PER-Lec-3

Pavement Evaluation and RehabilitationTE-507

Lecture-3

26-08-2015

Dr. Zia-ur-Rehman

Pavement Behaviour

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HMA Pavement Behaviour

•Wheel loading

• Temperature

• Moisture

Pavement Evaluation and Rehabilitation

Pavement Behaviour

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HMA Pavement Behaviour- Wheel loading


Pavement Behaviour

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•In the process of distribution of load, various states of

stress are built within different layers that can lead to the

overall deterioration of the pavement.

-Tensile strains develop at the bottom of the layer beneath the load,

results in cracking

-Vertical shear strains develop near the surface (at the edge of load),

results in permanent deformation (rutting) in the wheel paths


Pavement Behaviour

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•In the base and subbase layers, compressive stresses and

some tensile stresses will also develop.

-Compressive stresses can cause permanent deformation

-Unbound granular materials are not able to carry any tension

resulting from bending of the structure. If an applied load produces

excessive bending, the base /or subbase layers are likely to

decompose and lose some of their load carrying capacity.


Pavement Behaviour

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HMA Pavement Behaviour- Temperature

•Change in temperature results in varying compressive

and tensile stresses.-Compressive stresses results in rutting or permanent deformation

-If the tensile stress exceeds the tensile strength of the HMA layer,

cracks will develop

•Increase in HMA temperature can result in significant

decrease in stiffness and resistance to permanent

deformation

•Unbound material are not significantly affected by

temperature but freezing may result in ice formation and

eventually thawing.Pavement Evaluation and Rehabilitation

Pavement Behaviour

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HMA Pavement Behaviour-Moisture

• In unbound and some bound layers, moisture acts as a

lubricant. By permitting more movement between

aggregate particles, it effectively weakens the material and

reduces the load carrying capacity of the pavement.

• In certain asphalt bound layers, including HMA surfaces,

moisture can cause a separation of the asphalt from the

aggregate. This will also weaken the material and reduce

the overall load carrying capacity of the pavement.


Typical Distresses in HMA

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HMA Pavement Distress Mechanism

• Permanent deformation (Rutting)

• Fatigue cracking

•Thermal Cracking

•Top down cracking

•Stripping

•Reflection cracking

•Frost heave

•Intrusion of fines

•Soil swelling

•Oxidation



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HMA Pavement Distress Mechanism- Rutting

Permanent deformation (or Rutting) refers to a

progressive process whereby the accumulation of small

amounts of wheel load related permanent deformation in

one or more layers ultimately leads to a significant

depression of the pavement surface in its wheel paths (i.e.,

ruts).



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• If deformation exists only in the HMA surface layer

-The HMA surface was overloaded

-Loading was exerted during a hot period (when the HMA layer was

soft)

-Problem with the stability of mix or temperature susceptibility

-Material shoved to the side, indicates a problem with HMA surface

layer (either mix design or construction)



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•If permanent deformation exists only in the base/subbase

-The HMA surface layer was too thin

-The aggregate or aggregate blend in the base/subbase was unstable

or inadequately designed

-The layer was poorly constructed

-The layer was exposed to excessive or prolonged moisture



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•If the total rutting at the surface exists in all layers

-The pavement structure is too thin for the applied loads

-The soil is very weak or soil having high moisture content



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Severe rutting

Minor rutting


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HMA Pavement Distress Mechanism-Fatigue cracking

• Alligator or fatigue cracking is a series of interconnecting

cracks caused by the fatigue failure of an asphalt surface

or a stabilized base under repeated traffic loading.

• The cracking initiates at the bottom of the asphalt

surface or stabilized base, where the tensile stress or strain

is highest under a wheel load.

• The cracks propagate to the surface initially as one or

more longitudinal parallel cracks.

• After repeated traffic loading, the cracks connect and

form many-sided, sharp-angled pieces that develop a

pattern resembling chicken wire or the skin of an alligator.



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• Early stage



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• Intermediate stage



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• Advanced stage



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HMA Pavement Distress Mechanism-Thermal cracking

• Thermal cracking develops in HMA surface layers as the

pavement undergoes one or more temperature drops.

• Greater temperature drops produce greater potential for

contraction.

• Restraints to contraction is supplied by:

-Friction on the bottom of the HMA surface

-Continuity of the HMA layer itself

•Thermal cracking could be due to multiple temperature

drop cycles.



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• Wide Cracks

As crack width increases, the potential for moisture

infiltration from the surface increases. Moisture, in turn,

weakens the underlying layers.

•Spalling

If the transverse cracks are not sealed, they can be

infiltrated by incompressible fines (i.e., sand), as well as

moisture. These incompressible fines keep the crack from

closing as the temperature rises and the pavement

expands. Ultimately, the top edges of crack can chip, spall

or break away, resulting in poor ride quality.



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• Tenting

If the HMA material at the edge of the transverse crack

does not spall as the pavement expands, it can buckle

upwards and create a small tent and increased roughness.



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HMA Pavement Distress Mechanism- Top Down Cracking•Top-down (or surface-initiated) cracking is a distress

phenomenon that occurs in relatively thick (greater than

200mm[8in]) HMA surface layers (load related)



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HMA Pavement Distress Mechanism- Top Down Cracking•Mechanism is related to a combination of ageing (oxidation) at

the top of the HMA surface layer, the high pressures associated

with radial tyres and the high dynamic stresses that develop at

the perimeter of a moving wheel load.

•Top-down cracking usually does not penetrate more the 50 mm

(2 in) below the HMA surface, so it does not provide an avenue

for the ingress of moisture.

