WHERE DOES THE ASPHALT GO?

The asphalt requirement for pavements can vary from less that 4% to over 7%

Why?

A paper by Ilan Ishai and Egons Tons (Proc. AAPT 42, 160 (1973)) provides the answer. They show that the role that asphalt plays is divided into three areas; Flow asphalt, Rugosity asphalt and Absorbed asphalt. Each of these parts are necessary and play a role in preparing a mix design that works well in the field.

Flow Asphalt. From that paper one can calculate that about 2.5% of the asphalt is needed to glue the rocks together. This is the asphalt that flows as pavements stress relax.

Rugosity Asphalt. This asphalt is that which fills in the irregularities in the aggregate surfaces or Macro-Surface Voids. This can be quite variable resulting in the VMA being a questionable mix design parameter. The film thickness calculation includes both the flow and rugosity asphalt.

Absorption Asphalt. Generally the asphalt is absorbed quickly into what are called the Micro-Surface Voids, but not always. It may take considerable time. I was working with one aggregate for which absorption took considerable time. The Marshall mix design system was in use at that time which has no oven conditioning prior to compaction. The result was that the mix was designed with part of the flow asphalt actually becoming absorbed asphalt after a time. As a result the pavement initially looked good but would end up looking dry and cracking in less than a year. It was found that by allowing the mix to condition for one hour at 135° C prior to compaction, 90% of the absorption would take place.

In the same location there was another aggregate that had virtually no asphalt absorption, and might require only about 3.8% asphalt in the mix. On one project the project engineer thought the porous aggregate was to be used however in fact, the non-porous aggregate was actually being used and required less than 4% asphalt. The engineer insisted that the mix have over 7% asphalt rather than the 3+% less which would have been correct for the aggregate being used. It turned out to be quite a mess.

On one other occasion in the same location at an airport, the specification required 4.5% asphalt but the mix design called for only about 3.8%. However the consulting engineer would not budge. The contractor said that if Airport wished, they would add some of the highly porous aggregate, which was inferior, to increase the absorption. The final decision was to go with the lower % asphalt in spite of the consultant’s opinion.

Robert L. Dunning, chemistdunning@gmail.com, www.petroelumsciences.com

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