This research effort, conducted at the Illinois Center for
Transportation (ICT), evaluated the rutting potentials of unbound aggregate
materials commonly used in the state of North Carolina (NC) for pavement
subbase and base applications. Tests were conducted on different crushed
aggregate materials at quarry source gradations to determine moisture-density,
resilient modulus, shear strength and permanent deformation responses, and
predict field rutting performances of base courses constructed with these
materials. This study serves as a continuation of the Phase I study, which
tested sixteen NC aggregate materials at one engineered gradation, and a pilot
Phase II study, that tested four of those sixteen materials at the source
gradations.
The Phase I study successfully developed an improved rutting model
(known as the University of Illinois at Urbana-Champaign rutting model or UIUC
rutting model). The ability of the UIUC rutting model to accurately predict
permanent strain accumulation at different gradations was investigated by
evaluating the rutting performance of fifteen of the original materials at
their source gradations. To accomplish the overall objective of re-evaluating
the performance of the UIUC rutting model at different gradations, this study
focused on: (1) performing modified Proctor type moisture-density and resilient
modulus tests to establish maximum dry densities and optimum moisture contents
as well as the resilient modulus response characterization, (2) conducting a
full suite of shear strength and permanent deformation characterizations to
determine the permanent deformation trends influenced by aggregate material
properties, shear strength, applied stress states and stress to strength
ratios, and (3) developing the UIUC rutting damage model for all the aggregate
materials tested at both the engineered and source gradations.
The final product of this project was a
materials testing and characterization procedure to account for gradation and
aggregate property effects in assigning the UIUC rutting damage model
parameters in order to predict realistic rutting potentials of base course
aggregate materials in NC. A comprehensive database was established for all 16
NC aggregate materials characterized at both original source and engineered
gradations. The use of forced regression, to force the model parameters within
specified pre-determined ranges, resulted in reasonable predictions of
permanent strains for the aggregate materials at different gradations, while producing
reasonably controlled values of the model parameters. Next, a stepwise
regression approach was used to identify the most significant gradation and
material properties which influenced the values of the UIUC rutting model
parameters. The model parameters were then expressed as functions of these
material properties. Finally, a practical design approach was recommended for
the improved predictions of field pavement aggregate base rutting potentials
using the UIUC rutting model.
