Example R01.2: FDR and Overlay

The second design alternative to try is FDR and overlay. The FDR layer can be either FDR-FA or FDR-C depending on the characteristic of the underlying AB layer. Please refer to the "Guide for Partial- and Full-Depth Pavement Recycling in California".

For an FDR design, the questions being answered include the thickness of the FDR and the thickness of the HMA on top. Optimizing two independent variables can be a little bit challenging.

The strategy used here is to fix the HMA thickness at the minimum and combine that with the minimal and maximal FDR thickness respectively. These two option should indicate what to change next.

FDR thickness has a minimum of 0.70 ft and a maximum of 1.50 ft.

Note: The maximum thickness of FDR that is practical for a given project may be less than the general maximum of 1.50 ft. In this project for example, there is 0.65 ft of existing HMA and 1.0 ft of AB. To make room for the new HMA in order to maintain grade, some existing HMA needs to be removed before the FDR construction. If the minimum combined AC thickness is 0.40 ft, then this reduces the available HMA for FDR from 0.65 ft to 0.25 ft. As a result, the maximum FDR thickness in this case is 0.25 + 1.0  = 1.25 ft before considering the swell factor.

Option 1: Minimal HMA with minimal FDR-FA

The first option to try is with minimal HMA and minimal FDR-FA. Specifically, 0.2 ft of RHMA-G over 0.2 ft of HMA placed over 0.7 ft of FDR-FA. To maintain grade part of the existing HMA needs to be removed.

Note: one must consider the swell of FDR layer when determining the amount of existing HMA to be removed. In this particular option, using a swell factor of 1.07, a 0.7 ft of FDR-FA only needs 0.65 ft of raw material. The swell is 0.05 ft. Add this to the planned thickness of the new combined asphalt concrete layer (i.e., 0.40 ft), the total thickness that needs to be removed is 0.45 ft.

Since the excavation depth is 1.10 ft (0.45 ft + 0.65 ft), the remaining AB thickness is 0.65+1.0-1.10 = 0.55 ft.

Save the baseline trial as a new trial named "NBL1-493_538: FDR-FA and Overlay". Setup the structure for this option as shown below. Note the reduction in AB thickness compared to the Mill and Overlay option.

The CalME simulation results is shown below, indicating that this option meets the design requirements. In fact, it will last more than 30 years before failing in cracking.

Since minimal HMA combined with minimal FDR-FA works, there is no need for further adjustment.

Option 2: Minimal HMA with minimal FDR-C

Option 2 is the same as Option 1 except the FDR-FA is replaced with FDR-C. To evaluate this option, save a copy of the trial for Option 1 with a new name "NBL1-493_538: FDR-C and Overlay". The updated structure in CalME and simulation results are shown below, which indicate that this option satisfy the design requirement as well.