TxDOT Addresses Long-Term Durability of Asphalt

Written by  John Obr, PE, Construction Division Director, Texas Department of Transportation

Recently, TXDOT's construction division has undertaken several initiatives and material-requirement changes. The division has been trying to address hot mix asphalt (HMA) performance and long-term durability by working to get more asphalt into mixes. This is no easy task. One way we can do this is to encourage a shift from the TxDOT standard dense-graded mixtures (think Item 340 and 341) to dense graded Superpave mixtures (Item 344). Because of the changes in gradation and design, on average the result is mixture designs that contain higher asphalt contents. Higher asphalt content typically means higher durability.

Also, there is a national interest in the use of Recycled Engine Oil Bottoms (REOBs) in asphalt binders. While asphalt binder is generally the leftover materials from distilling crude oil, REOBs are the leftover materials from redistilling engine oil for recycling. They are soft, asphalt-like materials. Their addition to asphalt can help the PG binder blend achieve better low temperature properties.

There have been reports of poor performance using asphalt binders that contain excessive REOBs. Premature cracking and loss of surface fines have been reported, especially in the northeastern and upper mid-west states. These are areas that need binders with more low temperature properties than what Texas specifies.

We have taken a look at our binder supply and have determined that there are REOBs in some binders. While we believe a little can be useful and not detrimental, a lot may cause performance problems. Thus, we are taking steps, by Special Provision, to limit the amount of REOBs in our asphalt to no more than 5 percent.

This same Special Provision also addresses the use of Poly Phosphoric Acid (PPA). PPA can work to increase the high temperature properties of a PG binder. We are going to limit PPA content to no more than 0.5 percent.

These are interim temporary steps being taken until we get a handle on the real problem with why performance problems with PPA and REOBs are not identified by the specification. The current thinking at the national level is that longterm aging needs additional attention in the binder specification. Until then, our Special Provision follows the guidance of “all things in moderation.”

A mid-May look at Texas drought maps shows no “Exceptional Drought” conditions in Texas. There are still pockets of “Extreme Drought” and some moderate and severe drought. The rains have been good, as we needed some relief.

As you all know, weather impacts TxDOT construction work. Much work just can’t be done in the rain and even when the rain stops, it takes time for things to dry out. Dry weather is good for progress in construction, but can have its impact too if water is in short supply.

In the past few months, we have gone from water in short supply to water in general abundance. Hopefully the future will bring us the Goldilocks conditions we would really like: not too little and not too much, but just right.

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