Friday, May 13, 2011

Asphalt Binder

Over the last few years I've heard a lot of differing opinions about asphalt binder.  I say opinions because I don't think that most of these people are basing their information on a scientific study or experiences that they've had themselves.  The asphalt binder information floating around out there seems to be a lot of hearsay and myths.  Hopefully I can help set some of those myths to rest.

As background information I should probably tell you that there are several different types of paving grade (PG) asphalt binders.  In 2006, Caltrans switched to the PG grading system from the Aged Residue (AR) grading system.  If you see a grading such as AR-4000, this is the old system of asphalt gradings.  The standard asphalt binders used in dense grade asphalts in California today are known by the following names: PG 64-10, PG 64-16 &  PG 70-10.  The numbers in these names correspond to temperatures in degrees Celsius.  The numbers are a range at which the asphalt will maintain its' quality when laid down in pavement.  For example, PG 64-10 should be used in regions that the ambient temperature will be between 64 C and -10 C. 

To obtain different performances of asphalt binder, asphalt binder is made with different gradations of particles.  Yes, the gradations are similar to those that we use to define aggregates.  Asphalt binder is made up of varying sizes of particles and can be processed to achieve a specified gradation just as aggregates can.  For each of the Caltrans PG grades there are different specifications for the binder's gradation.    This brings me to myth #1.

Myth 1: PG 64-10 and PG 64-16 are the same thing and are interchangeable

The PG 64-10 and PG 64-16 have gradation specifications that overlap each other, just as the old section 39, 1/2" Maximum Medium and new section 39, 1/2" HMA specifications overlap each other.  However, this does not mean that in every case both PG 64-10 and PG 64-16 are the same oil.  This myth stems from the fact that several asphalt refineries produce a product that meets both specifications.  Not all refineries do this so be sure that if you are planning on using them interchangeably that your refinery does not make them as two different products.

Myth 2: PG 64-10 is the same no matter what producer you buy it from

Refineries have a specification range that they can produce their asphalt binder within so chances are that each refinery has slightly different gradations of the same product.  Slight changes in gradation might not bother you but there are other inherent properties that differ between refineries as well.  Refineries do not all make their products from the same crude oil source.  Similar to how aggregates vary in various rock properties depending on the deposit, crude oil can vary as well.  Even though all of the refineries may be producing within the same specification, their asphalt binders can give you varying performance results; some may last longer, others may be stronger, while still others may perform better in extreme temperatures. 

One thing to consider when you are choosing an asphalt binder supplier is how "clean" their product is.  Have you ever seen the Shell gasoline commercials where they show what an engine looks like with their product and what it looks like when using another brand?  In the commercial the Shell engine is relatively clean but the other company's engine has a build up of material or "engine gunk".  The same thing can happen with asphalt binders.  Over time your asphalt binder can deposit what the industry calls coke, in your asphalt tanks and lines.  This can slow down your disbursement from the tanks as well as reduce your ability to heat the asphalt efficiently.  Even though two different binders may meet the same specification, there are also other factors that might save you money as well.

Myth 3: You can use different producers' binders in the same mix design with no changes

Once again I'd like to point out that there is a specification range, not a specific target, that asphalt binder is produced around.  From personal experience I can tell you that the same grade of asphalt binder from different producers can give you varying asphalt content requirements in your mix designs.  These differences can be up to a 0.3% difference in oil content which is a considerable cost savings for your asphalt plant.  Oil contents in asphalt mix designs are determined by a variety of quality results but generally it is the percentage of asphalt binder the mixture needs in order to achieve 4% air voids in the mixture.  If one brand of binder takes up more volume than another then you will need less of that binder to achieve the same results. 

If you have the time and the money, take a look at your mix designs and do a couple of verification points with different producers' asphalt binders.  You might find that although one brand has the cheapest price, it is not the one that will save you the most money overall if you factor in reduced oil content in your mixes and reduced maintenance fees to your plant.   

I hope that I've helped to clear up a couple questions in your minds about asphalt binders.  What other myths are you curious about?  Do you have any myths that you can de-bunk for us?

1 comment:

  1. Very interesting to read about some of these myths. It's hard to believe that these do exist in our industry. But you're right, in the process of the Performance Grade (PG) classification, there is a chance of certain properties overlapping which allows the manufacturer to classify the binder in such a way that it could be used for two different low temperatures (either -10 and -16 or -16 and -22). For Myth 2 and 3, one must not forget the temperature-viscosity curve that might end up influencing both the mix design and the field compaction.