Jacob Uzan

Harmonized resilient modulus test method for unbound pavement materials

A new laboratory test protocol for the measurement of resilient modulus of unbound pavement materials is described. This harmonized protocol combines the best features from four state-of-the-art resilient modulus test procedures in current use. The harmonized procedure most closely follows the recommended protocol proposed in NCHRP Project 1-28, Appendix E, with some exceptions. The main modifications deal with revised and more rational stress sequences and a more accurate stress-dependent resilient modulus predictive equation. Thirteen different predictive models and 25 sets of resilient modulus test data were evaluated as the basis for the recommended predictive equation.

Characterization of clayey subgrade materials for mechanistic design of flexible pavements

The design of flexible pavements is based on the empirical approach and in some cases on the empirical-rational approach. Funds are being invested in developing new design methods that will principally be based on the mechanistic-rational approach. A procedure for characterizing clayey subgrade materials to provide the properties needed by the new methods is presented. The procedure addresses the following elements: first, the material should be brought to the condition that prevails under pavements. Sample preparation and a soaking procedure to simulate field condition are described. Second, after about 10 days of sample conditioning, the material is tested under repetitive loading to provide both resilient and permanent properties. In clayey soils, two samples are tested under different deviatoric stresses and for 10,000 to 100,000 load repetitions. The stress level should correspond to the stress induced by overburden and traffic load. Tests are conducted on a swelling clay, and the results are analyzed. It is seen that (a) the proposed procedure for sample conditioning is appropriate for Israeli climatic conditions, but it can be changed to fit other climatic conditions; (b) the stress level in subgrades is relatively low, so the behavior of the clayey materials can be fairly well described by linear elastic theory; and (c) the total deformation increases either linearly or exponentially with number of load repetitions (in a log-log scale).

PERMANENT DEFORMATION OF A GRANULAR BASE MATERIAL

An approach to the characterization of unbound pavement materials for mechanistic analysis of pavements is presented. To predict permanent deformation, it is necessary to evaluate the residual properties of the materials. A method for evaluation of both the resilient and residual properties from repeated load testing is recommended. The residual properties of the material are described by three functions: (a) the slope of the permanent strain versus number of load repetitions, S, on a loglog scale, (b) the loading stress-strain curve of the first cycle, and (c) the resilient properties of the material. The framework for evaluation of the permanent characteristics is presented. It is based on the theory of elastoplasticity. Two models developed by Vermeer and by Desai [hierarchical single-surface (HISS) model] are described. Test results conducted with a high-quality base course are presented and analyzed. It is found that (a) the slope S depends slightly on the confining pressure and (b) both models describe the dependence of total strain on stress. The HISS model with eight parameters fits the results better than the Vermeer model.

Backcalculation of layer moduli from nondestructive pavement deflection data using the expert system approach

None of the existing backcalculation procedures is guaranteed to give a reasonable, effective elastic layer modulus for every deflection basin measured. The differences of results among various backcalculation programs are magnified by the differences if distinct analysts using the same program but applying different input parameters. A more experienced analyst often obtain better results. Some of the difficulties, such as the deviation of pavement material properties from the linear elastic, isotropic, and homogenous assumptions, the uncertainty of input parameters (for example, the deflection measurement), and the limitation of basin matching techniques, may only be dealt with by informed engineering judgment. A knowledge-based expert system approach as described in this paper may be used to generate more consistent and better estimations of in situ pavement material properties by combining numerical computations with a pavement experts knowledge and judgment. A knowledge base which serves as an intelligent pre- and postprocessor to the backcalculation program is developed. The knowledge base may be revised easily as better understanding emerges.

GENERAL PROCEDURE FOR BACKCALCULATING LAYER MODULI

An efficient microcomputer program for back calculating moduli from nondestructive testing results is described. Its use is illustrated by analyzing a typical section where several deflection bowls were measured at the same location. sources of errors in the testing and backcalculating are presented and discussed. The error source is taken into consideration in the formulation of the objective function, the relative squared error between measured and computed deflections. The Hooke-Jeeves pattern search algorithm was selected for minimizing the objective function. The direct computation of the deflection at each move of the pattern search was replaced by the three-point Lagrange interpolation, using a data base generated ahead of time. The pattern search and interpolation were included in a microcomputer program named MODULUS, capable of performing and back calculation in 1 to 2 min per deflection bowl. It is highly efficient where several deflection bowls are measured along the pavement section or at the same location. The analysis of the typical section shows clearly that the measurement errors can be quite large. Averaging deflection measurements at each sensor cancels out some of the random errors and generally leads to acceptable values of the backcalculated moduli. The program MODULUS is especially suited toward these cases where multiple deflection bases are available for the same pavement geometry.

Linear and Nonlinear Backcalculation for Site 1 in Hanover, New Hampshire

Site 1 in Hanover, New Hampshire, is a test section with surface deflection, stress, and strain measurements. The data are used to perform linear and nonlinear backcalculation of pavement layer moduli and material parameters from the deflection bowls. The data show clearly that the pavement response is nonlinear and the following trends are observed: (a) the base material stiffens relative to the subgrade as the load increases and (b) the upper subgrade becomes softer as the load increases. The procedures and material characterization for linear and nonlinear analyses are presented. The nonlinear one uses an axisymmetric finite element program to compute displacements, stresses, and strains. The results of the backcalculation (using deflection bowls only) show better fit of the surface deflections with the nonlinear than with the linear approach. The linear procedure seems to introduce a systematic error in the fit. The nonlinear approach predicts a distribution of moduli in the structure that is acceptable from an engineering point of view. With the parameters obtained from backcalculation, displacements and stresses are computed and compared with the measured ones. It seems that the computed response differs from the measured one by a factor of about 1.5. The prediction of stresses and strains is slightly better with the nonlinear than with the linear procedure.