Air Entrainment: Achieving Optimal Concrete Quality

The air entrainment process is crucial for creating strong concrete that can weather the elements. Air entrainment prevents the expansion of water within hardened concrete from causing cracking, scaling, and other concrete damage during freeze-thaw cycles.

As such, the concrete air test is just as necessary as other common strength and slump tests.

This article will explore how to perform the concrete air entrainment test and how this can deliver durable slabs that can fight off freeze-thaw damage.

But before we get into how to perform a concrete air entrainment test, we should be familiar with what air entrainment is.

Understanding Entrained Air in Concrete

Entrained air in concrete describes the process of creating tiny air bubbles in a concrete paste by adding air-entraining agents when mixing or using cement that is already air-entrained.

An air-entraining agent (or air-entraining admixture) facilitates the development of billions of microscopic air bubbles within each cubic foot of concrete to relieve internal pressure during freeze/thaw cycles.

How Does Air and Concrete Mix?

Air and water do not mix. So, the air in concrete occurs as gas bubbles surrounded by a thin liquid film and suspended in the mix water. When the concrete hardens, the positions of the gas bubbles become air voids.

There’s always air in concrete. The mixing process causes a foaming action in the mix water, which generates and traps air bubbles within the concrete. This natural air in concrete is called entrapped air.

Thus, while entrapped air refers to air bubbles naturally occurring in concrete due to the mixing process, entrained air refers to microscopic air bubbles intentionally introduced into concrete by adding air-entraining admixtures.

Bubbles are closely spaced and usually better distributed within the concrete than entrapped air, providing a better system of capillary pores for water to expand into (and relieve pressure) during freeze-thaw cycles.

Thus, entrained air in concrete is more useful than entrapped air in preventing freeze-thaw damage.

What is Entrained Air in Concrete Used For?

Entrained air in concrete is used to construct concrete structures in cold weather climates to contain the damage caused by natural freeze-thaw cycles and those caused by using salt and deicers to fight ice in the wintertime.

In cold weather climates, the ambient temperature easily drops below freezing. When this happens, water within the concrete freezes and expands, exerting pressure within the hardened concrete that can cause cracking.

Air bubble entrainment in concrete is especially important where salt and deicers (even not corrosive ones) are used to fight ice in wintertime.

These kinds of chemicals can be hard on concrete because they lower the freezing point of water, which thaws existing ice.

But the water will often refreeze when the temperature drops below that new freezing point.

This result is even more freeze/thaw cycles that feature expanding ice within the hardened concrete that exerts enormous pressure that can cause cracking.

Why is Entrained Air in Concrete Important?

Pockets of entrained air in concrete act as tiny chambers for water within the concrete to expand into as it freezes.

With the ice having well-distributed tiny spaces to expand into, it does not exert pressure on the hardened concrete, preventing freeze-thaw damage like cracking and scaling.

Having the proper air entrainment ratio can help contain the damage caused by natural freeze-thaw cycles as well as those caused by people fighting ice in the wintertime.

Is There a Golden Ratio for Air Entrainment?

As with all other aspects of mixing strong concrete, there is a window for entrained air in concrete you should aim for.

Entrained air should fall between 4% and 7% of the concrete’s volume. For concrete that will be regularly exposed to freeze-thaw cycles, aim for 6%. Anything less than 4% would not have the durability needed to last through multiple winters.

However, be careful not to go overboard with air entrainment. Having too much air in your concrete can reduce its strength.

For every 1% increase in air entrainment, compressive strength can be reduced by 2% to 6%.

Performing concrete air entrainment testing will help you find the perfect air volume for your mixture.

Factors Influencing Air Entrainment in Concrete

Several factors can affect the incorporation of air bubbles into a concrete mix. These include:

Air-Entraining Admixtures

These agents reduce the surface tension between water and concrete, allowing tiny bubbles to form within the concrete.

Including these air-entraining agents in concrete during mixing increases the mix’s air content.

Water Cement Ratio

When the water-cement ratio is low, insufficient water films in the cement particles will produce inadequate foaming action, resulting in fewer entrained air bubbles in the concrete.

Increasing the water-cement ratio increases air entrainment. However, when you add excess water for an overly high water-cement ratio, many of the air bubbles produced will be lost with time.

