Thin Hot Mix Asphalt Overlays
PennDOT Research Findings
Pennsylvania Asphalt Pavement
Association
53rd Annual Asphalt Paving Conference
Hershey, PA
December 12, 2012
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Thin Overlay Development
Back GroundNEPPP: 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
2
Special Provision
6.3 mm – 100% passing 3/8 in.
Dense –graded ( 6 sieve sizes) - SRL
PG 76-22 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
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Use Guidelines
Only on structurally sound pavement
 Same as micro-surfacing
 For correcting surface distresses only
 Consider grinding PCC first

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Tack Coat Specification
SS – 1h or CSS – 1h
 RS – 1h or CRS – 1h
 60% min residue
 Low pen (40-90) asphalt residue
 0.05-0.06 gal/sy rate.

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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 76-226 binder (6.7%)

Mix Design (cont’d)

Volumetrics
- N design : 75 gyrations
- design voids: 4.0%
- VMA: 17.1%
- VFA: 77%
- TSR: 83% (without antistrip)
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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
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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

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Performance Evaluation - HWTD
Good Performance
Thin Asphalt Overlay Project
Hamburg Wheel Tracking Tests - 8/23/2012
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Rut Depth, mm
5
Right Track
4
Left Track
3
2
Temperature = 50ºC
1
0
0
2
4
6
8
10
12
14
Number of Wheel Passes (X1000)
10
16
18
20
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: 0.025 inches (0.6 mm)
Duration of each load cycle: 10 seconds
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Performance Evaluation – Overlay Tester
Good Performance
Cycles to failure > 500
High variability in data
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Tack Coat Evaluation
Notice Displacement of the Asphalt Layer
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Tack Coat Evaluation
Shear Strength = 44.5 psi (307 KPa) - Good Performance
THMAO Project
Tack Coat Evaluation - Core # 5
Interface Shear Stress, KPa
350
Deformation Rate: 1 mm/min
Test Temperature: 25.5 C
300
250
200
150
100
50
0
0
1
2
3
Interface Shear Deformation, mm
14
4
5
Emulsion Tack Coat Application
15
Smooth Mat Right Behind The Paver
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Mat Temperature
Mat Temperature, °F
THMAO Project - N. Cameron Street, WB, Passing Lane
Mat Temperature (Spot Measurment) - 7/23/12 - 7/24/12
330
310
290
270
250
230
210
190
170
150
130
Location 3
Location 2
Location 1
Pavecool Prediction
(Wind Speed = 5 mph)
0
5
10
15
20
Elapsed Time from Placement, minutes
Data is courtesy of Mr. Garry Hoffman
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25
30
Core Densities
18
Friction Improvement
Data: Courtesy of PennDOT BOMO
19
Ride Quality (Smoothness) Improvement
Data: Courtesy of PennDOT BOMO
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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: 280-315ºF
Rapid Mat Cooling
No Apparent Segregation
Coring Not A Problem
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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
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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.
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