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Three Layer System
The proposed design is provided in figure below. Three layers were added on top of the existing structure. A 0.2 ft rich bottom layer, an HMA layer with 25% RAP, and a final 0.20 ft HMA-PM surface course. The purpose of this design is to design the thickness of the HMA layer to carry the expected traffic.
Note: the reason for using the mix with up to 25% RAP as the intermediate layer is to take advantage of its high stiffness. Other options such as the mix with PG70-10 binder can provide similar benefit. Mix with softer binder may also be used, albeit with potentially thicker layer.
To evaluate this option, make a copy of the "Section 3: Baseline" trial under a new name "Section 3: Three-Layer Overlay". After adding the new layers, the project input screen is shown below:
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Note that the debonding within the old HMA layer can be accounted for in CalBack in two ways:
Explicitly: by splitting the old HMA layer into two lifts and indicate that they are debonded;
Implicitly: by treating the old HMA layer as a single layer, which should result in low back-calculated stiffness for the layer.
Judging the fact that there is only one layer for the old HMA, the debonding was accounted for implicitly in CalBack. This is confirmed by the relatively low stiffness imported from CalBack for the old HMA layer (see the screen shot above).
This design requires that reflective cracking be selected since the cracks in the existing HMA will not be removed. The simulation parameter screen is shown below:
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It is useful to enter a duration longer than the design life to determine how long this design will be able to carry the expected traffic if it does last longer than the design life.
For this example, three iterations were run to determine a suitable thickness of the intermediate HMA layer. To expedite the process, some preliminary Monte Carlo analysis were run with 20 simulations. The results are provided in Table 2. The results show that the thickness of the HMA layer should be at least 0.40’ to carry the expected traffic over the design life. Cracking is more critical with 0.30’ of intermediate layer, but both cracking and rutting are critical with thicker intermediate layer.
Table 2: Overlay CalME Results
|
Total AC Thickness (ft) |
HMA Thickness for Layer 2 (ft) |
Cracking Reliability (%) |
Rutting Reliability (%) |
Overall Reliability (%) |
Number of Simulations in Monte Carlo Analysis |
Years to failure (years) |
|
0.70 |
0.30 |
90 |
100 |
90 |
20 |
33.2 |
|
0.75 |
0.35 |
95 |
100 |
95 |
20 |
>40 |
|
0.75 |
0.35 |
90 |
98 |
90 |
60 |
38.0 |
|
0.80 |
0.40 |
98 |
98 |
98 |
60 |
>40 |
The simulation results for the final design is shown below:
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