Understanding VMA: Comparing Asphalt Mix Design to Production


Jul 18, 2019 11:00:00 AM / by Dan Ridolfi, PE



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Voids in mineral aggregates (VMA) is an asphalt property used in many specifications to ensure asphalt mixes will perform as designed. The trouble is that the VMA of an asphalt mix depends on a lot of different factors including aggregate gradation, aggregate shape, aggregate texture, and asphalt binder properties.

In this first video on asphalt VMA, we discuss a method of comparing mix design VMA to production VMA in order to understand why an asphalt’s VMA in production has changed from the original mix design. We explain why cumulative gradation doesn’t tell the whole VMA story, and why it’s important to focus on aggregate packing.

A demonstration is provided, and analysis can be duplicated using Excel or LASTRADA™.



After this video, you’ll be able to:
1. Process relevant aggregate test data in a report to determine where deviations occur
2. Identify where aggregate size distribution changes from a mix design correlate to changes in production VMA


In this video we're gonna look at how aggregate size distributions affect VMA. Some aggregate sizes in an asphalt mix will increase VMA by making space. Some aggregate sizes will decrease asphalt VMA by filling space. What we're going to do in this video is compare the aggregate size distribution during the mix design to the aggregate size distribution in production and see which aggregate sizes are increasing space, or VMA, and which aggregate sizes are decreasing space, or decreasing asphalt VMA.

Ok, let's see how we practically go about comparing a production sample’s gradation to a mix design.

Let's start here in LASTRADA. I'm going to first query in some data from a plant and a mix design. Let's start here at this Lambo Construction Plant. We'll search for some data, and we'll just pick a big group of data. Let's just select everything. Next, I'm going to run a report that we've pre-built to compare a production sample to its mix design.

So, I've selected my sample, I'm going to select my report, and now the report is being filled for me.
In the first page of this report, we're looking at traditional gradations, as we see at cumulative percent passing. You can see my sieve sizes across the top, and you can see my result in gradations here. One of the interesting things that you see in this data here is in the VMA column, you can see the VMA starting at 14.8, where the minimum is 15 and it trends all the way up to 16 and then starts to make its way back down. Actually, it trends its way all the way up to 17.

When we go to the next page of this same report, I went ahead and modified the report not to show me the cumulative percent passings, but the individual weight retained by each sieve. So, when we were looking earlier, we would have seen there was a 100% passing the one inch and a 100% passing the ¾ inch, and what we're seeing here is that there's nothing retained on that one sieve.

Then on the last page of this report, we're now subtracting what we retained on the sample from what was actually in the mix design. When you look at this sample point here, where you've got a -1.1, this is telling us that the production sample has 1.1% less retained on the ½” screen compared to its mix design.

Since we're comparing a sample to its mix design, I'm going to use this table as our reference for this video. What we see here in the VMA data is a trending from just under 15 all the way up to 17 and then we start to trend back down roughly. To show that trend, I went ahead and added a column where I'm actually showing the running average of 4, which helps smooth out some of the data. You can see that we've been under 15, we trend up to 16, then 16.4, and then we start to make our way down. So, if we're trying to understand which sizes are affecting VMA, why don't we look for that same trend where we start low in a value, we trend our way up, and then we make our way back down.

When you look at the data, you see that same behavior in the #8 screen. You can see roughly that we start to lose more and more of the #8 material and then we start to trend our way back down. And again, I went ahead and averaged the rolling average of 4 on the #8 screen because the #8 screen tends to be a very influential screen relative to VMA. When you do that, you see the rolling average is doing exactly what is going on with the VMA. So, if I were producing this mix design, I would be really careful to understand how much material I’m retaining on the #8 screen individually, because it does seem to have an influence on my VMA.

So, there seems to be a pretty good correlation here between the #8 screen and VMA. We keep saying that the material is losing the amount of material retained on the #8 relative to the mixed design. So where is it going? If we kind of keep our eyes down the line here we'll see in the #30 screen, we see that same trend where we're not actually getting more negative, we're actually retaining more material and then it starts to go back down again, which is the same trend we saw in the VMA.

So how do we use that in production?

What we need to do is look at the product that provides #eight and hopefully there's a second product that provides #30. And if that's the case, what we can do then is increase the amount of material that is providing the #8 to get this number up, and we can decrease the amount of material that's providing #30, essentially making room for the material that we're going to lose.

So why does VMA matter to cost? 

It's pretty simple, and this example really demonstrates the point. This mix design was designed at a 15.0. It looks like the specifications allow them to go all the way down to 14. This customer put a 14.3 warning limit internally so that they get warned as they start to approach the low limit of the spec. But what's important to know is that they've actually designed a 5.7 but what they're producing is actually much closer to 6. Why is that? That's because they're trying to keep their air voids close to 4, and to keep their air voids close to 4, they're having to add additional binder to fill the VMA space itself. So here's a really good example of why a good solid control of VMA helps you keep cost low. 

LASTRADA users can download a copy of the template used in this video at the LASTRADA Partners Customer Resource Center. Non-LASTRADA users can download a free Excel file, which will perform the
same analysis from our website. 

If you found this video useful, like the video and share the video. Subscribe to our YouTube channel where you will find more videos in the Leveraging LASTRADA series, the Lightning demo series, and other videos from LASTRADA Partners. 

For more information on how LASTRADA helps asphalt producers, visit our asphalt solutions page.

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