## Thursday, January 24, 2013

### Maximum Aggregate Size & Nominal Maximum Aggregate Size

If you're like me you have probably been confused about what "Maximum Aggregate Size" and "Nominal Maximum Aggregate Size" mean at least once or twice in your career.  The main reason for this is is that there are several different definitions.

In the August 2003 version of CT 382, nominal maximum aggregate size is defined as one sieve size smaller than the maximum aggregate size.  The maximum aggregate size is defined as the smallest sieve size that requires 100% passing.  For example, in the following gradation:

Percent Passing
3/4" : 100%
1/2" : 95%
3/8" : 89%
#4 : 63%
#8 : 39%

By the CT 382 definition, the maximum aggregate size is 3/4" and the nominal maximum aggregate size is 1/2".

In the November 2011 version of CT 202, nominal maximum aggregate size is defined as "one sieve size larger than the first size to retain more than 10%."  By this definition the nominal maximum aggregate size for the material example above is also 1/2".  However, if there were only 91% passing the 3/8" the nominal maximum aggregate size for that material would be 3/8", not 1/2".

Neither of these are the same definition as outlined in the Asphalt Institute's MS-2 guide for Mix Design Methods.  In the MS-2 definition the maximum particle size is defined as "two sizes larger than the first sieve to retain more than 10% of the material".  Although this definition uses the word "particle" instead of the word "aggregate" it can still be easily confused.  In the gradation example above, the nominal maximum particle size is 3/4" because the 3/8" is the first sieve to retain more than 10% of the material (11%) under the MS-2 definition.

In yet another definition, the Asphalt Institute's SP-2 guide for Superpave Mix Design defines maximum size as "one sieve size larger than the nominal maximum size".  The nominal maximum size is defined as "one sieve size larger than the first sieve to retain more than 10%".  By these definitions the material example above would have a maximum size of  3/4" and a nominal maximum size of 1/2".

So why does this matter?  What are these definitions even used for?  For the most part, in HMA, its just semantics and the definition will not affect your test results, just give you a whole lot of confusion in talking about it with other people.  For instance, nominal maximum aggregate size is usually a way to define the size of a mix when you're referring to it.  So when you're saying 1/2" HMA people generally know what that means.  Where you might see a problem is in drawing the maximum density line to help with designing a mix's gradation.  The maximum density gradation line represents the tightest arrangement that the mix's particles can fit together.  According to the Asphalt Institute's MS-2 this line is created by drawing a straight line from the origin to the desired maximum particle size which is defined for you.  However, if you use the wrong definition you could be setting yourself up for some major confusion and maybe even some unnecessary mix designs.

 Maximum Density Line for 25mm Maximum Particle Size
When designing your mix you will use the maximum density gradation line as a guide for increasing or decreasing VMA.  Moving the mix gradation away from the maximum density gradation line will increase your VMA and can reduce flushing.  However, if you're using the wrong maximum density line you may very well be making things worse.

What stories do you have where the definition of maximum aggregate size and/or nominal maximum aggregate size has made a difference on the quality of your mix?