Term: Air entrainment
**Benefits of Air Entrainment:**
– Improves workability of fresh concrete
– Prevents bleeding and segregation
– Strengthens concrete for freeze-thaw cycles
– Enhances resistance to cracking and fire damage
– Increases overall durability and strength
**Process of Air Entrainment:**
– Concrete porosity due to excess water evaporation
– Importance of controlling air entrainment process
– Impact of materials like fly ash, silica fume, and slag cement
– Influence of natural pozzolans on air-entrained concrete
– Adjustments in admixture dosage based on material properties
**Characteristics of Air Entrainment:**
– Air bubbles diameter ranges from 10 to 500 micrometers
– Introduced in the 1930s for concrete subjected to freezing temperatures
– Act as lubricant for aggregates and sand particles
– Void compression reduces stresses from freezing
– Enhances workability of concrete mix
**Issues with Entrapped Air:**
– Presence of unintentionally entrapped air in hardened concrete
– Larger bubbles creating honeycombing and uneven distribution
– Vibration during concrete placement to minimize entrapped air
– Essential in minimizing deleterious entrapped air
– Particularly crucial in wall forms
**Challenges and Future Directions:**
– Maintaining consistent air content in concrete during mixing and placement
– Factors affecting entrained air like long hauling durations and high temperatures
– Over-entrainment or under-entrainment impacting concrete performance
– Balancing air content with other concrete properties crucial for desired outcomes
– Future direction with superabsorbent polymers (SAP) potentially replacing traditional air-entraining agents (AEAs) in concrete
Air entrainment in concrete is the intentional creation of tiny air bubbles in a batch by adding an air entraining agent during mixing. A form of surfactant (a surface-active substance that in the instance reduces the surface tension between water and solids) it allows bubbles of a desired size to form. These are created during concrete mixing (while the slurry is in its liquid state), with most surviving to remain part of it when hardened.
Air entrainment makes concrete more workable during placement, and increases its durability when hardened, particularly in climates subject to freeze-thaw cycles. It also improves the workability of concrete.
In contrast to the foam concrete, that is made by introducing stable air bubbles through the use of a foam agent, which is lightweight (has lower density), and is commonly used for insulation or filling voids, air entrained concrete, has evenly distributed tiny air voids introduced through admixtures to enhance durability, workability, and resistance to freeze-thaw cycles without significantly reducing its overall density, and without negative impact on its mechanical properties, allowing to use it in objects such as bridges or roads built using roller compacted concrete. Another difference is manufacturing process: foam concrete involves the creation of a foam mixture separately, which is then mixed with cement, sand, and water to form the final product, while air entrained concrete is produced by adding specialized admixtures or additives directly into the concrete mix during mixing to create small air bubbles throughout the mixture.
Approximately 85% of concrete manufacturing in the United States contains air-entraining agents, which are considered the fifth ingredient in concrete manufacturing technology.