Воздушное вовлечение в бетон: повышение прочности и долговечности

Voluntary trapping tiny air bubbles in concrete
air bubble entrainment in hydraulicsAir 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. 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.

Air entrainment is beneficial for the properties of both fresh and hardened concrete. In fresh concrete, air entrainment improves workability and makes it easier to handle and pump. It also helps prevent bleeding and segregation, unwanted processes that can occur during mixing. In hardened concrete, air entrainment strengthens the material by making it better able to withstand freeze thaw cycles. It also increases its resistance to cracking, improves durability against fire damage, and enhances overall strength. Therefore, adding air to concrete when it's being made makes it easier to handle at first, but then later helps it stay strong even under tough conditions like freezing temperatures or fire exposure. Tiny air bubbles in air entrained concrete act as internal cushioning, absorbing energy during impact and increasing resistance to physical forces such as shock or vibration. This improved impact resistance helps minimize surface damage and prevent the propagation of cracks or breaks, thereby increasing overall durability. Additionally, the air voids, acting as pressure relief zones, allow water or moisture expansion during freeze thaw cycles without causing internal stresses and subsequent cracking. Various materials can impact the properties of air entraining admixture in several ways. Fly ash, a supplementary cementitious material, improves paste packing due to its smaller particles, resulting in better flow and finishing of the concrete. Fly ash's lower specific gravity increases the paste content for a given water to cementitious material ratio (w/cm) compared to ordinary Portland cement. Different types of fly ash require adjustments in air entraining admixture dosage due to variations in their chemical compositions and air loss characteristics. Class F fly ash typically demands higher levels of admixture to maintain desired entrained air levels compared to Class C fly ash.

The air bubbles typically have a diameter of and are closely spaced. The voids they create can be compressed a little, acting to reduce or absorb stresses from freezing. Air entraining was introduced in the 1930s and most modern concrete, especially if subjected to freezing temperatures, is air entrained. The bubbles contribute to workability by acting as a sort of lubricant for all the aggregates and large sand particles in a concrete mix.

In addition to intentionally entrained air, hardened concrete also typically contains some amount of entrapped air. These are larger bubbles, creating larger voids, known as "honeycombing", and generally are less evenly distributed than entrained air. Proper concrete placement, which often includes vibration to settle it into place and drive out entrapped air, particularly in wall forms, is essential to minimizing deleterious entrapped air.

Using fly ash, a byproduct of coal combustion, as an additive in concrete production, is a common practice due to its environmental and cost benefits. Still, residual carbon in fly ash can interfere with air entraining admixtures (AEAs) added to enhance air entrainment in concrete for improved workability and resistance against freezing and thawing conditions. This issue has become more pronounced with the implementation of low NOx combustion technologies. There are mechanisms behind the interactions between AEAs and fly ash in concrete mixtures, related to the effects of residual carbon. The amount of carbon and its properties, such as particle size and surface chemistry, impact the adsorption capacity of AEAs. The type of fuel used during combustion affects both the amount and properties of residual carbon present. Fly ash derived from bituminous coal generally has higher carbon content than those produced from sub bituminous coal or lignite but exhibits lower AEA adsorption capacity per mass of carbon. Different post treatment methods are used to improve fly ash quality for concrete utilization. Techniques such as ozonation, thermal treatment, and physical cleaning have shown promise in enhancing performance.

Air entrainment was discovered by accident in the mid 1930s. Air entraining agents (AEAs) have been developed and extensively studied to improve resistance against freezing and thawing damage caused by both internal distress and salt scaling.