Finally, the success of the revised SAMDM will be determined by the accuracy of the method in terms of the unbiased assessment of the structural capacity of different
pavement types. This accuracy can only be determined by evaluating the structural capacity estimates obtained from the method to a benchmark of observed structural capacities under in-service conditions for a range of different pavement types and environmental conditions. A performance based information system is therefore required to capture the actual structural capacity of different pavement types under in-service conditions, to provide a starting point for designs that may be refined using the ME-design method and to validate and, if required, calibrate the ME-design method.
6 CONCLUSION AND RECOMMENDATIONS
The current version of the South African Mechanistic-Empirical Design Method for flexible pavements is based on technology and models developed during the 1970s and 1980s.
Given the current level of concern regarding the validity of the results from the method and recent developments that are ready for implementation, it is an appropriate time for the revision of the design method. A framework has been created for the revision of the SAMDM. The core components of the ME-design process have been discussed in addition to a number of characteristics desired from the revised method and implementation challenges that will need to be addressed. The design method will have to apply to new and rehabilitation design, bridge the gap between ME-design and day-to-day pavement engineering practice, be unbiased in terms of estimating the structural capacity of different pavement types based on the true performance potential of these pavements and will have to yield realistic, validated design results. Design risk or reliability simulation as well as time-based performance simulation will be introduced using stochastic simulation and recursive analysis respectively.
Significant advances in the characterisation of traffic loading data have been noted.
Recent developments in the measurement of the tyre-pavement contact stress and statistical procedures for “fingerprinting” and correcting axle-load histograms for WIM error have been presented to improve the characterisation of traffic loading for the purpose of pavement design. It is strongly recommended that these improvements should be incorporated in the revised SAMDM.
A general discussion on resilient response modelling revealed that the resilient response characteristics of pavement materials, which is one of the main inputs to the SAMDM, are not yet fully understood and often incorrectly modelled. Yet, a design solution has to be put in place without delay for the initial revision of the SAMDM. A resilient modulus approach, calibrated for the appropriate parameters will therefore be adopted for the initial revision of the SAMDM but will require critical assessment during subsequent revisions. Three broad material types have been identified of which the mechanical properties and behaviour are sufficiently different to warrant separate resilient response models. These materials are hot-mix asphalt, stabilised material and unbound granular material. The resilient modelling options anticipated for these materials during the initial revision phase are presented and include the effects of the significant influential variables appropriate to each of the main material types.
An integral solution of a multi-layer, linear-elastic system will be retained as the primary pavement response model for the initial revision of the SAMDM. The following adaptations of the linear-elastic model are, however, required:
• A density, saturation and stress-dependent model for unbound material; and
• A temperature and load rate dependent model for hot-mix asphalt.
This will be achieved using sub-layering in the integral transformation solution of the multi- layer, linear-elastic system, effectively implementing vertical stress-dependency during the initial revision phase.
Given the problems highlighted in the discussion on the material resilient response models, the primary pavement response model will have to evolve to the point where the following material and analysis models can be accommodated:
• 3-dimensional stress dependency;
• Hypoelastic models calibrated from static test results;
• Perfect plasticity models calibrated from static test results;
• Contact non-linearity at pre-defined cracks; and
• Dynamic analysis including the effects of stiffness, damping and inertia.
It will only be possible to include these models in a finite-element solution. A finite element primary pavement response model will therefore have to be implemented in the subsequent revisions of the SAMDM. FE analysis tools will also have to be developed for the dynamic analysis of dynamic laboratory and field tests such as the FWD test.
A selection of damage models are presented for implementation during the initial revision of the SAMDM. The damage models will be calibrated for the appropriate critical response parameters appropriate to each of the material types as well as the significant influential variables. The method will make provision for the modelling of the plastic strain contribution from all pavement layers including HMA layers, stabilised layers, unbound structural layers and the pavement subgrade.
Elastic strain will be used as the critical response parameter for modelling the plastic strain of HMA layers during the initial revision. The validity of the assumed correlation between the elastic and plastic strain of HMA needs to be evaluated and if found to be incorrect, the use of a more appropriate critical response parameter will have to be investigated.
Stiffness reduction and fatigue damage models will be provided for stabilised and HMA material. The effects of long-term changes in material characteristics on the stiffness, yield strength and permanent deformation resistance of all materials will have to be addressed to enable realistic damage modelling.
The successful implementation of the revised SAMDM will rely on the availability of appropriate and accurate input to the design method. Some of the input to the design method will be project specific but certain of the required inputs will have to be obtained from general information systems. A number of information systems required for the successful implementation of the design method have been identified. These systems will have to be developed, populated and made available to engineering practice for design purposes and include:
• A design traffic information system;
• An environmental information system that also contains predictive relationships to derive field variables such as the temperature of HMA layers and moisture content of unbound layers from the environmental variables. This system also needs to provide input on the construction associated spatial variability of field variables such as layer thickness and density;
• A materials input information system. This system needs to contain all the pre- calibrated material and damage models used in the ME-design method in addition to the basic material properties of the materials used in the calibration of the models. The
system also needs to provide the necessary analysis tools required for the project specific calibration of the material models.
7 ACKNOWLEDGEMENTS
The sponsorship of the South African National Road Agency Ltd enabled the completion of the inception phase for the revision of the flexible pavement design method. Their support is gratefully acknowledged.
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