This confirms that the thesis entitled Evaluation of flexible Pavement Design Methods for Developing Countries:. For the evaluation of flexible pavement design methods for Bangladesh, two currently used methods have been selected, namely the AASFFI'O 1993 Guide for Design of Pavements Structures, developed in the USA.
List of Tabks
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Background
Over the past two decades, tremendous developments have been offered for more rational and rigorous pavement design procedures. These and other engineering data have enabled the development of the mechanistic-empirical pavement design procedure.
Statement of the Problem
This study will also consider the Indian Road Congress (IRC) method to compare the flexible pavement design with AASH1'O 1993 and Road Note 3 1.
Objectives
Project
This road is a vital link in the national highway network and forms part of the Asian Highway Network.
Organization of the thesis
General
- Structural Component of a Pavement
Subsoil: This is the top layer of natural soil, which can be intact local material or soil that has been excavated elsewhere and placed as a sponge. Overlay: This is the top layer of the pavement and will usually consist of a bituminous surface coating or a layer of pre-fixed bituminous material.
Review of Flexible Pavement Design Principles
- Mechanistic-Empirical Methods
However, most of the work was based on variants of the same two strain-based criteria. MNDOT (Timm, 1998) adopted a variant of Shell's fatigue crack model developed in Illinois and the Asphalt Institute's track model.
Pavement Distress Criteria
- Multilayer Elastic Computer Programs
- CIRCLY Software
- Maximum Stress, Strain and Deflection used in CIRCLY
If the cumulative damage is less than 1.0, the system has excess capacity and the cumulative damage represents the percentage of life consumed. If the cumulative damage is greater than 1.0, the system is predicted to fail before all design traffic is implemented.
Methods Based on Mechanistic-Empirical approach
- AASHTO Pavement Design Guide
- Indian Road Congress 37 Pavement Design Guide
- RHD Bangladesh Pavement Design Guide
The guide was based on the results of the AASHO Road Test, supplemented with the existing design procedure. The pavement designs are given for subsurface CBR values ranging from 2 percent to 10 percent and design traffic ranging from 1 msa to 150 insa for an annual average pavement temperature of 35 degrees Celsius. The layer thickness obtained from the analysis has been slightly adjusted to adapt the designs to the stage construction.
Materials Characterization
The ESAL design value depends on the traffic volume survey report conducted by RHD Bangladesh. The ESAL design value depends on the traffic volume survey report conducted by LGED, Bangladesh.
Traffic Volume
Other Material Variables
- Subgrade Properties
- Mechanistic - Elastic Design
- Asphalt
Asphalt stiffness is usually determined by the weighted average annual pavement temperature (WMAPT) of the project site. Selection of appropriate asphalt stiffness values also depends on the expected heavy vehicle speed for the project.
General
Selection of Highway/Project
Data Collection
- Material Properties Resilient Modulus of Roadbed
- Traffic Analysis
- Determination of Secondary and Other Input Variables Drainage
The elastic modulus for the base and subbase is taken as their respective elastic modulus. Specific instructions are given in the guide to select drainage coefficients under particular conditions. The effect of drainage on the performance of flexible pavements is considered in the AASI-ITO 1993 guideline in relation to the effect that water has on the strength of the base material and roadbed soil.
The recommended drainage coefficients for unbound bases and subbases used in flexible pavement design in Bangladesh are provided in Table 3.1 (Huang, 1993). The initial and terminal serviceability index must be established to calculate the total change in serviceability used in Equation 4.1. Such a low value of P should only be used in special cases on selected classes of motorways.
PAVEMENT DESIGN
General
- Subgradc Strength Evaluation
- Road note 31
- IRC: 37-2001
- Traffic
- Traffic Data Selection
- Design ESALs
- Selection of Representative ESAL Values
- Asphalt concrete (AC) and unbound granular material characteristics .1 Asphalt Concrete layer
- Unbound Granular Material Characterization
- Performance Criteria for Functional Classification
- Performance Period
The Effective Roadbed Resilient Modulus (MR) is used to characterize the soil strength of the roadbed under stress and moisture conditions for pavement design in the AASHTO 1993 design guide. The layer coefficients for wear surfaces are taken from the AASI ITO 1993 pavement design guide. These material properties relate to the following design classes in the Road Note 31 pavement design procedure.
For the structural design of the N4 highway, a reliability of 85% and a standard deviation (S0) of 0.45 is recommended, based on the guidelines of the AASHTO 1993 pavement design guide. The design of the pavement of the N4 assumes a performance period of 20 years before large-scale repair or reconstruction takes place. To date, the ranges for all input parameters required for pavement design have been established by AASHTO 1993 Design of Pavement Structures and Road Note 31 Pavement Design Procedure for Tropical and Subtropical Countries.
Comparison of Design Output for 3 rd
Comparison of results obtained by the Three Design Procedures
- Asphalt Concrete Layer
- Base Layer Thickness
Figure 4.7 shows that the AASHTO 1993 design procedure gives a much larger thickness for the asphalt concrete layer than is recommended in the Road Note 31 and IRC 37 design procedures. It can be seen that the higher base thickness is recommended by the Road Note 31 and IRC 37 Procedures roadway planning. This thicker value may be available to compensate for the lower amount of asphalt concrete layer given by the Road Note 31 and IRC 37 design methods.
