The Status of Superpave Binder Implementation

By Darren Hazlett and Gerald Peterson

TxDOT is implementing the Superpave binder specification on two different levels. These are "Sample and Approve at the Source" and a QC/QA Field Acceptance system.

Sample and Approve at the Source

"Sample and Approve at the Source" is the system that TxDOT has used for at least the last 30 years. In this system, materials are sampled at the source, tested at MAT and approved for state-wide use on any TxDOT project. Contractors are free to acquire asphalt binders from any source as long as the "source" has pre-approved material. Use of this system means that field samples are generally not taken because the materials have been already approved. Field samples may be taken any time at the discretion of the Engineer. Sample and Approve at the Source has some drawbacks. The system is generally an honor system because no one can be present at the source 24 hours a day. Additionally, the system can not detect problems which occur after approval and shipment. Some of these problems are: contamination in shipment, contamination in storage at distribution centers or at project sites, mistreatment of binder by overheating, and borderline materials changing with age and going out of specification.

"Sample and Approve at the Source" is transparent to contractors, since the materials they purchase have been pre-approved.

As of the July 1997 letting, all projects specify a PG binder instead of an AC binder for hot mixed bituminous products. The Sample and Approve at the Source system will be used for PG binders until a better system is developed and implemented. This system was used for implementation in July because this system was already in place and could result in a quick implementation of Superpave.

QC/QA Field Acceptance

TxDOT is conducting field trials of a QC/QA field acceptance program. This system makes detection of all the problems described above possible. It places the responsibility for securing a quality binder on the contractor. It requires partial Superpave testing of field samples by the district lab and complete Superpave testing of field samples by MAT. The specification has been used on several projects and will be rewritten, probably with significant changes, for use next construction season in additional trial projects. In trial projects we will be generating data to validate the sampling and testing rates as well as to check the agreement between contractor and TxDOT tests.

The development of this program is more related to getting what TxDOT is paying for rather than specifically to implement Superpave. Since we are implementing a new binder specification, it would also be a good time to change the way we do business.

In a Superpave Strategic Plan Roll Out Time Line we developed last November, we showed that QC/QA Field Acceptance would be put into place across the board by February 1998. This is obviously not going to happen on that schedule. We have tried the first version of a system, we will be making modifications and will need to conduct field trials using that revision. In order to replace the "sample and approve at the source" program with a field acceptance system, we are prepared to conduct field trials and make changes until we gain the necessary confidence that a new system will ensure that product specifications are met. System confidence, not implementation dates, will drive the system.

In addition to fine tuning the specification, there are some other things which need to occur prior to implementation. These are:

  1. Set up a certification program for binder testing at the THMAPA Center,
  2. Clearly define the binder training which is a prerequisite to certification,
  3. Define the relationship of binder penalties or bonuses with respect to the HMAC specification,
  4. Work on reducing the variability of the Superpave binder tests, and
  5. Conduct additional training in understanding the specification requirements, binder selection criteria, contractor responsibilities and materials handling concerns.

What is Superpave Doing for TxDOT ?

Every asphalt that TxDOT was using under the AC specifications will meet a Superpave Performance Graded binder. The only problem is that the supplier which historically served a geographic area may not currently supply, as an AC grade, the binder grade the district would now specify as a performance grade. There is good news, however. Some suppliers have already made changes in their refining or blending process to result in better binders.

As an example, in the past one asphalt supplier has provided AC-20 which at times would meet one of four different performance grades: PG 58-16, PG 58-22, PG 64-16 and PG 64-22. Of these materials, the PG 64-22 is the best binder and would be the predominant grade required by TxDOT state-wide. The supplier in question has made changes to consistently supply PG 64-22. This is a good thing!

There have been comments that Superpave has taken away the District's ability to specify products they have had success with, like AC-45P. Superpave has shown us that all binders are not created equal. The AC-45P's supplied to TxDOT projects have ranged from PG 64-22 to PG 70-28 depending on the supplier. Additionally, AC-20 w/ 3% Latex supplied to TxDOT has ranged from PG 64-28, PG 64-22, PG 70-22 and PG 76-22 depending on the supplier. This same difference between producers can be seen in virtually every grade of AC binder. Now, we can pinpoint the requirements in the PG binder we want.

There will be people who will say that the Superpave system is "no good" because it might not address some binder characteristics adequately. The AC specification only addressed road temperature properties and aging through the hot mix plant. The Superpave specification addresses road temperature properties, aging through the hot mix plant, long term aging, and low temperature properties. Superpave addresses some properties that have not ever been addressed in the AC specifications.

Other people will say that Superpave will allow them to put all manner of "modifiers" in the binder which will fool the system, but be detrimental. The reason we have chosen to implement Superpave with no additional requirements, like polymer type or content or additional non-Superpave tests targeting specific polymers, is that we want to address possible specification problems on a national basis and fix the system. If we see bad performance, we may add those non-Superpave requirements or change the Superpave requirements. If it isn't broke, we don't want to fix it.

Summary

We are looking at changes in the materials specification and possible changes in the way TxDOT accepts binders. The specification changes have resulted in binder suppliers making modifications to their production processes to generate "better" binders. The specification addresses properties which the old specifications did not even look at. We will monitor implementation for problems and try to address them on a national basis to "get it right" for everyone, not just TxDOT.

If you have comments, good or bad experiences, or questions, feel free to address them to Darren Hazlett (512-232-1902) or email DHAZLET@mailgw.dot.state.tx.us, or contact Gerald Peterson (512-232-1913) by phone or email gpeters@mailgw.dot.state.tx.us.

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