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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
Rehab & Maint. HMA Overlays 4
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
Rehab & Maint. HMA Overlays 6
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
Rehab & Maint. HMA Overlays 7
Sources of
Moisture in Pavements
Capillary movement
from
Vapor movements Seepage from
high ground
From edge
Through permeable surface
Water table
Rehab & Maint. HMA Overlays 8
Moisture-Related Distresses (AC)
• Stripping
• Rutting
• Depressions
• Fatigue cracking
• Potholes
Rehab & Maint. HMA Overlays 9
Rehab & Maint. HMA Overlays 10
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
Rehab & Maint. HMA Overlays 11
Rehab & Maint. HMA Overlays 12
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
Rehab & Maint. HMA Overlays 13
Rehab & Maint. HMA Overlays 14
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
Rehab & Maint. HMA Overlays 15
Stresses from Low Temperatures
Thermal stress
Lower temperatures
Old pavement
Overlay
Joint or crack
Subgrade
Higher temperatures
Rehab & Maint. HMA Overlays 16
Stresses from Traffic Loads
Shearing
stress
AC bending stress
Tip of the joint or working crack
AC overlay
Old AC pavementVoid
Stress at the tip of the crack
A
B
C
Rehab & Maint. HMA Overlays 17
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)
Rehab & Maint. HMA Overlays 18
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
Rehab & Maint. HMA Overlays 19
Proper Placement of
Geotextile Fabric
Stress concentration
Old
pavement
Overlay
Vertical differential
movement
Fabric
Horizontal opening
Subgrade
Rehab & Maint. HMA Overlays 20
Rehab & Maint. HMA Overlays 21
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
Rehab & Maint. HMA Overlays 22
Stress-Absorbing Membrane
Layer (SAMI)
Membrane
Old pavement
Overlay
Vertical differential
movement
Horizontal opening
Stress
concentration
Subgrade
Rehab & Maint. HMA Overlays 23
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)
Rehab & Maint. HMA Overlays 24
Crack-Arresting Interlayer
Old pavement
Overlay
Subgrade
Crack arresting
layer 20-30% voids
Crack arresting
layer 20-30% voids
Rehab & Maint. HMA Overlays 25
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
Rehab & Maint. HMA Overlays 26
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
Rehab & Maint. HMA Overlays 27
Recycling the Existing Pavement
• HMA surface recycling may be done with
an overlay
• Full-depth HMA or PCC recycling requires
new pavement design
Rehab & Maint. HMA Overlays 28
Benefits of Recycling
• Conservation of aggregate
• Conservation of binder
• Conversation of energy
• Preservation of the environment
• Lower initial project costs
• Lower life cycle costs
Rehab & Maint. HMA Overlays 29
Pavement Widening
• Design lives should be equal
• Match cross-sections closely
• Place reflection crack relief fabric along
longitudinal joint
• Place longitudinal subdrains, if needed
Rehab & Maint. HMA Overlays 30
Rehab & Maint. HMA Overlays 31
Shoulders
• Depending on shoulder condition
– Patch
– Mill HMA
– Remove and replace
• Shoulder and traffic lane elevations
should match
Rehab & Maint. HMA Overlays 32
Photo of lane-shoulder drop-off
Rehab & Maint. HMA Overlays 33
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
Rehab & Maint. HMA Overlays 34
Pavement Thickness
Along a Project
Station Number
PavementT
Rehab & Maint. HMA Overlays 35
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

