Overlay design requires that you have performed backcalculation generating
elastic moduli for the pavement layers. You can design based on the whole structure or part of the structure by removing top layers.
New design does not make use of FWD results or backcalculation. You can
define a structure and calculate the design thickness of the top layer.
11.1 Overlay Design
Sections with backcalculation results are listed in the grid. In the column Chosen you can mark sections to be included in the design by clicking in the field.
If parameter file and material types were already entered in the Structural input, then this information is shown in the grid, and the name of parameter file used. If the fields with the layer’s material type are empty, you have first to select which parameter file to use, and then by clicking in the fields identify the material type for each layer from the popup list.
If you have multiple sections, you can enter the material type for the first section, and following click the button Copy first structure to copy this to the other sections.
Base design on
Here you select if any existing layers should be removed before design of new overlay. When you select an option, you will see in the grid which layers are removed indicated by a greyed font color.
If you select Existing pavement then you have an option to mill a part of the top layer, by entering a value in the frame Milled from layer 1. The value entered must be smaller than the thickness of the top layer.
If you select Existing unbound layers then you can in the same way have a part of the first granular layer removed.
If you select Existing subgrade then you can specify an improvement to the subgrade moduli determined from the backcalculation by entering a percentual increase. Entering a value of 25 means the existing moduli will be multiplied with a factor of 1.25
Add new layers
Here you define which layer(s) to add on the top. In the usual case where you design for an overlay of the existing pavement, you will enter one bound layer type. Click in the combo box to specify the design layers material type. The reference modulus for this material will automatically be inserted in the Modulus column, and you have following possibility to change this value.
In the case where you wish to do a reconstruction based on the existing subgrade you would normally also add new granular layers on the top. Elmod will only calculate design values for the top layer, so you will have to enter both thickness and modulus of the other new layers added, like shown in this example:
The new construction must not have more than 6 layers, with maximum 2 bound layers included.
Traffic
Traffic must be entered as the total passages of all vehicles per year. The number of passages for each vehicle is then entered in the fields below as percentages of this value.
These are the values directly used in the deterioration functions defined in the parameter file. Issues regarding multilane roads, equivalence factors, traffic growth etc. must be considered before entering the value. Facilities are available for this through the buttons in the upper right corner of the Traffic frame. For help on this see details in the Elmod 6 Help file.
If multiple vehicles have been defined in the parameter file, only the Forecasting facility is available.
For airfields you have usually defined a fleet mix of the most critical aircrafts using the facility, as illustrated in this example:
If a normal distribution for the lateral wander of loads was selected in the
parameter file, the offset from the centerline to the line where the damage should be evaluated must be entered. Only one distance from the centerline will be evaluated in each design. In this case the critical aircrafts have a tread around 10.000 mm, and the offset should then be around 5.000 mm. You can repeat the design with various offset values to check if the design is sensitive to the change. If the structure has been defined with jointed concrete as top layer, k-values are calculated for both center and joints during the backcalculation. In the design window you will have an option for improving k-values at joints before overlaying:
If you select Subsealing the design calculation will assume the k-values at joints are improved to be the same as the values under slab center.
Earlier created designs will not be overwritten, meaning you can view design results in the Plot feature for various traffic input or other changes.
The button View Plot is a shortcut to the plot of the design results.
11.2 New Design
With this facility you can design a new pavement structure, and get the required thickness of the top layer calculated. No results from the FWD testing or backcalculations are used, so the facility is available even you have no file opened. You still have to create a database as usual.
Select parameter setup
Select the parameter file that is valid for your design calculation. You have to select your parameter file, before you can enter material types in the pavement structure. For setting up parameters, see 13.
Parameter Setup
Add Layers
Here you enter your pavement structure from bottom to top. Only the thickness of the top layer can be calculated. First layer to add is the subgrade material. The material’s reference modulus is automatically inserted, and if desired you can change this value. Next you can add the pavement layers with their respective thicknesses (remember to
enter the thickness to the right of the layer name), and at last the top layer for which the thickness is to be calculated.
Use Monte Carlo Simulation
Check this to base the design on a Monte Carlo Simulation. You will have the following input window next to the Add Layers window:
This will simulate a variation in the layer moduli according to a logarithmic normal distribution. Each simulation will generate a set of moduli randomly but with probabilities that follow the distribution given by the standard deviation factors. The standard deviation factor is 10 raised to the standard deviation of the logarithms, also called the geometric standard deviation. The number of runs in the simulation, and the standard deviation factor for each layer must be entered. The result for each simulation can be viewed from the Plot facility, and the average and standard deviation will be reported in the Results frame.
Select Method
Responses under the design loads can be calculated either according to
MET (Method of Equivalent Thicknesses) or LET (Linear Elastic
Theory).
Traffic
Please refer to description of input in previous chapter 11.1 Overlay Design.
Results
When you have clicked Start Design the calculation will be carried out, and the results can be viewed in the Results frame:
If a Monte Carlo simulation has been used, the thickness is given as the average over the number of simulations and with the corresponding standard deviation.