Example R01: Old Flexible Pavement

This example illustrates how to design for the rehabilitation of an existing flexible pavement. The project is located at:

District 3, Sacramento County, Route I-5, North and South bound, PM 15.9 to 16.7

At this location, there are four mainlines in each direction. The two inside mainlines are flexible pavements, while the two outside lanes are composite pavements. The rehabilitation of the flexible mainlines are demonstrated in this section, while the rehabilitation of the composite mainlines are demonstrated in the following section.

The following are the design requirements:

Must maintain grade

20 year design TI of 12.5 with a growth rate of 2%, specified by the Caltrans Office of Traffic Operations

According to site investigation (core logs and DCP data) and as-built plans, the existing structure for the inside lanes can be simplified as:

•0.1 ft of RHMA-O

•0.65 ft of HMA

•1.0 ft of aggregate base

•1.0 ft of lime stabilized subgrade

•CL subgrade

There was no cracking observed on the surface of the two inside lanes.

It is important to note that the 0.1 ft RHMA-O layer is only a functional layer. As is typically the case, it was combined with the underlying HMA layer during backcalculation in CalBack. This layer needs to be discounted by removing it first in CalME when setting up the design structure.

The file exported from CalBack can be found here, which includes backcalculation results for individual segments that are relatively uniform.

Step 1: Load CalBack file into CalME

Navigate to the Projects page by clicking on the “Projects” tab at the top of the screen. Click the “Choose File” button to select an exported CalBack file on the local computer’s file system. Select the “Upload” button to upload the selected files.  When the file has uploaded, CalME will create a new project and a trial for each section in the CalBack File. 

Note that the "Projects and Trials Stored in the Database" will be different from the screen shot below:

The screen should look like the following once the upload is successfully completed.

NOTE: the project name is generated by CalME and will be different if the CalBack file has been imported by the same user before.

Use the drop down list for "Loaded Trial" to see a list of loaded subsections:

The section names indicate the direction, lane number, the lateral position (centerline, inside wheelpath, outside wheelpath), and the range of the pavement covered by the particular subsection. The suffix "sect_1", "sect_2", or "sect_3" was added by CalBack whenever the data was divided into two or more relatively uniform subsections.

As shown above, there are at least one backcalculation results for each of the eight lanes. For some lanes, e.g., SB Lane 4, there are backcalculation results for both centerline and one of the two wheelpaths. Rehabilitation designs should be conduced using data along wheelpaths because they typically endured more damage from truck traffic than centerlines. The centerline data, if available, should nevertheless be used to confirm that the rehabilitation design based on wheelpath data works for the centerline as well.

To start the design process, select the trial "Section: SAC_05_NB_LANE_1_INSIDE_STA_493_538.dat_sect_1" from the list. As the name suggests, it is for part of the Northbound lane 1 along centerline between Station 493 and 538. This section is one of the weakest segment for the inside lanes based on the backcalculated layer stiffnesses.

Step 2: Fixing traffic and pre-rehabilitation structure inputs

Click the “Input” menu item on top of the screen to navigate to the “Project Information” menu, which is shown below:

CalME inputs the pavement structure that is recorded in the CalBack file. As indicated earlier, the RHMA-O layer was combined with the underlying HMA layer while conducting backcalculation.

Note 1:  CalBack converts thickness into mm using 1 inch = 25.4 mm, while CalME converts mm into feet using 1 ft = 300 mm (i.e., 1 inch = 25 mm). As a result, a 1.0 ft AB in CalBack becomes 1.02 ft in CalME. Although it seems awkward, this inconsistency does not affect pavement design because both CalBack and CalME uses mm for layer thickness internally.

Note 2: CalME automatically sets the project location based on the data exported from CalBack, and sets the climate zone and provides an initial guess of the design traffic based on the project location. The county name is case insensitive and can be in either the expanded or the abbreviated formats. The direction is also case insensitive and can be in either the expanded (North, South, East, and West) or the abbreviated formats (NB, SB, EB, or WB). For special counties please follow the rule explained here.

In this case the project location was entered in CalBack and exported, which allows CalME to setup the project location automatically. If this is not the case you will need to enter the appropriate project location. CalME will automatically enter the climate, traffic load distribution, and the growth rate data.

Enter the subgrade type noted in the site investigation.  The updated project input screen is shown below:

As noted in Design Inputs, the traffic inputs recommended by CalME based on project location is for the outside lane. Regardless of whether the design lane is the outside lane, these numbers need to be replaced with those provided by the Caltrans Office of Traffic Operations.

Overwrite the growth rate and TI with 2% and 12.5 respectively, and remove 0.10 ft from the old HMA layer that represents the RHMA-O layer. The screen shot should look like below:

Note: CalME rounds all layers to the nearest 0.05 ft when editing layer thicknesses. As a result the old HMA layer thickness becomes 0.65 ft rather than 0.66 ft.

To facilitate evaluation of multiple design alternatives, save this trial under a new name: "NBL1-495_538: Baseline":