Evaluation of Flexible Pavement Design Methods for Developing Countries: A Case Study in Bangladesh

Abstract

The analysis of data and design of flexible pavement in Bangladesh by conventional flexible design procedures is mostly based on the basis of test and field condition prevailing in western countries. These procedures do not comply with the environmental and material conditions of the region. These conditions generally do not match the parameters for design used in Bangladesh and south Asia. The result is often overdesigned being uneconomical or under-design resulting in heavy maintenance and rehabilitation costs. In this study climatic condition of the region is considered during the design. Coarse grained river sand shall be used as subgrade material. In this design mostly accepted conventional methods for flexible pavement design are compared to check their adaptability for design input condition prevailing in countries of Southeast Asia. For evaluation of flexible pavement design methods for Bangladesh, two currently used methods are selected, i.e., AASHTO 1993 Guide for Design of Pavements structures developed in the U.S.A. and Overseas Road Note 31 from Britain. Indian Road Congress (IRC 37) is also considered for this study for comparison. Pavement structures are designed by all the methods using same input parameters obtained from Bangladesh. The results indicate that both methods recommend nearly equal total thickness. However, AASHTO 1993 procedure suggests thicker concrete layer. whereas Road Note 31 and IRC 37 suggest thicker base and sub-base layers. Mechanistic responses in terms of stresses, strains and deflections at critical points in the pavement structure are calculated by computer program CIRCLY. The damaging factors are calculated using CIRCLY software. Mechanistic responses are usually related to failure modes in pavement. These failure modes generally are rutting, permanent deformation, fatigue and thermal cracking. The results of both methods show that structures designed by AASHTO 1993 procedure are safer against rutting, permanent deformation. fatigue and thermal crack failures. The thicker base and subbase layers suggested by Road Note 31 and IRC 37 to compensate for thinner asphalt concrete layer do not significantly contribute to lowering of stresses. The structures recommended by Road Note 31 and IRC 37 may cost less in the beginning due to lesser asphalt concrete layer thickness, but may fail permanently requiring higher maintenance and rehabilitation costs.