skidding, roughness.pptx

7/27/2019 skidding, roughness.pptx

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PAVEMENT SKIDDING,

PAVEMENT UNEVENESS,

PAVEMENT INSTRUMENTATION5TH MODULE


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Pavement skidding

Skid resistance propertyessential for pavement safety

40% of accidents are caused due to poor skid resistance of

pavement surface

Skidding is of three types:

Straight skidding

Impending skidding

Sideway skidding


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Types of skidding

Straight skidding

Occurs in the direction of travel when the suddenbrakes are applied

Impending skidding

Braking is gradual and wheel continues to revolve

Sideway skidding Occurs on curves

When sufficient super elevation is not provided


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Causes and remedies

Causes are

Presence of water, clay, dust, dry sand, oil andgrease on pavements

Presence of polished aggregates

Bleeding of excess bitumen, forming patches onthe road surface

Remedial measure

Spreading small aggregates on pavement surface

To renew the wearing course


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Factors affecting skid resistance

Type of pavement surface

Bituminous surface is more skidding than concrete

surface

Macrotexture of pavement and relative roughness

Macrotexture - large-scale texture of the pavement as a

whole due to the aggregate particle arrangement controls the escape of water from under the tire and

hence the loss of skid resistance with increased speed


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Condition of pavement

Coefficent of friction reduces when pavement issmooth or wet

Type and condition of tyre

Smooth and worn out tyres on dry pavementdevelops higher friction than new tyres

More area of contact

New tyres with good treads on wet pavements ismore safe than smoothened tyres

Lubricating effect of water is reduced, waterenters into the tread of tyres


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Speed of vehicle

As speed increases, friction coefficent decreases

Brake efficency

Increased brake efficency, friction coefficent decreases

Tyre pressure Coefficent of friction decreases slightly with increase

in tyre pressure


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Temperature of tyre and pavement

When temperature increases, friction coefficent

decreases

Type of skid, if any

Low value for transverse skid resistance

Essential to prevent lateral skid


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Skid resistance

Can be measured by

Pendulum type friction tester

Skid testing device attached to test vehicle

Instrument mounted dynamic skid resistance tester

towed by another vehicle


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Locked Wheel Tester

Basically, this method uses a locked wheel skiddingalong the tested surface to measure friction

resistance.


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A typical lock-wheel skid measurement system must have the

following:

A test vehicle with one or more test wheels incorporated

into it or as part of a towed trailer. A standard tire for use on

the test wheel

A means to transport water (usually 750 to 1900) and the

necessary apparatus to deliver it in front of the test wheel at

test speed

A transducer associated with the test wheel, senses the

force developed between the skidding test wheel and the

pavement

Electronic signal conditioning equipment to receive thetransducer output signal and modify it as required


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Suitable analog and/or digital readoutequipment to record either the magnitude of the

developed force or the calculated value of theresulting Skid Number (SN)

To take a measurement,

The vehicle (or trailer) is brought to the desiredtesting speed (typically 64 km/hr (40 mph))

Water is sprayed ahead of the test tire to create awetted pavement surface.

The test tire braking system is then actuated to lockthe test tire.

Instrumentation measures the friction force actingbetween the test tire and the pavement and reports theresult as a Skid Number (SN).


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Friction factor (like a coefficient of friction): f = F/L

Skid number: SN = 100(f)

where, F = frictional resistance to motion in planeof interface

L = load perpendicular to interface

Skid Number & Comments

< 30 : Take measures to correct 30 : Acceptable for low volume roads

3134 : Monitor pavement frequently

35 : Acceptable for heavily traveled roads


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Pavement unevenness/ Roughness

Expression of irregularities in the pavement surface

that adversely affect the ride quality of a vehicle

It affects not only ride quality but also vehicle delaycosts, fuel consumption and maintenance costs.


