Example R01.1: Mill and Overlay and Remove HMA and Replace

The first design alternative to try is mill and overlay with the same thickness. The question being answered is how much should be milled and in turn replaced. If milling and replacing all of the old HMA is not sufficient, part of the existing AB layer may also need to be replaced with new HMA.

For the given climate zone (Inland Valley), HDM requires to use mixes with PG70-10 binder for HMA, or PG64-16 or PG70-10 base binder for RHMA-G.

Note: the RHMA-G mix with PG70-10 base binder is currently not available from the CalME Standard Materials Library, it will be added to the library once enough data has been collected to properly characterize its performance.

Milling in CalME is simply achieved by reducing the thickness of the existing HMA layer. Deleting the existing HMA layer is equivalent to removing the whole existing HMA layer.

Option 1: Remove HMA and Replace

Given the relative thin existing HMA layer, the first option to try is to remove all the existing HMA and replace it with 0.2' of RHMA-G and 0.45' of HMA with PG70-10 binder. If this design is sufficient, then further explore the possibility of leaving some of the existing HMA in place.

To evaluate this design, save a copy of the trial named "NBL1-493_538: Baseline" as "NBL1-493_538: Mill and Overlay". Once the new trial is loaded, remove the existing HMA layer by using the "Delete" link. Add the RHMA-G and HMA layers to the pavement structure.

The project inputs and the simulation results are shown below. As one can see, the design is a valid option as it has higher than 95% reliability.

Note: The PG grade requirement specified in Table 632.1 in the HDM is meant for surface mix only. An HMA with PG64-16 base binder may also be used below 0.20 ft for Inland Valley.

Option 2: Keeping some existing HMA

Given that a complete removal of the existing HMA satisfy the design requirement, next step is to explore the option of keeping some existing HMA. A good option is to remove 0.4 ft of the existing HMA and replaced with 0.2' or RHMA-G and 0.2' of HMA. This leaves 0.25 ft of exiting HMA in place, which should provide sufficient cushion for typical construction variability in the existing HMA layer thickness.

Note that there is no need to activate reflective cracking for this design since there is no cracking observed on the existing pavement for the inside lanes.

To evaluate this option, save the previous trial under a new name "NBL1-493_538: Partial Removal".

The project inputs, simulation parameters, and simulation results are shown below, which indicated that this option is not viable as it fails in 8.4 years. The simulation results also suggests that the onset of surface cracking coincides with the significant drop in minimum stiffness of the existing HMA layer (the Min-E3 line).