Block 33b SP 13

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

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

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

Photo of old surface

Crack Sealing

• Reduce amount of moisture infiltration• Prevent intrusion of incompressibles• Generally not effective on pavements

exhibiting severe structural deterioration

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

Sources of Moisture in Pavements

Capillary movement from

Vapor movements Seepage fromhigh ground

From edge

Through permeable surface

Water table

Moisture-Related Distresses (AC)

• Stripping• Rutting• Depressions• Fatigue cracking• Potholes

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

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

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

Stresses from Low Temperatures

Thermal stress

Lower temperatures

Old pavement

Overlay

Joint or crack

Subgrade

Higher temperatures

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

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)

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

Proper Placement of Geotextile Fabric

Stress concentration

Old pavement

Overlay

Vertical differential movement

Fabric

Horizontal opening

Subgrade

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

Stress-Absorbing Membrane Layer (SAMI)

Membrane

Old pavement

Overlay

Vertical differentialmovement

Horizontal opening

Stress concentration

Subgrade

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)

Crack-Arresting Interlayer

Old pavement

Overlay

Subgrade

Crack arrestinglayer 20-30% voids

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

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

Recycling the Existing Pavement

• HMA surface recycling may be done with an overlay

• Full-depth HMA or PCC recycling requires new pavement design

Benefits of Recycling

• Conservation of aggregate• Conservation of binder• Conversation of energy• Preservation of the environment• Lower initial project costs• Lower life cycle costs

Pavement Widening

• Design lives should be equal• Match cross-sections closely• Place reflection crack relief fabric along

longitudinal joint• Place longitudinal subdrains, if needed

Shoulders

• Depending on shoulder condition– Patch– Mill HMA– Remove and replace

• Shoulder and traffic lane elevations should match

Photo of lane-shoulder drop-off

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

Pavement Thickness Along a Project

Station Number

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

Block 33b SP 13

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