Superpave Specifications for Surface Treatments?

By: Darren Hazlett, Branch Manager
TxDOT Asphalt & Chemical

There have been some initiatives to develop Superpave-like specification requirements for surface treatment binders, both asphalt emulsions and hot applied asphalt cements.

The Superpave binder tests certainly can be used to evaluate soft asphalts and emulsion residues which are used for surface treatments, but directly using the Superpave specification criteria is questionable. Superpave was developed to address, rutting, fatigue cracking and thermal cracking in HMAC.

1. Rutting resistance is not meaningful for surface treatments, but we want a stiff enough binder to retain aggregate chips under traffic ( not be dislodged, roll or move).
2. Thermal cracking of HMAC is not meaningful for surface treatments, but we don’t want the binder becoming brittle in cold weather, fracturing under load and loosing the aggregate.
3. Certainly the HMAC plant aging simulated by the RTFOT is not applicable, but surface treatment binders do age in the long term so the PAV might be applicable.

Some of the Superpave tests can apply to surface treatment binders, but the specification limits may not be correct to address surface treatment distresses.

In addition to the above complications in using the Superpave binder specification for surface treatment binders, there are some other considerations. The temperatures seen by surface treatment binders are higher than Superpave grade selection temperatures for HMAC. Also, traditionally the binders used have been less stiff than binders used for HMAC under the Asphalt Cement grading system.

ASTM Subcommittee D04.41, "Emulsified Asphalt Specifications" has begun looking at using the some of the Superpave tests for emulsion residues. The issue first arose as an attempt to develop specifications for polymer modified emulsions. Darren Hazlett with the Texas Department of Transportation is the chairman of a task force set up to address this issue. The task was broadened to address testing emulsion residues with some of the Superpave binder tests for all emulsions and cover both modified and unmodified systems. A proposed specification has been sent to ASTM D04.41 for ballot. The proposed specifications require nomenclature to describe the emulsion and the residue. The residue properties are termed SG for Surface Grade and use associated high and low temperature designations similar to PG binders. Some of the above complications and considerations have been addressed in the proposed ASTM specification, by the choice of tests and values. The actual numbers in the proposed specification are a starting point in the specification development process. The ballot should generate a lot of interest and constructive comments.

The two tables numbered as "Table 1" present the requirements for the Cationic Emulsified Asphalt andEmulsified Asphalt. Both of these tables refer to "Table 2" for emulsion residue properties.