Research Testing in 4 States Brings New Procedures & Offer Guidance for Using Bonded Whitetopping on Asphalt Pavements

Researchers developed procedures for selecting asphalt pavements for thin whitetopping based on site examination and lab testing. Test results do not offer definitive indications of future performance of overlaid asphalts (the next research step), but procedures offer recommendations on pre-overlay pavement treatment, testing protocols, and design considerations for bonded concrete overlay of asphalt. Tim Andersen, Pavement Design Engineer, Minnesota Department of Transportation (MnDOT), Office of Materials and Road Research said, “This research established a procedure for testing pavement cores. However, more performance data on whitetopping is needed to correlate pavement performance and asphalt properties.”

Dale Harrington, Principal Engineer, Snyder and Associates, Inc. said, “These procedures address collecting field data and testing pavement core samples in the lab. They also provide useful guidance for pavement repair and design considerations for overlays.”

What Was the Need?
Many counties throughout Minnesota, USA, have used bonded concrete overlays to rehabilitate asphalt pavement. A bonded concrete overlay, or whitetopping, normally involves milling a few inches of asphalt off the damaged surface and placing 4 to 6 inches of concrete over the asphalt pavement. A well-bonded overlay can add 20 years to a pavement’s service life.

Bonded whitetopping performance has not been carefully tracked, and correlation of its performance with the underlying pavement condition is not well understood. Before MnDOT can expand its use of bonded whitetopping, materials engineers needed a better understanding of:

• What asphalt pavement conditions are best suited to this type of overlay
• How asphalt behavior influences the concrete top layer
• What underlying pavement characteristics affect the expected lifetime and performance of bonded white-topping

Goal:
This project sought to:

• Develop an integrated selection procedure for analyzing existing distressed asphalt pavement
• Identify good candidates for bonded white topping
• Establish design considerations for site-specific effective concrete overlay

Investigators tested pavement core samples in the lab to identify asphalt pavement properties that correlate with distresses in concrete overlays that are 6 inches or less. They also sought specific recommendations for managing transverse cracking in asphalt to avoid reflective cracking into concrete overlays.

Execution:
Researchers began with a literature review of approaches for selecting pavements for bonded whitetopping to develop testing procedures in order to identify the volumetric properties of existing asphalt pavements. The researchers then applied these procedures to 22 pavement cores from six concrete overlay sites in Iowa, Michigan, Minnesota and Missouri. Selected projects entailed 4-inch to 6-inch overlays in fair to good condition that were built from 1994 through 2009. The research team:

• Compared roadway data—mix design, asphalt condition, pavement thickness, overlay thickness, site conditions, and other details—with falling weight deflectometer measurements from pavement cores to evaluate field performance and design recommendations suggested by the selection procedure.
• Evaluated volumetric asphalt characteristics for their potential influence on premature overlay cracking due to stripping, slab migration, and reflective cracking.
• Developed a detailed selection process that includes steps to identify and test asphalt pavements with potential for bonded whitetopping, repair asphalt before overlays, and establish design considerations for overlays based on the test results from the selected asphalt pavement.

Results
The selection procedure, which is based on recommended practices from the National Concrete Pavement Technology Center, has 6 steps:

• Perform a desk review of available site data, including design, repair and environmental conditions.
• Obtain pavement core samples.
• Conduct site visits to examine existing conditions.
• Obtain additional core samples for testing, when necessary.
• Prepare preliminary cost and materials estimates, if practical.
• Provide design recommendations

Investigators tested pavement cores for:

• Air voids
• Density
• Stiffness
• Fatigue
• Aging
• Stripping potential
• Additional distress parameters

The cores showed wide variation in material properties, but few of these distresses, although results were inconclusive in terms of identifying asphalt properties that lead to specific bonded concrete overlay failures or long-term performance of bonded white topping projects. Researchers framed the recommendations for testing volumetric properties in the format of MnDOT’s Pavement Design Manual, giving the agency an easily adoptable core testing protocol.

The selection procedures include information about the impact of transverse cracking, rutting, longitudinal cracking and other distresses on concrete overlays, and provide recommendations for treating various distresses before whitetopping. Design considerations for whitetopping are also provided based on site conditions and the results of core, ground penetrating radar and falling weight deflectometer testing.

Results of Additional Research to be Added to Manual:

The next step is evaluating tested overlay sections over time to determine if life expectancy is met or if asphalt stripping, slab migration, or reflective cracking has decreased overlay life. Also, further research is needed to identify mechanistic or field tests that could correlate asphalt properties with concrete overlay performance. Once this additional research is completed, the selection procedures identified could be refined and placed in the design guide. A life-cycle cost analysis of overlays would also be useful for decision-makers considering bonded concrete overlays of asphalt.

From Crossroads Minnesota Transportation Research Blog: https://mntransportationresearch.org/2017/10/02/new-procedures-offer-guidance-for-using-bonded-whitetopping-on-asphalt-pavements/

For the PDF of the MnDOT Thin Whitetopping Selection Procedures, Peter Taylor, Principal Investigator-National Concrete Pavement Technology Center Iowa State University Research Project Final Report 2017-24, please go to: http://www.dot.state.mn.us/research/reports/2017/201724.pdf