CAL/APT Program--Comparison of Caltrans and AASHTO Pavement Design Methods

This report compares the Caltrans and AASHTO pavement thickness design procedures. The design comparisons include pavement structures subjected to a range in traffic, as represented by Traffic Indexes of 7, 9, 11, 13, and 15, and a range in subgrade strengths, as measured by subgrade R-values of 5, 20, and 40. This report has four objectives: 1. Quantify the differences in pavement thickness resulting from use of the two methods. 2. Examine differences in predicted pavement performance for pavement designs considered equal within the Caltrans method. Related to this objective is the examination of the Gravel Factors for aggregate base and asphalt concrete. 3. Evaluate the effect of assumed drainage conditions on the pavement structures designed using the AASHTO method and relate this effect to the Caltrans method. 4. Demonstrate the flexibility of the mechanistic-empirical design procedure developed as part of the CAL/APT program to quantitatively, systematically, and rationally permit pavement designers to evaluate the performance of different pavement structures and different materials. This report illustrates that the AASHTO and Caltrans pavement thickness design procedures do not produce the same pavement structures for the same given inputs. The design procedures are based on different material properties determined in the laboratory: The Caltrans procedure uses the R-value test and AASHTO uses the resilient modulus test (MR). Generally, the pavement structures designed by the Caltrans procedure are thicker than those designed by the AASHTO procedure. This increase in thickness results in improved fatigue performance for the pavement designed according to the Caltrans procedure. The fatigue performance of the 2 pavements is extremely sensitive to the asphalt concrete thickness. In this report, it is shown that due to the differences between the design procedures they should not be used interchangeably. It is also shown that for the subbase, the procedures are sensitive to the conversion from one type of laboratory test to another.