United States Department of Transportation
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Evaluation of design and construction issues of thin HMA overlays.
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2015-04-01
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Abstract:While the overall implementation of thin HMA overlays in Texas has been successful, some issues need to be addressed:
appropriate blending of SAC A and SAC B aggregate to ensure adequate skid resistance; best practices to achieve adequate bonding
(surface prep and tack coats); and correct quality assurance test methods to achieve adequate compaction. The purpose of this
research, therefore, was to address these concerns through laboratory and field testing. In addition, preliminary work to refine a crack
propagation model for thin overlays was performed.
Laboratory friction testing considered samples with two gradation types, four aggregates types, and five levels of aggregate
blending. Samples were polished with simulated traffic in the lab and tested with the dynamic friction tester. Results show the terminal
polish value for all designs with 100 percent SAC B replacement failed, as had designs with 50 and 75 percent blending of one SAC
B+ and one marginal SAC B aggregate. SAC B replacement up to 25 percent was acceptable for all aggregates.
Shear and tensile strength tests were developed to measure interlayer bond strength. A computer model suggested the maximum
shear stress at a bonded thin-overlay interface is 120 psi. Bond strength tests were performed on laboratory samples made with two
base mix types, two thin overlay types, 5 tack types (including non-tracking tacks), 3 tack rates, simulated milling, and moisture
conditioning. Bond strength was most dependent on the mix type being bonded and compaction effort, and less on tack type and tack
rate. In the tensile strength tests and half the shear tests, non-tracking tacks had higher strengths than samples using CSS-1H or no
tack. No single non-tracking tack was found to have better performance than others. Variable tack rates of CSS-1H were only
significant on dense-graded mixes. Low and moderate levels of tack provided the best bond. Milled samples had higher strength than
unmilled samples in shear. A tack tracking test was developed to discern different non-tracking times during curing.
Four compaction quality assurance test methods were used on three thin overlay projects. Properties measured were flow time with
the current TxDOT permeability test, surface dielectric with high-frequency ground penetrating radar, mean profile depth (MPD) with
the circular-track meter, and bulk density from field cores. Correlations of the tests were strong on a project-by-project basis, but
generally not good when combining the data sets. Flow Time-MPD, Flow Time-Core Voids, and Surface Dielectric-Core Voids were
best correlations overall.
TTI provided support to TxDOT on many new thin overlay demonstration projects, ranging from mix design, performance testing,
construction method recommendations, and bonding testing. Hand working of on TOM-B project caused problems with mat thickness
and compaction uniformity. Using tack did not influence bond strength except for one fine-permeable friction course in shear testing.
Thermal segregation problems were noted on two projects.
Recommendations are contained in the draft specifications, including aggregate blending guidelines, bond strength testing, micromilling,
and minimum and maximum flow times for compaction quality control.
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