PerRoad: Pavement Structural Inputs
The Structural and Seasonal Information input dialog box,
pictured below, defines the pavement structure to be analyzed, the seasons to be
evaluated, the input variability associated with the stiffness and thickness of
each layer and the performance threshold criteria.
The particular inputs are outlined in the steps
below.
Determine the
number of pavement layers to include in the analysis by clicking the appropriate
number, up to five.
Select the seasons to evaluate, up to a total of 5. The summer or normal condition is chosen by default. For each season evaluated, enter the duration in weeks per year. The figure below shows the default values. Note that the number of weeks must add to 52. Also shown in the figure below, for each season, are the representative mean air temperatures for each season. These temperatures will influence the asphalt stiffness, if “Temperature Correction” is checked.
The Current Season drop-down box indicates the season that
the other boxes pertain to and may be used to view each of the other selected
seasons.
For example, choosing “Summer” will display the modulus of
each pavement layer for the summer season. The values will also be updated
when any of the Duration or Mean Air Temperature boxes are clicked.
Each pavement layer must be defined by its material type, seasonal layer modulus, Poisson Ratio and layer thickness as shown in the figure below. The drop down box allows you to select from a list of material types that will load default seasonal modulus values. Selecting asphalt concrete (AC) enables you to select the PG grade of the asphalt which modifies the temperature-modulus relationship that will predict the AC modulus from the Mean Air Temperature if the Temperature Correction box is checked. If you would like to enter your own values for AC modulus, be sure to uncheck the Temperature Correction box. If you have modulus values that fall outside the range allowed for the material type, simply select "Other" from the drop-down material type box which allows for a very wide range of modulus values.
The thickness
of each layer must be specified in terms of inches.
By default, layers not used in the problem will have a thickness of
999 inches to simulate and infinite amount of material.
When choosing number of layers greater than the previous number selected,
be sure to change the thicknesses of the new layers.
You should review, and modify if necessary, the modulus and thickness variability for each pavement layer. This is done by clicking the Variability button for each layer which brings up the input variability dialog box shown below. You may choose between Log-normal and Normal distributions and set the corresponding coefficient of variation. The default values were set to be consistent with values found in the literature but can be adjusted as needed. These values are used during the Monte Carlo simulation to generate pavement response distributions. Generally speaking, the amount of variation should increase deeper into the pavement structure.
Clicking on the Performance Criteria button will enable you to set the design criteria for each pavement layer as shown in the figure below. When the input box first comes up, no criteria have been selected and the input box is blank. You must decide which locations (Top, Middle and/or Bottom) require design criteria and what criteria to set as discussed below. Typically, you will want to add criteria to control bottom-up fatigue cracking and rutting. As explained below, bottom-up fatigue cracking is usually controlled by monitoring horizontal tensile strain at the bottom of the lowest new asphalt concrete layer. Rutting is often controlled by monitoring vertical compressive strain at the top of the subgrade layer.
Setting Bottom-Up Fatigue Cracking Design Criteria
(Horizontal Strain Distribution)
Recent research at
the National Center for Asphalt Technology has supported the use of strain
distributions for controlling bottom-up fatigue cracking.
The main idea of this design approach is to control the range of strain
values experienced by the pavement below a pre-defined range.
The range of values are quantified by their magnitudes and corresponding
percentiles. In PerRoad, the design
will be controlled by the 95th, 85th, 75th, 65th
and 55th percentiles, respectively.
The figure below
shows the results of a sample design where strain distributions were used.
The x-axis represents the tensile microstrain.
The values are negative to indicate tension.
The y-axis represents the cumulative percentile.
The red line labeled “Target” indicates the control
strain distribution.
Points above this line indicate a greater chance
that the strain levels are below the target values.
Points below this line indicate a poorer chance that
the strain levels are below the target values.
Therefore, the goal of the design is to select
thicknesses such that the resulting strain distribution percentiles fall
entirely above the target line.
If this is the case, then fatigue cracking should
not occur because the pavement is experiencing lower strain levels than those
expected to crack the pavement.
In the example
above, Trial 1 resulted in a failing distribution.
At the higher percentile and greater tensile strain levels, the results
were okay since the percentiles were above the target line.
However, at about the 78th percentile (about -160
microstrain), the percentiles fell below the target line indicating excessive
strain levels with the potential to develop fatigue cracking.
Trial 2 was then executed with additional asphalt concrete thickness and
the resulting percentiles, entirely above the target line, correspond to an
acceptable design since the strain levels are below the point where cracking is
expected. Additional trials could
be executed to bring the predicted strain distribution closer, without falling
below the target.
When you select
horizontal strain distribution from the drop down list of criteria, the input
boxes shown below will appear and must be populated.
This may be done three different ways as described below.
Method 1:
Use NCAT Default Values
Clicking the “Load
Default Distribution” button will automatically load strain criteria based
directly on results from the NCAT Test Track.
This selection assumes that the materials in this design are generally
consistent with those from the NCAT Test Track.
If you do not have any better information, this is an acceptable starting
point.
Clicking the “Enter
Endurance Limit” button will open the screen below and allow you to set a
laboratory-determined endurance limit.
This endurance limit will be used with NCAT Test Track based lab-to-field
ratios to establish the control strain levels.
This method is best used when you have a measured laboratory endurance
limit, but it still relies on Test Track data.
Method 3:
Enter Values Manually
You may simply enter
microstrain levels, at each percentile, rather than use NCAT Test Track
defaults.
Full documentation of the strain distribution approach is provided in the following report:
Setting Bottom-Up Fatigue Cracking Design Criteria
(Non-Horizontal Strain Distribution)
The figure below shows the main elements in selected design criteria. The layer position box (Top, Middle or Bottom) must first be checked. The criteria is then chosen from the drop-down box. Any choice except for the very last (Horizontal Strain Distribution) will enable you to enter a single threshold value and corresponding percentile control value. These are used in the PerRoad Monte Carlo simulation to act as a design control. In the example below, the design will require that the 50th percentile strain be at or below 150 microstrain in tension. If horizontal strain or vertical strain are selected as the criteria, the check box shown in the figure below is enabled and you can choose to add a transfer function to perform conventional M-E design, as described below.
Setting Rutting Design Criteria
(Vertical Strain Percentile)
Rutting is often controlled by monitoring vertical compressive strain at the top of the subgrade layer. This location serves as an indicator of how much deformation is reaching the subgrade and serves as a predictor of total rutting occurring throughout all the pavement layers. As documented in recent research, limiting the 50th percentile vertical compressive strain to less than 200 microstrain has been effective in controlling structural rutting. The figure below shows the relevant input screen and values.
Conventional M-E Design Criteria
If Horizontal or Vertical strain is selected as the design criteria, and the Transfer Function button is checked, you will be able to enter transfer function coefficients as shown in the figure below. The transfer function will be used to compute a damage accumulation rate, and the time until failure for a non-perpetual pavement. This is known as a conventional mechanistic-empirical (M-E) design. Note that the threshold strain must still be specified. When PerRoad executes the Monte Carlo simulation, strain responses less that the threshold do not contribute to damage accumulation. Strain values exceeding the threshold will be entered into the transfer function and used to compute damage over time. This approach should only be used for conventional M-E design where pavement failure (i.e., terminal cracking or rutting) is the desired outcome.
