The primary functions of a pavement are to:
- PROVIDE A REASONABLY SMOOTH RIDING SURFACE: A smooth riding surface (Low Roughness) is essential for riding comfort, and over the years it has become the measure of how road users perceive a road. Roughness can arise from a number of causes, most often however it is from pavement distress due to structural deformation. Roughness is the distress parameter used by the vehicle operating cost sub-models of a pavement management system, to estimate the timing, type and cost of maintenance needs.
- PROVIDE ADEQUATE SURFACE FRICTION (SKID RESISTANCE): In addition to a riding comfort, the other road user requirement is that of safety. Safety, especially during wet conditions can be linked to a loss of surface friction between the tyre and the pavement surface. A pavement must therefore provide sufficient surface friction and texture to ensure road user safety under all conditions.
- PROTECT THE SUBGRADE: The supporting soil beneath the pavement is commonly referred to as the subgrade, should it be over-stressed by the applied axle loads it will deform and lose its ability to properly support these axle loads. Therefore, the pavement must have sufficient structural capacity (strength and thickness) to adequately reduce the actual stresses so that they do not exceed the strength of the subgrade. The strength and thickness requirements of a pavement can vary greatly depending on the combination of subgrade type and loading condition (magnitude and number of axle loads).
- PROVIDE WATERPROOFING: The pavement surfacing acts as a waterproofing surface that prevent the underlaying support layers including the subgrade from becoming saturated through moisture ingress. When saturated, soil loses its ability to adequately support the applied axle loads, which will lead to premature failure of the pavement.
Types of Pavement
Pavements are typically divided into the following three general categories: flexible, rigid and unpaved (gravel or dirt). For the purpose of this document only flexible and rigid pavements are discussed.
Flexible (Bituminous) Pavements
Flexible pavements are constructed of several layers of natural granular material covered with one or more waterproof bituminous surface layers, and as the name implies, is considered to be flexible. A flexible pavement will flex (bend) under the load of a tyre. The objective with the design of a flexible pavement is to avoid the excessive flexing of any layer, failure to achieve this will result in the over stressing of a layer, which ultimately will cause the pavement to fail. In flexible pavements, the load distribution pattern changes from one layer to another, because the strength of each layer is different. The strongest material (least flexible) is in the top layer and the weakest material (most flexible) is in the lowest layer. The reason for this is that at the surface the wheel load is applied to a small area, the result is high stress levels, deeper down in the pavement, the wheel load is applied to larger area, the result is lower stress levels thus enabling the use of weaker materials.
Rigid (Concrete) Pavements
As opposed to a flexible pavement that develops its strength from a layer system, rigid pavements rely on the strength of the concrete surface typically ranging from 150 to 300 mm for strength. As the name states, rigid pavements are considered to be rigid, implying that the pavement will not flex (bend). This is not entirely accurate, since even a rigid pavement will flex, the amount is however so small that when compared with a flexible pavement, it is considered rigid.
The increased rigidity of concrete allows the concrete surface layer to bridge small weak areas in the supporting layer through what is known as beam action. This allows the placement of rigid pavements on relatively weak supporting layers, as long as the supporting layer material particles will not be carried away by water forced up by the pumping action of wheel loads.
Factors Affecting Pavement Performance
There are numerous factors influencing the performance of a pavement, the following five are considered the most influential:
- TRAFFIC: Traffic is the most important factor influencing pavement performance. The performance of pavements is mostly influenced by the loading magnitude, configuration and the number of load repetitions by heavy vehicles. The damage caused per pass to a pavement by an axle is defined relative to the damage per pass of a standard axle load, which is defined as a 80 kN single axle load (E80). Thus a pavement is designed to withstand a certain number of standard axle load repetitions (E80’s), that will result in a certain terminal condition of deterioration.
- MOISTURE (WATER): Moisture can significantly weaken the support strength of natural gravel materials, especially the subgrade. Moisture can enter the pavement structure through cracks and holes in the surface, laterally through the subgrade, and from the underlying water table through capillary action. The result of moisture ingress is the lubrication of particles, loss of particle interlock and subsequent particle displacement resulting in pavement failure.
- SUBGRADE: The subgrade is the underlying soil that supports the applied wheel loads. If the subgrade is too weak to support the wheel loads, the pavement will flex excessively which ultimately causes the pavement to fail. If natural variations in the composition of the subgrade are not adequately addressed by the pavement design, significant differences in pavement performance will be experienced.
- CONSTRUCTION QUALITY: Failure to obtain proper compaction, improper moisture conditions during construction, quality of materials, and accurate layer thickness (after compaction) all directly affect the performance of a pavement. These conditions stress the need for skilled staff, and the importance of good inspection and quality control procedures during construction.
- MAINTENANCE: Pavement performance depends on what, when, and how maintenance is performed. No matter how well the pavement is built, it will deteriorate over time based upon the mentioned factors. The timing of maintenance is very important, if a pavement is permitted to deteriorate to a very poor condition, as illustrated by point B in Error! Reference source not found., then the added life compared with point A, is typically about 2 to 3 years. This added life would present about 10 percent of the total life. The cost however of repairing the road at point B is minimum four times the cost if the road had been repaired at point A. The postponement of maintenance hold further implications, in that for the cost of repairing one badly deteriorated road (Point B), four roads at point A would have to be deferred, which would mean that in a few years the rehabilitation cost could be 16 times as much. Thus, postponing maintenance because of budget constraints, will result in a significant financial penalty within a few years.