Updated: Dec 19, 2022
Why would you ever want to specify the Super Air Meter?
Most people care about the bubble spacing inside their concrete, and one way to measure that is by the spacing factor. That's where you cut concrete, polish it, and count the bubbles on the surface.
*(This research was performed by Dr. Tyler Ley and the US Department of Transportation) under picture*
In this graph, we're plotting the spacing factor here on the Y-axis. And on the X-axis, we're plotting the Super Air Meter number or the SAM number.
Most experts agree that the spacing factor you want is 0.008 inches, which is kind of a mythical number!
In the Super Air Meter, if we want the same number, and the same agreement, we find that we need the SAM number of 0.20. This does a great job for these 227 mixtures, as seen in the graph, separating them into the quadrant we do and don’t want.
Of these 227 mixtures tested, there's about an 88% agreement. That means 88% of the time the data falls into one of these two quadrants!
Even though bubble spacing is extremely important, it isn't everything! Freeze-thaw durability is also really important!
In this plot, we're showing the SAM number on the X-axis and we're showing performance in the rapid freeze-thaw test on the Y-axis. This is the ASTM C666 test (definition anchor?).
The upper left-hand corner of the graph is where we want to be, and not the lower right-hand corner.
The vertical line at .32 is what we call the Cliff of Doom.
This is about a SAM number of 0.32 when failure starts to happen. And of these 98 mixtures, this Cliff of Doom, or 0.32 number, separates them to 91% agreement.
That means it does a good enough job 91% of the time, where things fall into either the “Yes” quadrant or the “No” quadrant, which is pretty good!
So, in summary, if you want to look at the SAM limits for a spacing factor of 0.008, you want a SAM number of 0.20.
And as for failure in the freeze-thaw test (definition anchor?), you want a SAM number of 0.32.
You might be thinking,
"Why are they different?"
Well, people know we don't want to live on the edge of danger. We want to be away from the edge, so we need a safety factor. And so historically, people have been using a safety factor of a spacing factor of about 0.008”, a SAM number of 0.20, and where failure starts to happen is about 0.32.
Going forward, we are just going to round that to 0.30 for our specifications, because it's a nice round number and it's a bit conservative.
If we compare the SAM number to these other measurements, as in the spacing factor or the performance in the rapid freeze-thaw test, we’re actually showing the coefficient of variation (definition anchor?). That's how many people compare tests to one another to see how variable they are, and we'd like that number to be as low as possible.
For 170 different SAM numbers, we found the coefficient of variation was about 15%.
The published coefficient of variation for the spacing factor is about 20%, and for the rapid freeze-thaw test is about 22%. So, the SAM number is a lower variance than these other tests.
The other great thing about the SAM is it can be completed in 10 minutes when the concrete is still wet and can still make changes to it, which is a real advantage!
You really must understand how variable your test method is, which is critical in how you set your limits or your bounds. This is so we can handle this in how we specify our concrete and design it.
We should be doing this with all of our tests!
AASHTO PP84 (anchor text to document) is a document that talks about SAM specifications. Which is a new document that actually covers the durability of paving concrete.
In the mixture design stage, we suggest that a SAM number should be less than 0.20 and your air content should be greater than 4%.
In the mix design stage, like in the lab, when you're putting all the materials together, you should make sure that your SAM number is 0.20 or lower at your working air content, at the air content you expect to use in the field.
In the field, we're going to loosen things up a bit. We know the variability, we know where the edge of the cliff is, we know where the cliff of doom is, and we're going to stay away from that!
Let’s dig a little deeper here!
If our SAM number is less than 0.25, the concrete is acceptable. And the large majority of the time, that's where you should be.
Sometimes you'll be between 0.25 and 0.30, which should be an action limit.
This should be something where the contractor increases the air content on the next truck. When you increase the air, the SAM number should go down. At this point, you should accept the concrete and place it.
If your SAM number ever gets above 0.30, reject the concrete. Because remember, that's the cliff of doom, and your air content always has to be greater than 4%.
This spec is designed to get people in the right place during the mix design so that it sets them up for success when they go and use this in the field. And then again, we'll give them a little bit of wiggle room to still accept concrete when it's okay, but ask them to increase the air content when we get close to our limits.
One question you might ask is,
"How do I improve my SAM number?"
Well, if you've got a truck and it's on the job site and it doesn't have the SAM number you want, just about the only thing you can do is add an Air-Entraining Admixture to the truck, usually with the Fritz-Pak (definition anchor), which are those things you throw in the back of the truck and dissolve and you mix it up to increase the air content.
That's about the only lever you have when you have a truck on site.
If you go back to the mix design stage, pay special attention to your mixture ingredients.
Some of those in there is causing you trouble and it's your job to figure out which one it is!
It could be your construction process, how the concrete is hauled, how it's treated, or something else that may be happening to it. So, it all depends on where you're at; these things can impact the SAM number.
But the cool thing is you now have a tool to investigate it and you're not chasing your tail as much!
You can really start to figure out what's real and what's not and get some real answers!
Editor's Note: This post contains information from Dr. Tyler Ley's YouTube Channel. See the full video here.