•Rehabilitation treatments for this type of distress include HMA

surface patching, mill and fill and hot in-place recycling.



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HMA Pavement Distress Mechanism- Top Down Cracking



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HMA Pavement Distress Mechanism- Top Down Cracking



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HMA Pavement Distress Mechanism-Stripping

•Asphalt bound layers with prolonged moisture condition

lead to the de-bonding of the binder from the aggregate

particles (acts like a unbound layer).

•Subsurface water through capillary action is another

source of water (surface seals can cause or accelerate the

problem).

•Mix design: use of anti-strip agent (typically lime).

•Permeable base courses to reduce prolonged exposure to

asphalt layer.



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•Surface stripping will result in reveling.

•Subsurface stripping more common form can

reach an advanced stage of development below

the surface without showing any significant signs

of distress at the surface (analogous to a termite

infestation).

•Core and NDT used for subsurface stripping.



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HMA Pavement Distress Mechanism-Reflection cracking•Reflection cracking refers to a type of cracking that results from

the reflection of pre-existing cracks in underlying structure.

•Forces acting in the HMA surface layer:•Thermal (tensile) stress related to a drop in the HMA surface layer

temperature.

•Tensile stress related to the horizontal contraction of the underlying

pavement as it is exposed to lower temperatures.

•Tensile stress related to the bending of the HMA surface as a wheel load is

applied above the existing crack.

•Shear stress related to the potential differential vertical movement between

one side of the crack and the other as a wheel load passes over the existing

crack.

•Remedial measures to reduce reflective cracking:•Crack sealing, fabrics (and other geosynthetic materials), stress-absorbing

membrane interlayers, and pre-overlay repairs.



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HMA Pavement Distress Mechanism-Reflection cracking



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HMA Pavement Distress Mechanism-Reflection cracking



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HMA Pavement Distress Mechanism-Frost Heave•The distress associated with combination of :

•Prolonged low-temperature exposure.

•Moisture availability.

•Frost susceptible materials.

•Randomly or differentially causes roughness.

•Uniform heaving (no roughness but problem at bridge

abutments).

•Formation of ice lenses result in upward-vertical movements at

the surface.

•Thaw weakening during ice melting.



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HMA Pavement Distress Mechanism-Frost Heave



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HMA Pavement Distress Mechanism-Frost Heave



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HMA Pavement Distress Mechanism-Frost HeaveProblem of frost heave is addressed through:

oInsulating layers and/or the replacement of fine-grained

natural materials with non-frost susceptible materials.

oIncreased overlay thickness will help insulate.

oImproved drainage solutions.



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HMA Pavement Distress Mechanism-Intrusion of Fines

•Unbound granular layers are placed directly on top of a

fine-grained soil. Natural soil will move vertically and

infiltrate the voids in the granular layer.

•Intrusion of these fine materials breaks down the

aggregate interlock in the granular layers.

•Control the intrusion of fines in the base layers:•Use of dense-graded, (but slow-draining) granular bases

•Use of a filter course

•Stabilizing a clayey subgrade



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HMA Pavement Distress Mechanism-Soil Swelling

•Soil swelling refers to a phenomenon whereby

the absorption of available moisture by and

underlying expansive natural material causes

that layer to swell.

•Usually, the swelling activity takes place very

rapidly during the first few years after

construction until equilibrium.



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HMA Pavement Distress Mechanism-Soil Swelling

•Fine-grained (clayey) soils with high plasticity (i.e.,

plasticity index values greater than 25) are the most

susceptible to swelling.

•Emphasis should be placed on providing good drainage.

•Reconstruction can destabilize the moisture equilibrium

in removal of the entire pavement surface layer. Under

certain conditions, this action could result in drying of the

expansive layer and, therefore, the possibility of swelling

recurrence.



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HMA Pavement Distress Mechanism-Oxidation•Chipseal is a pavement surface treatment that combines one or more layer(s) of

asphalt with one or more layer(s) of fine aggregate. Chipseals are typically used on

rural roads carrying lower traffic volumes and the process is often referred to as

"asphaltic surface treatment".

•A fog seal is a light application of a diluted slow-setting asphalt emulsion to the

surface of an aged (oxidized) pavement surface. Fog seals are low-cost and are used

to restore flexibility to an existing HMA pavement surface. They may be able to

temporarily postpone the need for a surface treatment or non-structural overlay.

•A Cape Seal is a chip seal covered with a slurry or micro-surface. The benefits

from using a cape seal include a very smooth surface with an increased durability

by sealing the subbase. Often the use of a chip seal is not popular with the public

because of the rougher ride and loose stones. With the addition of the top treatment,

a slurry seal or micro-surfacing, the road ends up with a smooth surface that binds

any loose aggregate, reducing stone loss.



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HMA Pavement Distress Mechanism-Oxidation

•With time, prolonged exposure of an HMA layer to

ultraviolet rays from the sun causes the asphalt (bitumen)

to oxidize, and become stiffer and more susceptible to

cracking.

•Various types of surface treatments (chip seals, fog seals,

cape seals, etc.) are effective at interrupting and slowing

down the oxidation process.

•Reflection cracking of oxidized pavement; recycling is an

option with recycling agent added to restore flexibility.



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Summary of Key Pavement Distress Mechanism


Distress type Wheel

load

Climate Materials

Temperature Moisture

Rutting PF SF SF SF

Fatigue cracking PF SF SF SF

Thermal cracking PF PF

Stripping PF PF

Reflection cracking PF PF SF

Top-Down cracking PF PF SF

PF: Primary Factor

SF: Secondary Factor

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