Concrete has evolved so much over the years. By adding various admixtures, and introducing new mixing techniques, it has undergone numerous changes. Furthermore, it has been optimised according to temperature conditions and various applications. One such type is air-entrained concrete, which utilises the mechanism of air bubbles.
In this blog, let’s explore further about this interesting concept and why we need air bubbles in the concrete mix.
What is Air Entrained Concrete?
Air-entrained concrete refers to a concrete mixture characterised by the intentional introduction of tiny air bubbles to enhance its properties. These air bubbles are created by adding air-entraining agents during the mixing process, which may include various admixtures, such as:
– Vinsol (resin-based admixtures)
– Darex (synthetic surfactants)
– Teepol (anionic surfactants)
– Polycarboxylate-based admixtures
The recommended air content in concrete exposed to freezing and thawing conditions is generally between 5% and 7% by volume. While some specifications may indicate a range of 4% to 8%, the most common standard for exterior concrete subjected to freeze-thaw cycles is indeed centred around 6%. It is essential to note that for some specific applications, such as decorative stamped concrete, a lower air content of around 4% is often recommended to ensure proper finishing without compromising durability.
Application of Air Entrained Concrete
Freeze-Thaw Prone Areas
Climatic conditions can weaken the strength of concrete. In regions where room temperature is unstable, frequently changing between extremely high and low temperatures, concrete loses strength. In these cases, when air bubbles are created in the concrete, it becomes more resistant to cracking and scaling. Thus, Air entrained concrete is ideal for:
– Northern climate construction (regions with harsh and cold climates)
– Coastal and marine structures (If freezing temperatures involved)
– Water treatment plants
– Freeze-thaw prone areas
Air-entrained concrete has the ability to combat damages caused by temperature fluctuations.
Infrastructure Developments
Compared to other types of concrete, air entrained concrete has better workability. This is due to air bubbles within concrete mix which act as a smoothing agent among ingredients. This results in better pouring of concrete and placement of concrete mix in the desired place. Due to its workability, increased resistance to cracks and scaling, it can speed up infrastructure projects and lower maintenance costs. Therefore, air entrained concrete is ideal for:
– Highway pavements
– Airport runways
– Parking garages
– Building foundations and walls
This ensures successful completion of infrastructure projects while maintaining structural uniformity.
Why Use Air Entrained Concrete
Air entrained concrete is specifically designed for regions prone to freeze-thaw cycles. The air bubbles within concrete act as a key to resistance against these cycles. Consider areas with extreme temperature fluctuations – scorching hot or bitterly cold, or constantly shifting between the two.
During freezing temperatures, water inside concrete expands by 7% as it turns to ice. This expansion creates internal pressure, causing cracks as water seeks to escape. When temperatures rise, the ice melts back into water. This constant freeze-thaw cycle damages concrete.
Air entrained concrete solves this issue. The air voids provide space for water to expand when it freezes, which relieves the internal pressure and prevents cracking. This makes air entrained concrete crucial for areas with poor and adverse climatic conditions.
Advantages of Air Entrained Concrete
Improved Freeze-Thaw Resistance
Air bubbles reduce damage from water expansion during freezing which increases durability.
Enhanced Workability
The incorporation of air bubbles in air entrained concrete facilitates smoother pouring and precise placement.
Increased Resistance to Scaling
Air entrained concrete can reduce water permeability and it prevents scaling and potential damages.
Reduced Shrinkage
Air bubbles compensate for shrinkage, thereby minimising overall shrinkage cracks.
Improved Durability
Air entrained concrete’s enhanced durability withstands chemical attacks, abrasion, and erosion.
Lower Maintenance Costs
By minimising repair needs and extended lifespan, air entrained concrete lowers long-term maintenance costs.
Increased Structural Integrity
Air entrained concrete ensures safer structures through improved resistance to cracking, scaling, and degradation.
Disadvantages of Air Entrained Concrete
Reduced Compressive Strength
Air bubbles in the concrete introduce numerous voids, which in turn affect concrete’s compressive strength. Hence, air entrained concrete is not ideal for heavy construction works.
Lower Unit Weight
Increased air content reduces concrete’s unit weight, potentially affecting structural calculations.
Increased Cost
Air entraining agents themselves are not particularly expensive, but quality control, proper mixing techniques, and ensuring uniform air void distribution during large-scale production can slightly increase costs. However, in regions requiring freeze-thaw protection, these additional costs are often justified by the longer lifespan and reduced maintenance.
Difficulty in Achieving Desired Air Content
Air bubbles must be of optimal diameter. If excessive, large voids form, it compromises concrete strength and structural integrity. So, air entrained concrete requires careful mixing and control.
Air Loss During Transportation
Air entrained concrete is susceptible to losing air content during transportation due to vibrations and pressure changes. Modern practices like the use of stabilising agents or continuous mixing help minimise air loss during transportation. Also, appropriate quality control measures ensure that the required air content is maintained.
How is Air Entrained Concrete Made
The four primary ingredients of air entrained concrete are cement, coarse aggregates, fine aggregates and water, which remain the same as normal concrete. However, an air-entraining agent (AEA) is added to create air bubbles to achieve its purpose.
Batching: Cement, coarse aggregates, fine aggregates, water, and air-entraining admixtures are measured and combined.
Mixing: Ingredients are blended precisely to create a uniform mixture.
Air Bubble Formation: AEA reduces surface tension, generating air bubbles (1-4 mm diameter).
Air Void Distribution: Mixing energy distributes air bubbles evenly throughout the mixture.
Vibration: Drum vibration ensures uniform air bubble distribution.
Casting: Mixed concrete is poured into forms.
Finishing and Curing: Standard concrete finishing and curing process follow.
Thus, the air-entraining agent is the main role player in air entrained concrete, which gives numerous benefits and helps mitigate damages due to adverse climate conditions.
Mechanisms of Air Entrainment in Concrete
Void Creation
Air entrainment in concrete begins with void creation. It starts with the addition of AEAs that facilitates air bubble formation in the mixture through reduced surface tension. The AEA ensures these bubbles remain stable, preventing collapse or merging.
Void Stabilisations
Stable air bubbles are essential in this concrete. AEAs make this happen by smoothing the water-air interaction. Air-entraining agents work by reducing the surface tension between water and cement, which creates stable, well-distributed air voids that don’t merge or collapse. The stabilising film around these bubbles prevents them from coalescing during mixing and placement.
Effects on Concrete Properties
Air entrainment concrete affects concrete’s physical properties. Notably, 1% air entrainment typically reduces compressive strength by 5-10%. However, it improves workability by enhancing mix consistency. Moreover, the air voids provide relief from freeze-thaw pressure, enhancing durability.
Key Factors Influencing Air Entrainment
Several factors influence air entertainment’s effectiveness. Optimal AEA dosage is crucial, as excessive or insufficient amounts compromise air void stability. Mixing energy also plays a vital role, ensuring even air bubble distribution. Additionally, water content affects air bubble stability, requiring careful control.
Conclusion
To conclude, air entrained concrete acts as a shield in protecting buildings from harsh cold climates. But it requires controlled mixing and techniques to achieve its full quality. This leads to infrastructure that lasts longer, performs better, and keeps communities safe.
FAQs
AEC air bubbles reduce internal pressure, thereby preventing cracks and damage. This increases its resistance to freezing, thawing, and de-icing salts.
Yes, AEC’s air bubbles slightly reduce compressive strength. However, its benefits in durability and resistance overcome this limitation.
AEC’s initial cost is slightly higher than regular concrete. However, its long-term durability and reduced maintenance needs compensate for the extra expense.