Topic 2 - Flexible Pavement Distress_2

7/30/2019 Topic 2 - Flexible Pavement Distress_2

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Topic 2 Flexible Pavement Distress

1. Alligator or Fatigue Cracking (cont.)




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2. Block Cracking (Thermal Cracking)

Block cracks divide the asphalt surface into approximatelyrectangular pieces. Blocks range from 1 to 100 ft2 in area.

Caused mainly by the shrinkage of hot mix asphalt Not load induced, although load can increase its severity Caused by low temperatures and improper grade of asphalt

2. Block Cracking (cont)



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2. Block Cracking (cont)



3. Joint Reflection Cracking from Concrete SlabOccur on pavements that have an asphalt surface over a jointedconcrete slab. Cracks occur over transverse and longitudinal

joints were pavement was widened. Why?

Caused by themovement of the slabbeneath because oftemperature andmoisture changes

Generally not load

initiated


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4. Lane/Shoulder Drop-off or Heave

Difference in elevation between the traffic lane and theshoulder.

Main Causes: Drop-off due to consolidation, settlement, or pumping of the underlying

granular or subgrade material Heave due to frost action or swelling soils Soil shoulder drop-off due to blowing away of material from passing trucks


5. Longitudinal CrackingLongitudinal cracks are running parallel to the pavementcentreline, while transverse cracks extend across the centreline.

Main Causes Shrinkage of asphalt surface (low

temperature cracking) Asphalt hardening Reflective cracks other than the

joints Poor construction of paving lane

joint

Usually not load associated


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5. Longitudinal Cracking Top Down Cracking

5. Longitudinal Cracking (cont)



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6. Water Bleeding & Pumping

Water bleeding occurs when water seeps out of joints or cracksor through an excessively porous HMA layer. Pumping occurswhen water and fine material is ejected from underlying layersthrough cracks in the HMA layer under moving loads.

Main Causes: Porous pavement as a result of inadequate compaction during

construction or poor mix design High water table Poor drainage

6.1 Water Bleeding



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The result of excessive consolidation of the pavement alongthe wheel path due to either reduction of the air voids in theasphalt concrete layer, or the permanent deformation of thebase or subgrade.

7.1 Consolidation Rutting




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7.2 Instability Rutting


7.2 Instability Rutting



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8. Bleeding

Bleeding is characterized by excess asphalt binder on thesurface of the pavement.

Main Causes: Excess prime or tack coats Excess asphalt content in mixture Drain-down Contamination with diesel or similar

8.1 Bleeding from Excess Prime



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8.2 Bleeding from Drain-down


8.2 Bleeding from Drain-down (cont)



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9. Slippage

Slippage is characterized by crescent or half-moon shapedcracks generally having two ends pointed into the direction oftraffic.

Main Causes: Thin Layers Insufficient/too much tack coat Wrong tack coat material Tracking of tack coat with equipment Contamination sand, dirt, organics Rolling process

9. Slippage (cont)



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9. Slippage (cont)


9. Slippage (cont)



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9. Other types of Distress

Swell upward bulge, swelling soils, PCC blowup

Corrugation ripples at truck/bus acceleration zones

Depression localized elevation difference; due to soil or built in

Potholes broken pavement due to fatigue cracking or freeze/thaw

Ravelling and weathering stripping of aggregates

Measurements by Smithers Scientific Services

Tire Contact Stresses

16 Transducers

Bed Motion

Tire RollingDirection

Bed

z, z

y, y

x, x

Tire

Coaxial Load andDisplacementTransducer Detail

16 Transducers

Bed Motion

Tire RollingDirection

Bed

z, z

y, y

x, x

Tire

Coaxial Load andDisplacementTransducer Detail


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Vertical Stress Distributions


-300

-250

-200

-150

-100

-50

0

0 2 4 6 8 10 12 14

Lateral Location, X (in)

VerticalContactStresses,zz

(psi)

Wide-base Radial

Radial

Bias ply

Transverse Stress Distributions


-80

-60

-40

-20

0

20

40

60

80

0 2 4 6 8 10 12 14

Lateral Location, X (in)

TransverseContactShearStresses,yy

(psi)

Wide-base Radial

Radial

Bias ply


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Effect of Tire StructureBias Ply TireRadial Tire

FlexibleTread

More RigidWall

PneumaticEffect

PoissonEffect

CompositeEffect

FlexibleWall

More RigidTread


Effect of Pavement Bending

Bias Ply TireRadial Tire


Overall Effectxx( + )

( - )

Truck TireTread Effect

Tension

Compression

PavementBending Effect

( + )

( - )

Tension

Compression

( + )

( - )

xx( + )

( - )

Topic 2 - Flexible Pavement Distress_2

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