Application of mechanistic empirical approach to predict rutting of superpave mixtures in Iraq

Qasim, Zaynab; Hamdou, Hamed; Alkawaaz, Namir · 2018 · Crossref

DOI: 10.1051/matecconf/201816201040

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Summary

This study addresses the critical issue of rutting in flexible pavements in Iraq, a distress exacerbated by high summer temperatures and increased axle loads. Rutting significantly reduces pavement service life and creates safety hazards. The research aims to predict the performance of Hot Mix Asphalt (HMA) mixtures against permanent deformation using a Mechanistic-Empirical (M-E) approach. The study integrates laboratory testing with computational modeling to develop predictive models for permanent strain and to validate these findings against the MnPAVE software, which simulates field loading conditions. The experimental design utilized local Iraqi materials, including asphalt binders (PG 64-16 and PG 58-22), aggregates, and Styrene Butadiene Rubber (SBR) additives. Researchers prepared 24 cylindrical specimens using a Superpave Gyratory Compactor for volumetric design and 108 slab specimens using a locally manufactured Roller Wheel Compactor for performance testing. These slabs were subjected to Wheel-Tracking Tests (WTT) following a full factorial design that varied asphalt content, binder type, air voids, temperature, and SBR additive percentage. Statistical analysis using SPSS software was employed to develop regression models correlating permanent strain with independent variables such as load cycles, temperature, and material properties. Additionally, the MnPAVE 2011 software was used to characterize rutting and predict allowable loading repetitions based on traffic, climate, and material inputs. The results indicate that permanent deformation is highly dependent on temperature, binder performance grade, and air voids, with moderate influence from polymer and asphalt content. Statistical models were developed for rut depth, permanent strain, and allowable repetitions, showing high correlation coefficients ($R^2$ values of 0.88–0.89). Analysis via MnPAVE revealed that increasing air voids from 4% to 7% decreased the dynamic modulus ($E^*$) by 11.2% and reliability by 12% for PG 64-16 binder. Switching from PG 64-16 to PG 58-22 binder resulted in a 5.5% decrease in $E^*$ and a 20% decrease in reliability at 40°C. Increasing asphalt content also reduced $E^*$, with decreases of 4.9% and 5.5% observed at 40°C and 70°C, respectively. A comparison between the laboratory-derived WTT models and MnPAVE outputs demonstrated a strong agreement, validating the mechanistic approach. The study concludes that the Mechanistic-Empirical approach, supported by MnPAVE software, effectively predicts rutting performance and allowable loading repetitions for local asphalt mixtures. The findings highlight the importance of controlling air voids and selecting appropriate binder grades to enhance pavement durability. By integrating laboratory statistical models with mechanistic software, the research provides a reliable framework for pavement design in Iraq, allowing engineers to optimize material selection and mix design to mitigate rutting under specific environmental and traffic conditions.

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• road surface friction