Permanent deformation and resilient modulus of unbound granular materials

Permanent strain and resilient modulus provide direct, quantifiable values that describe two types of deformation (irrecoverable and recoverable) of pavement foundation materials under repeated transient traffic loads. Although the permanent deformation is not directly used in pavement design calculation, it affects the long-term performances of pavement foundations. The resilient moduli are used in AASHTO 1993 pavement design and the current Mechanistic-Empirical Pavement Design Guide. This study investigated permanent deformation and resilient modulus characteristics of unbound granular materials in relation with relative densities, fines contents, material types, stress levels, and number of load applications. Materials tested in this study included crushed limestone and recycled aggregate materials (recycled asphalt pavement and recycled portland cement concrete). Laboratory prepared samples were tested to assess the influence of different conditions varied in situ (e.g., relative density and fines content). Varied stress levels and number of load cycles were applied to samples to assess their influence on different material types. Results from this investigation demonstrated that 1) accumulation of permanent deformations increased with deviator stress; 2) higher fines contents (12.4% 12.6%) result in lower permanent deformations than lower fines contents (0.8% 2.2%) for the materials with laboratory reconstituted fines contents; 3) relative densities in the range of 85% to 95% does not significantly affect the permanent deformation behavior for the materials tested in this study at low deviator stress (e.g., 68.9 kPa); 4) the crushed limestone material that was tested in this study has higher resistance to permanent deformation and higher resilient moduli compared to the recycled materials that were tested in this study; 5) two recycled materials generally have the similar resilient modulus values; 6) stress levels (i.e., confining pressure and deviator stress) significantly affect the resilient modulus values; and 7) relative densities and fines contents affect the resilient moduli but they are not clearly related to the resilient modulus values of the materials tested in this study. Due to the complexity of the test method used in this study, measurements errors related to data sampling, tests apparatus setup, and the analysis methods were studied. The analyses demonstrated that 200 readings which is the minimum value specified in AASHTO T307 99 are not sufficient in tracking the loading history and the selection of data points affects the calculated resilient modulus value at each load sequence in the resilient modulus tests