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SKID RESISTANCE ON PAVEMENTS


INTRODUCTION
Road engineers & users are concerned with the safe passage of vehicles that operate over them.
Safety on road is assessed by its skid resistance.
Slippery pavements are known to be the cause of many road accidents and fatalities.
Skid resistance involves the complex interaction of pavement , vehicle and environment factors.


SKID RESISTANCE
Skidding is the jerking of the wheels of the vehicles when it locked due to the sudden breaking , if the frictional resistance offered is too low.
The friction that can be mobilized is greater than the decelerating force , the vehicle comes to a safe halt within a certain distance.
If the friction developed is much smaller than the decelerating force , the vehicle cannot be controlled by the driver .
Skid resistance is the force developed when a tyre is prevented from rotating along the pavement surface.
Skid resistance is thought of as a pavement property ; it is the antonym of slipperiness.


CHARECTERISTICS OF SKID RESISTANT PAVEMENTS
Skid resistance is invariant whether wet or dry.
No decrease of the skid resistance with increase in speed.
No reduction of skid resistance with time.
Resistance to wear.
Structural durability.
Low noise generation.
Low cost.


FACTORS AFFECTING SKIDDING-PAVEMENT
Different types of pavements have different skid resistance properties.
Surface with open texture premix carpet have more skid resistance property than mastic asphalt.
The macro texture helps in providing a good drainage of the surface & the deformation of the tyres.
The macro texture property of the pavements which brings into play the hysteresis property of friction.
Aggregate which polish under traffic into smooth shape are not desirable for skid resistance .


FACTORS AFFECTING SKIDDING-EFFECT OF WATER
Danger of skidding is where the pavement is wet.
Contact surface divided into 3 zones
Zone1:Bulk displacement of water.
Zone2:Water carries a portion of load.
Zone3:Tyre in contact with surface.
At high speed only Zone1 is present.
Remedies are
Providing adequate camber .
Rough surface texture.


FACTORS AFFECTING SKIDDING-TYRE
Two components of friction which develop between the tyre and road surface
Adhesion component : Caused by the shear forces at the tyre road interface.
Hysteresis component: Caused by the deformation of tyres when it comes in contact with the pavement.
The thread depth and the pattern of tyres influence the skid resistance.
Drainage of surface water is done by the grooves of tyres.


METHODS TO IMPROVE SKID RESISTANCE
Influence of micro texture and macro texture:
composition of the wearing course must limit the binder content to a designed optimum level ; ensures safety and durability performance requirements
composition of the wearing course must aim at the upper limit for the recommended range ; reduces the tendency of bleeding due to compaction under the action of traffic and increase in temperature
Composition of the wearing course, the dimensions of the aggregate chips and the laying must be such that the aggregates form sufficiently sharp projections ; attain a gritty surface texture
Sand content should be increased but still within limits according to ASTM Standard D 3515 ; micro roughness of the pavement surface texture is ensured
Aggregate must be strong enough to withstand early fragmentation or wear from traffic.
Compromise between providing an adequate micro-roughness and macroroughness may be achieved by choosing a maximum grain size of 12.5mm

Composition of the wearing course must be designed to have sufficient stability ; Aiming at a value of a stability of 5.0 reaches a compromise between the skid resistance and resistance to cracking


MEASUREMENT TECHNIQUES
LOCKED WHEEL TESTER:
Commonly used method for testing skid resistance.
This method uses a locked wheel skidding along the tested surface to measure friction resistance.
A std. tyre for use is ,bias-ply G78x15 tyre with seven circumferential grooves.
The result of the test is the Skid Number(SN).


SPIN UP TESTER
The basics set up of the locked wheel test and the spin up test are the same.
At the desired speed here a locked wheel is lowered to the pavement surface.
The force can be calculated by knowing the test wheelâ„¢s moment of inertia and rotational acceleration.
It got 2 advantages over the Locked wheel tester
No force measurement is necessary.
Less wear and tear.


SURFACE TEXTURE MESUREMENT
This method seeks to measure the macro texture and correlate with skid resistance.
Two methods used are:
Road surface Analyzer(eg:ROSAN)
One drawback of this method is that pavement surface macro texture does not entirely determine its skid resistance
Sand patch test


MEASUREMENT TECHNIQUES
The resistance of the wet pavement to skidding can be checked by means of Portable Skid-Resistance Tester.
This method provides a measure of frictional property , micro texture of surface either in field or in the lab.
The quantity of measure with the portable tester is termed as Skid Resistance.


CONCLUSION
Skidding is a major cause for most of the road accidents.
In this seminar have gone through the different factors affecting skid resistance and pointed out some of the methods to improve skid resistance.
By adopting all these remedial measures we can reduce the rate of accidents.
A number of tests are available for measuring the skid resistance of the pavements.
The problem for those who design, build and maintain roads is to know enough the factors that contribute in skid resistance
To allow the later to be maintained at an adequate level under all conditions of traffic and environment