FDR and Overlay

The original site investigation has shown that the cracks are predominantly bottom-up distresses. Following the recycling guide (Guide for Partial- and Full-Depth Pavement Recycling in California), PDR is not an option since the distresses are deeper than 0.4 ft.

With 0.65 ft of old HMA thickness and 0.4 ft of AB below, FDR is an appropriate option. In this example, section 3 had less than 15% passing the # 200, and the PI was less than 6. FDR-FA is thus an appropriate strategy. Given the low stiffness in the SC subgrade, a 0.35 ft of AB is kept in place to help achieve good compaction for the FDR layer. The maximum thickness for FDR is 0.70 x 1.07 = 0.75 ft if no fresh AB is added on top of the old HMA before FDR operation.

Two options are evaluated here, one with 0.75 ft of FDR which does not require importing fresh AB, and the other with 1.0 ft of FDR and requires importing 0.30 ft of new AB.

Note: FDR-FA itself can be placed directly over subgrade to provide a good construction platform for the upper layers.

The proposed design shown in the figure below. In this example, the goal is to design the thickness of the HMA layer. The structure will be capped with a 0.20 ft HMA-PM overlay.

Load the "Section 3: Remove and Replace" trial and save a copy of it as “Section 3: FDR and Overlay“. Change the AB thickness to 0.35 ft, and add the FDR and HMA layers. The project inputs are shown below:

Several iterations were run to determine the thicknesses the FDR and HMA layer to carry the design traffic for the design life. Designs were completed with FDR thicknesses of 0.75 ft and 1.0 ft to determine minimum thickness for the HMA layer. The results are summarized in Table 6. The results show that the minimum thickness of HMA reduces as the FDR layer thickness increases, requiring a minimum of 0.45 ft and 0.30 ft of HMA for FDR thicknesses of 0.75 ft and 1.0 ft respectively. The type of failure is predominantly rutting as a result of compression in the HMA-PM and subgrade.

Table 6: CalME Results for FDR and Overlay Designs

                                                    *: shaded rows indicate optimal designs

The simulation results for 0.75 ft FDR option is shown below:

The simulation results for 1.0 ft FDR option is shown below: