HMA Module 03-06
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Transcript HMA Module 03-06
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.