Admixtures for Concrete - What is an Air Entraining Admixture?

Summary

In this video, Tyler Lai discusses the importance of air-entraining admixtures in concrete, especially in freeze-thaw environments. These admixtures are surfactants that stabilize small air bubbles within concrete, enhancing its durability. Tyler explains how these surfactants work at the molecular level, creating a shell around air bubbles that prevents them from coalescing or escaping. He demonstrates this with experiments showing how air bubbles behave in air-entrained and non-air-entrained cement paste. The video emphasizes that not only the volume but the distribution and size of the air bubbles are crucial for freeze-thaw resistance, highlighting the scientific basis and practical implications of air-entrained concrete.

Takeaways

• 🧼 Air-entraining admixtures are anionic surfactants or soaps that stabilize small air bubbles in concrete, essential for freeze-thaw durability.
• ❄️ In freeze-thaw environments, air-entraining admixtures are crucial for the concrete's ability to resist freezing and cracking.
• 💧 Surfactants have a hydrophilic head that loves water and a hydrophobic tail that hates water, which helps them align around air-water interfaces to stabilize bubbles.
• 🫧 The admixtures prevent bubbles from coalescing, forming a shell that maintains the bubble size and prevents air from escaping.
• 🔬 The user conducted experiments showing that air-entrained bubbles are stable over time and resist changes, unlike non-air-entrained bubbles that change size.
• 👁️ Air-entrained bubbles appear to have a visible shell that makes them more durable and prevents air exchange, crucial for concrete stability.
• 📉 Air-entrained bubbles show resistance to size changes under pressure, while non-air-entrained bubbles shrink or grow due to air transfer between them.
• 🔄 The shell around air-entrained bubbles can self-repair when cracked, showing the material's resilience under changing pressure conditions.
• 🧲 Cement particles adhere strongly to air-entrained bubbles, which enhances stability by preventing bubble floatation and maintaining their position within the concrete.
• 📏 The spacing factor, which measures the distance between air voids, is crucial for freeze-thaw protection, as well-distributed small bubbles provide more protection than large bubbles.

Q & A

What are air-entraining admixtures, and why are they important in concrete?

Air-entraining admixtures are anionic surfactants (soaps) added to concrete to stabilize small air bubbles during the mixing process. They are critical for providing freeze-thaw durability in environments where concrete may freeze and thaw repeatedly.

How do surfactants stabilize air bubbles in concrete?

Surfactants have hydrophilic (water-loving) heads and hydrophobic (water-hating) tails, which align at the air-water interface. This alignment helps stabilize air bubbles, making them thermodynamically possible, and preventing them from coalescing or escaping the concrete.

What is the role of cement grains in stabilizing air bubbles?

Cement grains, attracted by the negative charges of the surfactants, help hold the air bubbles in place. This prevents the bubbles from floating out of the concrete, similar to how foam forms on ocean waves but stays within the concrete due to air-entraining agents.

How do air-entrained bubbles differ from non-air-entrained bubbles in concrete?

Air-entrained bubbles are stable, maintain their size, and do not coalesce, thanks to a shell that forms around them. Non-air-entrained bubbles change in size over time, with larger bubbles getting larger and smaller ones shrinking due to air interchange between them.

What is the significance of the shell surrounding air-entrained bubbles?

The shell around air-entrained bubbles prevents air exchange between bubbles and helps resist the transfer of gas. This shell also helps the bubbles maintain their structure and size, contributing to the overall stability of the concrete.

What happens when the shell around an air-entrained bubble is damaged?

When the shell around an air-entrained bubble is damaged, air interchange can occur, and the bubble can shrink in size. However, these shells can self-heal over time, reforming to maintain the bubble's stability.

How does the size and distribution of air bubbles affect concrete's freeze-thaw durability?

Small, well-distributed air bubbles provide better protection against freeze-thaw cycles. The water in the paste expands when it freezes, and the bubbles provide space for the water to move into, preventing cracking and damage to the concrete.

What is the spacing factor, and how is it related to freeze-thaw protection?

The spacing factor measures the distance between air bubbles in the concrete. A lower spacing factor, with more closely spaced bubbles, provides better freeze-thaw protection because more of the paste is protected by the air voids.

How is the spacing factor determined in hardened concrete?

The spacing factor is determined through a linear traverse technique, where a polished section of concrete is examined under a microscope. The distances between air bubbles (chords) are measured, and these values are used to calculate the spacing factor.

How many air bubbles are typically present in air-entrained concrete, and why does their volume matter?

Air-entrained concrete typically contains 4-8% air by volume, which can translate to 10-15 billion bubbles per cubic yard. The volume and distribution of these bubbles are crucial for ensuring freeze-thaw durability, as they allow space for water to expand without damaging the concrete.