Upgrade Your Banking Experience with Advanced Core Banking Applications
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
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
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
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
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
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)
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
31. Rehab & Maint. HMA Overlays 31
Shoulders
• Depending on shoulder condition
– Patch
– Mill HMA
– Remove and replace
• Shoulder and traffic lane elevations
should match
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
The most common type of preoverlay treatment is the use of patching.
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.
Cold milling is also used extensively for the reasons listed in this slide.
This type of surface will need to be milled to removed the unstable material.
Cracks over ¼ inch in width should probably be sealed.
In many overlay projects the subsurface drainage of the pavement is fixed by installing subsurface drains.
The drains are used to reduce the pavement moisture which can reduce the strength of the underlying layers.
This moisture can cause the problems listed in this slide.
Moisture related distress.
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.
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.
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.
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.
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.
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.
This slide shows a listing of the various techniques used to control reflection cracking.
Photo of Petromat fabric being placed.
Photo of inadequate lane widening.
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.