1. Thin Hot Mix Asphalt Overlays PennDOT Research Findings
Pennsylvania Asphalt Pavement Association
53rd Annual Asphalt Paving Conference
Hershey, PA
December 12, 2012

2. Thin Overlay Development
Back Ground-
NEPPP: Smooth Seal in Ohio, THMAO in NY
PAPA Technical Sub Committee
Created draft specification
6.3mm PG 76-22, Polymer Modified, 75 Gyration mix
Research Project approved

3. Special Provision
6.3 mm – 100% passing 3/8 in. Dense –graded ( 6 sieve sizes) - SRL PG polymer modified N design = 75 gyrations Design voids = 4.0% Min. VMA = 16.5 No RAP or RAS Greater than 50 F Optimum Rolling Pattern

4. Use Guidelines Only on structurally sound pavement
Same as micro-surfacing
For correcting surface distresses only
Consider grinding PCC first

5. Tack Coat Specification
SS – 1h or CSS – 1h
RS – 1h or CRS – 1h
60% min residue
Low pen (40-90) asphalt residue
gal/sy rate.

6. Mix Design – Hempt Bros. 6 trial blends to get minimum VMA
Component materials
-baghouse fines ( 1.9%)
-Toland Conc sand Type A, SRL-E (39.4%)
-Toland ¼ in. SRL-E
- 4.0% passing #200
- 43% passing #8
- NuStar PG binder (6.7%)

7. Mix Design (cont’d) Volumetrics - N design : 75 gyrations
- design voids: 4.0%
- VMA: 17.1%
- VFA: 77%
- TSR: 83% (without antistrip)

8. Placed at 1 inch thickness
6.3 mm NMAS Mix
Placed at 1 inch thickness
Aggregate: Sandstone, Skid Resistance Level (SRL): E
Polymer Modified Binder: PG 76-22
Gyration Level: 75
Design Air Void: 4%, Design VMA: 17.1
Design Binder Content: 6.7%
NO RAP

9. Construction All night work 1/3 mile of four lanes (612 tons)
Including intersection and turn lane
1 in thick overlay on diamond ground PCC
CSS-1h tack coat
0.06 gal/sy tack application rate

10. Performance Evaluation - HWTD
Good Performance

11. Performance Evaluation – Overlay Tester
Test Temperature: 25ºC
# of load cycles: 1200
Or until load reduced to 93% of original
Repeated loading (triangular form) under constant deformation Deformation magnitude per load cycle: inches (0.6 mm)
Duration of each load cycle: 10 seconds

12. Performance Evaluation – Overlay Tester
Cycles to failure > 500
High variability in data
Good Performance

13. Tack Coat Evaluation
Notice Displacement of the Asphalt Layer

14. Tack Coat Evaluation
Shear Strength = 44.5 psi (307 KPa) - Good Performance

15. Emulsion Tack Coat Application

16. Smooth Mat Right Behind The Paver

17. Mat Temperature
Data is courtesy of Mr. Garry Hoffman

18. Core Densities

19. Friction Improvement
Data: Courtesy of PennDOT BOMO

20. Ride Quality (Smoothness) Improvement
Data: Courtesy of PennDOT BOMO

21. Summary/Findings Good Mix Lab Performance Permanent Deformation (SST)
Rutting and Moisture Resistance (HWTD)
Crack Resistance (Overly Test)
Good Tack Shear Resistance
Improved Ride and Friction
In-Place Average Density: 92.2% (based on 9 cores)
(Cores from Test Strip Only)
Could be better if compaction finished at higher temperature?
Mat Temperature After Placement: ºF
Rapid Mat Cooling
No Apparent Segregation
Coring Not A Problem

22. Conclusions Thin Overlays for Pavement Preservation
Improved Ride Quality
Improved Friction
Maintained Road Geometrics
Reduced Noise
Reduce Life Cycle Costs
Provide Long Lasting Service
Place before extensive rehab required according to use guidelines
Expected performance
10 years on asphalt
6 or more on PCC
Thin overlays for pavement preservation are popular due to a number of advantages that have been discussed throughout this presentation. Some of them include better smoothness, reduced surface distress, geometric considerations, noise reduction, lower life cycle cost, and long-lasting performance.
It is best to apply a thin asphalt overlay before extensive rehabilitation is required. This will maximize the performance and minimize the cost.
Thin asphalt overlays can be expected to perform 10 or more years on an existing asphalt pavement and from 6 to 10 years on an existing concrete pavement.

23. Thin HMA Overlays
Thin HMA overlays are an added solution to pavement preservation. They are economical, long-lasting, and effective in treating a wide variety of surface distresses to restore ride quality, skid resistance, and overall performance.