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 pro Vide a surface of acceptable riding quality, adequate skid resistance, favorable light reacting characteristics, and low noise pollution. 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.
One of the main purposes of pavement design is to produce a soil structure system that will carry traffic smoothly and safely with minimum cost. The increase in axle load and phenomenal growth of traffic warrant as much importance in design, construction and maintenance of roads.
Requirements of A Pavement
The pavement should meet the following requirements:
• Sufficient thickness to distribute the wheel load stresses to a safe value on the sub-grade soil
• Structurally strong to withstand all types of stresses imposed upon it
• Adequate coefficient of friction to prevent skidding of vehicles
• Smooth surface to provide comfort to road users even at high speed
• Produce least noise from moving vehicles
• Dust proof surface so that traffic safety is not impaired by reducing visibility.
• Impervious surface, so that sub-grade soil is well protected
• Long design life with low maintenance cost.
Flexible Pavement: A flexible, or asphalt, or Tarmac pavement typically consists of three or four layers. For a four layer flexible pavement, there is a surface course, base course, and sub base course constructed over a compacted, natural soil subgrade. When building a three layer flexible pavement, the sub base layer is not used and the base course is placed directly on the natural subgrade.
A flexible pavement's surface layer is constructed of hot-mix asphalt (HMA).Un stabilized aggregates are typically used for the base course; however, the base course could also be stabilized with asphalt, Foamed Bitumen, Portland cement, or another stabilizing agent. The sub base is generally constructed from local aggregate material, while the top of the subgrade is often stabilized with cement or lime.
With flexible pavement, the highest stress occurs at the surface and the stress decreases as the depth of the pavement increases. Therefore, the highest quality material needs to be used for the surface, while lower quality materials can be used as the depth of the pavement increases. The term "flexible" is used because of the asphalts ability to bend and deform slightly, then return to its original position as each traffic load is applied and removed. It is possible for these small deformations to become permanent, which can lead to rutting in the wheel path over an extended time.
The service life of a flexible pavement is typically designed in the range of 15 to 20 years. Required thicknesses of each layer of a flexible pavement vary widely depending on the materials used, magnitude, number of repetitions of traffic loads, environmental conditions, and the desired service life of the pavement. Factors such as these are taken into consideration during the design process so that the pavement will last for the designed life without excessive distresses.
Flexible Pavement Overlay Design
Over the service life of a flexible pavement, accumulated traffic loads may cause excessive rutting or cracking, inadequate ride quality, or an inadequate skid resistance. These problems can be avoided by adequately maintaining the pavement, but the solution usually has excessive maintenance costs, or the pavement may have an inadequate structural capacity for the projected traffic loads.
Throughout a highway‟s life, its level of serviceability is closely monitored and maintained. One common method used to maintain a highway‟s level of serviceability is to place an overlay on the pavement‟s surface.
There are three general types of overlay used on flexible pavements: asphalt-concrete overlay, Portland cement concrete overlay, and ultra-thin Portland cement concrete overlay. The concrete layer in a conventional PCC overlay is placed unbonded on top of the flexible surface. The typical thickness of an ultra-thin PCC overlay is 4 inches (10
cm) or less.
There are three main categories of flexible pavement overlay design procedures:
1. Component Analysis Design
2. Deflection-Based Design
Sub Base Course Construction
A sub base course is a layer designed of carefully selected materials that is located between the subgrade and base course of the pavement. The sub base thickness is generally in the range of 4 to 16 inches, and it is designed to withstand the required structural capacity of the pavement section.
Common materials used for a highway sub base include gravel, crushed stone, or subgrade soil that is stabilized with cement, fly ash, or lime. Permeable sub base courses are becoming more prevalent because of their ability to drain infiltrating water from the surface. They also prevent subsurface water from reaching the pavement surface.
When local material costs are excessively expensively or the material requirements to increase the structural bearing of the sub-base are not readily available, highway engineers can increase the bearing capacity of the underlying soil by mixing in Portland cement, foamed asphalt, or with emerging technologies such as the cross-linking styrene acrylic polymer that increases the California Bearing Ratio of in-situ materials by a factor 4 – 6.
Base Course Construction
The base course is the region of the pavement section that is located directly under the surface course. If there is a subbase course, the base course is constructed directly about this layer. Otherwise, it is built directly on top of the subgrade. Typical base course thickness ranges from 4 to 6 inches and is governed by underlying layer properties.
Heavy loads are continuously applied to pavement surfaces, and the base layer absorbs the majority of these stresses. Generally, the base course is constructed with an untreated crushed aggregate such as crushed stone, slag, or gravel. The base course material will have stability under the construction traffic and good drainage characteristics.The base course materials are often treated with cement, bitumen, calcium chloride, sodium chloride, fly ash, or lime. These treatments provide improved support for heavy loads, frost susceptibility, and serves as a moisture barrier between the base and surface layers.
Surface Course Construction
There are two most commonly used types of pavement surfaces used in highway construction: hot-mix asphalt and Portland cement concrete. These pavement surface courses provide a smooth and safe riding surface, while simultaneously transferring the heavy traffic loads through the various base courses and into the underlying subgrade soils.