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Pavement uneveness affects

Vehicle operating cost

Comfort

Safety

Fuel consumption Wear and tear of tyres

Wear and tear of other moving parts

Fatigue and accidents

All the above factors increase when unevennessincrease


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Factors causing unevenness

Inadequate or improper compaction of the fill,

subgrade and pavement layers

Un scientific construction practices including the use

of boulder stones and bricks as soling over loosesubgrade

Use of inferior quality pavement materials

Improper surface and subsurface drainage Use of improper construction machinery

Poor maintenance practices

Localized failures due to combination of causes


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Pavement unevenness Can be measured by

Bump integrator

Cumulative measure of vertical undulations of pavementsurface recorded per unit horizontal length of road

Less than 150cm/km : for high speed highways

Upto 250cm/km : for speed upto 100km/hr

More than 350cm/km : very uncomfortable even for

50km/hr

By using a straight edge

Extent of number of depressions or ruts along and across

the pavement


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Bump integrator/Automatic Road

Unevenness Recorder


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It comprises of a standard pneumatic wheel mounted

within a rectangular frame

Integrating unit, mounted on one side of the frame

integrates the unevenness in cm. For the measurement, it is towed by a jeep at a constant

speed of 32 kmph under standard tyre pressure of 2.1

kg/cm2 along the designated wheel path

Bumps in cm and corresponding road length in terms ofwheel revolution pulses are displayed / recorded on a

panel board

Unevenness index(UI) is calculated in terms of mm/km


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Vehicle Mounted Bump Integrator


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This Vehicle Mounted Bump Integrator consists of an

integrating unit which is mounted in the dicky on the

rear axle of a car/jeep.

Integrating unit, mounted on the dicky integrates theunevenness in cm or pulses.

For the measurement vehicle is run at a constant

speed of 32 kmph

Bumps in cm or pulses and corresponding road length

in meters are recorded on a computer based data

acquisition system


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Profilometer

a measuring instrument used to measure a surface's

profile, in order to quantify its roughness

They are of two types:

Contact profilometers

Non-contact profilometers


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Sketch of stylus profilometer


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A diamond stylus is moved vertically in contact with asample

Then moved laterally across the sample for a specified

distance and specified contact force.

A profilometer can measure small surface variations in

vertical stylus displacement.

A typical profilometer can measure small vertical features

ranging in height from 10 nanometres to 1 millimetre.


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The height position of the diamond stylus generates ananalog signal which is converted into a digital signal

stored, analyzed and displayed.

The horizontal resolution is controlled by the scan speed

and data signal sampling rate.


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Advantages of contact profilometers

Acceptance: Most of the world's surface finish

standards are written for contact profilometers

Surface Independence: Contacting the surface is oftenan advantage in dirty environments where non-

contact methods can end up measuring surface

contaminants instead of the surface itself

Direct Technique: No modeling required


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Non-contact profilometers

An optical profilometer is a non-contact method for

providing much of the same information

Optical profiling uses the wave properties of light to

compare the optical path difference between a testsurface and a reference surface

Inside an optical interference profiler, a light beam

is split, reflecting half the beam from a test materialwhich is passed through the focal plane of a

microscope objective, and the other half of the split

beam is reflected from the reference mirror.


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When the distance from the beam splitter to the

reference mirror is the same distance as the beam

splitter is from the test surface and the split beams

are recombined, constructive and destructiveinterference occurs in the combined beam

wherever the length of the light beams vary

This creates the light and dark bands known as

interference fringes


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Advantages of optical profilometers

Optical profilers are inherently three dimensional: theymeasure height (the Z-axis) over an area of X and Ylateral dimensions. Stylus profilometers are inherently

linear Every pixel in the imaging camera is a datum: its

optical path difference is calculated relative to eachadjacent pixel, by comparing the contrast between them

Stylii wear out, or need to be changed for varyingsurface conditions.

Accuracy of the optical path difference is essential togood non-contact profilometry


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The data collected by a profilometer is used tocalculate the International Roughness Index (IRI)which is expressed in units of inches/mile or mm/m.

IRI values range from 0 (equivalent to driving on aplate of glass) upwards to several hundred in/mi (avery rough road).

The IRI value is used for road management tomonitor road safety and quality issues.


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skidding, roughness.pptx

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