sir............ i am b.tech final year student in project so my project is design of flexible pavement so we
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literature review in flexible pavement design ppt

There are two types of pavements based on design considerations i.e. flexible pavement and rigid pavement. Difference between flexible and rigid pavements is based on the manner in which the loads are distributed to the subgrade.

Before we differentiate between flexible pavements and rigid pavements, it is better to first know about them. Details of these two are presented below:

Flexible Pavements:

Flexible pavement can be defined as the one consisting of a mixture of asphaltic or bituminous material and aggregates placed on a bed of compacted granular material of appropriate quality in layers over the subgrade. Water bound macadam roads and stabilized soil roads with or without asphaltic toppings are examples of flexible pavements.

The design of flexible pavement is based on the principle that for a load of any magnitude, the intensity of a load diminishes as the load is transmitted downwards from the surface by virtue of spreading over an increasingly larger area, by carrying it deep enough into the ground through successive layers of granular material.

Thus for flexible pavement, there can be grading in the quality of materials used, the materials with high degree of strength is used at or near the surface. Thus the strength of subgrade primarily influences the thickness of the flexible pavement.

Rigid Pavements:

A rigid pavement is constructed from cement concrete or reinforced concrete slabs. Grouted concrete roads are in the category of semi-rigid pavements.

The design of rigid pavement is based on providing a structural cement concrete slab of sufficient strength to resists the loads from traffic. The rigid pavement has rigidity and high modulus of elasticity to distribute the load over a relatively wide area of soil.

Definition of Flexible Pavement
A true flexible pavement yields “elastically” to traffic loading. It is constructed with a bituminous-treated surface or a relatively thin surface of hot-mix asphalt (HMA) over one or more unbound base courses resting on a subgrade. Its strength is derived from the load-distributing characteristics of a layered system designed to ultimately protect each underlying layer including the subgrade from compressive shear failure.

Progressively better materials are used in the upper structure to resist higher near-surface stress conditions caused by traffic wheel loads. These materials include an all-weather surface that is resistant to erosion by the environment and traffic action. The bituminous surface layer must also be resistant to fatigue damage and stable under traffic loads when pavement temperatures are in excess of 150ºF.

In this guide, hot-mix asphalt-surfaced pavements is a more generalized term used to describe any “black-topped” structure outside of HMA-overlaid concrete. These pavements are categorized as being flexible or semi-rigid (to include the full-depth or perpetual design). This chapter addresses the design of these types of structures.

The fundamental difference between a flexible, semi-rigid, and rigid pavement is the load distribution over the subgrade. The semi-rigid pavement has a higher composite modulus of elasticity than a flexible pavement and begins to resemble the rigid structure in terms of how the traffic loads are distributed over the subgrade. The elements contributing to the higher modulus may be:

increased thickness in asphalt concrete
chemical stabilization of the base, subbase, and/or subgrade layers
asphalt stabilization of the base course.
The higher modulus adds to the structural capacity of the pavement layers. As a result, the load is distributed over a wider area of the subgrade.

Types of Hot Mix Asphalt-Surfaced Pavements
These pavements may generally be placed into one of the following categories:

surface-treatment on a granular base
thin hot mix asphalt concrete (< 2 in.) on a granular base
intermediate hot mix asphalt concrete (2 - 5 in.) on a granular base
thick hot mix asphalt concrete (> 5 in.) (semi-rigid)
thin hot mix asphalt concrete on a chemically stabilized base or subbase (semi-rigid)
thin hot mix asphalt on an asphaltic bound base (semi-rigid).
Stabilization of the subgrade layer can apply to any of the above pavement types. Typical stabilizers include asphalt cement (for base only), lime, cement, fly ash, or lime-fly ash combinations.

Perpetual (HMA) Pavements

In 2001, the Flexible Pavement Design Task Force (FPDTF) studied structurally designed deep HMA pavement; a type of pavement typically associated with high assurance of long pavement life. The intent of the FPDTF study was to address the increased structural demands on heavy truck traffic facilities. The task force was composed of pavement and construction experts from TxDOT and industry.

As a result of the 2001 study, the following guidelines were established:

department guidelines for materials to be used
the general (“conceptual”) structural design format and
the locations where these structures should be considered (2001).
Current guidelines (2006) have taken into consideration design and constructability issues experienced in the structures designed under the original guidelines. Recommended structural layer composition (see Figure 2-2) for facilities with a projected 20-yr. one direction cumulative loading of at least 30 million ESALs was clarified.

When 30 million ESALs are exceeded, use Stone Matrix Asphalt (SMA), Item 346, and Performance-Designed Mixtures, Item 344, in lieu of conventional quality control/quality assurance dense-graded HMA specifications. When Items 346 and 344 are used with stiffer modified performance graded binders, these mixtures provide excellent resistance to permanent deformation. In addition, when Items 346 and 344 are used in a structure of at least 12” combined HMA thickness, or in conjunction with a high density rich bottom layer, the structure will be resistant to conventional bottom up fatigue cracking.

For the fatigue resistance characteristic to be effective, all HMA layers must be fully bonded to allow the system to act as a composite mass. The SMA and performance-designed mixes are specifically engineered to offer exceptional performance under heavy traffic loads. Dense-graded mixes under Item 341 may be used as an alternative to the stiff, rut-resistant HMA base (layer ‘C’). Prior approval by the Construction Division Director or designee is required for this substitution.

Special attention is required in designing a durable foundation by investigating the underlying soils to determine the appropriate type and level of stabilization needed. In lieu of subgrade stabilization, a high quality granular base, cement-treated base, or other engineered foundation should be used.