Flexible Pavement Material Behaviour Notes
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Transcript of Flexible Pavement Material Behaviour Notes
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Testing methods for materials used in flexible
pavements
Atul Narayan, S. P.
IIT Madras
September 6, 2015
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Outline
Introduction
Testing methods
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Outline
Introduction
Testing methods
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Subordinate learning objectives
To analyze the stress-strain distribution in pavements for given
loading conditions.
To estimate pavement distresses based on stresses and
strains in pavement structure.
To explain the effect of mechanical properties on
pavement behavior and performance.
To analyze the stresses and distresses caused by vehicle
loading.
To estimate the expected volume of traffic in design life
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Outline
Introduction
Testing methods
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Dynamic Modulus
Dynamic modulus is a viscoelastic property of bituminous
mixtures.
A sinusoidal or a haversine load is applied on the material,either in controlled stress or controlled strain.
If response is linear, the output, stress or strain, is also
sinusoidal.
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Dynamic Modulus (cont.)
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Dynamic Modulus (cont.)
Dynamic modulus is given by
G =
whereand are the amplitudes of stress and strainrespectively.
Dynamic modulus decreases with decrease in frequency.
Typical frequency used for testing is 10 Hz
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Dynamic modulusTesting equipment
Asphalt Mixture Performance Tester:
Source:PavementInteractive.org
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Resilient modulus
Resilient modulus is the elastic modulus to be used in layered
elastic theory for granular materials.
It is sometimes measured for bituminous mixtures as well.
Typically, in any testing procedure, the material is subjected toseveral cycles of load repetitions.
A confinement pressure is applied during the test.
Since granular materials are plastic in nature, a part of the
strain is irrecoverable.
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Resilient modulus (cont.)
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Resilient modulus (cont.)
The extent of irrecoverable strain in a cycle decreases as the
number of cycle increases.
After a certain number of cycles, the strain in the cycle is
completely recoverable.
The modulus is determined by dividing the applied stress by
the recoverable strain.
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Resilient modulus (cont.)
Resilient modulus varies with confinement pressure
It is a function of the stress state of the specimen.
The standard constitutive equation used is
MR = K1K2 (1)
where =11 + 22 + 33. (2)
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Resilient modulusTesting method
Samples are first subjected to conditioning.
Conditioning involves subjecting the material to differentconfinement pressures and applying cycles of deviatoric
stress of different magnitudes, each for 200 repetitions, at
each confinement pressure.
Coarse-grained aggregates are subjected to stronger
conditioning (higher confinement pressures and highermagnitudes of deviatoric stress) than fine-grained aggregates.
For determining resilient modulus, the specimen is again
subjected to different combinations of confinment pressure
and deviatoric stress. The recoverable strain in the 200th cycle of a deviatoric stress,
in a confinement pressure, is taken as the resilient strain at
those stress levels.
Resilient strains at different stress levels are used to calibrate
the constitutive equation.
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Flexural Fatigue TestEquipment
ASTM D7460 and AASHTO T321
Source: ASTM D7460
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Flexural Fatigue TestProcedure
Repeated oscillatory loading is applied on the beam.
Loading is either in constant stress or constant strain model
Procedure:
Apply haversine loading at a frequency of 5-10 Hz (Usually 10
Hz) Rest periods may be provided Calculate flexural stiffness in the 50th cycle. This is the initial
stiffness Measure flexural stiffness as a function of number of cycles.
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Flexural Fatigue TestFailure Criteria
For stress controlled test, failure point can be the number of
cycles at which the specimen fully fails.
AASHTO recommends strain controlled test at specified strainlevels for characterizing fatigue.
Point of failure is the number of cycles at which specimenstiffness reaches 50% of initial value
- 50% is completely arbitrary.
Fl l F i T
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Flexural Fatigue TestFailure Curve: Nfvs. Applied strain
Source: ASTM D7460
This curve is modeled using the equation
Nf = a1Ea2
a3
t (3)
Fatigue life decreases when modulus increases or when
applied strain increases
C lif i b i ti
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California bearing ratio
In a CBR test, a standard piston is allowed to penetrate the
soil by applying pressure on top of it.
The pressure required to penetrate the soil at a standard rate
is measured.
This pressure is expressed as a percentage of the pressure
required to penetrate a standard crushed rock.
The percentage is called the CBR value or ratio
For IRC 37, one may use a dynamic cone penetrometer
instead of CBR test to determine the CBR ratio.
CBR Test eq ipment
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CBR Test equipment