Post on 27-Jun-2015
Rehab & Maint. HMA Overlays 1
Bituminous Patching
• Replace areas with poor support• Two types
– Cold mixes– Hot mixes (HMA)
• HMA provides higher quality and consistency• HMAC recommended for permanent
rehabilitation projects
Rehab & Maint. HMA Overlays 2
Surface Leveling
• Provides level surface for overlay– Milling the surface– Removing the unstable HMA layer– Leveling course
• Surface irregularities often reappear in the overlay if not corrected
Rehab & Maint. HMA Overlays 3
Cold Milling
• Correct surface rutting• Maintain curb lines (don’t mill curb lines
without considering crown)• Remove hardened material• Create clean, roughened surface for bonding• Remove material for recycling• Maintain vertical clearances
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Photo of old surface
Rehab & Maint. HMA Overlays 5
Crack Sealing
• Reduce amount of moisture infiltration• Prevent intrusion of incompressibles• Generally not effective on pavements
exhibiting severe structural deterioration
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Subsurface Drainage
• Positive subsurface drainage is beneficial to overlay performance– Reduces erosion– Increases strength of base and subgrade– Reduces deflections– Slows stripping
• Subsurface drainage evaluation
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Sources of Moisture in Pavements
Capillary movement from
Vapor movements Seepage fromhigh ground
From edge
Through permeable surface
Water table
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Moisture-Related Distresses (AC)
• Stripping• Rutting• Depressions• Fatigue cracking• Potholes
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Reducing Moisture Damage
• Approaches– Keep the water out– Desensitize the pavement– Drain the pavement
• For rehabilitation projects, try to shorten drainage path (e.g., through longitudinal drains)
• Improve surface drainage
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Reflection Cracking
• Appear above joints or cracks in underlying pavement layer
• AASHTO design equations do not consider directly
• Additional steps must be taken to reduce the rate and severity
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Mechanisms of Reflection Cracking
• Causes– Low temperature cycles– Traffic loads
• Excessive tensile stresses developed in overlay due to movement of existing pavement
• Initiates at bottom of overlay
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Stresses from Low Temperatures
Thermal stress
Lower temperatures
Old pavement
Overlay
Joint or crack
Subgrade
Higher temperatures
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Stresses from Traffic Loads
Shearing stress
AC bending stress
Tip of the joint or working crackAC overlay
Old AC pavementVoid
Stress at the tip of the crackA
B
C
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Reflection Cracking Control Measures
• Fabrics• Stress-relieving interlayers• Crack-arresting interlayers• Preoverlay treatments• Increased overlay thickness (will not
eliminate reflective cracking, but may slow crack propagation)
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Fabrics
• Provide physical restraint (reinforcing layer) to resist formation of cracks
• Not as effective with substantial horizontal and vertical movements
• Most effective at longitudinal joints and in warm climates
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Proper Placement of Geotextile Fabric
Stress concentration
Old pavement
Overlay
Vertical differential movement
Fabric
Horizontal opening
Subgrade
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Stress-Relieving Interlayers
• Dissipates movements and stresses at joints and cracks
• Spray application of rubber- or polymer-modified asphalt binder and aggregate chips
• Most effective on AC pavements• Proprietary stress-relieving interlayers
– 1 or 2 fabrics and a modified asphalt layer– “Band-aid” treatments
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Stress-Absorbing Membrane Layer (SAMI)
Membrane
Old pavement
Overlay
Vertical differentialmovement
Horizontal opening
Stress concentration
Subgrade
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Crack-Arresting Interlayers
• Granular layers with little fines and large aggregates (75 mm [3 in] maximum size)
• Provide large void spaces that effectively blunt crack propagation
• Effective when properly constructed• Minimum thickness = 90 mm (3.5 in)
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Crack-Arresting Interlayer
Old pavement
Overlay
Subgrade
Crack arrestinglayer 20-30% voids
Crack arrestinglayer 20-30% voids
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Preoverlay Repair Treatments
• Any method that reduces movement at joints and cracks can potentially reduce reflection cracking
• Possible treatments– Heater-scarification– Slab stabilization– Full-depth PCC repair with dowels
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Increased Overlay Thickness
• Does not prevent the occurrence of reflection cracking
• Reduces the rate and severity of reflection cracking
• Cost-effectiveness must be considered relative to other techniques
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Recycling the Existing Pavement
• HMA surface recycling may be done with an overlay
• Full-depth HMA or PCC recycling requires new pavement design
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Benefits of Recycling
• Conservation of aggregate• Conservation of binder• Conversation of energy• Preservation of the environment• Lower initial project costs• Lower life cycle costs
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Pavement Widening
• Design lives should be equal• Match cross-sections closely• Place reflection crack relief fabric along
longitudinal joint• Place longitudinal subdrains, if needed
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Shoulders
• Depending on shoulder condition– Patch– Mill HMA– Remove and replace
• Shoulder and traffic lane elevations should match
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Photo of lane-shoulder drop-off
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Design of Overlay Along Project
• Uniform section approach– Use average inputs of section– Single overlay thickness
• Point-by-point approach– Overlay thickness at each point– Use mean thickness
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Pavement Thickness Along a Project
Station Number
Pavem
ent T
hick
ness
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Summary
• Examine the feasibility of an overlay as most effective alternative
• There is more to overlay design than just thickness design– Preoverlay repairs– Subdrainage– Reflection crack control