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Remove HMA and Replace
One of the alternative is to mill off the old HMA to remove the possibility of reflective cracking. Since the cracks originated from bottom, all the old HMA should be milled off. A new two layer system will then be placed on top of the exposed AB layer.
To evaluate this option, load the baseline trial for Section 3 and saved a copy as “Section 3: Remove HMA and Replace”.
The proposed design is shown in the figure below.
The project inputs are shown below with the layer 2 thickness set to 0.65 ft for one of the iterations tried.
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The simulation parameters are shown below. Note that the reflection cracking option has been deactivated.
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Several iterations were run to determine the minimum thickness of the HMA layer to carry the design traffic for the design life. The results are provided in Table 4. The results show that the thickness of the HMA should be at least 1.1 ft with a 0.20 ft HMA-PM overlay to carry the design traffic for 40 years.
Table 4: Mill and Overlay CalME Results
|
Total AC Thickness (ft) |
HMA Layer 2 Thickness (ft) |
Cracking Reliability (%) |
Rutting Reliability (%) |
Overall Reliability (%) |
Number of Simulations in Monte Carlo Analysis |
Years to failure (years) |
|
0.85 |
0.65 |
50 |
0 |
0 |
20 |
2.6 |
|
1.2 |
1.0 |
100 |
85 |
85 |
20 |
29.7 |
|
1.3 |
1.1 |
100 |
95 |
95 |
20 |
>40 |
|
1.3 |
1.1 |
100 |
98 |
98 |
60 |
>40 |
The simulation result for the final design is shown in the following figure:
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Table 4 suggests that the controlling failure mode is rutting. The layer compression summary is shown below, which suggests that the rutting mostly comes from the HMA-PM layer and the subgrade. Given the specific climate zone (High Desert), there is no other option for the surface mix. However, this suggests that strengthening the subgrade may be helpful.
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