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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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Factors affecting skid resistance
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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Factors affecting skid resistance
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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Factors affecting skid resistance
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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Contact profilometers
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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Contact profilometers
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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