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Flexible Pavement Maintenance & Rehabilitation

Distress and Causes

Senior/GraduateHMA Course

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• Drainage, drainage, drainage

• Sufficient thickness and internal strength to carry expected traffic loads

• Adequately dense to prevent penetration of moisture from underneath, sides, and surface

• Top surface which is smooth; waterproof; skid resistant; resistant to wear, distortion, and deterioration by weather and deicing chemicals

Basic Requirements for Flexible Pavements

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Distress ModeFracture

Distortion

Typical Distresses in Asphalt Pavements

Distress ManifestationCracking• load associated (fatigue)• non-load associated (thermal & block)• longitudinal• reflection

Permanent Deformation• densification• creep (rutting/shoving/ corrugations)• slippage

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Distress Mode• Disintegration

• Other

Typical Distresses in Asphalt Pavements (Cont.)

Distress Manifestation• Stripping• Raveling

• Reduced skid resistance• Riding quality-roughness• Swelling/frost

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Also called alligator or map cracking• Loads too heavy for the pavement structure• Too many repetitions of load• Inadequate support (pavement layers/subgrade)• Poor drainage• Very stiff binder in surface course• Occurs in wheel path, potholes develop in

advanced stage

Load Associated (Fatigue) Cracking

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Shoulder

Traffic

Moderate

Low

Center line

High

Fatigue Cracking Severity

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Transverse cracks occur at regular intervals• Asphalt too stiff at low service temperatures• Rapid chilling of the road surface• Highly temperature susceptible asphalt binder• Subgrade type (more on sandy)• Age of the pavement

Non-Load Associated (Thermal) Cracking

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• Both transverse and longitudinal cracking

• Most often on low volume roads/parking lots

• Thixotropic hardening (structuring) of asphalt binder

Block Cracking

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Occurs parallel to the center line of roadway

Center Line Joint • Joint between adjacent lanes• Density gradient across the joint• Low density - low tensile strength

Edge Cracking• Lack of lateral (shoulder) support• Settlement of widened pavement• Poor drainage (ditches)

Longitudinal Cracking

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Longitudinal cracking in the wheel path

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Streaks ofSegregation

Sources of Longitudinal Cracking

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Longitudinal edge cracking

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High severity

edge cracking

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Discontinuities in the underlying layers propagate through the HMA surface due to movement of cracks

Discontinuities • Cracks or joints in underlying PCC pavement• Cracks in soil-cement base course• All types of cracks in the existing HMA pavement

Causes• Thermal (movement of joints/cracks)• Load

Reflection Cracking

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Shearing and Bending Stresses in HMA Overlay

Stress at the tip of the crack

Shearing stress

AC bendingstress

Old PCC or AC pavement

AC overlay

Tip of the joint or working crack

Void

A

B

C

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• Lack of adequate subgrade support• Unstable HMA course(s)

- Poor mix design- Rounded aggregate- Excessive binder

• Stripping

Rutting

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Corrugations

• Lack of bond between HMA courses• Unstable surface course mix• Braking and accelerating action near

intersections

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• Associated with slippage mechanism

• Poor bond between the surface and underlying layer

• Too much or too little tack coat

• Steep grades/intersections

• Acceleration/braking exert surface traction forces

• U-shaped

Slippage

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High severity slippage cracking

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Breaking of adhesive bond between the aggregate surface and asphalt binder usually in presence of moisture

• Inadequate pavement drainage system• Mixes with excessive air voids (poor compaction), and low

asphalt content• Incompatible aggregate surface and asphalt binder, asphalt

binder displaced by water• Excessive pore pressure induced by traffic• Spontaneous emulsification of binder by water

Stripping

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Progressive disintegration of HMA surface as a result of dislodgment of aggregate particles in the mix

• Lack of sufficient cohesion due to low in-place density

• Lack of fines in the matrix, especially in coarse, segregated areas

• Aging of asphalt binder

Raveling

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Friction between the tire and road surface. Quantified by Friction Number (FN). Critical when surface is wet.

Causes for Low FNs • Flushing/bleeding of HMA surface• Inadequate microtexture

- aggregate prone to polishing• Inadequate macrotexture

- no drainage channels- causes hydroplaning- important at high speeds

Skid Resistance

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Riding quality/roughness

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Swelling/frost

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Questions?