Design and Construction Recommendations for Thin Overlays in Texas

Thin hot mix asphalt (HMA) overlays, laid at 1.0 inch or thinner, are cost-effective surface maintenance options. The primary focus of this research was to develop specifications for three such mixes: fine dense-graded mix (fine DGM), fine-graded stone matrix asphalt (fine SMA), and fine-graded permeable friction course (fine PFC). A number of slurry overlay systems were also evaluated, but to a lesser extent. Draft specifications for the three mix types were first developed based on the results of a literature/information search and a field investigation of 11 existing projects. The specifications included minimum material quality levels, laboratory performance criteria, and construction recommendations. To evaluate the design recommendations, extensive laboratory testing was performed on each of the three thin overlay mixes with five different aggregates. Of the 15 mixes attempted, 12 had acceptable designs in terms of the specified performance tests. For the most part, the draft specifications appeared to function well with minor alterations recommended. Testing also included two supplementary studies on the effects of screening type in fine SMA and the effects of recycled materials on both the fine SMA and fine PFC. Compaction of the fine SMAs was highly influenced by packing characteristics of the coarse and fine aggregates. Screening quality did not affect fine SMA rutting resistance, but did affect cracking resistance. Using recycled aggregates reduced rutting problems but increased cracking susceptibility; however, most mixes performed well suggesting that quality, well-engineered mixes can still have good performance when recycled materials are used in limited amounts. Concerning laboratory testing of slurry overlays, the applicability of the overlay tester/procedures and the three-wheel polishing device in testing should be further studied. In particular, a tie-in with actual field performance should be identified, perhaps with the Accelerated Pavement Test program. Six thin overlay projects, comprising 10 unique mix designs, were constructed and evaluated. Most projects were constructed without problems, though some encountered issues with over- and under-compaction. Initial performance has been very good, although, since all the sections are less than two years old, the long-term performance is still undetermined. The researchers recommend adapting the specifications accompanying this report, which require using high quality materials and passing strict laboratory performance tests on both the lab design and trial batch materials. They do not recommend incorporating recycled materials in these mixes, though preliminary results are promising. Guidelines for pavement evaluation and mix selection were also prepared, which recommend the use of certain thin overlay or slurry overlay options given the pavement, traffic, and climate conditions.