flexible pavement material behaviour notes

7/25/2019 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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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 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

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