# Why is the JPCP CTE default value in Pavement ME 4.9 when the ME Pavement Design Guide Manual of Practice recommends 5.5 for unknown?

The default value for JPCP CTE in v2.5.5 (and v2.3) is 4.9. However, per Table 10-5 on page 152 of the ME Pavement Design Guide Manual of Practice recommends 5.5 if unknown. Moreover, there is only one aggregate type with recommended CTE below 5.0, whereas most common aggregates are at least 5.2 or higher. So how could it even be that on average, the global calibration used 4.9 or that the default should be 4.9? Why was 4.9 selected?

The JPCP CTE values in the MOP seems to reflect the old default CTE values based on the AASHTO TP 60-00 test method. An error was found in the testing procedure which was corrected using the AASHTO T 336 test method. The LTPP standard data release 25 included the CTE values obtained from the AASHTO T336 test procedure and the rigid pavement performance prediction models were recalibrated using the corrected CTE values.

The default value of $$5.5\times10^{-6}$$ $$^{\circ}F$$ in the 2nd and 3rd edition of the Manual of Practice (MOP) is based on the old CTE values used in the NCHRP 1-40D rigid calibration. The table below shows the CTE values presented in the 2nd edition of the MOP, the values used in the NCHRP 1-40D and NCHRP 20-07 calibration efforts. The average values for each column is also shown. The difference between the old and new CTE values is approximately $$0.8\times10^{-6}$$ $$^{\circ}F$$.

I also found an FHWA techbrief summarizing the changes to the CTE testing procedure and the new average CTE values by aggregate type. The CTE values shown in Table 1 of the techbrief are similar to the table shown below.

+-------------------------+-----+-------------+----------------------+
| Aggregate Class or Type | MOP | NCHRP 1-40D | NCHRP 20-07 Task 327 |
+-------------------------+-----+-------------+----------------------+
| Andesite                | 5.3 |         5.3 |                  4.4 |
| Basalt                  | 5.2 |         5.2 |                  4.4 |
| Diabase                 | 4.6 |         5.2 |                  4.6 |
| Gabbro                  | 5.3 |             |                      |
| Granite                 | 5.8 |         5.8 |                  4.9 |
| Schist                  | 5.6 |         5.6 |                  4.7 |
| Chert                   |     |         6.6 |                  5.9 |
| Dolomite                | 5.8 |         5.8 |                  4.9 |
| Limestone               | 5.4 |         5.4 |                  4.4 |
| Quartzite               | 6.2 |         6.2 |                  5.3 |
| Sandstone               | 6.1 |         6.1 |                  5.2 |
| Expanded shale          | 5.7 |             |                      |
+-------------------------+-----+-------------+----------------------+
| Average                 | 5.5 |         5.7 |                  4.9 |
+-------------------------+-----+-------------+----------------------+