Bruce Steven

Relating Laboratory Foamed Asphalt Mix Resilient Modulus Tests to Field Measurements

ABSTRACT Resilient modulus of foamed asphalt materials is stress state dependent, but the field stress state cannot be fully characterized by any available laboratory test alone. Test results of the triaxial resilient modulus test and the flexural beam test, each of which characterizes one important stress state, were incorporated into a bilinear anisotropic constitutive model to calculate the pavement deflection under traffic loading. A virtual FWD back-calculation procedure was used to find the equivalent resilient modulus, which can be readily used as the input for most pavement response calculation programs for M-E design. FWD tests on four California FDR-foamed asphalt sections were analyzed, and the effects of temperature and moisture were quantified. Finally the field measurements were compared to the model predictions and recommendations for project level design were proposed.

Three-Dimensional Nonlinear Finite Element Model for Simulating Pavement Response: Study at Canterbury Accelerated Pavement Testing Indoor Facility, New Zealand

A three-dimensional nonlinear finite element model (3D-FEM) was developed as part of a study of the effect of increasing axle load and tire pressure on pavement deterioration. The measured strains, interface stresses, and deflections were collected from the instrumented Canterbury Accelerated Pavement Testing Indoor Facility in New Zealand. In addition, two multilayer elastic models were used to compare the values from the finite element simulation and the actual measurements. The first elastic multilayer model was developed with ELSYM5 software, and the second model was developed with CIRCLY software. CIRCLY differs from ELSYM5 in the ability to account for material anisotropy; ELSYM5 considers the pavement materials to be isotropic. The actual strains and deformations were measured by Emu strain gauges embedded at different depths in the base and subgrade materials. Both the unbound granular base and the subgrade materials were modeled in 3D-FEM as elastic plastic materials. The results showed that for the unbound base layer, the strains calculated from the two elastic models were in reasonable agreement with the values measured in the instrumented test track, while the 3D-FEM model tended to overestimate the strains at the bottom of the base. While none of the models provided a perfect fit to the measured strains in the subgrade layer because the subgrade is less homogenous than assumed, 3D-FEM provided the closest fit. Also, CIRCLY provided better results than ELSYM5, which underestimated the displacement values compared with values obtained with CIRCLY and 3D-FEM.

The influence of vehicle dynamics on pavement life

Element 1 of the OECD DIVINE international coordinated research programme was an accelerated pavement test undertaken at the Canterbury Accelerated Pavement Testing Indoor Facility (CAPTIF) in Christchurch, New Zealand in which parallel wheel paths on the same pavement were subjected to vehicle loads which were identical statically, but very different dynamically. Throughout the test extensive measurements were conducted on both the pavement and the vehicles. These included pavement material properties, the dynamic wheel forces applied, the longitudinal and transverse pavement profiles, tensile strains in the asphalt layer, vertical strains in the basecourse and subgrade layers, pavement deflections and cracking. In this paper, the changes in these various measured parameters throughout the test are analysed and presented. The correlation of dynamic loading patterns and construction variability with changes in profile, rutting and cracking are investigated. The effect of dynamic loading on the development of pavement distress and ultimately pavement life is estimated.

Elastic Nonlinear Finite Element Analysis of a Flexible Pavement Subjected to Varying Falling Weight Deflectometer Loads

A nonlinear finite element method (FEM) model has been developed to model thin-surfaced, unbound granular pavements. The FEM model incorporates a nonlinear anisotropic material model for the granular material and a nonlinear material model for the subgrade. The coefficients for the material models were determined from laboratory repeated load triaxial tests. A test pavement at the Canterbury Accelerated Pavement Testing Indoor Facility (CAPTIF) was instrumented with an inductive coil soil strain system to measure vertical compressive strains and pressure cells to measure the vertical compressive stresses. The test pavement was subjected to loading by a falling weight deflectometer (FWD) device at four different load levels. The initial FEM model configuration was undertaken at one load level using the surface deflection measurements from the FWD; however, there were significant differences between the computed and measured stress and strain values. The scalar coefficients for the granular and subgrade materials were adjusted so that the measured and computed stress and strain values matched within a reasonable tolerance (10%). After the calibration process was finished, it was found that the computed and measured surface deflections also matched. The model was rerun with the three remaining load cases; the computed and measured stress, strain, and surface deflection values were in close agreement. The range of computed stiffness values varied with both the pavement and load level. The results from this analysis showed the need to use a full nonlinear model to obtain realistic results when FWD data are modeled.

Comprehensive Laboratory Testing and Performance Evaluation of Recycled Pulverized Hot-Mix Asphalt Material

The California Department of Transportation is studying a pavement rehabilitation strategy that involves the pulverization of failed hot-mix asphalt (HMA) pavements and uses the pulverized material as a granular base material in the reconstruction of the pavement. Advantages of this technique include a reduction in the use of virgin aggregates, a reduction in the amount of construction traffic, and removal of the potential for reflective cracking from the existing pavement layers. The construction and performance of four pilot projects in District 2 in northeastern California were used to evaluate this rehabilitation strategy. The characteristics and performance of the pulverized material were evaluated by comprehensive laboratory and field testing. The laboratory testing covered basic index tests and comprehensive triaxial testing. Multistage repeated-load permanent strain triaxial tests on different material gradations, stress levels, moisture contents, with and without stabilizing agents (lime and cement), and the degree of compaction were used to investigate the resilient and permanent strain characteristics. Testing was conducted on saturated samples to evaluate the effect of saturation on the pulverized material. The effects of the number of load repetitions were also investigated. The performance of the pulverized material was also compared with two typical virgin aggregate base materials from California. A life cycle cost analysis of this rehabilitation technique when compared to a typical HMA overlay showed that the recycling option was cheaper in both the short term and over the life of the pavement.