ANALYTICAL TOOLS FOR DESIGN OF FLEXIBLE PAVEMENTS

This paper falls into two parts. In the first, the widely used analytical-empirical method of pavement design and evaluation is discussed and in the second two simulation models are presented. There are important differences between the assumptions on which the theoretical models are based, and the reality of pavement materials and structures, and these differences are important both for the determination of input values (elastic parameters) and for the calculation of pavement response. Linear elastic theory often results in incorrect moduli, when used for backcalculation of layer moduli from deflection testing, and in questionable stresses and strains, when used for forward calculation. Including non-linear materials characteristics may improve the theoretical model, but no theoretical model has yet been conclusively verified with experimental data. The empirical relationships used to predict pavement deterioration from critical stresses or strains, are equally problematic. In the second part of the paper two theoretical models are presented. The first model simulates the deterioration of a section of pavement over time using an incremental-recursive process. The model is stochastic and considers the spatial variation of pavement materials, layer thickness and traffic loads as well as seasonal variations. The second model deals with the forces on, and the displacements of, the individual grains in a particulate medium. This model does not rely on empirical relationships to predict permanent deformation or failure, but models this in the same process as forces and displacements. The input to this model includes the grain size distribution, the shape of the grains and the degree of compaction, parameters that are similar to those used for specification of materials and quality control. It is concluded that an interaction between development of theoretical models and experimental verification is needed to improve the understanding and predictability of the complex process of pavement deterioration.