Why Do We Need Air Bubbles in Concrete?
Updated: Dec 19, 2022
Today, we're going to talk about why we need air bubbles inside concrete.
And are all air bubbles created equal?
Finally, did you know that every specification when it comes to air-entrained concrete is totally flawed?
Air bubbles in concrete are one of our favorite topics to discuss. You may see the bubbles in concrete and think that somebody made a mistake. Maybe somebody didn't construct the concrete correctly.
You see these voids or bubbles on the surface, and some might think,
“They look ugly!”
But they're a critical part of the concrete!
And the smaller, tiny bubbles are much more important than the larger ones. They're the good ones.
Let's look at this project. Both are built by the same contractor, using the same materials and specifications.
The one on the right has fallen apart after about 5 years! The bubbles that were inside of them were mostly large.
The one on the left had those smaller bubbles, which we want! These bubbles have to do with the freeze-thaw durability of the concrete. Here’s what that means.
Take a look at this bubble drawing here.
You can see that it's surrounded by water pockets. As that concrete starts to freeze, that water tries to expand, and it'll try to escape. It tries to find those air bubble boundaries around it. That water can only go a certain distance, and if it makes it to that air bubble edge or that boundary, everything is saved!
But if that water pocket is too far away and can't reach a boundary, there's only a certain distance it can travel before it starts to cause cracks in the concrete.
So why do we add air to concrete?!
Well, these air-entrained bubbles are the key to the freeze-thaw resistance, but there are also other things to consider that are important.
You need to know that the volume of air in your concrete does not equal freeze-thaw performance and the smaller bubbles are much more effective at providing freeze-thaw resistance than the larger bubbles.
Typically, specifications look something like what is shown above.
Based on the nominal maximum aggregate size, they give you a certain volume of air, but most people make this even simpler and say,
“You need 6% of air in your concrete.”
We got that 6% number from a guy named Paul Klieger (Wikipedia anchor?). Paul Klieger, at the Portland Cement Association (site anchor), did a huge study using hundreds of concretes, freezing and thawing those concretes multiple times.
He found that the secret to making concrete last longer was to put air bubbles from soap inside the concrete! And we've been using that technology ever since.
Although Paul Klieger did some amazing things, the materials he looked at were very limited compared to today's mixtures. Concrete has changed so much since the 1950s. Paul Klieger’s mixtures were missing a few things:
And as far as the admixtures he used, he only used one, which was an air-entraining admixture (definition anchor?), which was the only admixture that existed back then!
How many concrete mixtures can you think of today that only have one admixture in them?
Almost all of them contain a whole cocktail of different chemicals that help make that concrete better.
Looking above, we've got two air void systems here.
Both of them have the same volume of air, but as concrete people, we'd much rather have the one on the right because there is much more protection from the air void system.
So how do you measure this? How do we understand what's going on?
Typically, we use a hardened air void analysis, also called an ASTM C457 analysis. This is where you cut the concrete, polish it, look at it underneath the microscope and trace over the surface in a line.
Every time that line intersects a bubble, you measure it. This measurement is called a chord.
Looking at the air void system on the left, we have a bunch of small bubbles and on the right, we have many large bubbles. We've taken this concrete, polished them, colored them black, and put white inside, all over the voids.
Looking at the one on the left, there are just not that many large voids. Now, look at all those large bubbles on the right. In examining them both, we would much rather have this air void system on the left.
Here's another way to look at this.
What you’re seeing are the sizes of the chords. This is showing the small and large voids, as well as the frequency or how often they occur.
The green line here is mainly made up of small bubbles, and the blue of large ones. If we had to pick, we would much rather have the green line.
Now, instead of having to look at this entire line, a guy named TC Powers (wiki anchor?), in 1949, came up with this really cool concept called the Spacing Factor.
He took all of these numbers and measurements, and said,
"I'm going to find an average size void, averagely spaced, uniformly inside the space."
And here is what he found:
The spacing factor is a magic number, which has something to do with the air void spacing. The number that we want is less than 0.008”, which is defined by ACI 201(definition anchor?).
Looking back at the line graph, we would say the green line here has a low spacing and the blue line has a high spacing factor.
Now let’s look at this another way:
Our air volume is shown on the X-axis, the spacing factor on the Y-axis, and our magic number of 0.008” is graphed.
We can see as our air content goes up; our spacing factor goes down. And as you're adding more and more air to the concrete, the spacing of the bubbles has to get closer and closer together.
Now, if you have small bubbles, with about 6% air, which is around most specifications, then you get the nice spacing factor we care about!
Now if you have large bubbles in your concrete, then you need much higher air content to get this magical spacing of your bubbles.
You cannot tell the size of your bubbles by just looking at the volume!
Now, here's another plot that shows air content versus durability factor.
(This is performed in the freeze-thaw test, where we make concrete, and freeze and thaw it multiples times over.)
In the freeze-thaw test, we send a stress wave through it and measure how many cracks form on the inside. The dashed line is what we call failure. Everything above this line is good and everything below the line is not what we want.
You can see with this “small bubbles” mixture that if I get enough air, or about 4% air in my concrete, everything is great!
But let’s take a look at this “large bubbles” mixture. You would need a lot more air- about 7%. This is quite a bit different.
The big takeaway here is that air content and volume alone do not let you determine if you're going to have freeze-thaw durable concrete. You must also know about the size of the bubbles.
Here's more data:
The air content is on the X-axis and the spacing factor is on the Y-axis. And here, we are looking at that magical number of 0.008".
These dots shown are all mixtures with small bubbles. Every filled diamond means that the freeze-thaw test was passed. The open ones are the ones that failed the freeze-thaw test. All of which were small bubbles.
Now, if we look at mixtures with larger bubbles, which happen all the time, you can see that there's a clear offset.
Looking at the two circled data points on the graph, notice that they’re not filled, having 7% air. These two data points would pass every specification in the world when it comes to freeze-thaw durability, and yet they're failing the freeze-thaw test.
Put simply, what Paul Klieger did was great, but it only worked great for his time in the ’50s. But the specifications we use today are still based on his research.
Our concretes are very different today and we now need to know the size of the bubbles in our concrete. Although we can run a hardened air void analysis, it's just not practical to run it regularly.
Editor's Note: This post contains information from Dr. Tyler Ley's YouTube Channel. See the full video here.