One of the primary objectives of the Strategic Highway Research Program (SHRP) Long-Term Pavement Performance (LTPP) studies is to improve the ability of highway engineers to design new and overlaid pavement structures. The specific analyses discussed in this report were aimed at evaluation of the American Association of State Highway and Transportation Officials (AASHTO) pavement design equations (for both new and overlaid pavements), based on data currently available. These initial analyses confirm that although improvements have been made to the AASHTO design equations over the years, the equations still do not fully explain data from North American pavements. The evaluation of the 1993 AASHTO overlay equation was severly limited by the lack of (1) distress and serviceability data prior to overlay, (2) knowledge as to design period and liability level sought, and (3) some of the input data requirewd for the design equation (which had to approximate required data based on other available data). The evaluations were inconclusive for asphalt concrete (AC) overlays of AC and unbonded portland cement concrete (PCC) overlays of PCC. For the five test sections used in the evaluation, however, the design equation for AC overlays of PCC appeared to work reasonable well. These analyses were accomplished on the LTPP data collected through December 1992. (A)
Abstract