Session 3-6
HMA Overlays
Learning Objectives
 Describe the characteristics of typical
Hot-Mix Asphalt (HMA) overlays
 Identify best applications
 Describe preoverlay repair need and
feasibility
 Describe thickness design approach
 Describe key construction issues
Introduction
 Most popular method
 Relatively fast and cost-effective means
for:
 Correcting
 Restoring
 Adding
deficiencies
user satisfaction
structural capacity
 Poor performance is NOT uncommon
Definitions
 Functional performance - Ability to
provide a safe, smooth riding surface
 Structural performance - Ability to carry
traffic without distress
 Empirical - Design based on past
experience or observation
 Mechanistic - Design based upon
engineering mechanics
Purpose and Applications
 Improve functional and/or structural
characteristics
 Wide range of applications

Road surface categories

Climate and support conditions
Characteristics of Typical
HMA Overlay
 Dense graded HMA
 Flexible or rigid surface
 25 to 200 mm (1 to 8 in) thickness
 Mill and Fill
Best Applications
What is the best
application for HMA
overlays?
Limitations and
Effectiveness
Why do we have premature failures?
 Improper selection
 Wrong type
 Inadequate design
 Insufficient preoverlay repair
 Lack of consideration of reflection
cracking
Limitations and
Effectiveness
What limits the effectiveness of HMA
overlays?
 Distress exhibited in HMA
 Intended design life of the overlay
 Availability of quality materials
Limitations and
Effectiveness
How can we improve our overlays?
 Preoverlay treatments
 Better materials and practices
 Sound engineering judgment
Overlay Selection to
Correct Deficiencies
Thin Overlay
Thick Overlay
Surface Defects
Structural Defects
What Are Considerations in
Overlay Selection?
 Construction feasibility
 Traffic
control
 Constructibility
 Vertical
clearances
 Utilities
 Performance period
 Funding
Preoverlay Treatment and
Repair
 Dependent upon:
 Type of overlay
 Structural adequacy of existing
pavement
 Existing types of distress
 Future traffic
 Physical constraints
 Cost
To Repair or Not to Repair?
Types of Preoverlay
Treatments
 Localized repair (patching)
 Surface leveling
 Controlling reflection cracking
 Drainage improvements
Localized Repair
Repair
cost
Overlay
cost
% Area repaired
Localized Repair
Total
Cost
Minimum Total Cost
Optimum
% of Area Repaired
Surface Leveling
 Purpose
 Rut
filling
 Restore
cross slope
 Improve
longitudinal profile
 Method
 Cold
milling
 Leveling
course
Controlling Reflection
Cracking
What do you use to control reflective
cracking?
 Geotextiles or fabrics
 Stress relieving or stress absorbing
membrane interlayers
 “Band aid” type crack sealants
Reflective Crack Treatments
 Geotextile: varied results on its
effectiveness
 SAMI: varied results also
 Aggregate Interlayers: effective when
designed correctly
 Route and seal: does not prevent but
effective at limiting deterioration
Drainage Corrections
 Drainage survey
 Identify moisture / drainage related
distresses
 Develop solutions that address moisture
problems
Two Aspects of
Overlay Design
 Asphalt mixture
 Fatigue
cracking
 Permanent
deformation
 Thermal
cracking
 Moisture
susceptibility
 Overlay Thickness
 Engineering
judgment
 Deflection
approach
 Structural
deficiency
 Mechanistic
approach
Mix Design
Superpave
 Goal – integrate mixture/structural design
 Performance Graded (PG) asphalt
cements
 Mix design
 Materials
 Aggregate structure
 Binder content
 Moisture susceptibility
Structural Design
Deflection Approach
Deflection,
(mm)
2.0
Original Surface
Deflection
1.6
THov
Simulated 80-kN
Axle Load
HMA Overlay
75 mm HMA Layer
1.2
Limiting
Deflection
0.8
150 mm Granular Layer
Natural Soil
0.4
0.0
0
50
100
150
200
250
Overlay Thickness, THov (mm)
Structural Design Structural
Deficiency
Overlay
PSI
2.5
1.5
SC
SCf
SCoL
SC
eff
Traffic
Structural Design
AASHTO Approach
 SCOL = SCf – SCeff
 Problem: Determining SCeff

Distress survey

Remaining life

Deflection testing
Structural Design
Mechanistic-Empirical
Thickness Young’s
Modulus
TH1
E1
TH2
E2
TH3
E3
TH4
E4
Simulated
Design Axle Load
ov
HMA Overlay
HMA
Original
HMA Surface
Layer
Base
σV
Subbase
Natural Soil
Structural Design
Mechanistic-Empirical
Allowable
Traffic
Critical Stress or Strain
Allowable Axle Load Applications
(80 - kN ESALs, millions)
Structural Design
Mechanistic-Empirical
4
3
THov
THhma
THbg
Simulated 80 - kN
Axle Load
ov HMA overlay
Original HMA layer (fatigued)
Base
Natural soil

2
Design 80-kN ESALS, W 80
1
Design HMA
Overlay
Thickness
25 50 75 100 125 150 175
HMA Overlay Thickness, THov (mm)
Design Overlay Thickness
Structural Requirement
Varies Along Roadway
2
1
3
Distance Along Roadway
Key Construction Issues
Lift Thickness
 Old – 2X maximum aggregate size
 New – 3X nominal maximum aggregate
size
 Lift thickness versus mix design
Compaction
Newer mixes more difficult to achieve density
 Consequences

Rutting

Raveling

Oxidation

Stripping
 Keys
 Proper
equipment
 Roller
immediately
behind paver
Ride Quality
 Public’s No. 1 issue
 Ride specification
 Method of measurement
 Incentive/disincentive
 Increased pavement
performance
 Key – continuous, steady
operation
Review
 What are the characteristics of an HMA
overlay?
 Where are HMA overlays applicable?
 What types of pre-overlay repair should
be considered?
 Name three structural design
approaches?
 What are some of the key construction
issues?
Key References
 Daleiden, J. F., A. Simpson, and J. B. Rahut.
1998. Rehabilitation Performance Trends:
Early Observation from Long-Term Pavement
Performance (LTPP) Specific Pavement
Studies (SPS). FHWA-RD-97-099. Federal
Highway Administration, Washington, DC.
 Brown, E. R. 1997. Superpave Construction
Guidelines. Special Report 180. National
Asphalt Pavement Association, Lanham, MD.
Key References (continued)
 Huber, G. A. 1999. Methods to Achieve RutResistant Durable Pavements. Synthesis of
Highway Practice 274. Transportation
Research Board, Washington, DC.
 Collura, J., T. El-Korchi, K. Black, M. Chase,
and L. Jin. 1997. Guidelines for Ride Quality
Acceptance of Pavements Final Report. New
England Transportation Consortium, University
of Connecticut, Storrs, CT.
Key References (continued)
 American Association of State Highway and
Transportation Officials (AASHTO). 1993a.
AASHTO Guide for Design of Pavement
Structures. American Association of State
Highway and Transportation Officials,
Washington, DC.