Aggregate Property

The type and grading of aggregates also affect air entrainment. For example, the quantity of air entrainment increases with an increase in the finest modulus of sand.

Thus, coarse aggregates will entrain more air than crushed stone aggregates, and excess of fine aggregates will reduce the amount of air entrained in concrete.

Mixing Time

Over-mixing reduces air entrainment in concrete. Mix concrete for 2 minutes in stationary mixers or about 15 minutes in transit mixers to reach the optimum air content.

Mixing past these times will reduce the amount of air entrained because of a loss in slump.

Cement Properties

Increasing the fineness of cement will decrease the amount of air entrained. For example, increasing the fineness of cement from 3,000cm2/g to 5,000cm2/g will cut the amount of air entrained by half.

The constituent of cement, especially the soluble alkali content, also affects air entrainment. Using cement with higher alkali results in more air entrainment.

Temperature

The temperature of the concrete when mixing affects air entrainment. Higher temperatures reduce the water films in the cement particles needed to produce foaming action, thereby decreasing the amount of entrained air.

Reduction in temperature increases air entrainment in concrete. However, extremely low temperatures will make it difficult to entrain air properly.

Compaction

Compaction destroys the air bubbles entrained within the concrete. In fact, vibration for 2½ minutes can reduce air bubbles by 50%.

So if the concrete mix vibrates a lot during transportation, the amount of air entrained reduces significantly.

Water Quality

Impurities (like salt) in the water used in the concrete mix can interfere with the formation and stability of air bubbles, reducing air entrainment.

Thus, using hard water in concrete reduces air entrainment.

How to Perform a Concrete Air Entrainment Test

The concrete air entrainment test is a test performed on plastic or fresh concrete to determine its air content. Perform the concrete air test according to the procedures outlined in the ASTM C 231 standards.

Equipment Needed for the Concrete Air Entrainment Test

The equipment you need to conduct the concrete air entrainment test is:

Pressure meter: A working pressure meter (concrete air meter), complete with an air pot and gauge, is used to carry the concrete sample and determine its apparent air content.
Scoop: A round bottom scoop is used to get concrete samples into the air pot.
Tamping rod: A big grout rod is used to tamp the concrete sample in the air pot. It should have a smooth hemispherical tip and measure 18 inches in length and 5/8 inches in diameter.
Mallet: A rubber mallet is used to strike the side of the air pot to get rid of air bubbles left by the tamping rod that could overinflate the air content value. It must weigh 1.25±0.50 lbs for pressure meters smaller than 0.5 cubic feet and 2.25±0.50 lbs for pressure meters larger than 0.5 cubic feet.
Strike-off plate or bar: A strike-off bar is used to strike off excess concrete from the air pot. It should be 1/8 inches thick, 3/4 inches wide, and 12 inches long.
Syringe/ water dropper: The syringe is used to fill the remaining space in the air meter.
Sponge or rag: Before using it, dampen the airpot and clean concrete off the pot’s rim.
Bucket of water: People also use it to dampen the air pot and clean.
Calculator: Subtract the aggregate correction factor from the apparent air content obtained from the pressure meter to calculate the accurate air content.

Also Read: Concrete Testing in Construction: 10 Must-Have Tools.

Procedures for the Concrete Air Entrainment Test

The specific procedures for doing the air test are as follows:

