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.
This could be your thickness design.
Block 33b SP 13
Rehab & Maint. HMA Overlays 1Bituminous Patching• Replace areas with poor support• Two types– Cold mixes– Hot mixes (HMA)• HMA provides higher quality and consistency• HMAC recommended for permanentrehabilitation projects
Rehab & Maint. HMA Overlays 2Surface Leveling• Provides level surface for overlay– Milling the surface– Removing the unstable HMA layer– Leveling course• Surface irregularities often reappear in theoverlay if not corrected
Rehab & Maint. HMA Overlays 4Photo of old surface
Rehab & Maint. HMA Overlays 5Crack Sealing• Reduce amount of moisture infiltration• Prevent intrusion of incompressibles• Generally not effective on pavementsexhibiting severe structural deterioration
Rehab & Maint. HMA Overlays 6Subsurface Drainage• Positive subsurface drainage is beneficial tooverlay performance– Reduces erosion– Increases strength of base and subgrade– Reduces deflections– Slows stripping• Subsurface drainage evaluation
Rehab & Maint. HMA Overlays 10Reducing Moisture Damage• Approaches– Keep the water out– Desensitize the pavement– Drain the pavement• For rehabilitation projects, try to shortendrainage path (e.g., through longitudinaldrains)• Improve surface drainage
Rehab & Maint. HMA Overlays 12Reflection Cracking• Appear above joints or cracks in underlyingpavement layer• AASHTO design equations do not considerdirectly• Additional steps must be taken to reduce therate and severity
Rehab & Maint. HMA Overlays 14Mechanisms of ReflectionCracking• Causes– Low temperature cycles– Traffic loads• Excessive tensile stresses developed inoverlay due to movement of existingpavement• Initiates at bottom of overlay
Rehab & Maint. HMA Overlays 15Stresses from Low TemperaturesThermal stressLower temperaturesOld pavementOverlayJoint or crackSubgradeHigher temperatures
Rehab & Maint. HMA Overlays 16Stresses from Traffic LoadsShearingstressAC bending stressTip of the joint or working crackAC overlayOld AC pavementVoidStress at the tip of the crackABC
Rehab & Maint. HMA Overlays 18Fabrics• Provide physical restraint (reinforcinglayer) to resist formation of cracks• Not as effective with substantial horizontaland vertical movements• Most effective at longitudinal joints and inwarm climates
Rehab & Maint. HMA Overlays 21Stress-Relieving Interlayers• Dissipates movements and stresses at joints andcracks• Spray application of rubber- or polymer-modifiedasphalt 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 23Crack-Arresting Interlayers• Granular layers with little fines and largeaggregates (75 mm [3 in] maximum size)• Provide large void spaces that effectivelyblunt crack propagation• Effective when properly constructed• Minimum thickness = 90 mm (3.5 in)
Rehab & Maint. HMA Overlays 25Preoverlay Repair Treatments• Any method that reduces movement at jointsand cracks can potentially reduce reflectioncracking• Possible treatments– Heater-scarification– Slab stabilization– Full-depth PCC repair with dowels
Rehab & Maint. HMA Overlays 26Increased Overlay Thickness• Does not prevent the occurrence ofreflection cracking• Reduces the rate and severity of reflectioncracking• Cost-effectiveness must be consideredrelative to other techniques
Rehab & Maint. HMA Overlays 27Recycling the Existing Pavement• HMA surface recycling may be done withan overlay• Full-depth HMA or PCC recycling requiresnew pavement design
Rehab & Maint. HMA Overlays 28Benefits 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 29Pavement Widening• Design lives should be equal• Match cross-sections closely• Place reflection crack relief fabric alonglongitudinal joint• Place longitudinal subdrains, if needed
Rehab & Maint. HMA Overlays 31Shoulders• Depending on shoulder condition– Patch– Mill HMA– Remove and replace• Shoulder and traffic lane elevationsshould match
Rehab & Maint. HMA Overlays 32Photo of lane-shoulder drop-off
Rehab & Maint. HMA Overlays 33Design 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 35Summary• Examine the feasibility of an overlay asmost effective alternative• There is more to overlay design than justthickness design– Preoverlay repairs– Subdrainage– Reflection crack control