Figure 4.9 is obtained by comparing the subbase thickness recommended by the two pavement design procedures. The same trend is also shown in the case of underlayment as is the case for underlayment, i.e. Road Note 31 and IRC 37 Pavement Design Procedures recommend greater thickness for underlayment. To date, it has been found that the lower asphalt layer thickness achieved in the Road Note 31 and IRC 37 design procedures is compensated by a thicker base and subbase.
Total Design Thickness
The evaluation of these approaches was performed by analyzing the three design procedures in terms of their mechanistic responses.
Summary
General
- Permanent deformation and Rutting
- Fatigue Cracking
- Thermal Cracking
To control permanent deformation, the vertical compressive stress reaching the top of each layer must be kept within the allowable limits of the material. Fatigue cracking at the bottom of the asphalt layer depends on the cumulative action of repeated traffic loads. The fatigue effect associated with this horizontal tensile strain at the bottom of the asphalt concrete layer.
Thermal cracking is accompanied by occasional very heavy loads, especially at low temperatures, causing cracks to develop on the underside of the asphalt layer. At low temperatures, incipient cracks at the bottom of the asphalt layer are very stiff and a heavy load can cause a large tensile force at the bottom of the asphalt layer. This is checked by checking the horizontal tensile stress at the bottom of the asphalt concrete layer.
Traffic Loading
- Load Condition as per AASHTO 1993 Guide
- Loading Condition for RHD Guideline of Bangladesh based on AASHTO 1993 and Road Note 31 Guide
The total number and axle load of heavy vehicles that will use the road during its design life should be taken into account. The total number of vehicles that will use the road during the projected period must be expressed by the cumulative number of equivalent standard axles (esa). In the standard axle method, the traffic is defined according to the total number of standard axles (8160 kg) that must be implemented during the planned lifetime of the road.
Its calculation involves estimating the initial volume of commercial vehicles per day, lateral distribution of traffic, the growth rate. For pavement design purposes, all heavy commercial vehicles are expressed in terms of the equivalent number of standard axles. Using the estimated AADT for the vehicle categories together with their equivalence factors shown in Table 2.2, estimates must be made of the current daily ESAs for the road.
Multilayer Elastic Analysis Computer Program
In all cases, underground material with a CBR value of less than 2% must be removed and replaced with filling material.
Evaluation of Design Procedures by Distress Modes Considered
- Permanent Deformation
- Fatigue Cracking
It is seen from Figure 5.2 that in case of deflection the AASI-ITO 1993 recommends less value of deflection than that of Road Note 31 and IRC 37 Pavement Design Procedures at the top of subgrade layer. This is because the smaller asphalt concrete layer thickness recommended by Road Note 31 and IRC 37 than that of AASHTO 1993 Pavement Design Procedures. It is seen from Figure 5.3 that in the case of vertical deformation, the AASFITO 1993 recommends less value of vertical deformation than that of Road Note 31 and IRC 37 Pavement Design Procedures at the top of subgrade layer.
Road Note 31 and IRC 37 recommend higher value of thickness for base and sub-base layers than that of AASI-ITO 1993 Pavement Design Procedure. This is because the higher asphalt concrete layer thickness is recommended by AASI-ITO 1993 than that of Road Note 31 and IRC 37 Pavement Design Procedures. From Figure 5.7 it is seen that the vertical stress at the top of base as designed by Road Note 31 and IRC 37 rejection methods is more than AASHTO 1993 design method.
Conclusions
Four major distress modes, i.e., abrasion, permanent deformation, fatigue, and thermal cracking are associated with stresses, strains, and defects at critical locations in a pavement. It was found that smaller values of stresses, strains and deflections were shown in pavement structures designed by AASHTO 1993 Pavement Design Procedure. While much higher values of mechanical responses were obtained in the pavement structure designed by Design Procedure Road Note 31.
But the values designed by IRC 37 were found to be less than those of Road Note 3 I. It concluded that pavement structures designed by AASHTO 1993 design procedure are relatively safer against rutting, fatigue and thermal cracking failures, than the structures designed by the Road Note 3 was designed. 1 and IRC 37 design procedures. Pavement structures designed by Pathnote 3 I Pavement design procedure can be cost-effective in construction due to less AC layer thickness, but they can fail prematurely requiring high maintenance and rehabilitation costs. It is concluded that the pavement structures supporting heavy traffic in Bangladesh and other developing countries can be designed by AASIITO 1993 Pavement Design Procedure and checked by multi-layer elastic computer software for mechanistic pavement design.
Recommendations
- Recommendations for RHD
- Recommendations for further study
Estimation of equivalent axle load per vehicle (EV) followed in this design in accordance with RH[) recommendation which can be accurately assessed if a Weigh-in-Motion facility to weigh and count vehicles is available in the country . AASI-ITO 1993 pavement design procedure leads to more conservative design for design input conditions of pavements in Bangladesh. The soil strength parameter CBR used in Bangladesh for pavement design should be replaced by effective roadbed soil resilient modulus as suggested by AASHTO Pavement Design Guide 1993.
The following efforts can be conducted to further improve the evaluation of flexible pavement design methods in developing countries. The compatibility of other design methods such as Asphalt Institute, NAASRA, SHELL pavement design methods etc. can also be evaluated for their application in developing countries. The evaluation of the design methods used in this study can be improved if other important highways of Bangladesh are included in the analysis.
GRANULAR ROADBASE I STRUCTURAL SURFACE
A maximum of 100mm of sub-base may be replaced by a selected fill provided that the sub-base is not reduced to less than the thickness of the road base or 200mm, whichever is greater.
If( 37-2Aft0 I pAv1:MENT I)ISIGN cA'IALo(;JI