Dampen the inside of the air pot that will receive the concrete.
Take the concrete sample and fill one-third of the air pot.
Tamp the concrete 25 times throughout its depth but without forcefully striking the bottom of the pot. Distribute the blows uniformly throughout the concrete’s surface.
Tap the outside of the pot 10 – 15 times with a mallet to close the voids left in the sample by the tamping rod.
Use the scoop to take more concrete and fill two-thirds of the air pot.
Tamp the concrete 25 times. Distribute the blows throughout the surface of the concrete, and ensure each blow penetrates the first layer by about one inch.
Tap the outside of the pot 10 – 15 times to close the voids left by the tamping rod.
Take more concrete and fill the air pot completely, ensuring the concrete is slightly above the rim of the pot.
Tamp the concrete 25 times, ensuring each blow penetrates the second layer by about one inch. Also, make sure you spread the blows throughout the surface of the concrete.
Tap the outside of the pot 10 – 15 times to close the voids left by the tamping rod.
Use the strike-off plate to clear off excess concrete from the pot’s top, leveling the concrete surface with the pot’s rim. Place the plate in the middle of the pot and use a back-and-forth sawing motion to clear excess concrete off the pot’s top. Incline the plate and use its edge to sweep the pot’s top, producing a smooth surface.
Clean the edges of the pot’s rim.
Take the cover of the air pot. Dampen the insides that will make contact with the concrete surface. Then, use it to cover the pot. Fasten the clamps and ensure you have an air-tight seal.
Close the main air valve and open the petcocks on the air meter by pushing the levers upright.
Use the syringe/ water dropper to inject water into one petcock until the water squirts out from the opposite petcock. This expels all the air in the chamber, so the only air in the meter is the air inside the concrete.
Pump air into the pressure meter until the hand on the dial gauge is on the initial pressure value. Slightly exceed the initial pressure line, then gently tap the side of the gauge with your finger to stabilize it at the correct initial pressure.
Close both petcocks.
Release the pressure in the pot by opening the main air valve between the air pot and the air chamber. Then, use your mallet to tap the side of the air pot.
Lightly tap the side of the pressure gauge with your finger to stabilize the reading.
Record the reading. This is your apparent air content. Subtract the aggregate correction factor from this apparent air content to get the actual air content in the concrete.
Open each peacock slowly and release the remaining air in the chamber. Then, remove the air pot cover and dump the concrete sample.

A good air entrainment meter will produce results within ±1% of the target value in about 10 – 15 minutes.

If the results fall outside of the 4% to 7% range, we will need to either increase or decrease the volume of air entrainment to ensure the final product has good compressive strength and freeze/thaw resistance.

What are the Benefits of Air-Entrained Concrete?

Air-entrained concrete offers benefits such as:

Increased Resistance to Freeze-Thaw Damage

One of the benefits of air-entrained concrete is that it resists freeze-thaw damage.

When the water in concrete freezes during freezing temperatures, the air bubbles serve as expansion chambers for the ice to expand into. Thus, the expanding ice does not exert excessive pressure within the concrete to cause damage.

Increased Durability

The expansion of water in concrete during freeze-thaw cycles causes concrete failure. This ranges from cracking to scaling.

Air entrainment concrete helps prevent the damage caused by freeze-thaw cycles, resulting in more durable concrete structures.

Improved Workability

The air bubbles within the concrete act as lubricants within the concrete mix. This increases the fluidity of the concrete, making it easier to mix, pump, and place.

Reduced Bleeding of Water

During concrete pouring, denser materials settle, causing water to rise to the surface, resulting in bleeding.

When using entrained concrete, the air bubbles within the concrete obstruct and delay the sedimentation of the denser particles, thereby retaining water where it should be and preventing bleeding.

When Should I Avoid Air-Entrained Concrete?

While air-entrained concrete offers many advantages, you may want to avoid it in certain situations, including:

Interior Floors

Air entrainment is often unnecessary for interior floors because freeze-thaw resistance is not a concern for indoor applications.

Smooth Finishes

The mico air bubbles within entrained concrete reduce the hard-trowel finishability of concrete. They result in surface imperfections, ranging from blisters to delaminations.

Thus, air-entrained concrete may not be ideal if you desire a concrete surface with a smooth and polished finish.

High Strength Concrete

Air entrainment reduces the compressive strength of concrete. So, it may not be ideal for certain high-strength concrete applications like high-rise structures, highway pavements, hydropower structures, etc.

Takeaway: Conduct Concrete Air Entrainment Testing to Get Quality Concrete for Durable Structures

Air entrainment is the process of intentionally incorporating microscopic air bubbles into a concrete mix by adding air-entraining agents.

Air entrainment helps concrete structures weather freeze-thaw cycles that would otherwise damage them.

When the water in concrete freezes, the entrained air bubbles provide pockets for the ice to expand into, relieving pressure on the hardened concrete and preventing cracking.

Thus, the concrete air entrainment test is one of the important tests you should carry out on freshly prepared concrete. If you follow the detailed procedures above, you should have no problem doing the test.

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