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Block 33b SP 13

  • 1. 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
  • 2. 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
  • 3. 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
  • 4. Rehab & Maint. HMA Overlays 4 Photo of old surface
  • 5. 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
  • 6. Rehab & Maint. HMA Overlays 6 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
  • 7. Rehab & Maint. HMA Overlays 7 Sources of Moisture in Pavements Capillary movement from Vapor movements Seepage from high ground From edge Through permeable surface Water table
  • 8. Rehab & Maint. HMA Overlays 8 Moisture-Related Distresses (AC) • Stripping • Rutting • Depressions • Fatigue cracking • Potholes
  • 9. Rehab & Maint. HMA Overlays 9
  • 10. Rehab & Maint. HMA Overlays 10 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
  • 11. Rehab & Maint. HMA Overlays 11
  • 12. Rehab & Maint. HMA Overlays 12 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
  • 13. Rehab & Maint. HMA Overlays 13
  • 14. Rehab & Maint. HMA Overlays 14 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
  • 15. Rehab & Maint. HMA Overlays 15 Stresses from Low Temperatures Thermal stress Lower temperatures Old pavement Overlay Joint or crack Subgrade Higher temperatures
  • 16. Rehab & Maint. HMA Overlays 16 Stresses from Traffic Loads Shearing stress AC bending stress Tip of the joint or working crack AC overlay Old AC pavementVoid Stress at the tip of the crack A B C
  • 17. Rehab & Maint. HMA Overlays 17 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)
  • 18. Rehab & Maint. HMA Overlays 18 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
  • 19. Rehab & Maint. HMA Overlays 19 Proper Placement of Geotextile Fabric Stress concentration Old pavement Overlay Vertical differential movement Fabric Horizontal opening Subgrade
  • 20. Rehab & Maint. HMA Overlays 20
  • 21. Rehab & Maint. HMA Overlays 21 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
  • 22. Rehab & Maint. HMA Overlays 22 Stress-Absorbing Membrane Layer (SAMI) Membrane Old pavement Overlay Vertical differential movement Horizontal opening Stress concentration Subgrade
  • 23. Rehab & Maint. HMA Overlays 23 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)
  • 24. Rehab & Maint. HMA Overlays 24 Crack-Arresting Interlayer Old pavement Overlay Subgrade Crack arresting layer 20-30% voids Crack arresting layer 20-30% voids
  • 25. Rehab & Maint. HMA Overlays 25 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
  • 26. Rehab & Maint. HMA Overlays 26 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
  • 27. Rehab & Maint. HMA Overlays 27 Recycling the Existing Pavement • HMA surface recycling may be done with an overlay • Full-depth HMA or PCC recycling requires new pavement design
  • 28. Rehab & Maint. HMA Overlays 28 Benefits of Recycling • Conservation of aggregate • Conservation of binder • Conversation of energy • Preservation of the environment • Lower initial project costs • Lower life cycle costs
  • 29. Rehab & Maint. HMA Overlays 29 Pavement Widening • Design lives should be equal • Match cross-sections closely • Place reflection crack relief fabric along longitudinal joint • Place longitudinal subdrains, if needed
  • 30. Rehab & Maint. HMA Overlays 30
  • 31. Rehab & Maint. HMA Overlays 31 Shoulders • Depending on shoulder condition – Patch – Mill HMA – Remove and replace • Shoulder and traffic lane elevations should match
  • 32. Rehab & Maint. HMA Overlays 32 Photo of lane-shoulder drop-off
  • 33. Rehab & Maint. HMA Overlays 33 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
  • 34. Rehab & Maint. HMA Overlays 34 Pavement Thickness Along a Project Station Number PavementT
  • 35. Rehab & Maint. HMA Overlays 35 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

Editor's Notes

  1. The most common type of preoverlay treatment is the use of patching.
  2. Surface leveling is generally required – it can be done by the use of a thin leveling course or through the use of a milling machine.
  3. Cold milling is also used extensively for the reasons listed in this slide.
  4. This type of surface will need to be milled to removed the unstable material.
  5. Cracks over ¼ inch in width should probably be sealed.
  6. In many overlay projects the subsurface drainage of the pavement is fixed by installing subsurface drains.
  7. The drains are used to reduce the pavement moisture which can reduce the strength of the underlying layers.
  8. This moisture can cause the problems listed in this slide.
  9. Moisture related distress.
  10. This picture shows the installation of a drainage pipe along side an existing pavement. Generally a trench is cut, the pipe often along with a fabric liner is placed in the trench in a drainable crushed stone material. The water flows out of the base layer into the drainage and then into the ditch.
  11. Reflection cracking is a major problem with HMA overlays. The purpose of the overlay is to smoothen the surface – but if the cracks reflect through the pavement the roughness can return soon after the overlay.
  12. Photo of transverse reflection cracking at joints. Generally if an overlay is placed on a cracked pavement (either AC or PCC) the cracks will reflect through the overlay. There have been a number of techniques proposed for the mitigation of the problem.
  13. The cause of reflection cracking is that low temperatures & traffic loads will cause stresses in the asphalt layer that exceed the strength of the mix.
  14. As the pavement cools the underlying layer will shrink at the crack induce cracks in the new overlay due to friction at the interface. Two approaches have been used to solve the problem – one to provide a slippery layer to break the bond and the other is to strengthen the overlay so that it can handle the stresses. The first technique is the most common approach.
  15. In addition to the thermal stresses – the weakest point is the point right above the old crack. Therefore the deflection is the highest at this point and the stresses are also the highest. To solve this problem – additional thickness of overlay is used and/or the joints/cracks are sealed.
  16. This slide shows a listing of the various techniques used to control reflection cracking.
  17. Photo of Petromat fabric being placed.
  18. Photo of inadequate lane widening.
  19. When the analysis is done for a project it may say that you need different thicknesses throughout the project length. But, this can create a nightmare for the construction process. Therefore, the project may be designed with a uniform thickness (thus most likely thicker than need be) or designed in large segments.
  20. This could be your thickness design.