To achieve expected pavement performance, it is important that asphalt concrete (AC) have adequate density. A critical factor to achieving this density is lift thickness to nominal maximum aggregate size (t/NMAS) ratio. Several other AC and environmental factors also influence this compactability, but lift thickness is critical because it interacts with both the mixture and environment. For example, thin lifts can cool down much faster than thicker lifts leaving less time available to compact the pavement. Lift thickness has a direct effect because compaction pressure from a roller is greatest at the top surface of the layer but decreases with depth. Since the exact change with depth is roller dependent (i.e. weight, drum diameter, drum width, vibratory amplitude, frequency, tire pressure, and operating speed) there is a lift thickness where insufficient energy exists at the bottom of the lift to achieve compaction. Conversely, if lift thickness is too thin, there is insufficient room for aggregate particles to re-orient and densify.
NCHRP Project 531 “Relationship of Air Voids, Lift Thickness, and Permeability in Hot-Mix Asphalt Pavements” recommended optimum t/NMAS ratios for both fine and coarse graded materials and others have made similar recommendations. However, agencies may specify t/NMAS less than these recommendations, for example when contractors request exceptions to meet smoothness requirements. Many agencies have also attempted to stretch limited funds by using thinner pavements. Thus, even though robust structural and material designs may exist, agencies can fail to deliver long-lasting pavements due to the simple fact that structural design, pavement design, construction phasing, and construction quality management are planned in different divisions within agencies.
The objective of this synthesis is to document agency policy for lift thickness and minimum compaction requirements on resultant asphalt pavement quality.
Information to be gathered will include, but not be limited to:
• Whether or not State DOT have formal policies or guidelines for lift thickness
• Mix types that DOTs use, including size, corresponding allowable lift thickness range and variations in this range depending on mix location within the asphalt pavement structure (base versus binder versus surface), density targets, and construction restrictions.
o Mix types including open graded friction coarse, stone matrix asphalt, and fine coarse dense grade.
o Effects of environmental factors (e.g. air temperature, mix temperature).
o Effects of the materials used including binder type, modifiers, compaction aids (or mix additives), and recycled materials.
• Compaction practices (e.g. roller types, vibratory restrictions).
• Tests for examining workability, density, and permeability of the mixes
• Reasons for state DOT policies and minimum criteria.
• Exceptions that may exist to established policy, guidelines, or contract requirements after design or during construction.
• Documented or anecdotal evidence of lift thickness as it relates to structural performance (e.g. rutting, cracking, smoothness).
• If State DOTs have data to correlate lift thickness to pavement performance, and if they are documenting their findings.
• Recorded service life, both with respect to structural performance and smoothness.
• Steps being incorporated to tie pavement quality with construction data and design.
• FHWA research modeling project by Eyad Masad and Tom Scarpus
• National Asphalt Pavement Association
The work will include a literature review and a survey of U.S. state DOTs and Canadian provinces (via the AASHTO Committee on Materials and Pavements) to determine their current mix type selection and pavement lift thickness standards and how they relate to structural design and construction specifications. The survey will include questions relative to what changes typically occur post project design which lead to changes in t/NMAS and how to prevent this from happening. The study will also document successful strategies through no less than three case examples, as well as all other factors that need to be considered when specifying materials and construction. Information gaps and research needs will be documented.
• Relationship of Air Voids, Lift Thickness, and Permeability in Hot-Mix Asphalt Pavements, NCHRP Report 531, 2004
• Evaluation of Hot Mix Asphalt (HMA) Lift Thickness, Technical Report Documentation Prepared for Mississippi Department of Transportation, L. Allen Cooley, Jr., Kevin L. Williams, P.E., October, 2009
• Determining Minimum Lift Thickness for Hot Mix Asphalt (HMA) Mixtures (With Discussion), Brown, E Ray; Hainin, M Rosli; Cooley Jr, L Allen, Journal of the Association of Asphalt Paving Technologists, Volume: 74, ISSN: 0270-2932
• Analysis of HMA Field Compactibility Using the Accumulated Compaction Pressure (ACP) Concept, Leiva, F. and R. C. West, TRB 87th Annual Meeting Compendium Papers, Paper #08-1222. Washington, D. C. 2008.
• HMA Pavement Mix Type Selection Guide, National Asphalt Pavement Association Information Series 128, Lanham, Maryland. 2001.
• CoolPave Software
Lyndi D. Blackburn, Alabama DOT
Ramamohan Bommavaram, California DOT
John Garrity, Minnesota DOT
Timothy Kelley, Maine DOT
Howard L. Moseley, Florida DOT
Roger C. Olson
B. Shane Underwood, North Carolina State University
Matthew Corrigan, Federal Highway Administration
Jack Youtcheff, Federal Highway Administration
Nelson H. Gibson, Transportation Research Board
First Panel: October 17, 2017, Washington, DC
Teleconference with Consultant: November 17, 2017, 3:00 p.m., ET
Second Panel: July 10, 2018, Woods Hole, MA