Description of incremental recursive procedure

The incremental recursive procedure works in increments of time and uses the output from one increment, recursively, as input to the next increment. The procedure predicts the pavement conditions, in terms of layer moduli, crack propagation, permanent deformation and roughness (with Monte Carlo simulation), as a function of time, but it does not carry out an automatic design, where the needed layer thicknesses to achieve certain pavement conditions at the end of the design life, are determined. It may be used to check a design done using the Caltrans present methods or the Classical Mechanistic-Empirical method, and to modify that design if it is found to be unsatisfactory.

The method also allows the user to include one or more Maintenance & Rehabilitation actions, either at fixed points in time or triggered based on the predicted pavement condition.

The default duration of each increment is 30 days, but this may be changed by the user. The program will select the day in the middle of the first increment as being representative for the climatic conditions during that increment. The representative day is divided into periods. The default division is into 5 periods of 4, 4, 5, 5, and 6 hours, starting at 13 hours (1 pm), but this can be changed in the database table "Daily time periods" by the user. By doing this it is possible to use time increments form one hour and upwards. For calibration using HVS or track tests an increment of one hour is used.

The temperatures at different depths of the pavement structure, over the simulation period, are first calculated. The temperature at the surface is read from the EICM database (with 30 years of data) and the temperatures at different depths are  calculated using the surface temperature, a constant deep soil temperature and the previous temperatures. It is done using a 1-D Galerkin Finite Element formulation with a finite difference time step. Calculation over one year is used to initialize the system.

For the first period of the first time increment the program applies the load spectrum, read from the WIM data table, one load at a time. If the calculation considers wheel wander, the load is applied at the first lateral position. The temperature at one third depth of each asphalt layer is determined. The master curve for each asphalt material is used with this temperature and the loading time (depending on the vehicle speed and the depth in the structure) to determine the modulus of each asphalt layer. The modulus may also be influenced by existing damage to the layer and by aging/hardening. For the unbound materials the moduli are modified according to season and to the time elapsed after thawing (if relevant). The moduli of the unbound materials may also be influenced by the stiffness of the pavement layers above the material and by the load level.

For each load, at each load position, the critical stresses and strains in the materials are calculated at a reference line. As the default the reference line is assume to be at the center line of the single wheel and of one wheel in the dual wheel, but the wheels may be offset with respect to the reference line. For each layer the increase in damage and in permanent deformation is calculated using the time  hardening procedure. For the next load or load position the new conditions of the pavement layers are used for determining the moduli and the increase in damage and permanent deformation.

When all load positions for all loads during the first period have been completed, the temperatures and moduli for next period are calculated and the loads of the period are applied, and so on until all periods of all increments in the desired analysis period have been completed.