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Fatigue and rutting strain analysis of flexible pavements designed using CBR methods
Abstract
The layered elastic analysis of pavements designed using three known CBR methods; the Asphalt Institute, the National Crushed Stone Association and the Nigerian CBR methods were carried out to evaluate their fatigue strain and rutting deformation characteristics. The elastic properties of the materials were determined. Structural thickness requirement of the pavements were carried out using their respective design charts for a traffic volume of 3000 vehicles/day and expected load repetition of
3.2 x 106. Stresses, strains and deflections due to 80 kN single axle load having a tyre pressure of 690.78 kPa were computed by analyzing the effect due to 20 kN single axle load spaced 30.5 mm centre to centre. Strain evaluation was carried out at the underside of the asphalt bound layer and at the top of the subgrade 15.25 mm (midway) between the 20 kN axle loads. The Heukelom and Klomp Model, and the Asphalt Institute Model were used to evaluate pavement response. Results showed that the horizontal tensile strains on the underside of the asphalt bound layer were 355.50, 355.34 and 371.91 ì for Asphalt Institute CBR, NCSA CBR and the Nigerian CBR methods respectively. Similarly, the vertical compressive strains at the top of the subgrade were found to be -924.033, -906.7 and -774.24 ì for Asphalt Institute CBR, NCSA CBR and the Nigerian CBR methods respectively. The resulting fatigue
and rutting strains were compared with the permissible values using the Heukelom and Klomp Model, and it was found that the computed vertical compressive strains were more than permissible values. In terms of fatigue, the damage factors were found to be less than 1.0 while in terms of rutting, the damage factors were greater than 1.0 for both models. It was concluded that flexible pavements designed using the three known CBR methods are prone to failure due rutting deformation and
recommended the use of mechanistic procedures in the design of flexible pavements in developing tropical countries. The study was carried out with the layered elastic analysis software EVERSTRESS.
3.2 x 106. Stresses, strains and deflections due to 80 kN single axle load having a tyre pressure of 690.78 kPa were computed by analyzing the effect due to 20 kN single axle load spaced 30.5 mm centre to centre. Strain evaluation was carried out at the underside of the asphalt bound layer and at the top of the subgrade 15.25 mm (midway) between the 20 kN axle loads. The Heukelom and Klomp Model, and the Asphalt Institute Model were used to evaluate pavement response. Results showed that the horizontal tensile strains on the underside of the asphalt bound layer were 355.50, 355.34 and 371.91 ì for Asphalt Institute CBR, NCSA CBR and the Nigerian CBR methods respectively. Similarly, the vertical compressive strains at the top of the subgrade were found to be -924.033, -906.7 and -774.24 ì for Asphalt Institute CBR, NCSA CBR and the Nigerian CBR methods respectively. The resulting fatigue
and rutting strains were compared with the permissible values using the Heukelom and Klomp Model, and it was found that the computed vertical compressive strains were more than permissible values. In terms of fatigue, the damage factors were found to be less than 1.0 while in terms of rutting, the damage factors were greater than 1.0 for both models. It was concluded that flexible pavements designed using the three known CBR methods are prone to failure due rutting deformation and
recommended the use of mechanistic procedures in the design of flexible pavements in developing tropical countries. The study was carried out with the layered elastic analysis software EVERSTRESS.
