Texas Considers Ultra-Thin HMA Alternatives to Seal Coats

Pavement Preservation Journal, Spring 2015, Vol. 8, No. 1

By Yetkin Yildirim, P.E.

As a result of numerous complaints about noise from seal coats and flying stone in recent years, TxDOT has requested the development of new asphalt mixtures that can be built in thinner lifts as an alternative to seal coats, and can be used to pave thinner overlays with overall reduced life-cycle cost.

These thinner overlays must be built at or less than 0.5 in. thick to realize the economic benefits without compromising their ability to restore the pavement surface.

The design and testing of some of the thin overlays is mostly done using currently available equipment and procedures e.g. Indirect Tensile Strength (ITS), Hamburg Wheel Tracking Device (HWTD), and Overlay Tester (OT).  These tests were originally developed for mixes paved with a thickness of at least 2 to 4 in.

The state of stress that a 4-in. overlay is subjected to under the action of highway traffic is significantly different from the state of stress in a 0.5-in. overlay, particularly shear stresses under the action of braking and acceleration forces. For this reason, it is necessary to develop mix designs for such mixes using evaluative tools and testing procedures that are appropriate for ultra-thin overlays.

The Texas Pavement Preservation Center at The University of Texas at Austin – together with the University of Texas at El Paso – have put together a team to work on cost effective alternatives to seal coats.

The first step of this project is to develop a number of potential mix designs for ultra-thin overlays. This process will include developing mixes by varying factors such as aggregate gradation, aggregate class, binder grade, and binder content. The project will evaluate the most popular materials used for thin overlays in Texas alongside newly developed, polymer-based materials in order to determine how current mix design methods should be modified to provide optimal field performance for ultra-thin overlays.

The second step will be to identify the most suitable set of tests to evaluate the performance of the selected asphalt mixtures intended to be used in ultra-thin overlays.  Finally, the study will produce guidelines, curriculum for personnel, and specifications for the design and testing of these mixtures. The second but equally important objective of the research includes the development of surface preparation guidelines for ultra-thin overlays to address underseals, micro-milling applications, and the bond strength of these pavements.

Based on the findings of this research project, the researchers will provide a methodology to design ultra-thin HMA along with necessary material requirement specifications, and construction guidelines including the use of underseals and tack coats.

Deliverables will also include a test method to verify the bond strength of ultra-thin overlays with existing pavement surface and laboratory test methods to evaluate the performance of ultra-thin layer mixes. However, it is likely that these tests may not be needed on a routine basis for the design and use of the ultra-thin HMA mixes but only as a contingency to evaluate new materials or reduce risks for higher-value projects.

The skid resistance of ultra-thin overlays also is addressed as a part of this study. Preliminary implementation will begin as a part of this study in the form of a program to monitor the skid resistance on three occasions over a one-year period.

However, this period may not be sufficient to capture the long-term evolution of skid resistance for these overlays and make adjustments to the design procedure and specifications as needed.

In order to address this need, researchers will provide TxDOT with a detailed implementation plan to track the performance of the ultra-thin surface mixes. This will include a plan to systematically document the materials (e.g. aggregate class, aggregate source, and binder grade), mix designs (e.g. JMF), and results from laboratory tests (if any) for mixes used in different construction sites over a period of several years.

The plan will also include details on how TxDOT can collect and use field performance data for ultra-thin HMA mixes (e.g. skid resistance) in order to refine the mix design procedures and requirements.

The main goal of TxDOT’s research program is applied research that can be implemented to address the needs identified by districts and divisions that benefit the state.  The University of Texas at Austin and El Paso recognize this important fact, and consequently the products and reports from this study are aimed at helping TxDOT make informed decisions for the design and construction of ultra-thin asphalt layers.

Yildirim is director, Texas Pavement Preservation Center at the University of Texas-Austin

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