Two Layer System

The three layer system introduces an extra mix type so can potentially induce extra cost. The two layer overlay structure is the same as the three layer overlay except without the rich bottom layer.

Note: the rich bottom mix may offer other benefits that are not accounted for in CalME.

The update project input window is shown below with a 0.55 ft layer of HMA with up to 25% RAP.

Compared to the three layer plain overlay, the two layer system performance is practically the same.

The simulation parameters are shown below:

Note that the cracked layers have to be changed to "from 3 to 3". Otherwise CalME will issue a warning and no analysis is allowed because Layer 4 is now the existing AB, which can not be served as the cracked layer.

Several iterations were tried before reaching the final design, which requires 0.65 ft for the second layer and a total thickness of 0.85 ft. In this case, using the rich bottom mix can reduce the total AC thickness required by 0.05 ft. The results of the CalME analyses are summarized below in Table 3. The simulation result of the final design is shown in the figure below.

Table 3: Two-Layer Plain Overlay CalME Results