Permanent deformation prediction model of unbound granular materials for flexible pavement design

Constitutive prediction models in flexible pavement are used to predict the permanent deformation (PD) of the unbound granular materials (UGMs). The most recent flexible pavement structural design guide in Australia does not consider the response of the UGMs layers as a design criterion. Additionally, it referred to the absence of an appropriate prediction model. The available prediction models have complex equations with many variables; the use of some of which has been found to be unsuitable in Australia. Thus, the aim of this study was to develop a new simple constitutive model based on an empirical regression equation, which has two predictor variables and four regression parameters. Repeated load triaxial test data covering many loading cycles and stages of stresses were used in order to validate the proposed model by calibration. The tested materials, specified by VicRoads as high-quality base and subbase crushed rocks, were used to prepare 46 samples with a range of moisture contents, gradations, densities, and stress conditions. Additional validation was conducted by predicting the PD response of recycled crushed concrete for different stress conditions. The proposed model excellently matches the experimental PDs with the sum of squared error less than 0.060 and R2 more than 0.96 over the tested matrix for both the crushed rocks and recycled concrete. This high prediction accuracy could lead to a better design of pavement materials and their thicknesses. A new update is proposed for the current Austroads flexible pavements design procedure by involving two more critical locations as performance criteria in the base and subbase layers based on the developed constitutive model. Adopting the proposed model can better generalise the design procedure to determine the accumulated vertical PD for the whole layer system.