Introduction
A highway pavement is a structure consisting of superimposed layers of processed materials above the natural soil sub-grade, whose primary function is to distribute the applied vehicle loads to the sub-grade. The pavement structure should be able to provide; a surface of
acceptable riding quality, adequate skid resistance, favorable light reflecting characteristics, low noise pollution.
A Highway Pavement is a structure made up of carefully selected and well proportioned materials in different layers designed to transfer loads applied to the surface so that the underlying subgrade is not overstressed.
The ultimate aim is to ensure that the transmitted stresses due to wheel load are sufficiently reduced, so that they will not exceed bearing capacity of the sub grade. Two types of pavements are generally
recognized as serving this purpose, namely flexible pavements and rigid pavements.
Types of pavements
- Rigid pavements
- Flexible pavements.
Rigid pavements.
RIGID: applied to wearing surfaces constructed of Portland cement concrete.
A pavement constructed of concrete is assumed to possess considerable flexural strength that will permit it to act as a beam and allow it to bridge over minor irregularities, which may occur in the base or subgrade on which it rests; hence the term rigid. A concrete base that supports a brick or block layer might also be described as rigid.
Flexible Pavement.
On the other hand, a flexible pavement is a structure that maintains intimate contact with and distributes loads to the subgrade and depends on aggregate interlock, particle friction and cohesion for stability.
Thus the classification of flexible pavements includes primarily those pavements that are composed of a series of granular layers topped by a relatively thin – quality bituminous wearing course. Typically, the highest – quality materials are at or near the surface.
Requirements of a Highway Pavement
An ideal pavement should meet the following requirements:
1. Sufficient thickness to distribute the wheel load stresses to a safe value on the sub-grade soil,
2. Structurally strong to withstand all types of stresses imposed upon it,
3. Adequate coefficient of friction to prevent skidding of vehicles,
4. Smooth surface to provide comfort to road users even at high speed,
5. Produce least noise from moving vehicles,
6. Dust proof surface so that traffic safety is not impaired by reducing visibility,
7. Impervious surface, so that sub-grade soil is well protected, and
8. Long design life with low maintenance cost.
FLEXIBLE HIGHWAY PAVEMENT
Factors affecting the performance of flexible pavements
⦿Traffic
⦿Soil and pavement materials
⦿Environment
⦿Construction and maintenance practice.
⦿Traffic
Traffic has a major effect on pavement performance. Traffic characteristics that affect performance are traffic load, traffic volume, tyre pressure, and vehicle speed.
Traffic load produces stresses and strains within the pavement structure and the subgrade, which gradually contribute to the development of pavement distresses.
For example, heavier loads result potential for fatigue cracking and rutting.
Traffic volume affects pavement performance since larger number of load repetitions increases the chance for fatigue
cracking.
Higher tyre pressure produces higher stress concentrations at the pavement surface that could result in rutting and shoving in the
HMS layer.
Vehicle speed affects the rate of applying the load. Since asphalt concrete is a visco-elastic plastic material, its response is
affected by the rate of load application.
Slow or stationary vehicles have more chances of developing rutting and shoving than high-speed vehicles.
On the other hand, high travel speeds cause more severe bouncing of vehicles, and result in larger dynamic loading and increased roughness.
Design Vehicle
◾Length – capacity of a lane
◾Width – overtaking and capacity
◾Height – clearance at bridges, overheads etc
◾Weight – load to be imposed on to the pavement
⦿Soil and highway pavement materials
Soil and pavement materials significantly affect pavement performance. High quality materials are needed to provide good support to traffic loads under various environmental conditions.
Important material properties include mechanical properties such as elasticity, visco-elasticity, plasticity, temperature susceptibility, durability and aging characteristics.
These properties affect how the material responds to traffic loads and environmental conditions such as temperature, freeze thaw
effect, and rain.
⦿Environment/Climate
Environmental conditions that affect pavement performance include moisture, temperature, and their interaction.
