-entrained concrete retains more than 80% of its strength because it does not crack, and its long-term strength is more stable.

5. Adaptable to special raw materials and scenarios: Enhancing applicability

(1) Improve the workability of lightweight aggregate and poor aggregate gradation concrete: Lightweight aggregate has strong water absorption, and air bubbles can compensate for the lack of fluidity caused by it; in poorly graded aggregate (such as lack of fine materials), air bubbles can fill the voids and avoid the concrete being dry and difficult to mix.

(2) Reduce surface defects caused by water bleeding: reduce sanding and dusting caused by water rising, improve the hardness and smoothness of concrete surface , and reduce the cost of later repairs.

6. Key Usage Details (Mitigating Negative Impacts)

(1) Air content control: The suitable air content is 3%-6% (ordinary concrete) and 5%-8% (antifreeze concrete). If it is too high, the strength will be reduced excessively. It needs to be precisely controlled by the amount of air-entraining agents (usually 0.005%-0.02% of the mass of cementitious materials).

(2) Bubble quality requirements: It is necessary to ensure that the bubbles are small (0.02-0.2mm in diameter), uniformly distributed, closed and not interconnected. If the bubbles are too large or interconnected, it will reduce the impermeability and freeze resistance. Therefore, it is necessary to use high-quality air-entraining agents (such as rosin and alkylbenzene sulfonates) and reasonable vibration process.

The core value of air-entraining agents lies in their ability to significantly improve the workability and durability of concrete by introducing a suitable amount of stable micro-bubbles, resulting in a controlled, slight strength concession. Especially under conditions of freezing and thawing, humidity, chemical corrosion, or complex construction, this technology can effectively alleviate internal stress, inhibit crack development, and enhance impermeability and freeze-thaw resistance, thus becoming a key measure to ensure the long-term safety and durability of concrete structures in harsh environments.

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