The features of the cross-section of the pavement influences the life of the pavement as well as the riding comfort and safety. Of these, pavement surface characteristics affect both of these. Camber, kerbs and geometry of various cross-sectional elements are important aspects to be considered in this regard. For safe and comfortable driving four aspects of the pavement surface are important; the friction between the wheels and the pavement surface, smoothness of the road surface, light reflection characteristics of the top of pavement surface and drainage to water.
1) Friction
Friction between the wheel and the pavement surface is a crucial factor in the design of horizontal curves and thus the safe operating speed. Further, it also affects the acceleration and deceleration ability of vehicles. Lack of adequate friction can cause skidding or slipping of vehicles. Skidding happens when the path travelled along the road surface is more than the circumferential movement of the wheels due to friction. Slip occurs when the wheel revolves more than the corresponding longitudinal movement along the road. Various factors that affect friction are given below.
- Type of the pavement (like bituminous, concrete or gravel)
- Condition of the pavement (dry or wet, hot or cold etc.)
- Condition of the tyre (new or old)
- Speed and load of the vehicle
The frictional force that develops between the wheel and the pavement is the load acting multiplied by a factor called the coefficient of friction and denoted as “f”. The choice of the value of this is a very complicated issue since it depends on many variables. IRC suggests the coefficient of longitudinal friction as 0.35 - 0.4 depending on the speed and coefficient of lateral friction as 0.15. The former is useful in sight distance calculation and the latter in horizontal curve design.
2) Unevenness
It is always desirable to have an even surface, but it is seldom possible to have such a one. Even if a road is constructed with high quality pavers, it is possible to develop unevenness due to pavement failures. Unevenness affect the vehicle operating cost, speed, riding comfort, safety, fuel consumption and wear and tear of tyres.
Unevenness index
It is a measure of unevenness which is the cumulative measure of vertical undulations of the pavement surface recorded per unit horizontal length of the road. An unevenness index value less than 1500 mm/km is considered as good, a value less than 2500 mm/km is satisfactory up to speed of 100 kmph and values greater than 3200 mm/km is considered as uncomfortable even for 55 kmph.
3) Light Reflection
White roads have good visibility at night, but caused glare during day time. Black roads have no glare during day, but has poor visibility at night. Concrete roads has better visibility and less glare. It is necessary that the road surface should be visible at night.
4) Drainage
The pavement surface should be absolutely impermeable to prevent seepage of water into the pavement layers. Further, both the geometry and texture of pavement surface should help in draining out the water from the surface in less time.
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