For example, moisture may reduce subgrade support and weakens various pavement layers.
High temperatures soften asphalt concrete and could create rutting within the surface layer. Temperatures below freezing have a bad effect on pavement performance, especially cycles of freeze and thaw.
⦿Construction and maintenance practice.
Poor construction procedure will almost always ensure poor pavement performance.
For example, poor compaction of subgrade or any pavement layer allows excessive further compaction by traffic, which appears in the form of rutting and surface cracking.
Poor placement of Hot Mixed Asphalt (HMA) during construction may result in weak transverse or longitudinal construction joints that are susceptible to early cracking and deterioration.
Excessive air voids in the HMA layer due to poor compaction will result in fast aging followed by cracking.
In contrast, too much compaction of HMA will result in too small amount of air voids that could create rutting or bleeding.
Lack of smoothness of the pavement during construction increases the dynamic impact of traffic, and consequently, speeds up the rate of developing roughness during service.
Design Procedures For Flexible Pavements
Design of flexible pavement entails two steps
- Mix Design
- Structural design
Mix design involves the determination of the relative proportions of a mixture, while structural design involves the determination of the layer thicknesses for the components of a pavement structure.
In structural design of pavements, the objective is to determine for the given set of materials, the layer thicknesses required for the pavement to serve.
Methods of design
- AASHTO
- Road Notes developed by
- TRL Method
- CBR method
- MoWT method
- Group index method.
- Westergard method.
- CBR method.
- Benkelman beam method.
Design input factors
- Traffic
- Material characteristics
- Environmental considerations
- Costs to be involve
Traffic
Traffic is considered in terms of axles.
Types of axles include:
- Single axles
- Tandem axles
- Tridem or triple axles
Typical cross-section of a flexible pavement
Types of Flexible Pavements
1. Conventional layered flexible pavement,
2. Full – depth asphalt pavement
3. Contained rock asphalt mat (CRAM).
Conventional flexible pavements are layered systems with high quality expensive materials are placed in the top where stresses are high, and low quality cheap materials are placed in lower layers.
Full – depth asphalt pavements are constructed by placing bituminous layers directly on the soil sub grade. This is more suitable when there is high traffic and local materials are not available.
Contained rock asphalt mats are constructed by placing dense/open graded aggregate layers in between two asphalt layers. Modified dense graded asphalt concrete is placed above the sub-grade will significantly reduce the vertical compressive strain on soil sub-grade and protect from surface water.
Layers of a flexible highway pavement
1/Surface course/wearing course
Surface course is the layer directly in contact with traffic loads and generally contains superior quality materials.
They are usually constructed with dense graded asphalt concrete (AC).
Functions of the surfacing course
- It provides characteristics such as friction,
smoothness, drainage, etc. - Also it will prevent the entrance of excessive quantities of surface water into the underlying base, sub-base and
sub-grade, - It must be tough to resist the distortion under traffic and provide a smooth and skid- resistant riding surface,
- It must be water proof to protect the entire base and sub-grade from the weakening effect of water
2/Base course
The base course is the layer of material immediately beneath the surface of binder course and it provides additional load distribution and contributes to the sub-surface drainage.
It may be composed of crushed stone, crushed slag, and other untreated or stabilized materials.
3/Sub-Base course
The sub-base course is the layer of material beneath the base course and the primary functions are;
1. To provide structural support
2. Improve drainage,
3. Reduce the intrusion of fines from the sub-grade in the pavement structure
4. If the base course is open graded, then the sub-base course with more fines can serve as a filler between sub-grade and the base course.
A sub-base course is not always needed or used. For example, a pavement constructed over a high quality, sub-grade may not need the additional features offered by a sub-base course. In such situations, sub-base course may not be provided.
4/Sub-grade
The top soil or sub-grade is a layer of natural soil prepared to receive the stresses from the layers above. It is essential that at no time soil sub-grade is overstressed. It should be compacted to the desirable density, near the optimum moisture content.
