Example N01: HMA/AB/SG

This example illustrates the design of a new flexible pavement for a TI that is not listed in Table 1. The project location and design requirements are listed below:

•Project location: District 3, Yolo County, Route 5, Northbound, PM 10.0 to 11.0

•Design life: 20

•Design TI: 13.5

•Subgrade type: CL

The first design alternative to be evaluated is HMA/AB/SG.

Step 1: Define a Project and an Empty Trial

Navigate to the Projects page by clicking on the “Projects”  tab at the top of the screen.

Click the “Add Project” button to bring up the project definition screen and enter information as shown below. This screen also asks for the title of a first trial, which is given as “Flexible Pavement with a Starting Thickness” in this case. A “trial” represents a pavement design problem. A Project can have multiple Trials, corresponding to different segments within the project limit.

Click the “Save” button to return to the project page and it should look like below. As indicated in the screen shot, CalME has created a new Project along with an "Trial" using the names given in the dialog above.

Step 2: Define the Trial

At this point, the "trial” has not been defined. It only has a name and a description. To proceed with the design, one needs to input traffic, climate, structure, etc. (see Design Inputs).

To provide design inputs, click the “Input” tab item on top of the screen to navigate to the input page. CalME automatically enters a default project location, climate, traffic load distribution, growth rate, design life and traffic index (TI). While automatically setting these defaults were intended to make it easier to try the program, this information usually needs to be changed. After entering the correct project location, the screen will resemble the screenshot below:

The traffic suggested by CalME for 20 year design life is 12.7 for this location. After changing this to the 13.5, and choose "CL" in the “Create a Trial Structure” box, then click on the Generate button, the input page looks like below:

CalME uses the Starting Point Table to find a starting structure with necessary interpolation or extrapolation, and load it into the Pavement Structure window as the starting point as shown above.

Several notes:

•The aggregate base layer was set to the minimum specified thickness for this example.  Increasing the AB layer will affect the required thickness of the surface HMA.

•CalME automatically picks the surface mix that meets the requirement on PG grade listed in Table 632.1 of the HDM. For High Desert and High Mountain, the HMA is divided into two layers by default because it is not recommended to use a mix with polymer modified binder for depths below 0.20 ft.

•The name of the materials also serve as links to the corresponding entries in the Standard Materials Library (SML), which provides the list of models as well as the model parameters for each material in the library.

•Use the “Edit” links in the “Pavement Structure” grid to change materials, thicknesses, or any other parameters as needed.

•The layer numbers can be used to navigate to a page to change some additional properties such as Poisson's ratio

•Use the "Delete" links in the "Pavement Structure" grid to delete layers

•Use the "Insert" links in the "Pavement Structure"grid to insert layers

•CalME will use the starting structure to replace any existing structure input, so make sure that is what you intended before hitting the "Generate" button.

Click the “Input” tab on top of the screen to navigate to the simulation parameters page, which is shown below:

Select the desired number of simulations for the Monte Carlo simulations. The default number is 60. You may use 20 simulations to expedite the process of finding the optimal design and then use 60 to confirm the final design. Select the construction variability option, which is enabled by default.

Step 3: Run CalME on the Selected Trial

Use the “ME Design” tab to navigate to the simulation window. Use the “Run Simulation” button at the upper left part of the screen to run CalME. The simulations will run in the background. The cracking/rutting chart and stiffness chart will update in real time as the simulations are running. The Layer Compression and Expected Life summaries will be generated after the simulations are completed. The result is shown below:

As expected, the starting structure satisfies the design traffic. If this is not the case, minor adjustment may be needed. Scroll down the window to review the Layer Compression summary:

Use the scroll bar to right of the "Layer Compression" summary to scroll down to the "Expected Life" summary. The end of the Expected Life report lists the number of simulations that failed and the percent reliability.  In general, designs should target a minimum of 95% reliability.

Keep scrolling down until the overall summary of Expected Life becomes visible:

This design achieved 98% reliability. To ensure that the pavement was not over-designed, iterative trials may be used to determine the minimum HMA thickness required to satisfy the 95 % reliability.

Step 4: Update the Structure and Run CalME

Return to the input screen and update the HMA thickness. For the next iteration, reduce the HMA thickness from 1.05 ft to 1.00 ft. Return to the simulation window and run the next trial. The results of the new trial are listed below.

The messaging area lists that the overall reliability of the structure is less than 95%. This design is projected to fail in year 18.6 due to cracking.

This confirms that 1.05 ft was the minimum required thickness to achieve 95% reliability.

It should be noted that Monte Carlo is probabilistic analysis, and may not return exactly the same reliability for every set of simulations run. One can use either deterministic analysis or Monte Carlo analysis with 10 to 40 simulations as the design is being refined. The final design must be done with Monte Carlo simulations. It is recommended that final design reliability be checked with Monte Carlo analysis using 60 simulations.

When the failure life is close to the design life (e.g., within 1 year), it is not uncommon for a 60-simulation Monte Carlo run to contradict a 20-simulation Monte Carlo run regarding whether a given design is sufficient.

Step 5: Generate a Report

Change the HMA layer thickness back to 1.05 ft and re-run the Monte Carlo simulation so that the results can be used to generate design report.

Use the “Reports” button to bring up the report window. Choose “Mechanistic Empirical” from the list at the top left corner of the window. Use the “Generate” button to generate the report, and the “Download Report” button at the top right of the window to download a PDF copy of the report. See below for the report generated for this example:

Note that the report window does not close automatically. You may wish to close it once you are done downloading the report.

Step 6: Explore Alternatives

Given the weak subgrade (CL) encountered in this example, there may be ways to improve the support for the HMA layer and hence reduce its thickness and reduce the overall project cost. These alternatives are demonstrated in the examples included in the next subsections.