Effect of Design and Site Factors on Structural Rutting ofFlexible Pavements in the LTPP SPS-1 Experiment (With Discussion)

The research described herein was conducted as a part of NCHRP Project 20-50 (10/16), LTPP Data Analysis: Influence of Design and Construction Features on the Response and Performance of New Flexible and Rigid Pavements. While there is considerable research on factors affecting flexible pavement rutting, they are limited in scope because of lack of field validation and no consideration for interactions of structural and site factors. This research addresses the relative influence of design and site factors on structural rutting of in-service flexible pavements. The data from the SPS-1 experiment of the LTPP program were used. This experiment was designed to investigate the effects of asphalt surface layer thickness, base type, base thickness, and drainage on performance of flexible pavements constructed in different site conditions (subgrade type and climate). Since there has been no comprehensive study of the SPS-1 experimental data, a thorough methodology involving analysis of variance (ANOVA), logistic regression and discriminant analysis was developed. Although most of the findings from the analysis of the SPS-1 experiment data support the existing understanding of pavement rutting performance, the results from this study provide an outline of the interactions between design and site factors. The results showed that while pavement structural and asphalt mix designs are crucial, pavement construction quality plays a vital role in achieving better rutting performance for flexible pavements. This was clearly apparent in the SPS-1 experiment where 56 out of 212 sections had construction-related problems leading to premature rutting failures. Among the sections that did not show premature rutting, the effects of design and site factors on structural rutting were marginal. This is mainly due to the fact that these sections are still fairly young. In general, pavements built on fine-grained soils have shown more rutting. Also, sections in warmer climates have shown slightly more rutting. Also, in general, regardless of site conditions, providing a thicker surface layer or an asphalt treated base layer will improve the structural rutting performance provided the materials are handled appropriately in design and during construction. Performance will be further enhanced if in-pavement drainage is provided, especially when the pavement structure is to be constructed on fine-grained soils. Also, analysis of transverse surface profiles showed that most of the rutting in the test sections is confined to the upper layers of the pavement structure. Therefore, using the mechanistic-empirical prediction models that incorporate the contribution of each pavement layer to surface rutting is a better approach to characterize the rutting mechanism in flexible pavements.